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date: 21 February 2018

Nonassociative Learning in Invertebrates

Abstract and Keywords

Nonassociative learning has been extensively studied in many invertebrate species for several decades. Habituation and sensitization are ubiquitously observed in different species, and observations of these behavioral plasticities have greatly contributed to the theoretical work to characterize nonassociative learning. Here, we review the rich body of literature in invertebrate nonassociative learning research. Investigations of the underpinnings of nonassociative learning in a variety of invertebrate species have demonstrated that conserved neural and molecular mechanisms are common despite the diversity of phylogeny. Nonassociative learning appears to be biologically essential and evolutionarily adaptive. Mechanisms of learning and memory uncovered in the studies of nonassociative learning in invertebrate species reviewed here have also been shown to be conserved in vertebrate systems and also play important roles in more complex forms of learning.

Keywords: nonassociative learning, habituation, sensitization, invertebrate, behavior, mechanism

Theorists characterize animal learning into two categories: associative learning and nonassociative learning. Associative learning, including classical conditioning (Pavlovian conditioning) and operant conditioning, refers to learning that involves establishing associations between stimuli (sensory experiences) and/or stimuli and responses. Nonassociative learning, on the other hand, does not require associations to be established: Animals learn to modulate their responses when single stimuli are presented alone or repeatedly.

Nonassociative learning has traditionally been divided into two forms: habituation and sensitization. Habituation is the decrement in response resulting from repeated presentation of a stimulus; sensitization is the increase in the magnitude and/or the likelihood of responding following the presentation of a strong or novel stimulus. The focus of this chapter is nonassociative learning in invertebrates.

Habituation

Observations of habituation have been documented for over 120 years. Simply termed “learning,” the first published behavioral account of habituation was described by Peckham and Peckham (1887), who showed that a spider would move a progressively smaller distance along its web in response to the sound of a tuning fork that was repeatedly presented. Habituation was first reviewed by Harris in 1943 and then the common behavioral characteristics were described by Thompson and Spencer (1966). It is defined as a form of nonassociative learning in which animals show decreased response to repeatedly presented stimuli. This decrement is not caused by fatigue or sensory adaptation, but by learning. Animals are constantly bombarded with various stimuli in the environment, and they cannot equally attend to all sensory inputs; thus, habituation is thought to help them selectively attribute their attention, a limited resource. Habituation is widely studied in humans and many animal species, including a variety of invertebrates.

In 2009, a group of habituation researchers revisited the nine classic behavioral characteristics of habituation proposed by Thompson and Spencer (1966), and they enriched the content with additional observational and experimental evidence. The nine characteristics were revised and a tenth one was added. The 10 characteristics of habituation are summarized as follows:

  1. 1. Habituation: Repeated application of a stimulus results in a progressive decrease in one or more parameters of a response to an asymptotic level.

  2. 2. Spontaneous recovery: if stimulation is stopped the response recovers at least partially.

  3. 3. Potentiation of habituation: repeated blocks of habituation training and spontaneous recovery leads to progressively deeper and more rapid habituation.

  4. 4. The interstimulus interval (ISI) effect: with the same stimulus properties high frequency stimulation leads to more rapid and/or more pronounced response decrement, and more rapid spontaneous recovery.

  5. 5. The intensity of stimulus effect: the less intense the stimulus, the more rapid and/or more pronounced the behavioral response decrement. Very intense stimuli may show no response decrement.

  6. 6. Below-zero habituation: Even after the response has decremented to no response or to asymptotic level, the effects of stimulation may continue to accumulate.

  7. 7. Stimulus specificity: the response decrement shows some stimulus specificity.

  8. 8. Dishabituation: if a novel or noxious stimulus is inserted into a series of habituating stimuli there is an increase of the decremented response to the original stimulus.

  9. 9. Habituation of dishabituation: if a response is repeatedly habituated and dishabituated, the magnitude of the dishabituation decreases.

  10. 10. Long-term habituation: with extended or patterned training memory for the response decrement can last for hours, days, or weeks.

In this section, we will review studies on habituation in invertebrates and categorize them by phylogeny. It should be pointed out that because different species are able to perform different types of behavior, and experimenters are limited by experimental designs and equipment, not all 10 characteristics of habituation are always observed or tested.

Cnidaria

There are over 10,000 Cnidarian species, and a large proportion of them lives in marine environments. Cnidarians appear in either one of two forms of living: free-swimming medusae or sessile polyps. Cnidarians have a radially symmetrical body plan; thus, they lack cephalization and a centralized nervous system. To support this type of body plan, Cnidarians have a ring-shaped “nerve net.” This nerve net is able to execute sensory and motor functions, such as detecting light and vibration in the environment, and producing muscle contractions for feeding and locomotion.

In the laboratory, habituation has been studied in the sessile polyp stage, mainly because of the difficulties with tracking moving transparent soft-bodied animals. Contraction is often investigated because it appears to be a robust and universal defensive behavior in Cnidarians. The polyps of Hydra pirardi contract their bodies in response to nonlocalized mechanical stimulation (e.g., shaking; Rushforth et al., 1963). The rate of responding (defined as the proportion of animals producing responses to a stimulus) decreases to an asymptotic level after animals are repeatedly stimulated for 8 hours. Hydra are photosensitive and they also respond to light stimulation by contracting. Animals habituated to mechanical stimulation still readily contract to light stimulation, ruling out the possibility of fatigue. Thus, the stimulus specificity of habituation is in line with the parametric characteristics of habituation.

Habituation has also been examined in the sea anemone Anthopleura elegantissima. A. elegantissima responds to a stream of fresh water by contracting its oral disk, and the change in the diameter of the disk is measured as the magnitude of response. A. elegantissima clearly showed characteristics of habituation in a three-phase experiment (Logan, 1975). In phase 1, the intensity of the stimulus (duration of exposure to freshwater stream) was manipulated, and responses reached different asymptotic levels that were positively correlated with stimulus intensity. In phase 2, when these previously habituated animals were retrained 2 hours later, their initial response had spontaneously recovered to the original level; however, they habituated to the same asymptotic level with fewer trials. In phase 3, animals habituated to a water stream were stroked with direct tactile stimulation to the oral disk, and virtually all of them responded with significantly larger magnitude responses, thus eliminating fatigue as the cause of the decrement. In a similar study, A. elegantissima also displayed retention of habituation up to 72 hours (Logan & Beck, 1978).

Habituation has also been demonstrated in the polyps of the jellyfish Aurelia aurita (Johnson & Wuensch, 1994). Animals were trained with a probe (defined as gentle touch to the base of tentacles) and the effects of ISI in habituation were examined. A 2-min ISI resulted in a lower asymptotic level and faster rate of habituation of responding probability than a 6-minute ISI. Using a water stream or a mechanical shake as a novel dishabituating stimulus, the habituated response increased in probability, indicating that fatigue was not preventing animals from responding.

The take-home message from studies in Cnidaria is that habituation learning can occur in the absence of a central nervous system to modulate behavior. The observed characteristics of habituation in these invertebrates conform to the parametric characteristics of habituation.

Platyhelminthes

Platyhelminthes (flatworms) is a phylum classified as the “lowest” bilateria—animals that are bilaterally symmetrical. In this phylum one can find the most primitive organ systems including a centralized nervous system, or a brain. The plexiform nervous system of Platyhelminthes consists of a pair of ganglia in the anterior end and a pair of nerve cords running through the body with ladder-like lateral connections.

Over a half of Platyhelminthes species are parasitic, rendering them hard to culture in the laboratory. Habituation has been studied in a few free-living planarian species. Some Platyhelminthes are able to survive harsh preparations such as severance of body and dissection. Studies have taken advantage of this feature and explored the neurobiology of habituation.

Habituation in Platyhelminthes was first reported by Applewhite and Morowitz (1966). The flatworm Stenostomum responds to both mechanical and electrical stimuli by contracting its body, and the magnitude of the response decreases with repeated stimulation. However, Stenostomum that were habituated to either mechanical or electrical stimuli still responded to the other stimulus as well as naïve animals, and the number of stimuli required for the animal to habituate to the second stimulus was unchanged (Applewhite, 1971). If an animal habituated to stimuli applied to the anterior body, a fewer number of stimuli were required for the posterior part to habituate, and vice versa. If a habituated flatworm was cut in half, both halves required significantly fewer number of stimuli to habituate; two halves of an untrained flatworm required the same number of stimuli to habituate as a naïve whole organism. Further dissection to remove the brain of the flatworm revealed no difference in the number of stimuli required to habituate compared to an intact animal. Taken together, these findings suggest that in different sensory modalities, habituation is governed by different underlying neural circuits; within the same modality, the neural representation of habituation appears to be diffusely distributed in the nervous system.

To further investigate the cellular activity in the brain of a habituated planarian, Koopowitz performed a series of electrophysiology experiments in Freemania litoricola (1975a, b). He found that the number of evoked spikes recorded from the brain decreased in a manner correlated with the decreased behavioral output in response to repeated touch to the margin. Similarly, using air puffs to introduce vibrations in the bathing medium, evoked activity could be recorded from anterior and posterior regions of the brain in a dissected preparation. With repeated administration of air puffs, the number of spikes decreased, and the rate of decrement was positively correlated with the intensity of the stimulus (i.e., volume of air puffs) (1975b). The habituated response to vibration could be dishabituated by either a more intense vibratory stimulus or a tactile stimulus, as reflected by the observation that the activity evoked by the succeeding stimulus contains more spikes than the habituated neural response (1975b). Intriguingly, habituation was absent in preparations in which Mg2+ was added to the bathing medium, suggesting that the mechanisms underlying habituation involve synaptic activity that may be disrupted by Mg2+.

Studies in Platyhelminthes suggest that habituation can occur at both central and peripheral sites in the nervous system. The neural representation of habituation can diffuse across the whole organism, and different sensory modalities habituate independently of one another. The effect of habituation appears to be specific to sensory modality, but generalizable to other parts of the body within the modality.

Nematoda

Nematoda is a greatly diverse group of invertebrates: They can be found in every niche on the planet. A large number of nematode species are extremophilic or parasitic, making many species difficult to study in the laboratory. Among all nematodes, the free-living species Caenorhabditis elegans is the most extensively studied. It was originally chosen as a model to study genetics and development by Sydney Brenner (1974), and throughout the years, many discoveries and breakthroughs have been made in this animal. The cell lineage of the organism is invariant (Sulston et al., 1983), and the synaptic connections between its 302 identified neurons are largely conserved (White et al., 1986). Together with its fully sequenced genome, C. elegans has become the most understood multicellular organism. Because of its determinant development, the question was then raised: Is this “hard-wired” organism capable of learning?

Rankin, Beck, and Chiba (1990) were the first to characterize learning, in the form of habituation, in C. elegans. Worms respond to nonlocalized mechanical taps to the side of a Petri dish by moving backward. With repeated stimulation, the reversal distance decreases to an asymptotic level. This decremented response spontaneously recovers to initial level when further stimulation is withheld (Rankin & Broster, 1992), and applying an electrical shock to the substrate can dishabituate the response (Rankin et al., 1990). With a series of follow-up studies, the effects of ISI, stimulus intensity, and long-term habituation were investigated. Worms habituated faster at a 10-second ISI compared to at a 60-second ISI (Rankin & Broster, 1992), and weaker stimuli increase the rate and magnitude of habituation (Timbers et al., 2013). With worms trained on a distributed training paradigm or with a contextual reminder, retention of habituation up to 48 hours is observed (Beck & Rankin, 1995; Rose & Rankin, 2006; Lau et al., 2013). Tap habituation is specific to mechanosensory modality since a thermal stimulus still elicits a robust reversal response (Wicks & Rankin, 1997). All these observations match the parametric characteristics of habituation described in Thompson and Spencer (1966) and Rankin et al. (2009).

With the fully annotated wiring diagram of C. elegans nervous system, researchers can isolate behavioral components and investigate the possible loci of habituation with laser and genetic ablation of neurons. The tap response is mediated by two competing subcircuits: One consists of three mechanosensory neurons (two ALMs and AVM) in the anterior part of the body, and the other contains two mechanosensory neurons (PLMs) in the posterior. In both subcircuits, sensory neurons connect to a network of premotor interneurons, including AVA and AVB, the “pattern generators” corresponding to backward and forward locomotion, through electrical and chemical synapses, to generate responses to mechanical stimuli (Chalfie et al., 1985; Wicks & Rankin, 1995). Laser ablation of mechanosensory neurons in forward and backward movement circuits revealed that responses mediated by the two circuits habituated at different rates (Wicks & Rankin, 1996). Wicks and Rankin (1997) showed that tap habituation training did not change spontaneous reversal or reversal to thermal stimuli that are mediated by the same interneurons; ablating mechanosensory neurons does not alter spontaneous or temperature-evoked reversals either. These findings suggest that the possible loci of tap habituation are within the mechanosensory neurons and/or the synapses between touch neurons and command interneurons, as the synaptic changes associated with tap habituation should be downstream of touch neurons but upstream of interneurons.

The advanced genetic manipulations in C. elegans also allow one to uncover the molecular underpinnings of habituation. Through mutant analysis, glutamate is implicated in short-term tap habituation, as mutant worms missing a vesicular glutamate transporter EAT-4 are not able to maintain their response, showing rapid decrement and slow spontaneous recovery from habituation. Short-term habituation can also be modulated by dopamine as mutations in genes involved in dopamine neurotransmission lead to altered habituation depending on whether worms are in the presence or absence of food (bacteria; Kindt et al., 2007). Tap habituation also shows intermediate and long-term memory depending on the training protocol and time of testing. Intermediate-term memory is seen 12 hours after massed training and is mediated by the FMRFamide-related neuropeptide FLP-20 released by the mechanosensory neurons (Li et al., 2013). Protein synthesis–dependent long-term memory is mediated by CREB and by a downregulation in the number of postsynaptic glutamate receptors on premotor interneurons (Rankin & Wicks, 2000; Rose et al., 2003; Rose & Rankin, 2006; Timbers & Rankin, 2011).

With powerful research tools, nematode habituation studies provide insight into habituation at both anatomical and molecular levels. Habituation appears to be mediated by changes in sensory neuron excitability and/or changes in the synapses between sensory neurons and interneurons, and many signaling molecules are responsible for the behavioral plasticity.

Annelida

Members in the phylum Annelida are defined by their segmented bodies: Each segment contains an independent set of organs. The nervous system of annelids is typically composed of a nerve ring and a pair of ventral nerve cords. The nerve ring is located in the anterior segments of the animal, and in some species, the division of forebrain, midbrain, and hindbrain is obvious. The ventral nerve cords innervate each segment along the body, with a pair of ganglia in each segment. Sensory organs that detect light and mechanical force can be found throughout the body, allowing animals to respond to a wide range of stimuli.

Habituation in different sensory modalities has been studied in the soil of free-living oligochaete Lumbricus. Ratner and Stein (1965) reported that Lumbricus terrestris, the common earthworm, reduced its withdrawal response frequency to light at a 6-second ISI but not at an 88-second ISI, showing that the behavioral plasticity occurs in an ISI-dependent manner (the authors labeled this behavioral plasticity as adaptation at the time). Later, Gardner (1968) investigated habituation to vibration in L. terrestris. A vibratory stimulus to the body produces a dual-component response: rapid contraction of the anterior portion and ventral hooking in the posterior portion. Repeated stimulation caused the animal to habituate, and interestingly, these two components of the response appeared to habituate at different rates. Retention of habituation persisted at least 96 hours, as rehabituation in previously habituated animals was significantly faster. However, overtraining the animal after it appeared to habituate to the stimulus did not accelerate rehabituation 24 hours later, either because this time window was long enough to allow spontaneous recovery to occur even with overtraining, or below-zero habituation does not apply to all invertebrates. Worms also respond to an air puff with progressively smaller likelihoods and magnitudes when repeatedly stimulated (Ratner & Gilpin, 1974). At a 30-second ISI, subjects eventually decreased the likelihood of responding to zero, and fewer trials were required for worms to reach the same habituation level when trained 24 and 48 hours later, demonstrating potentiation of habituation. In addition, it took significantly more trials for animals to cease responding to stimulation at a 10-second ISI than that at a 60-second ISI. If the suprapharyngeal ganglion was ablated, the number of trials for worms to habituate remained unchanged compared to control animals.

Habituation has also been explored in marine polychaete species. The bristle worm Nereis diversicolor responds to sudden increase or decrease in illumination by body contraction. When such stimuli were repeatedly presented, not only did the proportion of animals responding drop, the length of contraction also decreased, until both reached their asymptotic levels (Evans, 1969a). In addition to body contraction, N. diversicolor also displays a second behavioral component, inhibition of irrigating movement, in response to sudden decrease in illumination—this component habituates independently at a much faster rate than body contractions (Evans, 1969a). Next, habituation to stimuli of different modalities was compared between three polychaetes, N. diversicolor, N. pelagica, and P. dumerilii (Evans, 1969b). A variety of constituent stimuli ranging from changes in illumination to nonlocalized and localized mechanical stimuli, as well as the complex stimuli of light and shock were used. All three species showed habituation to all stimuli. The rate of habituation to a certain stimulus seemed to be related to the significance of the stimulus in terms of how much the stimulus predicted danger such as an approaching predator. Animals habituated to complex stimuli more slowly than to simple stimuli, probably because complex stimuli containing more sensory cues are more salient. Surprisingly, worms with the supraesophageal ganglion surgically removed continued to respond to light stimuli, even though this procedure included removal of the eyes. Decerebrate and intact worms habituated to stimuli in the same manner except in decerebrate worms, response magnitude was smaller and inhibition of irrigating movement was no longer synchronized with anterior body contraction.

Among other Annelids, leeches have also received great research attention. The nervous system of leech is well characterized: T (touch), P (pressure), and N (nociceptive) sensory cells in the body wall detect mechanosensory stimuli and send signals to S cells—fast-conducting nerve fibers—to execute motor behavior (Nicholls & Baylor, 1968). Based on this anatomical knowledge, the plasticity can be scrutinized at both behavioral and electrophysiological levels.

Many leech species show behavioral plasticity in different sensory modalities. Macrobdella decora responds to light with a dual-component response: release anterior sucker and reattach the posterior sucker. The two components of this response habituated at different rates in a 20-second ISI, 30-trial habituation assay (Ratner, 1972). When the same leeches were tested 24 and 48 hours later, they showed retention of habituation, as animals were less likely to respond in the testing sessions. M. decora also responds to water current in similar pattern, and leeches tested at a 30-second ISI require fewer trials to stop responding to water pulses than those tested at a 60-second ISI. Response topography for habituation of the two response components resembles that of light habituation. Medicinal leeches Hirudo medicinalis exhibit a shortening reflex by longitudinally contracting the body in response to photic or tactile stimuli. Lockery et al. (1985) investigated habituation of this response to photic stimuli. Short-term habituation of light-induced shortening was obvious as the number of responses decreased when the stimulus was presented every 20 seconds for 40 trials; however, retention of habituation is not evident in their experiment, possibly confounded by using well-fed leeches in this experiment. By applying a brief dishabituating electric shock at the end of the thirtieth trial, response likelihood immediately increased and latency decreased. Boulis and Sahley (1988) reported that in both intact and semi-intact leeches the shortening reflex to tactile stimuli habituates. In the semi-intact preparations, the magnitude of contraction decreased with repeated tactile stimulation to the body wall, while an electrical shock to the tail of a habituated leech facilitated/dishabituated the response. Intriguingly, the opposite effect of ISI is observed: The decrement of response in leeches is larger at longer ISIs. It may be that the surgical procedure alters this aspect of learning, or that tactile stimuli with different temporal frequencies carry different levels of significance for leech.

Intracellular recording methods have also been used to study habituation in leeches. Habituation of touch-elicited swimming in H. medicinalis was examined in dissected preparations (Debski & Friesen, 1985). The neural correlates of touch-elicited swimming behavior were recorded from the dorsal posterior nerves that reflect impulses of excitatory motor neurons. With repeated stroking to the body wall, characteristics of habituation are observed in the recordings. The probability and length of evoked activity became smaller and shorter with repeated stimulation, and eventually activity could not be elicited anymore; this reduction in responding showed spontaneous recovery and could be dishabituated by pinching the middle section of the body. Below-zero habituation was also obvious as additional stimuli after habituation delayed spontaneous recovery. Burrell and Sahley (1998) developed a technique to simultaneously score behavior and record cellular activity in a quasi-intact medicinal leech. The generalizability of habituation was investigated with this preparation. If habituation is trained with repeatedly stimulation of one location in segment 8, a stimulus applied to another position in the same segment elicited a decremented response compared to a naïve response, and the size of the response was proportional to the distance between two sites of stimulation, suggesting that habituation in one sensory modality is generalizable to other sites to some extent. With this quasi-intact experimental setup, it was later found that both behavioral response and neuronal activity decreased correspondingly and that the excitability of S cells, the interneurons, declined in habituated animals (Burrell et al., 2001). Both the size of shortening and the peak amplitude of elicited activity in S cells in response to a touch substantially decreased with repeated administration of the stimuli; strong correlation was seen between the decrements in behavioral and neural responses. After an animal appeared to habituate, a larger amount of injected current was needed to generate firing in S cells, suggesting that habituation training reduced the excitability of the interneurons.

More recently, studies have taken molecular and pharmacological approaches to elucidate signaling pathways underlying habituation in leeches. Serotonin has been shown to delay habituation in medicinal leeches by increasing the activity in interneurons (Alkatout et al., 2007). In a reduced preparation of leech, perfusing the head ganglia with 5-HT increased the responsiveness of the swim-generating motor neurons, and it required more trials to cease responding compared to a nonexposed preparation. Furthermore, cAMP was identified as a key component in the signaling pathway, because adding either adenylyl cyclase inhibitor or cAMP-activated protein kinase A to the bath depleted the effect. These observations suggest that habituation may be partially mediated by dampening serotonin signaling in peripheral interneurons. Ca2+ has also been shown to play a role in the formation of memory for habituation (Zaccardi et al., 2012). Repeated low-frequency electrical stimulation to the body increased the latency for leeches to initiate swimming; habituation of the response was absent when either Ca2+ influx or Ca2+ release from intracellular stores was blocked. Two downstream targets activated by Ca2+, phospholipase A2, and arachidonic acid metabolites were identified as important for this effect.

From the studies of habituation in Annelida, it is of particular importance to note that there are often multiple behavioral components in response to a single stimulus, and these components can habituate interdependently as well as independently, such that the animal’s survival and fitness are promoted by a suite of coordinated behavioral changes. Additionally, many studies demonstrate that habituation can occur without a central nervous system, yet the central nervous system undoubtedly plays a vital role in regulating and coordinating an animal’s behavior in habituation. Studies employing electrophysiology techniques substantiate the usefulness of invertebrate research, as associations between behavioral and neural plasticity in the same animal can be established. These studies indicate that both synaptic transmission and postsynaptic excitability can be involved in habituation.

Arthropoda

Arthropods widely inhabit various environments and display heterogeneous behaviors. Segmentation of the body dictates the form of the nervous system arthropods have. As in annelids, the paired ventral nerve cords innervate the whole body with pairs of ganglia in each segment. Arthropod abdominal ganglia and nerve fibers are of particular interest in many studies because these neurons are largely in control of motor behavior, whether flying or swimming. The anterior brain is highly differentiated, with observable lobes executing various functions. Because habituation research has been conducted in a variety of species and habituation of different forms is studied in Arthropoda, this section is subdivided into three parts based on taxonomical classification.

Crustacea

The most commonly studied crustacean for learning and memory is crayfish. When one attempts to capture or disturbs crayfish, often an escape reflex is elicited. This reflex is accomplished by two nearly simultaneous actions: rapid tail flip to generate repulsion and forward thrusting of appendages to form body streamline; together they result in an episode of rapid backward movement. This reflexive behavior in Procamborus clarkii was shown to habituate with repeated stimulation (Krasne & Woodsmall, 1969). Pinching crayfish at either abdominal segment 2 or 4 induces this reflex, which then undergoes habituation if stimulated every 5 minutes for 10 trials. Typically, in the initial three responses, the tail flips several times, the fourth response has only one tail flip, and animals respond to later responses by merely bending the tail slowly without generating swimming, and they eventually cease responding. The effect of habituation appeared to be generalizable to some extent, because animals habituated to lateral compression at one segment also exhibited lower responsivity when another segment was stimulated. Habituation of tail flip showed time-dependent spontaneous recovery, because subjects became gradually more responsive when tested 2, 8, and 22 hours after habituation training.

Because the neural compartments mediating the tail flip were well characterized by early work (Kennedy & Takeda, 1965; Zucker, et al., 1971), showing that the lateral giant (LG) fibers in the ventral nerve cord conduct excitatory potentials to trigger the motor response, the neurophysiology of habituation of the escape reflex could be inspected in dissected preparations of crayfish (Krasne, 1969). First, the electrophysiological response in the lateral giant fibers was carefully characterized. Applying electrical stimulation to the dorsal root of sensory endings led to activity being recorded from the LG. Closer examination of the firing pattern identified three major components: an immediate and small depolarization “alpha,” followed by a large action potential-like depolarization “beta,” and later during the down phase, a third small and slow depolarization “gamma.” Krasne (1969) showed that repeated electrical stimulation to the sensory afferent of dorsal root at ISIs ranging from 2.7 seconds to 5 minutes caused these three components to habituate at different rates; beta and gamma components habituated with large decrements in magnitude, whereas the alpha component was relatively stable. The habituated beta component could be dishabituated by doubling the stimulus intensity, or it spontaneously recovered with time.

Zucker (1972) tapped the tail of the crayfish repeatedly and recorded from the tactile sensory afferents, the interneurons, and the LG fibers. He found that while elicited activity in the afferents remained stable across many trials, the typical decrementing pattern of habituation was observed in the excitatory postsynaptic potentials (EPSPs) in the interneurons and the LG fibers. He concluded that the primary locus of habituation is in the sensory-to-interneuron synapses. Sensory afferents can also directly synapse onto LG and mediate a distinguishable discharge pattern at the boundary between the alpha and beta components in the EPSP that habituated with repeated stimuli. Pharmacologically blocking the nicotinic receptors on the LG fibers decreased this component while leaving the alpha and beta components largely unaltered (Araki & Nagayama, 2003). This suggested that the sensory-LG cholinergic synapses could also play a partial role in LG-mediated habituation.

Outside the local sensory-to-interneuron circuit, several other mechanisms were found to modulate habituation. By comparing habituation in intact crayfish and crayfish with severed ventral nerve cords, GABAergic input from the cephalic ganglia was revealed to play a substantial role in inhibiting the response (Krasne & Teshiba, 1995). Habituation of LG-mediated tail flips in crayfish could also be mediated by neuroplastic changes in the neuromuscular junction (Bruner & Kennedy, 1970). Electrically stimulating the motor giant neurons at a 1-minute ISI could cause marked decrement in the excitatory junctional potentials. This decrement could be restored if stimulation of a higher frequency was applied, potentially correlated with dishabituation.

The decrement of EPSPs in LG associated with habituation of the tail flip could also be inhibited if the motor circuit was stimulated within 100 msec before sensory stimulation (Krasne & Bryan, 1973; Bryan & Krasne, 1977a, b). This inhibition is thought to protect the reflex from habituating to the sensory input generated by the animal’s own movement. Altogether, these findings illustrated that habituation of a single response can be a complex product of multiple habituation processes—both facilitatory and inhibitory—at different levels in the nervous system.

More recent work discovered some additional biological variables that affect habituation. Fricke (1984) found that tail flip in small crayfish did not habituate, and during postnatal development the gap junction between sensory neurons and LG fibers selectively weakened; thus, habituation of tail flip was thought to be largely mediated by the chemical synapses. Araki et al. (2013) found that social status could modulate the rate of habituation. Subordinate animals habituated at a slower rate compared to the dominant counterparts, and decreased excitability of LG in habituated subordinates was observed. Decreased habituation was thought to help these animals maintain a robust response to escape from being attacked. Habituation in crayfish in different behavioral states was explored (Kellie et al., 2001). Larger crayfish tended to habituate more readily, and amputation of the claws and changing the light and water levels could all affect habituation. In the blind species O. australis packardi that have evolved to live in darkness, tail flip habituation was very slow, possibly compensating the loss of vision. These data suggested that state-dependent modulation of habituation plays an important role in an animal’s adaption to the environment.

The crab Chasmagnathus granulatus exhibits an escape response when a shadow is cast onto it. This reflex was shown to habituate with repeated presentation of shadow, and at least five parametric characteristics of habituation were displayed (Brunner & Maldonado, 1988). Lozada et al. (1990) reported long-term habituation of the escape response, and PKA (Romano et al., 1996), protein, and RNA synthesis (Pedreira et al., 1996) were shown to play key roles in the formation of long-term memory. A great deal of research has been done on associative context-dependent habituation in this crab (as reviewed in Tomsic et al., 2009).

Insecta

The fruit fly Drosophila melagangster was first chosen as an experimental model to study genetics. Flies are capable of responding to a variety of stimuli, and habituation has been observed in many responses. When a flying Drosophila approaches a surface, it conducts a stereotypical response, including leg extension and reaching. Experiments simulated flight by tethering a fly’s back to a suspended wire, and approaching objects were presented on a screen in the animal’s visual field to elicit only the leg extension (Fischbach, 1981). Repeated presentation of the stimuli caused the probability of the landing response to decrease and latency of the response to increase (Fischbach, 1981; Rees & Spatz, 1989). If the landing response was allowed after training, the decrement was dishabituated (Fischbach, 1981). Fischbach and Bausenwein (1988) examined the generalization of habituation of the landing response. Flies were habituated to landing-evoking stimuli in one region in the visual field, and test stimuli were presented in different regions. Habituation only partially generalized to other regions in the visual field, and the degree of generalization was correlated with the amount of overlap between the habituated and test regions. These data suggested that habituation occurred downstream of receptor and motion-detection neurons, but upstream of neurons integrating information across the visual field.

Another visually evoked response in flies is the jump response. Visual stimuli such as the sudden appearance of shade, probably representing an approaching predator, trigger a jump response, and the probability of this response gradually diminishes with repeated presentation of the stimuli (Engel & Wu, 1996). The jump response is largely mediated by the giant fiber, the major neural output to the motor neurons (Trimarchi & Schneiderman, 1995a). To investigate the neural correlates and loci of habituation, Engel and Wu (1996) stimulated the visual sensory afferents to the giant fiber and recorded the action potentials in the leg muscle. The muscle activity showed similar decrement as the behavior. Higher intensity stimulation that directly activated the giant fiber did not produce any decrement in the firing, and decremented muscle potentials could be recorded on the contralateral side, where bifurcated giant fibers synapse onto the motor neurons. From these data it was concluded that the site of the plasticity was between the sensory afferents and the giant fiber.

The jump response could also be elicited by olfactory stimuli, although the neural component mediating the jump does not appear to be the same giant fiber as in the visually evoked response (Trimarchi & Schneiderman, 1995b). Repeated exposure to the odor decreased the probability of responding, and centrifuging the flies dishabituated the response (Boynton & Tully, 1992). Exposure to odorants also increased locomotion in flies in the first 30 seconds, and this effect gradually became smaller if animals were exposed repeatedly (Cho et al., 2004). A mechanical stimulus could reverse the decrement in habituated flies. Habituation to one odorant could be generalized to other odorants detected by different olfactory sensory neurons, and ablation of the mushroom bodies disrupted habituation, suggesting a portion of the plasticity was mediated by this structure. Soluble chemicals can be detected by the sensory organs on the forelegs in flies. If sucrose water is tasted, the fly extends its proboscis to attempt to drink. This reflex, if repeatedly elicited at a 1-minute ISI, showed habituation of response probability (Duerr & Quinn, 1982). The decrement generalized to contralateral legs, indicating that this habituation was mediated by a central mechanism.

Flies also habituate to localized mechanosensory stimuli. Repeatedly stimulating the thoracic bristles with air puffs reduced the likelihood and magnitude of the cleaning reflex elicited by the stimuli (Corfas & Dudai, 1989). Interestingly, stimulation of single bristles led to no generalization of habituation. A proprioceptive reflex was also shown to habituate (Jin et al., 1998). When a leg is deflected by external force, the extensor muscles contract to maintain the position of the leg. Repeatedly bending the leg caused a decrease in the muscle electrophysiological response. This proprioceptive behavior showed many parametric characteristics of habituation, including the ISI effect, spontaneous recovery, and dishabituation.

Insights into the molecular mechanisms of habituation have been gained by mutant analysis and genetic manipulation in Drosophila. Several genes were found to play a role in habituation, with both cyclic-AMP signaling and metabolism implicated, but with differing effects on different responses. For example, rutabaga mutants that are deficient in adenylyl cyclase activity showed altered habituation in different paradigms: Habituation was slower in visually evoked jump (Engel & Wu, 1996), olfactory evoked jump (Asztalos et al., 2007), locomotory startle (Cho et al., 2004), and the proboscis extension reflex (Duerr & Quinn, 1982), whereas the landing response habituated more rapidly (Rees and Spatz, 1989). Dunce mutants lacking cAMP-specific phosphodiesterase activity habituated more quickly to visual stimuli (Engel & Wu, 1996) but more slowly to chemical stimuli (Duerr & Quinn, 1982).

Genetic approaches have also assisted in dissecting the mechanisms of habituation at the circuit level. Das et al. (2011) specifically expressed rutabaga gene in the multiglomerular local interneurons (LNs) and rescued a null phenotype for habituation to aversive odorants in rutabaga mutants; expression in the olfactory sensory neurons or glomeruli-specific projection neurons did not produce any effects. Thus, for this paradigm, the locus for olfactory habituation was pinpointed to be in the LNs.

Another class of molecules examined in habituation in Drosophila is potassium channels. K+ channels are largely responsible for the hyperpolarization of neurons, regulating their excitability. Several mutants for K+ channels showed variations in habituation. Slowpoke encodes a calcium-activated K+ channel subunit; mutants for this gene showed slow decrement of giant fiber responses (Engel & Wu, 1998) and decreased odor-evoked jump habituation (Joiner et al., 2007). Two genes encoding the pore-forming subunits of voltate-gated K+ channel, ether-a-go-go and shaker, were shown to affect habituation (Engel & Wu, 1998; Joiner et al., 2007). Two alleles of hyperkinetic, a beta-subunit that interacts with shaker, also altered habituation (Engel & Wu, 1998; Joiner et al., 2007).

To date, a number of genes have been implicated in habituation in Drosophila. Among them, CaMKII (Jin et al., 1998); Shaggy, a glycogen synthase kinase-3 homolog (Wolf et al., 2007); foraging, a cGMP-dependent protein kinase (Engel et al., 2000; Scheiner et al., 2004); and fickle, a tyrosine kinase (Asztalos et al., 2007), the molecules involved in protein phosphorylation were found important for habituation. Mutations in other genes such as the transcription factor gene period (Megighian et al., 2001), a neuropeptide precursor gene amnesiac (Rees & Spatz, 1989), and a synaptic protein gene synapsin (Godenschwege et al., 2004) all were shown to affect habituation. Multiple genes of diverse functions playing a role in habituation suggest that habituation is mediated by a complex cascade of molecular mechanisms.

Habituation has also been studied in the honeybees. Braun and Bicker (1992) examined habituation of a proboscis extension reflex. Similar to the proboscis extension reflex in Drosophila, a droplet of sugar water to one antenna can trigger the response. Braun and Bicker showed that habituation of this reflex displayed several of the consistent parametric characteristics of habituation: the ISI effect, the intensity effect, potentiation of habituation, spontaneous recovery, and dishabituation. The impact of the state of the animal and the effect of neurotransmitters were also explored in their study. They showed that the rate of habituation was modulated by the hunger level; hungry bees habituated to food-related cues at a slower rate. Using reserpine to deplete monoamines in the nervous system decreased the responsiveness, and octopamine or tyramine could restore the depression. These findings suggested that both biological states and neuromodulators influence habituation. Additionally, habituation resulting from stimuli applied to one antenna did not generalize to the contralateral half, suggesting that a local circuit was mediating the habituation effect.

In caterpillars of Manducca sexta, habituation of the proleg withdrawal reflex was examined. The force with which the leg was withdrawn to a tactile stimulus was decreased with repeated stimulation in an ISI-dependent manner: with a 30-second or 60- second ISI, magnitude of the response habituated, whereas a 5-minute ISI did not produce noticeable decrement (Wiel & Weeks, 1996). Spontaneous recovery and dishabituation were also demonstrated. Furthermore, they found that decrement in the withdrawal force could be reliably produced in a preparation disconnected from the central nervous system. This reduced preparation was then used to investigate the cellular mechanism of proleg withdrawal reflex habituation (Wood et al., 1997). Significant decrement in the number of spikes in the proleg motor nerve was seen following habituation training. These neural correlates of habituation also showed analogs of spontaneous recovery and dishabituation. Both direct electrical stimulation of the sensory neurons and natural stimuli caused the decrement, indicating that sensory adaptation was not playing a major role in the decrement, and that habituation of this reflex was primarily mediated by a central mechanism in the sensorimotor pathway.

Habituation has been studied in several other insects: Crickets show habituation to ultrasound stimuli (May & Hoy, 1991). The patterns of the response decrement were congruent with a number of the parametric characteristics of habituation. The mantis Stagmatoptera biocellata decreased the likelihood and magnitude of behavioral response to visual cues of either a prey item (Maldonado, 1972) or a predator (Balderrama & Maldonado, 1971). The decrement appeared to be specific to the cues, suggesting that object recognition could modulate the response. Decremented motor discharge was recorded in isolated abdominal ganglion of cockroaches in response to repeated stimulation (Zilber-Gachelin & Chartier, 1973). These decremented potentials exhibited at least five of the characteristics of habituation.

Other Arthropods

Although the number of studies is few, arthropods other than crustaceans and insects have been shown to habituate to various stimuli. Szlep (1964) characterized habituation of a response to web vibrations in spiders. He found that spiders gradually decreased the likelihood and speed of the response if the web was vibrated repeatedly. After the response waned in a habituated animal, if vibration occurred at a different place, it remained responsive to that stimulus. Some evidence for potentiation of habituation and long-term habituation was also found, as spiders habituated much faster and spontaneously recovered less if trained on successive days.

In the horseshoe crabs Limus polyphemus, a tactile stimulus, such as a puff of air, to the gill elicits a telson movement. The neural correlates of habituation of this reflex were examined in the abdominal ganglia (Lahue & Corning, 1971). Repeated stimulation caused a rapid decrease in the magnitude of the extracellular recordings. This decrement spontaneously recovered to the initial level if the preparation was unstimulated. A local neural circuit appeared to mediate the decrease in activity, as no differences in the rate of the decrement were found in preparations with an intact nervous system, with the ventral nerve cord attached, or with only one ganglion. Using the same preparation, it was found that a faster ISI produced faster decrement, and recovery from faster ISIs required less time (Lahue & Corning, 1973). Habituation of telson reflex was also studied by recording the muscle activity (Lahue et al., 1975). The rate of the decrement in the muscle discharge was shown to inversely correlate with the length of ISI and the intensity of the stimuli. Potentiation of habituation was also demonstrated in repeatedly trained animals.

In summary, many valuable findings have come from studies in arthropods. Many genes appear to be important for the memory of habituation. The roles of second messengers and molecules involved in cell excitability regulation are strongly implicated. Habituation appears to be the behavioral manifestation of the sum of synaptic activity at both local and global levels in the organism. Habituation can be modulated by the organism’s internal states and external environment to large extent. The omnipresence of habituation in many different species lends support to the notion that habituation is an evolutionarily conserved learning mechanism.

Mollusca

These soft-bodied invertebrates comprise another diverse group in the animal kingdom, from terrestrial to aquatic to parasitic species. Despite large differences in overall morphology, the nervous system in mollusks is relatively similar. In species such as gastropods and cephalopods, the anterior nerve ring is thought to be analogous to a brain that executes central integration function; in other parts of the body, several pairs of large ganglia innervate muscle and internal organs. Many of these compartments in the mollusk nervous system are identifiable by their anatomy; thus, studies take advantage of the knowledge to study learning and memory at cellular and molecular levels.

Habituation has been observed in several cephalopods. In the squid Lolliguncula brevis, repeated exposure to a plastic model of a predator causes the escape responses (inking and jetting) to habituate (Long et al., 1989). Habituation to the visual cues recovered after a 1-hour rest, and they could be dishabituated by a noxious visual stimulus. Interestingly, animals habituated to a model of one type of fish remained partially responsive to a model of another type of fish, suggesting that a top-down mechanism involving object recognition can modulate habituation. Long-term habituation for 24 hours was produced if a spaced training paradigm was used. The cuttlefish Sepia officinalis habituate to acoustic stimuli; habituation to weaker stimuli was more rapid than to stronger stimuli (Samson et al., 2014). Habituation in Sepia was age dependent: A stage 30 embryo habituated to repeated visual stimuli by contracting the mantle less and less, and this decremented response could be dishabituated by mechanical stimuli; a stage 25 embryo had robust responses to the same stimuli but did not habituate (Romagny et al., 2012). This finding pointed to the importance of development in habituation.

In Aplysia, a number of reflexes have been shown to habituate. With repeated mechanical stimulation to the head, the animal exhibits a progressively smaller likelihood to retract its tentacles (Bruner & Tauc, 1965). Similarly, if a tactile stimulus was repeatedly applied to the siphon, the gill-withdrawal response decremented (Pinsker et al., 1970). The rate of decrement was faster with a shorter ISI (1 min vs. 3 min) or with a weaker intensity, and it showed time-dependent spontaneous recovery and dishabituation following a strong stimulus.

In both the tentacle-withdrawal reflex and gill-withdrawal reflex (GWR), decremented EPSPs were recorded in the L7 motor neurons in habituated preparations. In either spontaneous recovery or dishabituation of the reflexes, the behavioral recovery was accompanied by a recovery in the magnitude of EPSPs (Bruner & Tauc, 1966; Kupfermann et al., 1970; Pinsker et al., 1970). Habituation of the GWR did not produce any changes in the magnitude of spontaneous gill withdrawal nor the EPSPs associated with it (Pinsker et al., 1970), and no obvious contribution of the sensory organs and musculature to habituation was found (Kupfermann et al., 1970). The mechanosensory neurons were shown to make significant contribution to the EPSPs in the motor neurons (Byrne et al., 1978), while peripheral sensory neurons could play a modulatory role depending on the property of the stimulus (Carew et al., 1979). These data suggest that habituation primarily involves a central process in which the synaptic transmission decreases at the synapses between the sensory neurons and the motor neurons. Further experiments demonstrated that this monosynaptic depression underlying short-term habituation is largely regulated by presynaptic plasticity. In a quantal analysis, it was shown that fewer synaptic vesicles were released by the mechanosensory neurons in habituated animals (Castellucci & Kandel, 1974). This presynaptic theory was supported by anatomical evidence that there was a decrease in the number of synaptic vesicles in the readily releasable pool of neurotransmitter vesicles after habituation training (Bailey & Chen, 1988c). Armitage and Siegelbaum (1998) demonstrated that stimulating the presynaptic sensory neurons at a 1-minute ISI could induce such synaptic depression, even when the postsynaptic glutamate receptors were chemically blocked during training.

The GWR also exhibits long-term habituation lasting for weeks. A spaced training paradigm was more effective for the acquisition of long-term habituation than a massed training paradigm (Carew et al., 1972). Surprisingly, as few as 40 spaced trials could produce enduring decrement in the duration of gill withdrawal a week later (Carew & Kandel, 1973). In animals trained for long-term habituation, decremented EPSPs in the motor neurons could be recorded 3 weeks after training (Castellucci et al., 1978).

Although depression of the same sensorimotor synapse was implicated in long-term habituation of GWR, the cellular and molecular mechanisms of long-term habituation appear to be different from those of short-term habituation. In long-term habituated animals, electron microscopy studies of the presynaptic sensory neurons showed a decreased number of active zones and vesicles compared to control animals (Bailey & Chen, 1983, 1988c). In addition, postsynaptically, the LTD-like synaptic plasticity associated with long-term habituation also requires activation of glutamate receptors, as blocking NMDA or AMPA receptors with APV or DNQX abolished long-term habituation (Ezzeddine & Glanzman, 2003). The postsynaptic AMPA receptor downregulation is dependent on protein synthesis and gene expression, as administration of anisomycin (Ezzeddine & Glanzman, 2003) or actinomycin-D (Esdin et al., 2010) blocked the formation of long-term habituation. In animals trained for long-term habituation, there was an upregulated transcription level of an Aplysia homolog of cornichon, a protein implicated in glutamate receptor trafficking (Holmes et al., 2015). Thus, both presynaptic and postsynaptic mechanisms are implicated in long-term habituation.

The rich research findings in mollusks make these organisms the frontier for studying the neurobiology of learning and memory. Habituation of a single response can involve different neural mechanisms depending on the time course of the memory. In short-term habituation, presynaptic depression occurs, whereas long-term habituation relies on both pre- and postsynaptic changes to modulate behavioral response to the stimulus. Additionally, glutamatergic signaling, gene expression, and protein synthesis are particularly important for long-term memory of habituation in Aplysia.

Habituation Summary

Taken together, the research on habituation in invertebrates highlights several key points. It reinforces the ubiquity of habituation across phylogeny—wherever it has been looked for, it has been found. Invertebrates from jellyfish to cephalopods all habituate, and they do so with similar behavioral characteristics. The mechanisms of long-term memory for habituation appear to involve many of the same processes as more complex memory in mammalian systems, suggesting conservation of memory mechanisms. Finally, the lack of understanding of the mechanism of short-term habituation in “simple” invertebrates, despite many years of study, should have readers reconsider the definition of habituation as a “simple” form of learning.

Sensitization

Another form of nonassociative learning is sensitization. It refers to the enhancement of a response following the presentation of another strong stimulus or the repetition of a moderate to strong stimulus.

Sensitization is similar to habituation in that its acquisition does not require stimulus–response associations and that it has a long-lasting form, that is, long-term sensitization. Sensitization is distinguished from habituation in several aspects. Whereas habituation becomes more profound with more trials, sensitization can be produced with a single trial. Habituation appears to be relatively specific to the stimulus and occur in the stimulus response pathway, whereas the effects of sensitization involve increased arousal and are seen across all responses in an organism. Habituation is better induced with weak stimuli; in contrast, sensitization is better induced with strong stimuli.

Groves and Thompson (1970) proposed the dual-process theory in which they suggested that a stimulus simultaneously elicits both habituation and sensitization processes in the organism, and the animal’s observed behavior is the net outcome of two processes. The extent of sensitization is positively correlated with the intensity of the stimulus, and sensitization also habituates with repeated stimuli. Experimental evidence supports this theory. In habituation studies (e.g., Koopowitz, 1975b; Lockery et al., 1985), it is not uncommon to observe signs of sensitization in the first few trials following the initial presentation of the stimulus, before animals fully habituate.

A second facilitatory process has been observed—that is, facilitation of a habituated response, termed dishabituation. In dishabituation a novel strong stimulus induces facilitation of the habituated background back to or above baseline levels. Research has demonstrated that sensitization and dishabituation are two separate processes that emerge at different developmental stages (Rankin & Carew, 1988), and they depend on molecular pathways that are partially shared but with distinct components (as reviewed in Byrne & Hawkins, 2015).

A great amount of work contributing to our understanding of the mechanisms of sensitization has been done in two major model systems, Aplysia and leeches. The mechanism of short-term sensitization in Aplysia was the first ever described molecular mechanism for learning (Castellucci et al., 1982). Research in leeches and other species has also been a great source of knowledge on sensitization. Sensitization is arguably the best understood learning and memory process today.

Kandel and colleagues first studied the sensitization of the gill-withdrawal reflex in Aplysia. An electrical shock to the tail or the body wall significantly increased the magnitude of the elicited reflex (Carew et al., 1971). Similar to habituation of the GWR, sensory neurons are at least one site of the plasticity because sensitization is correlated with increases in the EPSPs at the sensorimotor synapse in the abdominal ganglion.

The effect of a single shock wanes after minutes, and it is called short-term sensitization. A form of sensitization that persists longer can also be induced. Training Aplysia with five spaced shocks in 1 day or over 4 days produced long-term sensitization lasting for days to weeks (Pinsker et al., 1973; Frost et al., 1985). An increase in the synaptic transmission was seen in animals given long-term sensitization training (Frost et al., 1985).

Several studies in behaving Aplysia (Carew et al., 1971; Walters et al., 1983) and isolated ganglia (Hawkins et al., 1981) indicated that short-term sensitization of defensive withdrawal reflexes is mediated by the heterosynaptic facilitation of sensorimotor synaptic transmission. Such facilitation altered the properties of presynaptic sensory neurons. Shocking the animal or electrically stimulating the interneurons in the isolated ganglion resulted in a decreased outward K+ current (Klein & Kandel, 1980), broadened action potentials (Walters et al., 1983), and increased neurotransmitter release (Castellucci & Kandel, 1976) in the sensory neurons, all of which led to presynaptic facilitation, suggesting that presynaptic plasticity underlies the memory for short-term sensitization.

In search for the biomolecules producing sensitization, several lines of evidence implicated the neurotransmitter serotonin. Levenson et al. (1999) found that the level of 5-HT increased in the hemolymph in tail-shocked Aplysia, and the increase was correlated with the magnitude of the sensitizing stimuli; nerve stimulation also produced elevated 5-HT release (Marinesco & Carew, 2002). Injecting 5-HT into naïve animals sensitized their response (Philips et al., 2011). Similarly, perfusing the isolated abdominal ganglion with 5-HT produced facilitation of the sensorimotor synapse (Brunelli et al., 1976; Walters et al., 1983). In contrast, depleting endogenous 5-HT pharmacologically abolished tail shock–induced sensitization (Glanzman et al., 1989). Further confirming the role of 5-HT, application of 5-HT to sensory neurons was shown to reduce the outward K+ currents (Klein et al., 1982; Siegelbaum et al., 1982), and application of a 5-HT receptor antagonist blocked the spike broadening in the sensory neurons (Mercer et al., 1991). Anatomical studies also supported this view when serotonergic interneurons were found to make contact with sensory neurons (Zhang et al., 1991).

Serotonin signaling primarily occurs through its G protein–coupled receptors. One of the main downstream effectors of 5-HT, cyclic AMP, was strongly implicated in sensitization. Intracellular injection of cAMP into the sensory neurons exerted effects similar to those of administration of 5-HT: Decremented K+ currents (Klein et al., 1982; Siegelbaum et al., 1982) and enhanced evoked potentials (Brunelli et al., 1976) were observed. Lowering endogenous cAMP levels in the animal inhibited both short-term sensitization and presynaptic facilitation (Belardetti et al., 1983). The effect of cAMP is mediated by cAMP-dependent protein kinase (PKA), as injecting the catalytic subunit of PKA into sensory neuron was sufficient to produce the facilitation of neurotransmitter release (Castellucci et al., 1980), and injecting an inhibitor of PKA blocked the presynaptic facilitation (Castellucci et al., 1982).

These data led to the first proposed cellular mechanism for learning. Behavioral short-term sensitization is mediated by presynaptic facilitation following the activation of 5-HT signaling pathway. A sensitizing stimulus causes the heterosynaptic interneuron to release 5-HT onto the sensory neuron. Binding of 5-HT to sensory neuron receptors activates a G protein alpha subunit, adenylyl cyclase, and PKA, in a series of biochemical reactions. The catalytic subunit of PKA then can phosphorylate specific proteins to alter the functional properties of a subset of K+ channels in the sensory neuron causing broadening of the action potential. The increased duration of depolarization causes a greater influx of Ca2+ into the sensory neuron, leading to more neurotransmitter release from that sensory neuron. Thus, presynaptic facilitation is achieved, leading to the sensitized response.

However, this model could not fully explain some observations in later experiments. At least three other ion conductances were shown to be modulated by 5-HT (Baxter & Byrne, 1990; Braha et al., 1990; Walsh & Byrne, 1989). Besides the K+ conductances exclusively modulated by 5-HT (Hochner & Kandel, 1992), another second messenger pathway was identified to modulate a Ca2+ current and a voltage-dependent K+ current. Spike broadening was contributed to by the cAMP/PKA pathway as well as a G protein signaling pathway involving diacylglycerol and protein kinase C (PKC) (Sugita et al., 1994). The relative contribution of two processes is adjusted depending on preexisting depression of the facilitated synapse, likely making the differentiation between dishabituation and sensitization (Sugita et al., 1997).

Gingrich and Byrne (1985, 1987) mathematically modeled a sensory neuron undergoing sensitization, and they predicted the existence of a spike-duration independent process. Experimental data supported their model, and an increased number of neurotransmitter vesicles moving to the readily releasable pool was hypothesized to be the underlying mechanism (e.g., Braha et al., 1990; Pieroni & Byrne, 1992; Klein, 1993). Angers et al. (2002) showed the Aplysia homolog of synapsin (apSyn), a synaptic protein known to interact with vesicles, was phosphorylated by 5-HT-induced PKA and MAPK activity. Serotonin promoted dissociation of apSyn and synaptic vesicles to mobilize vesicles into the readily releasable pools.

The revised model of short-term sensitization was proposed based on the new findings (Angers et al., 2002). In addition to increasing the spike duration trough modulating K+ and Ca2+ conductances, 5-HT also increases mobilization of vesicles into the readily releasable pools by phosphorylating synapsin. Both processes are PKA dependent, while PKC also plays a role. Through these mechanisms short-term sensitization is mediated by facilitated synaptic release in the sensory neuron.

Because short-term sensitization is mediated by presynaptic facilitation, the search for the cellular mechanism for long-term sensitization began with a presynaptic bias. As expected, similar presynaptic plasticity was observed. For example, long-term sensitization altered the K+ conductance (Scholz & Byrne, 1987) and excitability (Cleary et al., 1998) of sensory neurons. Anatomical changes of sensory neurons also occur in long-term sensitization. Bailey and Chen (1983, 1988a, b, 1989) observed increases in the number and size of active zones and vesicles and more varicosities contacting the gill motor neurons at EM level in animals trained for long-term sensitization. Neuronal outgrowth of sensory neurons in culture was also stimulated by long-term sensitization training (Wainwright et al., 2002), and cAMP was required for the presynaptic morphological changes (Nazif et al., 1991).

Later studies revealed that postsynaptic plasticity is also involved in the mechanism for long-term sensitization. Cleary et al. (1998) found increased excitability of motor neurons in long-term sensitized animals. The presynaptic structural growth was absent if the sensory neuron was isolated from its postsynaptic partner (Glanzman et al., 1990). More excitatory receptors were found in the motor neuron after induction of long-term facilitation (Trudeau & Castellucci, 1995). The emerging evidence suggested that whereas short-term sensitization is primarily presynaptic, long-term sensitization requires a concert of biochemical events at both pre- and postsynaptic sites.

Another difference between the mechanisms of short- and long-term sensitization is that long-term sensitization is dependent on protein synthesis and changes in gene expression. Inhibiting protein and RNA synthesis were shown to block long-term facilitation of sensorimotor synapses and sensitization of withdrawal reflexes (Castellucci et al., 1986, 1989; Schacher et al., 1988). Protein synthesis in long-term facilitation is modulated by cAMP-responsive element-binding proteins (CREBs), a class of transcription factors regulating gene expression (Kaang et al., 1993). Two forms of CREB, CREB1 and CREB2, were experimentally shown to selectively enhance or repress long-term facilitation, respectively (Dash et al., 1990; Bartsch et al., 1995, 1998).

During investigation of 5-HT-induced synaptic facilitation, a third form of facilitation was identified (Ghirardi et al., 1995). Sutton et al. (2001) confirmed this observation in vivo. This form of memory remains after short-term memory but declines before long-term memory for sensitization, and it appears to be RNA synthesis independent; this form of sensitization was named intermediate-term facilitation. The concentration of 5-HT and the exposure duration determine the form of facilitation (Ghirardi et al., 1995). Different concentrations of 5-HT activate PKA only; PKA and protein synthesis; or PKA, protein, and RNA synthesis, to induce synaptic facilitation of different durations. These data suggest that short-, intermediate-, and long-term sensitization have different cellular and molecular underpinnings, and different forms of memory can develop, at least partially, in parallel.

It became clear that behavioral sensitization depends on synaptic facilitation that is mediated by several second messenger pathways depending on the intensity of sensitizing stimuli (or the amount of 5-HT release) and the time course. More work from several groups has discovered a number of important molecules in this pathway responsible for the expression and maintenance of synaptic facilitation.

Carew and colleagues identified two possible mechanisms for intermediate-term presynaptic facilitation. Depending on how animals were trained, either MAPK and PKA (Sutton & Carew, 2000; Sutton et al., 2001) or MAPK and PKC (Zhao et al., 2006) could mediate the facilitation. Glanzman’s group found that Ca2+ transients increased postsynaptic glutamate AMPA receptor production and insertion (Li et al., 2005; Villareal et al., 2007), and protein kinase M, a cleaved form of the atypical Aplysia PKC, was required for this effect (Bougie et al., 2012). Hawkins and colleagues showed that CAMKII and protein synthesis in both sensory and motor neurons were required for the facilitation (Antonov et al., 2010).

In long-term facilitation, structural plasticity in the presynaptic neuron requires the presence of the postsynaptic neuron. Experiments showed that ApCAM, an Aplysia NCAM-related adhesion molecule, in the sensory neuron decreased and translocated in response to 5-HT treatment (Mayford et al., 1992). These changes presumably increased the morphological plasticity and promoted the outgrowth. Presynaptic neurexin and postsynaptic neuroligin were also implicated (Choi et al., 2011).

Gene expression regulation is another important aspect of long-term facilitation. For instance, Aplysia ubiquitin hydrolase (ApUch) was found to be upregulated in sensory neurons; ApUch increases PKA catalytic activity by degrading its regulatory subunits (Chain et al., 1999). Transcription factors such as ApAF (Bartsch et al., 2000) and ApLLP (Kim et al., 2003) were also implicated. Growth factors are yet another group of molecules that are regulated by learning. An endogenous Aplysia neurotrophin (ApNT) was shown to activate the Aplysia tyrosine kinase receptor (ApTrk) (Kassabov et al., 2013), which then acts through the MAPK/ERK pathway to regulate CREB2 (Ormond et al., 2004). Aplysia tolloid/BMP-like protein (ApTBL-1) in the TGF-β pathway also increases during facilitation to activate ERK (Zhang et al., 1997). These findings added extra layers of complexity to the molecular mechanisms underlying sensitization.

Taken together, studies in Aplysia show that sensitization, depending on the time course of the memory, is mediated by heterosynaptic facilitation with different cellular mechanisms. Short-term sensitization is expressed in the presynaptic cells in a covalent modification manner, whereas intermediate- and long-term sensitization involve both pre- and postsynaptic changes, and often rely on more complex second messenger pathways, including protein synthesis and gene expression.

Research on sensitization has also been done in medicinal leech Hirudo medicinalis. Boulis and Sahley (1988) studied sensitization of the shortening reflex in semi-intact leeches. A noxious stimulus did not increase the baseline response. However, habituation of the reflex was prevented, and a dishabituating stimulus evoked a response above the baseline. Lockery and Kristan (1991) examined the leech local bending reflex. Leeches will bend several adjacent segments if a sensory cell for pressure (P cell) is repeatedly stimulated or a nociceptive cell (N cell) is stimulated in a local segment. Behavior and electrophysiological recordings were done simultaneously. Sensitized animals bent their bodies with larger tension, and evoked motor neuron responses contained more spikes for local bending. Swim induction could also be sensitized by a noxious mechanical stimulus, as the latency for the leech to initiate swim shortened (Zaccardi et al., 2001). Interestingly, with repetition, this sensitizing stimulus became less and less effective, showing habituation of sensitization.

Search for the locus of sensitization in leech has identified the S cell—the same interneuron implicated in habituation (Sahley et al., 1994). Increases in S cell activity were recorded in sensitized animals, and ablating S cells eliminated sensitization. Separating S cells from one another in the ventral nerve cord disrupted sensitization while the shortening reflex remained intact, suggesting the connections between S cells are important for sensitization (Modney et al., 1997). S cell ablation only reduced dishabituation, suggesting that dishabituation and sensitization are two separate processes mediated by partly shared cellular pathways.

The same neurotransmitter as in Aplysia, 5-HT, was shown to be critical for sensitization in leech. Intracellular stimulation of serotonergic neurons mimicked the sensitizing effect of shocks (Lockery & Kristan, 1991). Bathing the abdominal ganglion in 5-HT also increased the excitability of motor neuron (Burrell et al., 2001). Depletion of 5-HT disrupted sensitization (Ehrlich et al., 1992). In leech, 5-HT signaling cascade also recruits cAMP to induce sensitization (Zaccardi et al., 2004). These findings conform to the well-established Aplysia model.

One novel aspect of sensitization was demonstrated in leech. Burrell and Sahley (1998) observed a type of behavioral facilitation that showed proximity-dependent generalization: The extent of the facilitation is proportional to the distance between sensitized and tested sites. Later they found that this process was not mediated by 5-HT (Burrell & Sahley, 1999). It pointed to a possible serotonin-independent form of sensitization.

Sensitization has also been studied in Crustacea. Krasne and Glanzman (1986) studied the sensitization of the lateral giant (LG) fiber-mediated escape response in crayfish. Following a strong electric shock to the body, the threshold for a test stimulus to elicit the LG response was significantly decreased. Crab escape responses could be sensitized by 5-HT in a dose-dependent fashion: Low dose induced short-term sensitization and high dose induced 24-hour long-term sensitization (Aggio et al., 1996).

Several insect species, including Drosophila melanogaster (Duerr & Quinn, 1982), honeybee (Mauelshagen, 1993), and cockroach (Zilber-Gachelin & Paupardin, 1974), show sensitization.

In summary, many invertebrates show sensitization in multiple behaviors for various durations. The enhancement in behavioral responses is mediated by synaptic facilitation. Memories for sensitization of different time courses are mediated by different signaling pathways, many of which use serotonin as the primary neurotransmitter.

Conclusion

This chapter has reviewed nonassociative learning in invertebrates. Both habituation and sensitization are universally found across phylogeny, and many conserved mechanisms have been found in different species. These observations firmly support the notion that nonassociative learning is biologically essential and evolutionarily adaptive.

The expression of nonassociative learning is largely modulated by a variety of environmental factors. Organisms learn to modulate their responses to subtle differences in the environment. The learning processes are also influenced by the animal’s biological states. These complex interactions illustrate that even the simplest forms of learning do not have simple explanations. It is also interesting to note that many of the mechanisms and molecules uncovered in studies of nonassociative learning in invertebrates are also involved in associative learning in vertebrates and invertebrates. Despite the decades of rich literature on invertebrate learning and memory, more is yet to be discovered.

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