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Positron Emission Tomography: Ligand Imaging

Abstract and Keywords

This chapter describes the application of positron emission tomography (PET) technology to study neurotransmitter systems. The process of developing radiotracers to study the neurotransmitter systems, namely radioligands and the technical aspects of utilizing these radiotracers in PET imaging, is discussed. Normal distribution of neurotransmitters including dopamine, serotonin, opioids, and γ-aminobutyric acid (GABA), as well as abnormalities of these systems in various neurological and psychiatric disorders, are highlighted. The chapter provides evidence that radioligand imaging has been useful not only in delineating pathophysiological processes in psychiatric disorders, but also in contributing to the diagnosis, prognosis, and disease course, and in assessing drug effects. Recent advances in receptor imaging that are rapidly gaining clinical relevance are also discussed.

Keywords: neurotransmitters, receptors, dopamine, serotonin, opioids, acetylcholine, GABA, amyloid, neurological disorders, psychiatric disorders.

Positron emission tomography (PET), along with an array of biomolecules labeled with positron-emitting isotopes (radiotracers), has been utilized to study many physiological and pathological states throughout the body. An extensive array of tracers have been developed over the past 30 years that explore many different molecular processes, including amino acid metabolism, blood flow, and neurotransmitter systems (see Table 5.1). Specifically, PET used to the study of the brain, both for research and for diagnostic purposes, has resulted in some important findings. The previous chapter described the use of PET in studying cerebral blood flow and cerebral metabolism. This chapter describes the application of PET to the study neurotransmitter systems, namely dopamine, serotonin, opioids, and γ-aminobutyric acid (GABA), using radiotracers that bind to various receptors (also called radioligands). The radioligands have been useful not only in delineating abnormalities in the neurotransmitter systems in several neuropsychiatric disorders, but also in contributing to their diagnosis and prognosis and in assessing drug effects. The following sections focus on the development and technical aspects of the utilization of radioligands in brain PET imaging and provide some examples of clinical and research applications of neurotransmitter imaging.

The fundamentals of radioligand PET imaging are the same as those applied in imaging cerebral blood flow (CBF) and metabolism (described in the previous chapter). The principles of use of positron-emitting nuclides, the detection of annihilation radiation, PET instrumentation, and image reconstruction are common to CBF, metabolic, and radioligand PET imaging. However, there are some differences in the process of developing the radioligands and their quantification, and these are discussed in the following sections.

Overview of Receptors, Synaptic Transmission, and Radioligand Selection

Neuronal activation consists in the development of an action potential and its propagation along the axon of the neuron to its terminal where it initiates (p. 82) the release of calcium, and, in turn, the release of neurotransmitters. The space between the axon terminal of one neuron, termed the presynaptic terminal, and a dendrite or cell body of another neuron, called the postsynaptic terminal, is known as a synapse. The released neurotransmitters diffuse across this space (also called the synaptic cleft) to interact with protein complexes present in the postsynaptic membrane (receptors) in an interaction termed receptor activation. Activation of receptors results in the release of ions, enzymes, or second-messengers in the postsynaptic terminal that in turn alters the membrane potential and the ionic concentration across the postsynaptic membrane. The combined process of neurotransmitter release and receptor activation is called synaptic transmission (Young, Frey, & Agranoff, 1986). The next event involves the dissociation of the neurotransmitter from the receptor site and its inactivation. The inactivation can occur by the removal of the neurotransmitters from the synaptic cleft either by their reuptake into the presynaptic membrane or by enzymatic degradation.

Neurotransmitters administered peripherally do not cross the blood–brain barrier (BBB), and therefore radiolabeled neurotransmitters cannot be used in PET. However, various aspects of presynaptic and postsynaptic function can be assessed using precursor amino acids; that is, radiotracer molecules out of which neurotransmitters are made in the brain. An example of this is the L-3,4,-dihydroxyphenylalanine (L-DOPA), a precursor of the neurotransmitter dopamine that can be radiolabeled with 18F ([18F]FDOPA). [18F]FDOPA administered intravenously readily crosses the BBB and is incorporated into the dopamine synthesis pathway in the neurons. The PET scanner can follow the fate of dopamine that is now labeled with 18F and thereby index the presynaptic dopamine. Additionally, with PET imaging, one can examine other aspects of presynaptic functions by measuring the activity of enzymes that synthesize the neurotransmitters, neurotransmitter reuptake, and its metabolism. Postsynaptic events such as receptor activation, receptor density, and synthesis of enzymes and second-messengers can also be quantified using PET. Examples of this include measuring the receptor density by using molecules structurally similar to the neurotransmitters that bind to the receptors and can either mimic the neurotransmitter action (agonists) or inhibit the neurotransmitter action (antagonists). Accurate measurement of these synaptic events, once possible only in animals, can now be performed in vivo in humans mainly due to the advent of PET. The versatility of radiolabeling allows for the characterization of the interaction of receptors not only with neurotransmitters, but also with various drugs.

PET has been used to examine the normal function of neurotransmitters and their abnormalities, such as the location and density of neurotransmitter receptors in neurological and psychiatric disorders and the changes in those systems following treatment.

Establishing the usefulness of radioligand PET for human applications requires the expertise of several different scientists, including radiochemists, pharmacologists, pathologists, and clinicians working together in an interdisciplinary environment. The process of developing a radioligand is complex, and several important factors have to be addressed. Usually, the initial issue to be settled is the determination of the molecular target for the new radiotracer. Selection of the radioligand is largely determined by the pathophysiology of the disease under study. For example, if the disorder is Parkinson disease (PD), the dopaminergic system is a likely target since it is well established that dopamine plays a primary role in this disease. The next step is determining what specific molecular target will be the focus of radiotracer development. Continuing with the PD example, the dopamine synthesis pathway, postsynaptic receptors, and dopamine transporter that mediates the dopamine reuptake might be reasonable targets. The next step is then to explore existing molecules or drugs that are precursors of dopamine or those that bind to the dopamine receptors or the dopamine transporter (ligands). Once a tracer is identified, radiolabeling must be attempted with a variety of isotopes that would be ideal for imaging.

The selection of a particular radioisotope that will be used to label the selected ligand depends on a variety of factors including its half-life, positron energy, and other chemical properties for the creation of a proper radioligand. Several varieties of isotopes and ligands might be tried to determine the radiotracer with the best yield and imaging characteristics. It is also important to confirm that the radioligand binds to the receptor in vivo and that it can cross the BBB, a specialized brain capillary endothelial cell layer that limits movement of water-soluble substances into the brain. Since the endothelial cell membrane is a lipid bilayer, lipid-soluble molecules can readily traverse across the BBB. As mentioned earlier, the neurotransmitters do not (p. 83) cross the BBB, but their precursor amino acids do, and therefore any radioligand used in brain imaging must be capable of crossing the BBB. For a radiotracer to cross the BBB passively, it must be relatively small and moderately lipid soluble (Waterhouse, 2003). However, radiotracers with high lipid solubility can become extensively bound to blood proteins and reduce the fraction of radiotracer that is freely available to traverse the BBB. Molecules that are large or not lipid soluble are moved across the BBB by specific active transport systems.

Table 5.1. Partial list of radiotracers used in PET imaging

Radiotracer

Application

[15O] H2O

Blood flow

[18F] fluorodeoxyglucose

Glucose metabolism

[15O] O2

Oxygen metabolism

[11C] l-methionine

Amino acid metabolism

[11C] raclopride, [11C] methylspiperone

Dopamine receptor activity

6-[18F] fluoro-L-DOPA, 4-[18F] fluoro-m-tyrosine (FMT)

Presynaptic dopaminergic system

[carbonyl-11C]WAY100635 and [(18)F]altanserin

Serotonin receptor activity

[11C] (N,N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine (DASB)

Serotonin transporter binding

[11C] carfentanil, [11C] etorphine

Opiate receptor activity

[11C] flunitrazepam

Benzodiazepine receptor activity

[11C] scopolamine, [11C] quinuclidinyl benzilate

Muscarinic cholinergic receptors

[11C] ephedrine, [18F] fluorometaraminol

Adrenergic terminals

[18F] Florbetapir, [11C] Pittsburgh Compound B

Amyloid detection

Unlike [15O]water, 18FDG, and 15O2, which freely diffuse into the brain and are taken up by all neurons uniformly, it is important to verify that the radioligand used in neurotransmitter mapping has a strong tendency to bind to the receptor (i.e., that it has high affinity). Considering that the concentration of available receptor binding sites for the ligands is rather low (nano- to femto-moles per milligram tissue), PET radioligands should have high binding affinities so that there is a substantial amount of binding to allow for accurate imaging results.

A radioligand must also be developed so that it has high “specific” radioactivity. Specific activity refers to the amount of radioactivity per unit mass of a radioligand. Radioligands used in brain receptor imaging must be prepared with high specific activity so that sufficient radioactivity can be detected while binding to a small fraction of available receptors and thus preventing pharmacological effects and saturation of the binding sites. An example where the specific activity of a tracer is important is [11C] carfentanil. Carfentanil, a synthetic opioid, is approximately 10,000 times stronger than morphine. If the specific activity of the [11C] carfentanil is not high enough, too much nonradioactive or “cold” carfentanil is injected with each scan. This can result in subjects feeling morphinelike effects such as dizziness, lightheadedness, or fatigue.

Developing a radioligand also requires attention to its metabolism or breakdown in the body. Even if the radioligand effectively crosses the BBB and binds to its target with high affinity, the total amount of it entering the brain is decreased if it is metabolized before reaching the brain. A good example of this is [18F] FDOPA, which is rapidly metabolized by the peripheral enzymes dopadecarboxylase and catechol-O-methyl transferase. To overcome this drawback, prior to the administration of the radioligand, subjects are given carbidopa and entacapone to block these enzymes in the periphery and allow a sufficient amount of the [18F] FDOPA to enter the brain. It is also important to ensure that the administered radioligand is not metabolized to other compounds that can then enter the brain and bind to receptors because PET cannot discriminate between radioactive signals coming from the parent radioligand and its radiolabeled metabolites (Zoghbi et al., 2006).

Other factors that are also important in ligand selection include its clearance rate from the blood and whether it binds to plasma proteins. A low binding of radioligand to plasma proteins is essential because only the unbound fraction of radioligand in the plasma is available for diffusion across the BBB. Finally, whether or not the radioligand binds reversibly and redistributes over time, whereby it is no longer interacting with receptors and results in an underestimation of the receptor activity, plays an important role in determining its usefulness as an imaging tool. (p. 84)

Positron Emission TomographyLigand Imaging

Figure 5.1 Whole body positron emission tomography (PET) images at various time points (L→R: starting at 0 min, 10 min, 20 min, 40 min post injection) associated with estimation of radiation dose of [11C]carfentanil. The inset shows the time activity curves, where the concentration of [11C] carfentanil is plotted over time in the urinary bladder and the liver.

Preclinical Evaluation of Radioligand Imaging

Once a potential radioligand is developed in the laboratory setting that meets the criteria just described, it must go through a series of preclinical evaluations prior to testing in humans. Such evaluations include studying its distribution and metabolism, its physiological and/or pharmacological properties, adverse effects, and toxicity in rodent and primate models. Distribution studies evaluate the concentration of the radioligand in plasma and tissues, its metabolism, and excretion. Because radioligands combine a radioactive atom with a probe that follows some aspect of body physiology, their distribution in the body and the brain can be readily evaluated through a series of scans in which the radioactivity emitted from the radioligand is detected at various time points after administration. This also allows for estimating the degree of exposure to radiation, or the radiation dose an individual receives from the radioligand or radiotracer, and thereby determines the dose of that radiotracer to be administered.

Rarely, there may be reasons to forego animal studies, as in the case of a drug that targets a process for which there is no appropriate animal model. For example, recent studies of radiopharmaceuticals designed to bind to the amyloid plaque in patients with Alzheimer disease (AD) did not require a study in nonhuman primates because there is no good model for AD in these animals. Once a radiotracer is determined to be relatively safe in animal studies, the next step is to appropriately design the phase I safety evaluation in human subjects, to evaluate the metabolic and pharmacologic actions of the study drug, its side effects, and, if possible, its efficacy in producing useful images.

Human studies include a series of preclinical evaluations through various phases of clinical trials to verify the distribution, safety, adverse effects, radiation dose, and physiological and/or pharmacological effects of the radioligand. These studies are also used to optimize the imaging protocol and define the specificity and sensitivity of the radioligand. An example of determining the radiation dose in humans by monitoring the radioactivity distribution within the whole body at multiple time points after the administration of a radioligand is shown in Figure 5.1. Images of the whole body are typically acquired throughout the period in which (p. 85) radioactivity is expected to reside in the subject (see Figure 5.1). Images are then analyzed by measuring the concentration of the radioligands over time (time activity curves) in organs of interest (e.g., liver and urinary bladder in Figure 5.1). The time activity curves in various organs including the brain are input into mathematical models that describe the uptake, retention, and clearance of the radioactivity from each organ of the body and the interaction of radiation with tissues. The radiation dose to critical organs is estimated and used to compare the overall radiological risk associated with other low-radiation exposures, such as diagnostic radiology procedures (see Table 5.2).

Table 5.2. Comparison of the Radiation Doses for Selected Positron Emission Tomography Radiopharmaceuticals

Radiopharmaceutical

Effective Dose Equivalent (mSv/MBq)

18F FDG

0.030

18F NaF

0.023

18F Fluorodopa

0.018

11C IMP

0.018

11C Iomazenil

0.015

18F FPCIT

0.012

11C DASB

0.007

11C Carfentanil

0.005

Radioligand imaging with PET not only provides in vivo evidence of accumulation of a specific radioligand, it can also quantify specific features of the binding sites, such as regional receptor density and the affinity of the radioligand to bind to the receptor. However, the exact in vivo distribution of a radioligand at the time of imaging is influenced by factors other than its affinity and the number of receptors. These include binding to nonspecific sites, BBB permeability, and blood flow. These parameters are included into physiological compartmental models to quantify the radioligands in a manner similar to that applied to CBF, cerebral metabolic rate of oxygen (CMRO2), and cerebral metabolic rate of glucose (CMRGlu) quantification (described in the previous chapter). As described previously, a compartment is a physiological or biochemical “space” in which the tracer concentration is assumed to be homogeneous at all times. Typically, physiological models of neurotransmitter systems consist of three or four compartments, with each compartment presumed to have its own tracer concentration along with the ability of the radiotracer to move back and forth between them at a particular rate. For example, a four-compartment model includes the plasma compartment, the intracerebral compartment in which the tracer is free, a compartment where the radioligand is nonspecifically bound, and a compartment representing the radioligand bound specifically to the receptor of interest. This could be reduced to a three-compartment model by combining the free and nonspecifically bound tracer compartments. This is believed to be justified on the basis of the assumption that the equilibration between free and nonspecifically bound tracer in the brain is rapid compared to the kinetics of specific binding. The transfer of the radiotracer between the compartments is determined by rate constants reflecting movement in each direction (i.e., into or out of the compartments; see Figure 5.2). The rate constants k1 and k2, characterizing the rate of influx and efflux across the BBB, depend on blood flow and permeability. Most radioligands used in PET bind to the neurotransporters or receptors in a reversible manner, therefore requiring the terms k3 (association rate constant) and k4 (dissociation rate constant). The accuracy of radioligand assays depends on the correct estimation of the rate constants and the concentration of the radioligand in the different compartments. Both experimental and mathematical methods are used to derive these parameters.

The classical approach to determining the various parameters for the compartmental models is to perform imaging of the brain at several time points, beginning at the time of administration of the tracer, to measure the concentration of the radioligand in the different compartments, along with a simultaneous determination of the arterial concentration of the radiotracer (ideally obtained by arterial blood sampling). Similar to CBF, CMRO2, and CMRglu quantification, model parameters are solved by applying nonlinear least-squared fit to the experimental data. Despite rigorous data collection and modeling, the total number of available binding sites and the binding affinity of the radioligands cannot be quantified separately in humans. The two terms are collectively represented as the binding potential (BP), which is equivalent to the product of the total number of binding sites and the binding affinity (Mintun, Raichle, Kilbourn, Wooten, & Welch, 1984). BP reflects the capacity of a given (p. 86) tissue, or region of a tissue, for ligand-binding site interaction.

Positron Emission TomographyLigand Imaging

Figure 5.2 Example of a three-compartment model used to describe radioligand kinetics.

In some instances, identification of the most appropriate compartment configuration might be difficult. For example, it may be the case that there are both rapid and slow components to the nonspecific binding. In such cases, accurate measurement of arterial concentration of the radiotracer becomes critical. However, arterial blood sampling is logistically complicated and frequently difficult in the case of human subjects. To avoid this requirement, another approach, called the reference tissue method, has been devised as an alternative. The reference tissue method generally makes the assumption that the concentration and rate constants in specific brain regions such as cerebellum are representative of the plasma compartment (Lammertsma & Hume, 1996). Such reference regions are assumed to exhibit nonspecific binding exclusively (i.e., do not exhibit specific binding for the receptor being studied). In such cases, accurate estimates of radiotracer activity can be obtained by fitting the data using the reference tissue compartment in the model. Using reference tissue in models can be very useful, but unfortunately does not work for all radiopharmaceuticals.

Another general approach in receptor quantification is the use of graphical methods. Logan et al. (1990) proposed this model-independent and mathematically simple approach to quantify tracers that undergo rapid dissociation and efflux from tissue. In this method, different variables can be estimated from the slope of the nearly linear part of the activity curve or graph determined from a series of PET images taken over time. Because this method relies mostly on PET data, it can be applied to radioligand data obtained without arterial sampling.

Ultimately, whatever approach is used, it is important to compare the results from more than one method so as to confirm that the quantification is, in fact, as accurate as possible. However, each radiopharmaceutical can present different challenges and problems with each of these models, thus rendering true quantification of radioligand studies difficult.

Applications of Neurotransmitter Imaging

Once all of the above-mentioned issues are addressed with regard to performing and analyzing radioligand imaging with PET, it becomes crucial to understand what the “normal” distribution is if we are to assess its deviation in neurodevelopmental, neurological, and psychiatric disorders. It is well known that the brain changes substantially throughout the life cycle, therefore an evaluation of these radioligands across the lifespan is essential if they are to be effectively utilized for evaluating specific disorders. For example, studies of schizophrenia occur when patients are in their early adulthood, whereas PD occurs late in life. Knowing how the dopamine system differs in normal subjects of different ages is necessary to accurately evaluate how the dopamine system deviates from the age-appropriate normal state in these disorders. In the following pages, the main neurotransmitter systems studied using radioligand PET are described. For each neurotransmitter system, its synthesis, normal distribution, and age-related changes, if any, are detailed.

Dopaminergic System

The dopaminergic neurons are found predominantly in the retrobulbar areas, substantia nigra (SN), and the ventral tegmental area (VTA) of the mesencephalon. The dopaminergic system plays a major role in several functions including motor functions, working memory, and learning, and it is affected in several neurological and psychiatric disorders such as PD, Huntington disease, tardive dyskinesia, schizophrenia, and substance abuse. Knowledge of normal and altered dopamine synthesis and receptor (p. 87) densities is important for understanding the mechanisms underlying the pathogenesis and therapy in these diseases. Dopamine is synthesized from the amino acid tyrosine in the neurons, which is then stored in the presynaptic vesicles. Axonal depolarization results in the release of the dopamine from the vesicles into the synaptic cleft, where it binds to the receptors. Dopamine receptors exist both in the presynaptic and the postsynaptic membranes. The presynaptic receptors are few in number and regulate dopamine synthesis (by negative feedback), whereas most of the receptors are postsynaptic and mediate the effects of dopamine. After mediating its effect, dopamine is released from the receptor and is then taken up by membrane transporters back into the presynaptic terminal. Here, the dopamine is either metabolized or stored once again in the presynaptic vesicles. Presynaptic dopaminergic function can be assessed by examining dopamine synthesis, storage, and transport. Dopamine synthesis is most commonly studied using [18F]FDOPA and fluorinated m-tyrosine analogs. The vesicular monoamine transporter type 2 (VMAT2), which is responsible for packaging dopamine into the presynaptic vesicles, has been studied using [11C]dihydrotetrabenazine, and the membrane dopamine transporter (DAT) has been examined using [11C]methylphenidate and [11C]cocaine. The latter two radioligands inhibit the reuptake of dopamine by DAT (DAT antagonists). Postsynaptic dopamine receptors can also be studied with PET. There are two main dopamine receptor subtypes: D1-like family (includes D1 and D5 subtypes) and D2-like family (includes D2, D3, and D4 subtypes). D1-like family of receptors when activated (i.e., when they bind to the neurotransmitter dopamine), release the second-messenger cyclic adenosine monophosphate (cAMP) and activation of D2-like receptors results in the inhibition of cAMP. To investigate the role of each receptor subtype, selective and high-affinity PET radioligands are required. Dopamine D1 subtype receptors are most commonly studied using receptor antagonists [11C]Schering 23390 or [11C]NNC 112, whereas [11C]raclopride is a receptor antagonist used to examine the D2/D3 receptor subtypes. [18F]Fallypride is a PET tracer used in the investigation of extrastriatal D2-receptors. Additionally, because of their competitive binding, these radiolabeled receptor antagonists reveal the synaptic concentration of endogenous dopamine and can also measure the effect of drugs that alter the dopamine concentration. In the former case, increases in endogenous dopamine results in decreased binding of the exogenously administered radioligand to the receptors and is seen as decreased concentration of the radioligand during PET imaging. Dopamine metabolism by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) enzymes can also be examined by radioligand PET. By binding to the enzyme MAO, its inhibitors, such as L-deprenyl labeled by [11C], can localize its distribution in the brain.

The neurons from the substantia nigra project to the striatum and the VTA neurons project to limbic areas. D1 and D2 receptor subtypes are predominantly present in the striatum. D1 receptors have been shown to be most concentrated in the basal ganglia, followed by the neocortex, and they are least present in the cerebellum. Striatal-to-cerebellar concentration ratios of D1 receptors have been shown to be 5.77 ± 0.31 (Halldin et al., 1998). Striatal dopamine is released (inferred by a decrease in the binding of D2 antagonist [11C] raclopride) during performance of motor tasks, executive tasks, and tasks associated with rewards. In fact, the placebo effect in several neurological and psychiatric disorders that is thought to reflect an expectation of a reward or benefit appears to be mediated by increased dopamine release in the striatum (de la Fuente-Fernandez, Schulzer, & Stoessl, 2002).

Several PET studies have shown that the uptake of [18F]FDOPA, a dopamine analogue, decreases with age. Cordes et al. (1994) found a 21% decrease in [18F]FDOPA uptake when comparing the uptake in grandparents (ages range from 70 to 80 years) to that in their grandchildren (ages range from 18 to 29 years). The authors suggest that this decrease is consistent with the decline in the number of nigral dopaminergic neurons with age. In fact, the average decrease per year in [18F]FDOPA uptake is similar to the mean decrease in nigral neurons per year (Gibb & Lees, 1991; McGeer, McGeer, & Suzuki, 1977). However, some studies have not found a decrease in [18F]FDOPA uptake with age (Eidelberg et al., 1993; Sawle et al., 1990). In fact, a recent study by Kumakura et al. (2010) has demonstrated that the reason for apparent decreased [18F]FDOPA uptake in the elderly is in part due to a decrease in the vesicular storage of dopamine, thereby leading to an increased breakdown of dopamine (see Figure 5.4). The influx of [18F]FDOPA into the striatum was not different between the young and the old in this study. Another study reported increased MAO activity with increasing age, supporting the notion of increased dopamine breakdown with age (Fowler et al., 1997). (p. 88) (p. 89)

Positron Emission TomographyLigand Imaging

Figure 5.3 Normal distribution of dopamine in the human brain A: D1 subtype receptors mapped using [11C]NNC (adapted from Halldin et al., 1998 with permission from Society of Nuclear Medicine and Molecular Imaging). B: D2/D3 subtype receptors mapped using [11C]FLB 457 (adapted from Cselényi et al., 2006, with permission from the publisher, Elsevier). C: Dopamine mapped by [18F]FDOPA. The basal ganglia have the greater number and cerebellum has almost none. The color scale indicates the maximum concentration in red/yellow and the minimum concentration in blue/black.

Positron Emission TomographyLigand Imaging

Figure 5.4 Average magnitude of FDOPA storage capacity in the basal ganglia groups of healthy younger subjects (n = 14, mean age 32 ± 6 years) shown in the left column and healthy older subjects (n = 14, mean age 55 ± 9 years) shown in the middle. The color scale indicates maximum storage capacity as white and minimum storage capacity as magenta. The Montreal Neurological Institute stereotaxic brain atlas is illustrated in the right column at three planes in the Z-axis at intervals of 10 mm (Z = −15, −5, +5).

Adapted from Kumakura et al., 2010, with permission of the publisher, Elsevier.

Positron Emission TomographyLigand Imaging

Figure 5.5 Loss of D2 dopamine receptors in aging. Transaxial planes at the level of the caudate nucleus demonstrating the relative uptake of [11C]FLB 457, a dopamine D2-like receptor ligand, in a young healthy woman (age 25) and an old healthy woman (age 70), left and right, respectively. The images have been scaled to an equal radioactivity level in the reference region (the cerebellum). The color scale indicates maximum uptake as red and minimum uptake as black.

Adapted from Kaasinen & Rinne, 2002, with permission from the publisher, Elsevier.

Neuroimaging studies measuring D1 receptor numbers have found inconsistent results, with some suggesting a decrease, some no change, and some an increase with age. However, a recent imaging study demonstrated significant declines in D1 receptor binding of approximately 7% per decade in both the striatum as well as in several cortical areas (Wang et al., 1998). Several studies have found age-related decreases in D2 receptor binding (Inoue et al., 2001; Kaasinen et al., 2000) and a decrease in D2 receptor density (Antonini & Leenders, 1993; Rinne et al., 1993; Volkow et al., 1996). After 30 years of age, there appears to be a 0.6% decline per year in [11C]raclopride binding. Figure 5.5 shows the images of the uptake of a D2 receptor-binding radioligand in a young woman and an older woman (Kaasinen et al., 2002). This study also demonstrated that the decrease with age in D2 receptor binding not only occurs in the striatum, but in the extrastriatal regions as well (Kaasinen et al., 2000; see Figure 5.5). Indeed, these age-related reductions in D2 receptor availability of the caudate nucleus are associated with impairments in both motor and cognitive functions (Volkow et al., 1998). One PET study by Tedroff et al. (1988) showed a decline with age of the dopamine transporter using [11C]nomifensine. Collectively, these findings suggest that there is a decline in storage and binding of dopamine with age. These abnormalities in the dopaminergic pathway may explain some of the motor and behavioral symptoms that occur in the elderly.

Changes in the Dopaminergic System in Neuropsychiatric Disorders

Parkinson Disease

The hallmark radioligand PET finding in PD is a decrease in the presynaptic dopamine activity, demonstrated by reduced [18F]FDOPA uptake (Figure 5.6). Specifically, the decrease forms a rostral-caudal gradient in the basal ganglia: the putamen is more affected than the caudate nucleus (Stoessl, Martin, McKeown, & Sossi, 2011). The reduction in tracer uptake correlates with the degree of motor dysfunction (see Figure 5.6). DAT binding is downregulated or reduced in early PD, possibly as a compensatory mechanism, in an attempt to decrease the reuptake of dopamine and thereby increase the level of synaptic dopamine, whereas striatal D2 receptors and D1 receptor binding appear to be spared in PD. In addition, D2 binding is increased in early PD, possibly by an increase in the number of D2 receptors, a mechanism termed upregulation, which returns to normal or slightly below normal following treatment (Stoessl et al., 2011). (p. 90)

Positron Emission TomographyLigand Imaging

Figure 5.6 [18F]FDOPA PET scans of the basal ganglia show the dopamine levels in the striatum in one healthy individual and in three patients with Parkinson disease (PD) of varying severity. The color scale indicates maximum uptake as red and minimum uptake as blue. In mild PD, the decrease is more in the right striatum, and there is a rostral-caudal gradient in which the dopamine decrease is greater in the putamen than in the caudate (white arrow). In severe PD, the decrease in dopamine level is observed in both hemispheres, as well as in both putamen and caudate.

Adapted from Lang & Obeso, 2004, with permission of the publisher, Elsevier.

Radioligand imaging has an additional application in that it can be used to evaluate the changes in dopamine release following interventions such as pharmacotherapy, placebo administration, stem cell therapy, and transcranial magnetic stimulation (Nandhagopal, McKeown, & Stoessl, 2008). [18F]FDOPA imaging has demonstrated an increase in dopamine turnover in the putamen following levodopa treatment in de novo PD (Storch et al., 2013). [18F]FDOPA PET imaging has demonstrated the effects of drugs used in treating PD. Ropinirole, a D2/D3 dopamine agonist resulted in a significantly slower loss of dopamine than levodopa (Whone et al., 2003). D2 receptor imaging can also be used to monitor endogenous dopamine release following an intervention. In such cases, an increase in endogenous dopamine results in a reduction in the striatal binding of a D2 antagonist radioligand. An 1% reduction in tracer binding is thought to reflect a several-fold (8–40 times) increase in synaptic dopamine levels as estimated by microdialysis (Nandhagopal et al., 2008). A recent study demonstrated no alterations in dopamine release during subthalamic nucleus stimulation in patients with PD in a [11C]raclopride PET study (Thobois et al., 2003).

PET imaging in PD using radioligands has helped in differentiating PD form other movement disorders such as multiple system atrophy (MSA) and Huntington disease. Thus, radioligand PET imaging can differentiate PD, mainly a presynaptic disorder, from Huntington disease, which is mainly characterized by damage of postsynaptic striatal neurons, and MSA, in which both pre- and postsynaptic neuronal degeneration is observed.

Schizophrenia

The hypothesis that symptoms of schizophrenia result from increased dopaminergic neurotransmission in the brain first postulated by Van Rossum (1966) has received strong conformation by radioligand PET studies. In a study using 11C labeled (p. 91) L-DOPA, significantly higher influx rate of DOPA were found in the schizophrenic patients compared to the controls in the caudate nucleus, putamen, and medial prefrontal cortex, indicating an elevated synthesis of dopamine in these brain regions in schizophrenia (Lindstrom et al., 1999). Figure 5.7 demonstrates this finding. The increased dopaminergic function has been found even in persons with prodromal signs of schizophrenia (Howes et al., 2009). Antipsychotic drugs compete with endogenous dopamine and bind to D2 receptors, thereby preventing the action of dopamine (Farde, 1997). The magnitude of blockage of receptors by antipsychotic drugs has been shown to correlate with symptom relief (Farde, Wiesel, Halldin, & Sedvall, 1988). Early studies also demonstrated a curvilinear relationship between receptor occupancy and concentration of the drug in the blood. At a high degree of receptor occupancy (>80%), as during an antipsychotic drug treatment, a twofold increase in drug concentration resulted in only a small increase in receptor occupancy (Farde et al., 1988). This phenomenon explains the extrapyramidal adverse effects of the antipsychotic medications such as muscle spasms or dystonia (inability to initiate movement or akinesia) and akathisia (an inability to remain still) at higher concentrations. This finding has directly influenced the clinical management of acute schizophrenic episodes where lower doses of antipsychotics are given without compromising the efficacy of the drug (Jones & Rabiner, 2012). Recently, a meta-analysis study identified a total of 44 studies that collectively compared 618 patients with schizophrenia with 606 controls using PET or single-photon emission computed tomography (SPECT) to measure in vivo striatal dopaminergic function (Howes et al., 2012). The main finding of this meta-analysis was a highly significant elevation (P < 0.001) in presynaptic dopaminergic function in schizophrenia, with a large effect size (Cohen d = 0.79). Dopamine transporter and D2/D3 receptor availability were not observed to be significantly impaired in patients with schizophrenia. Increased D2 receptor density in schizophrenia has been shown to be, in fact, a side effect of prolonged neuroleptic treatment (Farde, 1997; Howes et al., 2012). Thus, the main defect in schizophrenia is in the presynaptic dopaminergic system affecting dopamine synthesis capacity, baseline synaptic dopamine levels, and dopamine release (Howes et al., 2012). Studies have indicated a significant decrease in D1 receptor density in the striatum and the frontal cortex of patients with bipolar affective disorders and in the prefrontal cortex of patients with schizophrenia (Heiss & Herholz, 2006).

Positron Emission TomographyLigand Imaging

Figure 5.7 Group averaged images of the rate of influx of [11C]DOPA: Left: schizophrenic patients (n = 12); right: healthy volunteers (n = 10). Images were averaged over all subjects in each group after matching of images to a standard stereotaxic brain atlas. The color scale indicates maximum uptake as red and minimum uptake as blue. The following anatomical structures are outlined by color code: yellow = outline of the brain, caudate nucleus, putamen; white = prefrontal cortex; black = temporal cortex; and red =caudal medial prefrontal cortex. The white arrow points to the increase in the rate of influx of DOPA in the putamen and caudate regions in schizophrenic patients.

Adapted from Lindstrom et al., 1999, with permission from the publisher, Elsevier.

Addiction

Radioligand PET studies have examined various aspects of addiction including intoxication, (p. 92) withdrawal, and abstinence (Jones & Rabiner, 2012). The main finding in this field is the blunted striatal dopaminergic response at pre- and postsynaptic levels in a range of addictions. [11C]Raclopride PET imaging demonstrated the decreased dopamine release in chronic cocaine abusers compared to controls (Volkow, Wang, & Fowler, 1997), contrary to an early notion that repeated drug exposure resulted in increased dopamine levels (Jones & Rabiner, 2012). Decreases in dopamine levels in the basal ganglia are reported in alcohol and cocaine addictions.

Dementia

In addition to detecting metabolic abnormalities in clinical dementia such as AD and dementia with Lewy bodies (DLB) with [18F]FDG imaging, radioligand PET can differentiate AD from DLB. In AD, presynaptic dopaminergic integrity is maintained, in contrast to DLB (Jones & Rabiner, 2012). Recently abnormalities in the cholinergic system have been identified and are being investigated with appropriate radioligands (see later section). The advent of radioligands that directly bind to the amyloid deposits has been a major breakthrough in the in vivo diagnosis of AD. This is discussed in more detail later.

Serotonergic System

The serotonin or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter that plays an important role in many central nervous system functions including mood, sleep, eating, cognition, learning and memory, and motivation. Serotonergic dysfunction has been implicated in the etiology of many psychiatric disorders including depression, bipolar disorder, obsessive compulsive disorder, anxiety, drug addiction, schizophrenia, and neurological disorders such as PD, AD, and epilepsy (Paterson et al., 2013). Knowledge of normal and altered serotonin synthesis and receptor densities is important for understanding the mechanisms underlying the pathogenesis and therapy of these disorders. The serotonergic system originates in the neurons in the raphe nucleus, located along the midline in the brainstem. Axons from the raphe nuclei terminate in almost all parts of the cerebral cortex, cerebellum, and the spinal cord. Serotonin is synthesized in the neurons from the amino acid L-tryptophan. Similar to dopamine, serotonin is released into the synaptic cleft during synaptic transmission, binds to 5-HT receptors, and initiates the release of second-messengers. There are seven major classes of 5-HT receptors (1–7), and more than 16 subtypes, but suitable PET radioligands are developed for four of the subtypes: 5-HT1A, 5-HT2A, 5-HT1B, and 5-HT4. The 5-HT1 receptors are inhibitory in their function, whereas 5-HT2 receptors are excitatory. The function of 5-HT1A subtype can be studied using the antagonist radioligand [carbonyl-11C]WAY-100635 and the function of 5-HT1B subtype can examined by [11C]P943, also an antagonist. [18F]altanserin is a radioligand used to map the distribution of 5-HT2 type receptors. Psychedelic drugs like mescaline and LSD act as agonists at 5-HT2A/2C receptors. As in the case of dopamine, the action of serotonin is terminated by reuptake of the neurotransmitter by serotonin transporter (SERT). Drugs such as amphetamine, cocaine, and antidepressants inhibit serotonin reuptake. One PET ligand applied in studying SERT is [11C]DASB.

Many imaging and postmortem studies have identified the presence of 5-HT1A receptors in high density in the hippocampus, septum, amygdala, hypothalamus, and neocortex of the human brain. 5-HT2A receptors are found in all neocortical regions, as well as in the hippocampus, the basal ganglia, and the thalamus. The cerebellum, the striatum, and the brainstem do not appear to have 5-HT2A receptors. SERT is mainly found in the midbrain, the thalamus, and the striatum. Recently, Savli et al. (2012) undertook an ambitious but difficult task of developing a normative database of the serotonergic system in healthy subjects and successfully mapped the 5-HT1A, 5-HT2A, 5-HT1B receptors and SERT in 95 healthy adults and generated a whole-brain map of the distribution of these receptors (Figures 5.8 and 5.9). There was good concordance between the regional distribution of the serotoninergic receptors obtained in this study and the receptor density distribution measured in postmortem studies (Savli et al., 2012). This database provides a normal receptor distribution pattern in a standardized template for comparison across different studies and other imaging modalities. Developing such normative maps is an important step toward understanding the normal distribution and enabling proper interpretation of radioligand imaging findings in diseases.

Several studies have examined the changes in the serotonergic system with age. For example, a study of the serotonin reuptake receptors demonstrated a decline in binding of 9.6% per decade in the thalamus and 10.5% per decade in the midbrain (Yamamoto et al., 2002). Another study demonstrated a slightly lower rate of decline in women with age (Kuikka et al., 2001). An age-related decrease in activity of the 5-HT1A receptor has been reported, (p. 93) with a decline of approximately 10% per decade in a number of cortical areas except for the medial temporal cortex (Tauscher et al., 2001). However, another report suggested that this finding is much more prominent in men and not significant in women (Meltzer et al., 2001). Similar decreases have been reported for the 5-HT2A receptors, with an overall 42% decline between the ages of 23 and 60 years (Baeken et al., 1998). Furthermore, a more recent study has reported that this decline is nonlinear, with most of the decrease occurring through mid-life (Sheline, Mintun, Moerlein, & Snyder, 2002), and the decrease remains significant even when corrected for age-related cerebral atrophy (Meltzer et al., 1998).

Positron Emission TomographyLigand Imaging

Figure 5.8 Distribution of the 5-HT1A receptor (A), the 5-HT2A receptor (B), 5-HT1B receptor (C), and the serotonin transporter (SERT) (D) in the human brain measured with PET using the selective radioligands [carbonyl-11C]WAY-100635, [18F]altanserin, [11C]P943, and [11C]DASB, respectively. Color tables indicate receptor binding potentials. Maps are mean PET images based on healthy controls superimposed on a T1-weighted magnetic resonance imaging (MRI) in standardized space. Sagittal view 1 mm left of midline.

Adapted from Savli et al., 2012, with permission from the publisher, Elsevier.

Changes in the Serotonergic System in Neuropsychiatric Disorders

Depression and Anxiety

PET imaging has played an important role in advancing the understanding of the pathophysiology of affective disorders and the vulnerability traits that make some individuals with these traits prone to having affective disorders, and in assessing their treatment efficacy (Jones & Rabiner, 2012). The levels of monoamine neurotransmitters such as serotonin, dopamine, and norepinephrine are decreased in major depressive disorder (MDD). In fact, elevated levels of one enzyme, MAO-A, which metabolizes these monoamines, has been identified to be the primary abnormality in MDD; Jones & Rabiner, 2012). The increased MAO-A enzyme causes greater metabolism of the monoamines and hence their decreases their levels. A substantial reduction in the binding potential of 5-HT1A was found in the raphe (42% less than controls) and in the mesotemporal cortex (25–33% less than controls) in patients with depression (Drevets et al., 1999). The decreased binding potential of 5-HT1A is thought to reflect either a downregulation of 5HT1A receptor gene expression or a reduction in the number (p. 94) of cellular processes expressing 5HT1A receptors (Drevets et al., 1999). These findings have helped in the development of new pharmacological treatments in MDD. Radioligand PET imaging has also led to the discovery that at least 80% occupancy of the SERT by serotonin reuptake inhibitors (SSRI) is critical for therapeutic effects. Abnormalities in the serotonergic neurotransmitter system have been demonstrated also in persons who have an environmental (e.g., depressed mood during postpartum) or genetic (e.g., healthy twin of a depressed patient) vulnerability to MDD (Jones & Rabiner, 2012). Another disorder that has been studied with radioligand PET is anxiety. The salient finding in anxiety disorder has been a reduction in midbrain SERT and 5-HT1A receptors (Nikolaus, Antke, Beu, & Müller, 2010).

Positron Emission TomographyLigand Imaging

Figure 5.9 Distinct topological pattern of the 5-HT1A (blue), 5-HT2A (green), 5-HT1B (yellow) receptors and the serotonin transporter (red) across normal human brain listed according to the Brodmann area organizational scheme, including 41 regions of interest. The radial axis indicates binding potential (BP) values measured by positron emission tomography (PET). BP values are averaged for left and right hemisphere regions. Each shaded arc represents Brodmann areas that share a functional attribute. For example, Brodmann areas 4, 6, 8, and 32 are motor regions; and Brodmann areas 1, 2, 3, 5, 31, and 40 have somatosensory functions. 5-HT1A (blue pattern) is the most widely distributed serotonergic receptor in the brain.

Adapted from Savli et al., 2012, with permission from the publisher, Elsevier.

Parkinson Disease

Recently, in PD, abnormalities in other neurotransmitter systems such as serotonin are also being investigated. These abnormalities may explain the nonmotor complications seen in PD such as cognitive decline, depression, and anxiety. A decrease in cholinergic activity is observed in PD patients with dementia (Stoessl et al., 2011).

Opioid Receptors

The opioid family includes endogenous opioids like endorphin and enkephalin and exogenous opioids like morphine, codeine, heroin, and pethidine. The action of these drugs is mediated by opioid receptors (Henriksen & Willoch, 2008). To date, four types of opioid receptors are known: the mu (μ), kappa (κ), delta (δ), and the nociception (p. 95) receptors. The opioid receptors are distributed widely in the central nervous system, peripheral sensory system, and the autonomic nerves. Activation of opioid receptors most commonly results in inhibitory effects. Opioid peptides often are present in neurons that also contain glutamate, a fast-acting neurotransmitter, and, in such cases, opioids act as co-transmitters. Opioids also result in an indirect excitation of neurons by inhibiting the presynaptic release of GABA, termed “disinhibition.” In general, agonists selective for μ or δ receptor are analgesic and rewarding, whereas κ receptor selective agonists produce aversive effects like dysphoria and hallucinations. Opioids are important in signaling mechanisms in pain and analgesia, mood, tolerance and dependence, epilepsy, neurodegeneration, and movement disorders.

Positron Emission TomographyLigand Imaging

Figure 5.10 [11C] Carfentanil positron emission tomography (PET) brain scan in a health subject showing uptake in the basal ganglia and thalamus and much of the cortex. There is minimal uptake in the occipital lobe. Images are normalized into the Montreal Neurological Institute (MNI) stereotaxic brain atlas space, voxel-size is 2 × 2 × 2 mm3. X, Y, and Z denote the MNI coordinates, and color bar indicates binding potential, with greater binding shown in red and minimal binding represented in blue/black.

Adapted from Tuominen et al., 2013, with permission from the publisher, Wiley.

Exogenous opioids like morphine, codeine, heroin, and pethidine can be labeled with [11C] but, because of the complex metabolism of these compounds and nonspecific binding, these tracers are not well suited for opioid receptor imaging (Heiss & Herholz, 2006). Successful PET tracers have been obtained with [11C]carfentanil, a potent synthetic μ opiate antagonist, and 11C-diprenorphine (DPN) and 18F-cyclofoxy, which bind to all types of opioid receptors.

Using [11C]carfentanil, the μ opioid receptors are identified to have the highest concentration in the basal ganglia and thalamus (Frost et al., 1985; see Figure 5.10). Intermediate levels of μ receptors were found in the frontal and parietal cortex, with the occipital cortex and cerebellum having low concentrations. Delta receptors are in high concentrations in the frontal cortex and putamen, but are in low levels in the thalamus and absent in the cerebellum (Henriksen & Willoch, 2008). In a recent study in nonhuman primates, the κ receptor was mapped in high concentrations in the cingulate cortex, the striatum, frontal cortex, temporal cortex, and the parietal cortex (Talbot et al., 2005). Intermediate levels of the κ receptors were reported in the thalamus and medial temporal lobe, and low levels were seen in brainstem and occipital cortex.

The concentration of the opioid receptors appears to vary with age and gender. Mu receptor binding increases with age in the neocortical areas, caudate, and putamen. Women have higher binding of μ receptors in thalamus, amygdala, and cerebellum (Henriksen & Willoch, 2008). However, in postmenopausal women, the μ binding in thalamus and amygdala is decreased to levels below that in men.

Changes in the Opioid Receptors in Neuropsychiatric Disorders

Pain

Jones et al. (1991) studied the distribution of the opioid receptors in regions of the brain involved in processing pain using [11C]DPN. Higher levels of opioid receptor binding were seen in the cortical projections of the medial pain system (a functional subdivision of brain areas mediating the affective-motivational components in pain perception that includes areas like the cingulate and frontal cortex). Focally decreased binding was also observed in the primary sensorimotor cortex, a part of the lateral pain system (areas mediating sensory discriminative components in pain perception). The opioid receptor system plays a central role in perception and emotional processing of pain, and reductions in receptor availability are observed in chronic pain, suggesting either an increased endogenous opioid release or a downregulation of the opioid receptors. (p. 96) It has also been shown that the activation of the μ receptor system was associated with reductions in the sensory and affective ratings of the pain experience. Interestingly, related to this observation, placebo effects have been demonstrated to be mediated by the μ receptors in brain areas involved in pain and affective regulation and motivated behavior, such as the anterior cingulate cortex, orbitofrontal and dorsolateral prefrontal cortex, anterior and posterior insula, nucleus accumbens, amygdala, thalamus, hypothalamus, and the periaqueductal gray (Zubieta et al., 2005). Thus, both dopaminergic and opioid neurotransmitters respond to expectation of symptom amelioration during placebo administration.

Epilepsy

In temporal lobe epilepsy, [11C]carfentanil binding was increased in the amygdala and the temporal cortex, suggesting an upregulation of μ opioid receptors or decreased occupancy by endogenous peptides (Frost et al., 1988). Delta opioid receptors also demonstrate an increase in binding, but κ receptors are found to be normal (Heiss & Herholz, 2006) in epilepsy. A recent radioligand study indicates that there may be an increased availability of μ and δ receptors during the ictal phase, but no identifiable asymmetry was observed in the overall distribution of opioid receptors in patients with epilepsy (Heiss & Herholz, 2006). Hence, the μ and δ receptors may play a role in an anticonvulsive mechanism by limiting the spread of electrical activity from the epileptogenic focus.

Movement Disorders

A decrease in the concentration of endogenous opioids in the basal ganglia, normally present in high concentrations, is thought to mediate levodopa-induced involuntary movements or dyskinesias in PD. Similarly, decreased levels of endogenous opioids and loss of opioid receptors in the basal ganglion are reported in another movement disorder, Huntington disease (Heiss & Herholz, 2006).

Psychiatric Disorders

Abnormalities in the opioid system have been identified in affective disorders, eating disorders, and addiction using radioligand PET imaging methods (Henriksen & Willoch, 2008).

The Cholinergic System

In addition to being present ubiquitously in the autonomic and peripheral nervous systems, acetylcholine (ACh) is synthesized by neurons in the brainstem and the basal forebrain. These neurons project to the subcortical regions, the cerebellum, thalamus, hippocampus, limbic areas, and most of the neocortex. The ACh in the presynaptic terminal is stored in vesicles, and activation of the cholinergic neurons results in its release into the synaptic cleft. The action of ACh is mediated by two types of receptors—nicotinic and muscarinic—and is deactivated by the enzyme acetylcholinesterase (AChE). The activity of this enzyme can be measured by radiolabeled ACh analogs such as N-methyl-3-piperydyl-acetate (MP3A) and N-methylpiperidin-4-yl-acetate (MP4A). Nicotinic receptors can be mapped by [11C]nicotine, and muscarinic receptors can be imaged using [11C]scopolamine. The highest binding for these ligands is usually found in the cortex, the striatum, thalamus, and the pons. Several PET studies of the muscarinic cholinergic receptor ligand in healthy volunteers from 18 to 82 years old have demonstrated a decrease of 45–50% over the age range (Dewey et al., 1990; Lee et al., 1996; Suhara, Inoue, Kobayashi, Suzuki, & Tateno, 1993). Studies in patients with AD have demonstrated a widespread reduction of AChE activity in the cerebral cortex and the hippocampus (see Figure 5.11).

The γ-Aminobutyric Acid (GABA) System

The amino acid GABA is the most important of the inhibitory neurotransmitters in the brain. GABA is synthesized from glutamate in a reaction mediated by the enzyme glutamic acid decarboxylase. The neurons that have GABA are present almost universally in the brain, mainly as inhibitory interneurons. Actions of GABA are mediated through GABAA and GABAB receptors. The GABAA receptor complex consists of a central benzodiazepine receptor, which specifically mediates all the pharmacological effects of the benzodiazepines (sedative, anxiolytic, anticonvulsant, and muscle relaxant effects; Heiss & Herholz, 2006). GABA transporter molecules shuttle GABA into presynaptic vesicles, and the free GABA is metabolized by the enzyme GABA transaminase. [11C]Flumazenil (FMZ) is a radioligand that binds to the GABAA and benzodiazepine receptor complex. Flumazenil binding is age dependent, with highest values reached at approximately 2 years of age, followed by a steady decrease until adult values are reached at age 14–22 years (Heiss & Herholz, 2006). The inhibitory networks in epilepsy have been investigated by PET using [11C] (p. 97) Flumazenil, and decreases in the benzodiazepine-GABAA receptor complex expression have been reported in cortical areas that appear normal in magnetic resonance imaging (MRI). In fact, brain areas where the benzodiazepine-GABAA receptor complex expression is decreased can be identified to be epileptogenic foci (Jones & Rabiner, 2012). In Huntington disease, the density of benzodiazepine receptors is decreased in the caudate and putamen (Heiss & Herholz, 2006). Additionally, benzodiazepine receptors have been shown to be abnormal in AD, ischemic stroke, and schizophrenia.

Positron Emission TomographyLigand Imaging

Figure 5.11 [11C]MP4A PET in Alzheimer disease (bottom panel) demonstrates decreased acetylcholinesterase (AChE) activity in cortex and amygdala (red arrows in the bottom right image) but preserved activity in basal forebrain (blue arrows in the bottom images) compared to a health individual (top panel). AChE, acetylcholinesterase; nbM, nucleus basalis of Meynert present in the basal forebrain region.

Adapted from Heiss & Herholz, 2006, with permission from Society of Nuclear Medicine and Molecular Institute.

Other Neurotransmitters

The adenosine receptors play a role in neuromodulation and are altered in epilepsy, stroke, movement disorders, and schizophrenia. Their distribution in the human brain can be studied using radiolabeled xanthine analogs. These tracers might have a potential for predicting the severity of tissue damage in early states of ischemic stroke (Heiss & Herholz, 2006).

The involvement of the norepinephrine transporter (NET) in the pathophysiology and treatment of attention deficit hyperactivity disorder, substance abuse, AD, PD, and depression has long been recognized. Therefore, the ability to examine the NET system in humans is being explored with [11C]-labeled methylreboxetine.

The amino acid glutamate interacts with three types of receptors: N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainate (KA). There is growing evidence for glutamatergic abnormalities in schizophrenia, specifically in the NMDA receptor (Stone, 2009). At present, NMDA receptor binding can be examined by SPECT using the radioligand [123I]CNS-1261. (p. 98)

Positron Emission TomographyLigand Imaging

Figure 5.12 Increased [11C]PK-11195 binding indicating widespread microglial activation in Parkinson disease (PD). Transaxial (A and C) and coronal (B and D) sections of binding potential maps co-registered to the individual magnetic resonance image (MRI) are shown. In the PD patient (A and B), binding is increased in the basal ganglia, pons, and frontal regions, while the healthy control person (C and D) only shows constitutive [11C]PK-11195 binding in the thalamus and pons. The color bar denotes normalized binding potential values from 0 to 1.

Adopted from Gerhard et al., 2006, with permission of the publisher, Elsevier.

Recent Advances in Radioligand PET

The process of discovering new radioligands for use in PET is an ongoing process, and recent developments have made a significant impact on advancing our understanding of the pathophysiology of several neurological and psychiatric disorders.

Microglia Mapping

Translocator protein (TSPO) is a membrane-bound protein located in the mitochondria that regulates cholesterol movement into the mitochondria in the periphery and is thought to mediate apoptosis or cell death in the brain. This protein is expressed extensively in the periphery, but to a much lesser extent in the healthy human brain. However, TSPO expression is increased in cases of brain injury and inflammation, and this increased expression correlates with the extent of microglial activation (Scarf & Kassiou, 2011). Therefore, using high-affinity, selective TSPO ligands in PET, it is possible to study the process of microglial activation in the living brain (Scarf & Kassiou, 2011). A selective ligand that binds to TSPO, called PK-11195 and labeled with [11C], has been used as a marker of disease activity in inflammatory diseases such as multiple sclerosis and as an indicator of glia–macrophage activation in ischemia, AD, schizophrenia, PD (see Figure 5.12), progressive supranuclear palsy, and brain tumors.

Amyloid Imaging

The presence of amyloid lesions composed of senile plaques and neurofibrillary tangles is a neuropathological hallmark of the AD brain. Previously, the confirmation of AD diagnosis was only made postmortem. However, recent advances in radioligand imaging now permits the in vivo premortem diagnosis of AD. A major breakthrough in amyloid imaging was developing ligands that could efficiently bind to the amyloid fibrils. The first PET radioligand that successfully imaged amyloid lesions in living AD patients was 2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene) (p. 99) alononitrile (FDDNP). There is greater retention of FDDNP in brain regions that have the amyloid lesions as compared to regions with no amyloid deposits. Unfortunately, specific signals of [18F]FDDNP in amyloid-rich regions of AD brains were not very much higher than those in the cerebellar reference region (Mori et al., 2012), thereby making it difficult to use it in the diagnosis of AD. A more widely used amyloid tracer [11C]-6-OH-BTA-1, or Pittsburgh Compound-B (PIB) has better affinity to amyloid lesions. Recently, PIB has been proven to be very useful in detecting amyloid at early symptomatic and even presymptomatic stages of AD (Mori et al., 2012). The [11C]PIB retention is increased in cortical areas known to be affected in AD, primarily parietal and temporal cortex (see Figure 5.13).

Positron Emission TomographyLigand Imaging

Figure 5.13 Representative amyloid positron emission tomography (PET) images showing standardized uptake value for Pittsburgh Compound-B (PIB). Increased uptake values (in yellow and red) reflect greater PIB binding in many cortical areas of AD than in normal control. AD, Alzheimer disease; PIB, Pittsburgh Compound-B.

Adapted from Mori et al., 2012, with permission from the publisher, Wiley.

Because 11C has a short half-life (20 minutes), new amyloid binding ligands were developed that were labeled with 18F. [18F]florbetapir ([18F]AV-45), one such radioligand, was submitted to the US Food and Drug Administration (FDA) for diagnostic use approval. It was demonstrated that [18F]AV-45-PET data correlated with the presence and density of amyloid lesions identified by postmortem pathological analysis. In 2012, the FDA approved PET imaging using of [18F]AV-45 as tool for use in the diagnosis of AD.

Blood–Brain Barrier Imaging

P-glycoprotein (P-gp) at the BBB has an important function of actively pumping out a wide range of molecules from the brain. This action is important for protecting the brain against toxic substances in the blood. Recently, P-gp function has shown to be altered in neurodegenerative diseases and various neurological and psychiatric disorders (Bartels, de Klerk, Kortekaas, de Vries, & Leenders, 2010). Verapamil, a substrate for P-gp, labeled with [11]C allows the in vivo measurement of P-gp function at the human BBB.

Using [11C]verapamil PET, Feldmann and Koepp (2012) showed that patients with epilepsy who are resistant to antiepileptic drugs (AED) showed decreased uptake of the substrate [11C]verapamil in PET when compared to patients whose seizures are controlled by AEDs, indicating an overexpression of P-gp at the BBB in the AED-resistant group. This overexpression of P-gp is thought to make it harder for the AED to pass the BBB. These findings provide new insight into possible mechanisms of drug resistance.

In another exciting study using [11C]verapamil PET, decreased P-gp function was demonstrated in patients with AD (van Assema et al., 2012). This is the first direct evidence that the P-gp transporter at the BBB is compromised in AD, suggesting that it may be involved in AD pathogenesis.

In conclusion, radioligand PET is a unique imaging method that enables the in vivo 3D imaging of the human neurotransmitter systems not feasible by any other imaging method. Overall, radioligand PET imaging has been utilized to assess (p. 100) neurotransmitter systems in normal individuals and in a wide variety of disorders. For the first time in the history of noninvasive neuroimaging, the normal distribution pattern of several neurotransmitters in humans is possible because of radioligand PET. Furthermore, radioligand PET has contributed greatly to the understanding of the physiological basis for the placebo effect, as well as the pathophysiology of disorders like PD, schizophrenia, and drug addition. Most of these imaging results, although aiding the diagnosis of several neuropsychiatric disorders and the evaluation of the treatment effects, still lie mainly in the realm of research. Future studies are needed to explore how the growing number of neurotransmitter ligands can be applied clinically in the diagnosis and follow-up for many of the neuropsychiatric disorders discussed in this chapter. However, the recent advances are rapidly gaining clinical relevance, as evidenced by the FDA’s approval of [18F]Florbetapir-PET as a diagnostic tool. Using this technique, AD can now be diagnosed early in its course. Ultimately, identifying and validating additional clinical applications will be necessary so that PET imaging continues to play a key role in the management of several neurological, psychiatric, and neurodegenerative disorders.

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