An apparatus and behavioral training protocol to conduct positron emission tomography (PET) neuroimaging in conscious rhesus monkeys

Individually customisable non-invasive head immobilisation system for non-human primates with an option for voluntary engagement

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Non-invasive head immobilisation for neuroscience experiments in monkeys.

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Individually customised system combining functionality of previous systems.

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Allows access for auditory and visual stimulation.

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Has the option for voluntary engagement to assist habituation.

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Systematically evaluated against scientific and animal welfare needs.

Background

Head immobilisation is often necessary for neuroscientific procedures. A number of Non-invasive Head Immobilisation Systems (NHIS) for monkeys are available, but the need remains for a feasible integrated system combining a broad range of essential features.

New method

We developed an individualised macaque NHIS addressing several animal welfare and scientific needs. The system comprises a customised-to-fit facemask that can be used separately or combined with a back piece to form a full-head helmet. The system permits presentation of visual and auditory stimuli during immobilisation and provides mouth access for reward.

Results

The facemask was incorporated into an automated voluntary training system, allowing the animals to engage with it for increasing periods leading to full head immobilisation. We evaluated the system during performance on several auditory or visual behavioural tasks with testing sessions lasting 1.5–2 h, used thermal imaging to monitor for and prevent pressure points, and measured head movement using MRI.

Comparison with existing methods

A comprehensive evaluation of the system is provided in relation to several scientific and animal welfare requirements. Behavioural results were often comparable to those obtained with surgical implants. Cost–benefit analyses were conducted comparing the system with surgical options, highlighting the benefits of implementing the non-invasive option.

Conclusions

The system has a number of potential applications and could be an important tool in neuroscientific research, when direct access to the brain for neuronal recordings is not required, offering the opportunity to conduct non-invasive experiments while improving animal welfare and reducing reliance on surgically implanted head posts.

Functional connectivity in frontal-striatal brain networks and cocaine self-administration in female rhesus monkeys

RATIONALE

Cocaine addiction is characterized by alternating cycles of abstinence and relapse and loss of control of drug use despite severe negative life consequences associated with its abuse.

OBJECTIVE

The objective of the present study was to elucidate critical neural circuits involved in individual vulnerabilities to resumption of cocaine self-administration following prolonged abstinence.

METHODS

The subjects were three female rhesus monkeys in prolonged abstinence following a long history of cocaine self-administration. Initial experiments examined the effects of acute cocaine administration (0.3 mg/kg, IV) on functional brain connectivity across the whole brain and in specific brain networks related to behavioral control using functional magnetic resonance imaging in fully conscious subjects. Subsequently, these subjects were allowed to resume cocaine self-administration to determine whether loss of basal connectivity within specific brain networks predicted the magnitude of resumption of cocaine intake following prolonged abstinence.

RESULTS

Acute cocaine administration robustly decreased global functional connectivity and selectively impaired top-down prefrontal circuits that control behavior, while sparing connectivity of striatal areas within limbic circuits. Importantly, impaired connectivity between prefrontal and striatal areas during abstinence predicted cocaine intake when these subjects were provided renewed access to cocaine.

CONCLUSIONS

Based on these findings, loss of prefrontal to striatal functional connectivity may be a critical mechanism underlying the negative downward spiral of cycles of abstinence and relapse that characterizes cocaine addiction.

PET studies in nonhuman primate models of cocaine abuse: translational research related to vulnerability and neuroadaptations

The current review highlights the utility of positron emission tomography (PET) imaging to study the neurobiological substrates underlying vulnerability to cocaine addiction and subsequent adaptations following chronic cocaine self-administration in nonhuman primate models of cocaine abuse. Environmental (e.g., social rank) and sex-specific influences on dopaminergic function and sensitivity to the reinforcing effects of cocaine are discussed. Cocaine-related cognitive deficits have been hypothesized to contribute to high rates of relapse and are described in nonhuman primate models. Lastly, the long-term consequences of cocaine on neurobiology are discussed. PET imaging and longitudinal, within-subject behavioral studies in nonhuman primates have provided a strong framework for designing pharmacological and behavioral treatment strategies to aid drug-dependent treatment seekers. Non-invasive PET imaging will allow for individualized treatment strategies. Recent advances in radiochemistry of novel PET ligands and other imaging modalities can further advance our understanding of stimulant use on the brain. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Environmental modulation of drug taking: Nonhuman primate models of cocaine abuse and PET neuroimaging

The current review highlights the importance of environmental variables on cocaine self-administration in nonhuman primate models of drug abuse. In addition to describing the behavioral consequences, potential mechanisms of action are discussed, based on imaging results using the non-invasive and translational technique of positron emission tomography (PET). In this review, the role of three environmental variables - both positive and negative - are described: alternative non-drug reinforcers; social rank (as an independent variable) and punishment of cocaine self-administration. These environmental stimuli can profoundly influence brain function and drug self-administration. We focus on environmental manipulations involving non-drug alternatives (e.g., food reinforcement) using choice paradigms. Manipulations such as response cost and social variables (e.g., social rank, social stress) also influence the behavioral effects of drugs. Importantly, these manipulations are amenable to brain imaging studies. Taken together, these studies emphasize the profound impact environmental variables can have on drug taking, which should provide important information related to individual-subject variability in treatment responsiveness, and the imaging work may highlight pharmacological targets for medications related to treating drug abuse. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The helmet head restraint system: a viable solution for resting state fMRI in awake monkeys

In monkey neuroimaging, head restraint is currently achieved via surgical implants. Eradicating such invasive head restraint from otherwise non-invasive monkey studies could represent a substantial progress in terms of Reduction and Refinement. Two non-invasive helmet-based methods are available but they are used exclusively by the pioneering research groups who designed them. In the absence of independent replication, they have had little impact in replacing the surgical implants. Here, we built a modified version of the helmet system proposed by Srihasam et al. (2010 NeuroImage, 51(1), 267-73) and tested it for resting state fMRI in awake monkeys. Extremely vulnerable to motion artifacts, resting state fMRI represents a decisive test for non-invasive head restraint systems. We compared two monkeys restrained with the helmet to one monkey with a surgically implanted head post using both a seed-based approach and an independent component analysis. Technically, the helmet system proved relatively easy to develop. Scientifically, although it allowed more extensive movements than the head post system, the helmet proved viable for resting state fMRI, in particular when combined with the independent component analysis that deals more effectively with movement-related noise than the seed-based approach. We also discuss the pros and cons of such device in light of the European Union new 2013 regulation on non-human primate research and its firm Reduction and Refinement requests.

Anaesthesia for positron emission tomography scanning of animal brains

Positron emission tomography (PET) provides a means of studying physiological and pharmacological processes as they occur in the living brain. Mice, rats, dogs, cats, pigs and non-human primates are often used in studies using PET. They are commonly anaesthetized with ketamine, propofol or isoflurane in order to prevent them from moving during the imaging procedure. The use of anaesthesia in PET studies suffers, however, from the drawback of possibly altering central neuromolecular mechanisms. As a result, PET findings obtained in anaesthetized animals may fail to correctly represent normal properties of the awake brain. Here, we review findings of PET studies carried out either in both awake and anaesthetized animals or in animals given at least two different anaesthetics. Such studies provide a means of estimating the extent to which anaesthesia affects the outcome of PET neuroimaging in animals. While no final conclusion can be drawn concerning the 'best' general anaesthetic for PET neuroimaging in laboratory animals, such studies provide findings that can enhance an understanding of neurobiological mechanisms in the living brain.

Nonhuman primate models of addiction and PET imaging: dopamine system dysregulation

This chapter highlights the use of nonhuman primate models of cocaine addiction and the use of positron emission tomography (PET) imaging to study the role of individual differences in vulnerability and how environmental and pharmacological variables can impact cocaine abuse. The chapter will describe studies related to the dopamine (DA) neurotransmitter system, and focus primarily on the D2-like DA receptor, the DA transporter and the use of fluorodeoxyglucose to better understand the neuropharmacology of cocaine abuse. The use of nonhuman primates allows for within-subject, longitudinal studies that have provided insight into the human condition and serve as an ideal model of translational research. The combination of nonhuman primate behavior, pharmacology and state-of-the-art brain imaging using PET will provide the foundation for future studies aimed at developing behavioral and pharmacological treatments for drug addiction in humans.

Neuroimaging and drug taking in primates

RATIONALE

Neuroimaging techniques have led to significant advances in our understanding of the neurobiology of drug taking and the treatment of drug addiction in humans. Neuroimaging approaches provide a powerful translational approach that can link findings from humans and laboratory animals.

OBJECTIVE

This review describes the utility of neuroimaging toward understanding the neurobiological basis of drug taking and documents the close concordance that can be achieved among neuroimaging, neurochemical, and behavioral endpoints.

RESULTS

The study of drug interactions with dopamine and serotonin transporters in vivo has identified pharmacological mechanisms of action associated with the abuse liability of stimulants. Neuroimaging has identified the extended limbic system, including the prefrontal cortex and anterior cingulate, as important neuronal circuitry that underlies drug taking. The ability to conduct within-subject longitudinal assessments of brain chemistry and neuronal function has enhanced our efforts to document long-term changes in dopamine D2 receptors, monoamine transporters, and prefrontal metabolism due to chronic drug exposure. Dysregulation of dopamine function and brain metabolic changes in areas involved in reward circuitry have been linked to drug taking behavior, cognitive impairment, and treatment response.

CONCLUSIONS

Experimental designs employing neuroimaging should consider well-documented determinants of drug taking, including pharmacokinetic considerations, subject history, and environmental variables. Methodological issues to consider include limited molecular probes, lack of neurochemical specificity in brain activation studies, and the potential influence of anesthetics in animal studies. Nevertheless, these integrative approaches should have important implications for understanding drug taking behavior and the treatment of drug addiction.

Acute brain metabolic effects of cocaine in rhesus monkeys with a history of cocaine use

Cocaine addiction involves an escalation in drug intake which alters many brain functions. The present study documented cocaine-induced changes in brain metabolic activity as a function of cocaine self-administration history. Experimentally naive rhesus monkeys (N = 6) were given increasing access to cocaine under a fixed-ratio schedule of intravenous (i.v.) drug self-administration. PET imaging with F-18 labeled fluorodeoxyglucose (FDG) was used to measure acute intramuscular (i.m.) cocaine-induced changes in brain metabolism in the cocaine-naïve state, following 60 sessions under limited-access conditions (1 h/day), following 60 sessions under extended-access conditions (4 h/day), and following 4 weeks of drug withdrawal. In the cocaine-naïve state, cocaine-induced increases in brain metabolism were restricted to the prefrontal cortex. As cocaine exposure increased from limited to extended access, metabolic effects expanded throughout the frontal cortex and were induced within the striatum. Conversely, cocaine-induced activation was far less robust following withdrawal. The results highlight a progressive expansion of the metabolic effects of cocaine to include previously unaffected dopamine innervated brain regions as a consequence of cocaine self-administration history. The identification of brain regions progressively influenced by drug exposure may be highly relevant toward efforts to develop treatments for cocaine addiction.

Nonhuman primate positron emission tomography neuroimaging in drug abuse research

Positron emission tomography (PET) neuroimaging in nonhuman primates has led to significant advances in our current understanding of the neurobiology and treatment of stimulant addiction in humans. PET neuroimaging has defined the in vivo biodistribution and pharmacokinetics of abused drugs and related these findings to the time course of behavioral effects associated with their addictive properties. With novel radiotracers and enhanced resolution, PET neuroimaging techniques have also characterized in vivo drug interactions with specific protein targets in the brain, including neurotransmitter receptors and transporters. In vivo determinations of cerebral blood flow and metabolism have localized brain circuits implicated in the effects of abused drugs and drug-associated stimuli. Moreover, determinations of the predisposing factors to chronic drug use and long-term neurobiological consequences of chronic drug use, such as potential neurotoxicity, have led to novel insights regarding the pathology and treatment of drug addiction. However, similar approaches clearly need to be extended to drug classes other than stimulants. Although dopaminergic systems have been extensively studied, other neurotransmitter systems known to play a critical role in the pharmacological effects of abused drugs have been largely ignored in nonhuman primate PET neuroimaging. Finally, the study of brain activation with PET neuroimaging has been replaced in humans mostly by functional magnetic resonance imaging (fMRI). There has been some success in implementing pharmacological fMRI in awake nonhuman primates. Nevertheless, the unique versatility of PET imaging will continue to complement the systems-level strengths of fMRI, especially in the context of nonhuman primate drug abuse research.

Effects of cocaine rewards on neural representations of cognitive demand in nonhuman primates

RATIONALE

Investigations of the neural consequences of the effects of cocaine on cognition have centered on specific brain circuits including prefrontal cortex, medial temporal lobe and striatum and their roles in controlling drug dependent behavior and addiction. These regions are critical to many aspects of drug abuse; however recent investigations in addicted individuals have reported possible cognitive deficits that impact recovery and other therapeutic interventions.

OBJECTIVES

Therefore a direct assessment of the effects of cocaine as a reward for cognitive function provides a means of determining how brain systems involved such as prefrontal cortex are affected under normal vs. conditions of acute drug exposure as a precursor to the final impaired function in the addicted state.

METHODS

Nonhuman primates (NHPs) were tested in a delayed-match-to-sample decision making task to determine effects of high vs. low cognitive load trials on single neuron activity and fluorodeoxyglucose-positron emission tomography (FDG-PET) determined metabolic activation of prefrontal cortex when juice vs. intravenous cocaine were employed as rewards for successful performance.

RESULTS

Cognitive processing in prefrontal cortex was altered primarily on high load trials in which cocaine was randomly presented as the signaled and delivered reward on particular trials. The detrimental actions of cocaine rewards were also shown to persist and impair task performance on subsequent juice rewarded trials.

CONCLUSIONS

The findings indicate that one of the ways in which cocaine use may disrupt performance of a cognitive task is to alter neural processing in prefrontal cortex when involved in discriminating circumstances on the basis of low vs. high cognitive demand.

Development of an apparatus and methodology for conducting functional magnetic resonance imaging (fMRI) with pharmacological stimuli in conscious rhesus monkeys

Functional magnetic resonance imaging (fMRI) is a technique with significant potential to advance our understanding of multiple brain systems. However, when human subjects undergo fMRI studies they are typically conscious whereas pre-clinical fMRI studies typically utilize anesthesia, which complicates comparisons across studies. Therefore, we have developed an apparatus suitable for imaging conscious rhesus monkeys. In order to minimize subject stress and spatial motion, each subject was acclimated to the necessary procedures over several months. The effectiveness of this process was then evaluated, in fully trained subjects, by quantifying objective physiological measures. These physiological metrics were stable both within and across sessions and did not differ from when these same subjects were immobilized using standard primate handling procedures. Subject motion and blood oxygenation level dependent (BOLD) fMRI measurements were then evaluated by scanning subjects under three different conditions: the absence of stimulation, presentation of a visual stimulus, or administration of intravenous (i.v.) cocaine (0.3mg/kg). Spatial motion differed neither by condition nor along the three principal axes. In addition, maximum translational and rotational motion never exceeded one half of the voxel size (0.75 mm) or 1.5 degrees, respectively. Furthermore, the localization of changes in blood oxygenation closely matched those reported in previous studies using similar stimuli. These findings document the feasibility of fMRI data collection in conscious rhesus monkeys using these procedures and allow for the further study of the neural effects of psychoactive drugs.

Cortical activation during cocaine use and extinction in rhesus monkeys

RATIONALE

Acute re-exposure to cocaine or drug cues associated with cocaine use can elicit drug craving and relapse. Neuroimaging studies have begun to define neurobiological substrates underlying the acute effects of cocaine or cocaine cues in cocaine-dependent subjects.

OBJECTIVE

The present study was the first to use functional brain imaging to document acute cocaine-induced changes in brain activity during active drug use in nonhuman primates.

MATERIALS AND METHODS

Positron emission tomography imaging with O15-labeled water was used to measure drug-induced changes in cerebral blood flow. The acute effects of cocaine administered noncontingently were characterized in four drug-naïve rhesus monkeys. The same subjects were trained to self-administer cocaine under a fixed ratio schedule during image acquisition. Subsequently, three subjects with an extensive history of cocaine use were trained to self-administer cocaine under a second-order schedule. The same subjects also underwent extinction sessions during which saline was substituted for cocaine under the second-order schedule.

RESULTS

Noncontingent administration of cocaine in drug-naïve subjects induced robust activation of prefrontal cortex localized primarily to the dorsolateral regions. In contrast, the pattern of brain activation induced by self-administered cocaine differed qualitatively and included anterior cingulate cortex. Moreover, drug-associated stimuli during extinction also induced robust activation of prefrontal cortex.

CONCLUSIONS

The effects of cocaine and associated cues extend beyond the limbic system to engage brain areas involved in cognitive processes. The identification of neural circuits underlying the direct pharmacological and conditioned stimulus effects of cocaine may be highly relevant toward efforts to develop treatments for cocaine addiction.

Noninvasive functional MRI in alert monkeys

Functional magnetic resonance imaging (fMRI) is now widely used to study human brain function. Alert monkey fMRI experiments have been used to localize functions and to compare the workings of the human and monkey brains. Monkey fMRI poses considerable challenges because of the monkey's small brain and naturally uncooperative disposition. While training can encourage monkeys to be more obliging during scanning, the usual procedure is to hold the monkey's head motionless by means of a surgically implanted head post. Such implants are invasive and require regular maintenance. In order to overcome these problems we developed a technique for holding monkeys' heads motionless during scanning using a custom-fitted plastic helmet, a chin strap, and a mild suction supplied by a vacuum blower. This vacuum helmet method is totally noninvasive and has shown no adverse effects after repeated use for several months. The motion of a trained monkey's head in the helmet during scanning was comparable to that of a trained monkey implanted with a conventional head post.

Neuroscientific approaches and applications within anthropology

Many of the most distinctive attributes of our species are a product of our brains. To understand the function, development, variability, and evolution of the human brain, we must engage with the field of neuroscience. Neuroscientific methods can be used to investigate research topics that are of special interest to anthropologists, such as the neural bases of primate behavioral diversity, human brain evolution, and human brain development. Traditional neuroscience methods had to rely on investigation of postmortem brains, as well as invasive studies in living nonhuman primates. However, recent neuroimaging methods have made it possible to compare living human and nonhuman primate brains using noninvasive techniques such as structural and functional magnetic resonance imaging, positron emission tomography, and diffusion tensor imaging. These methods are providing an integrated picture of brain structure and function that was not previously available. With a combination of these traditional and modern neuroscience methods, we are beginning to explore and understand the neural bases of some of the most distinctive cognitive and behavioral attributes of the human species, including language, tool use, altruism, and mental self-projection, and we can now begin to propose plausible scenarios by which the neural substrates supporting these human specializations evolved from pre-existing neural circuitry serving related functions in common ancestors we shared with the living nonhuman primates. Consideration of the process of neurodevelopment suggests plausible mechanisms by which the highly encephalized human brain might have evolved. Neurodevelopmental studies also demonstrate that experience can shape both brain structure and function, providing a mechanism by which people of different cultures learn to act and think differently. Finally, not only can anthropologists benefit from neuroscience, neuroscience can benefit from the more sophisticated concept of evolution that anthropology offers, including an appreciation of evolutionary diversity as well as consideration of the process by which the human brain was formed during evolution.

Synthesis, radiosynthesis, and biological evaluation of carbon-11 and fluorine-18 labeled reboxetine analogues: potential positron emission tomography radioligands for in vivo imaging of the norepinephrine transporter

Reboxetine analogues with methyl and fluoroalkyl substituents at position 2 of the phenoxy ring 1-4 were synthesized. In vitro competition binding with [(3)H]nisoxetine demonstrated that 1-4 have a high affinity for the norepinephrine transporter (NET) with K(i)'s = 1.02, 3.14, 3.68, and 0.30 nM, respectively. MicroPET imaging in rhesus monkeys showed that the relative regional distribution of [(11)C]1 and [(11)C]4 is consistent with distribution of the NET in the brain, while [(18)F]2 and [(18)F]3 showed only slight regional differentiation in brain uptake. Especially, the highest ratios of uptake of [(11)C]1 in NET-rich regions to that in caudate were obtained at 1.30-1.45 at 45 min and remained relatively constant over 85 min. Pretreatment of the monkey with the selective NET inhibitor, desipramine, decreased the specific binding for both [(11)C]1 and [(11)C]4. PET imaging in awake monkeys suggested that anesthesia influenced the binding potential of [(11)C]1 and [(11)C]4 at the NET.

Nonhuman primate neuroimaging and the neurobiology of psychostimulant addiction

Neuroimaging techniques have led to significant advances in our understanding of the neurobiology and treatment of drug addiction in humans. The capability to conduct parallel studies in nonhuman primates and human subjects provides a powerful translational approach to link findings in human and animal research. A significant advantage of nonhuman primate models is the ability to use drug-naïve subjects in longitudinal designs that document the neurobiological changes that are associated with chronic drug use. Moreover, experimental therapeutics can be evaluated in subjects with well-documented histories of drug exposure. The in vivo distribution and pharmacokinetics of drug binding in brain have been related to the time-course of behavioral effects associated with the addictive properties of stimulants. Importantly, the characterization of drug interactions with specific protein targets in brain has identified potential targets for medication development. Neuroimaging has proven especially useful in studying the dynamic changes in neuronal function that may be associated with environmental variables. Last, neuroimaging has been used effectively in nonhuman primates to characterize both transient and long-lasting changes in brain chemistry associated with chronic drug exposure. Although there is some evidence to suggest neurotoxicity in humans with long histories of stimulant use, parallel studies in nonhuman primates have not identified consistent long-term changes in such neurochemical markers. Collectively, the results of these studies of nonhuman primates have enhanced our understanding of the neurobiological basis of stimulant addiction and should have a significant impact on efforts to develop medications to treat stimulant abuse.

Nonhuman primate neuroimaging and cocaine medication development

Given the important role of the dopamine transporter (DAT) in the addictive properties of cocaine, the development and use of compounds that target the DAT represents a reasonable approach for the pharmacological treatment of cocaine abuse. The present report describes a series of studies conducted in nonhuman primates that evaluated the effectiveness of DAT inhibitors in reducing cocaine self-administration. In addition, drug substitution studies evaluated the abuse liability of the DAT inhibitors. Positron emission tomography neuroimaging studies quantified DAT occupancy at behaviorally relevant doses, characterized the time-course of drug uptake in brain, and documented drug-induced changes in cerebral blood flow as a model of brain activation. Selective DAT inhibitors were effective in reducing cocaine use but high (>70%) levels of DAT occupancy were associated with significant reductions in cocaine self-administration. The selective DAT inhibitors were reliably self-administered but rates of responding were lower than those maintained by cocaine even at higher levels of DAT occupancy. A profile of slow rate of drug uptake in brain accompanied by a gradual increase in extracellular dopamine may account for the more limited reinforcing effectiveness of the DAT inhibitors. Selective serotonin transporter (SERT) inhibitors were also effective in reducing cocaine use and blocked cocaine-induced brain activation and increases in extracellular dopamine. Coadministration of SERT inhibitors with a selective DAT inhibitor was more effective than the DAT inhibitor administered alone, even at comparable levels of DAT occupancy. The results indicate that combined inhibition of DAT and SERT may be a viable approach to treat cocaine addiction.

Modelling human drug abuse and addiction with dedicated small animal positron emission tomography

Drug addiction is a chronically relapsing brain disorder, which causes substantial harm to the addicted individual and society as a whole. Despite considerable research we still do not understand why some people appear particularly disposed to drug abuse and addiction, nor do we understand how frequently co-morbid brain disorders such as depression and attention-deficit hyperactivity disorder (ADHD) contribute causally to the emergence of addiction-like behaviour. In recent years positron emission tomography (PET) has come of age as a translational neuroimaging technique in the study of drug addiction, ADHD and other psychopathological states in humans. PET provides unparalleled quantitative assessment of the spatial distribution of radiolabelled molecules in the brain and because it is non-invasive permits longitudinal assessment of physiological parameters such as binding potential in the same subject over extended periods of time. However, whilst there are a burgeoning number of human PET experiments in ADHD and drug addiction there is presently a paucity of PET imaging studies in animals despite enormous advances in our understanding of the neurobiology of these disorders based on sophisticated animal models. This article highlights recent examples of successful cross-species convergence of findings from PET studies in the context of drug addiction and ADHD and identifies how small animal PET can more effectively be used to model complex psychiatric disorders involving at their core impaired behavioural self-control.

Developing functional magnetic resonance imaging techniques for alert macaque monkeys

Functional magnetic resonance imaging (fMRI) has developed rapidly into a major non-invasive tool for studying the human brain. However, due to a variety of technical difficulties, it has yet to be widely adopted for use in alert, trained non-human primates. Our laboratory has been developing techniques for such fMRI studies. As background, we first consider basic principles of fMRI imaging, experimental design, and post-processing. We discuss appropriate MRI system hardware and components for conducting fMRI studies in alert macaques, and the animal preparation and behavior necessary for optimal experiments. Finally, we consider alternative fMRI techniques using exogenous contrast agents, arterial spin labeling, and more direct measures of neural activation.

Evaluation of carbon-11-labeled 2beta-carbomethoxy-3beta-[4'-((Z)-2-iodoethenyl)phenyl]nortropane as a potential radioligand for imaging the serotonin transporter by PET

The nortropane cocaine analogue, 2beta-carbomethoxy-3beta-[4'-((Z)-2-iodoethenyl)phenyl]nortropane (ZIENT), is a high affinity, selective serotonin transporter (SERT) ligand that has shown promise as a SERT imaging agent for single photon computed tomography (SPECT) when labeled with I-123. Synthesis of the labeling precursor, radiosynthesis of [(11)C]ZIENT, and in vivo evaluation in anesthetized and awake monkeys have been performed to determine the suitability of [(11)C]ZIENT as a PET agent for SERT imaging.

Effects of Dopamine Transporter Inhibitors on Cocaine Self-Administration in Rhesus Monkeys: Relationship to Transporter Occupancy Determined by Positron Emission Tomography Neuroimaging

The dopamine transporter (DAT) is a critical recognition site for cocaine and contributes to its significant abuse liability. Accordingly, the development of compounds that target the DAT represents a logical approach in the pharmacological treatment of cocaine abuse. The present study characterized the effects of DAT inhibitors as pretreatments in rhesus monkeys trained to self-administer cocaine under a second-order schedule of i.v. drug delivery. The drugs also were substituted for cocaine to characterize their effectiveness in maintaining drug self-administration. Positron emission tomography neuroimaging with [(18)F]8-(2-[(18)F]fluoroethyl)-2beta-carbomethoxy-3beta-(4-chlorophenyl) nortropane established the DAT occupancy associated with behaviorally relevant doses of each drug. The drugs studied included a selective DAT inhibitor, [1-(2[bis(4-fluorophenyl-) methoxy]ethyl)-4-(3-phenylpropyl)piperazine] bimesylate hydrate (GBR 12909); an inhibitor with equal potency at dopamine and norepinephrine transporters, [3beta-(4-chlorophenyl)tropane-2beta-(3-phenylisoxazol-5-yl)] HCl (RTI-177); and a nonselective inhibitor of dopamine, norepinephrinem and serotonin transporters, [(-)-3beta-(3'-methyl-4-chlorophenyl)tropane-2beta-carboxylic acid methyl ester] tartrate (RTI-112). All drugs produced dose-related reductions in cocaine self-administration. Doses of GBR 12909 and RTI-177 that reduced responding by 50% (ED(50)) resulted in DAT occupancies of 67 +/- 5 and 73 +/- 5%, respectively. In contrast, DAT occupancy was below the limit of detection for the ED(50) dose of RTI-112. Both GBR 12909 and RTI-177 reliably maintained drug self-administration, and DAT occupancies at doses that maintained peak rates of responding were 57 +/- 1 and 92 +/- 7%, respectively. In contrast, RTI-112 failed to maintain robust drug self-administration in any subject. The results indicate that selective DAT inhibitors may require high DAT occupancy to reduce cocaine self-administration and maintain drug self-administration. Moreover, the behavioral profile of DAT inhibitors may be influenced by actions at other monoamine transporters.