Nonhuman primate neuroimaging and the neurobiology of psychostimulant addiction

Updated Perspectives on the Neurobiology of Substance Use Disorders Using Neuroimaging

Substance use problems impair social functioning, academic achievement, and employability. Psychological, biological, social, and environmental factors can contribute to substance use disorders. In recent years, neuroimaging breakthroughs have helped elucidate the mechanisms of substance misuse and its effects on the brain. Functional magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance spectroscopy (MRS) are all examples. Neuroimaging studies suggest substance misuse affects executive function, reward, memory, and stress systems. Recent neuroimaging research attempts have provided clinicians with improved tools to diagnose patients who misuse substances, comprehend the complicated neuroanatomy and neurobiology involved, and devise individually tailored and monitorable treatment regimens for individuals with substance use disorders. This review describes the most recent developments in drug misuse neuroimaging, including the neurobiology of substance use disorders, neuroimaging, and substance use disorders, established neuroimaging techniques, recent developments with established neuroimaging techniques and substance use disorders, and emerging clinical neuroimaging technology.

Methamphetamine and Designer Stimulants Modulate Tonic Human Cerebrovascular Smooth Muscle Contractility: Relevance to Drug-Induced Neurovascular Stress

To avoid criminal prosecution, clandestine chemists produce designer stimulants that mimic the pharmacological and psychoactive effects of conventional stimulants, such as methamphetamine. Following persistent or high-dose exposure, both acute vasoconstriction and loss of vascular homeostasis are reported dangers of conventional stimulants, and designer stimulants may pose even greater dangers. To compare the effects of a conventional stimulant and two designer stimulants on vascular contraction, this study examined the direct effects of 1,3-benzodioxolylbutanamine (BDB) and N-butylpentylone in comparison to methamphetamine on the function of human brain vascular smooth muscle cells (HBVSMCs). HBVSMCs suspended in collagen gels were exposed to varying concentrations of each drug, and the degree of constriction was assessed over one week. The MTT assay was used to measure the impact of the three drugs on the cellular metabolic activity as a marker of cellular toxicity. The highest concentration tested of either methamphetamine or N-butylpentylone produced a loss of HBVSMC contractility and impaired cellular metabolism. BDB showed a similar pattern of effects, but, uniquely, it also induced vasoconstrictive effects at substantially lower concentrations. Each drug produced direct effects on HBVSMC contraction that may be a mechanism by which the cardiovascular system is damaged following high-dose or persistent exposure, and this could be exacerbated by any sympathomimetic effects of these compounds in whole organisms. BDB appears to impact HBVSMC function in ways distinct from methamphetamine and N-butylpentylone, which may present unique dangers.

Transcranial Stimulation for the Treatment of Stimulant Use Disorder

The increasing prevalence of stimulant use disorder (StUD) involving methamphetamine and cocaine has been a growing healthcare concern in the United States. Cocaine usage is associated with atherosclerosis, systolic and diastolic dysfunction, and arrhythmias. Furthermore, approximately one of every four MIs is cocaine-induced among patients aged 18 to 45. Methamphetamine use has been associated with nerve terminal damage in the dopaminergic system resulting in impaired motor function, cognitive decline, and co-morbid psychiatric disorders. Current treatment options for StUD are extremely limited, and there are currently no FDA-approved pharmacotherapies. Behavioral interventions are considered first-line treatment; however, in a recent meta-analysis comparing behavioral treatment options for cocaine, contingency management programs provided the only significant reduction in use. Current evidence points to the potential of various neuromodulation techniques as the next best modality in treating StUD. The most promising evidence thus far has been transcranial magnetic stimulation which several studies have shown to reduce risk factors associated with relapse. Another more invasive neuromodulation technique being studied is deep-brain stimulation, which has shown promising results in its ability to modulate reward circuits to treat addiction. Results showing the impact of transcranial magnetic stimulation (TMS) in the treatment of StUD are limited by the lack of studies conducted and the limited understanding of the neurological involvement driving addiction-based diseases such as StUD. Future studies should seek to provide data on consumption-reducing effects rather than craving evaluations.

Transdermal Delivery of the Free Base of 3-Fluoroamphetamine: In Vitro Skin Permeation and Irritation Potential

This work aimed to continue our effort in establishing the feasibility of 3-fluoroamphetamine (also known as PAL-353) to be a transdermal drug candidate by studying the delivery of the base form through the human cadaver skin in lieu of the previously investigated salt form, and for the first time using an EPIDERM™-reconstructed human epidermal model to predict the skin irritation potential of PAL-353, in support of development for a matrix-type transdermal delivery system. Passive and enhanced (with chemical permeation enhancers) transdermal delivery were investigated via in vitro permeation studies that were performed on Franz diffusion cells with dermatomed human cadaver skin. After 24 h, PAL-353 free base revealed high passive permeation of 417.49 ± 30.12, 1577.68 ± 165.41, and 4295.16 ± 264.36 μg/cm², with applied formulation concentrations of 5.5 (F1), 20 (F2), and 40 (F3) mg/mL, respectively. Oleyl alcohol produced an approximately threefold steady-state flux enhancement at 5% or 10% w/w but may not be needed as the free base alone provided therapeutically relevant permeation. Further, it was predicted that therapeutically relevant delivery would be unlikely to cause skin irritation using the EPIDERM™-reconstructed human epidermal model. In conclusion, the present study further supported the development of PAL-353 transdermal delivery systems.

The pharmacokinetics of 3-fluoroamphetamine following delivery using clinically relevant routes of administration

3-Fluoroamphetamine (also called PAL-353) is a synthetic amphetamine analog that has been investigated for cocaine use disorder (CUD), yet no studies have characterized its pharmacokinetics (PK). In the present study, we determined the PK of PAL-353 in male Sprague Dawley rats following intravenous bolus injection (5 mg/kg). Plasma samples were analyzed using a novel bioanalytical method that coupled liquid-liquid extraction and LC-MS/MS. The primary PK parameters determined by WinNonlin were a C0 (ng/mL) of 1412.09 ± 196.12 and a plasma half-life of 2.27 ± 0.67 h. As transdermal delivery may be an optimal approach to delivering PAL-353 for CUD, we assessed its PK profile following application of 50 mg of transdermal gel (10% w/w drug over 5 cm2). The 10% w/w gel resulted in a short lag time, sustained delivery, and a rapid clearance in plasma immediately after removal. The rodent PK data were verified by examining in vitro permeation through human epidermis mounted on Franz diffusion cells. An in vitro-in vivo correlation (IVIVC) analysis was performed using the Phoenix IVIVC toolkit to assess the predictive relationship between rodent and human skin absorption/permeation. The in vitro permeation study revealed a dose-proportional cumulative and steady-state flux with ~ 70% of drug permeated. The fraction absorbed in vivo and fraction permeated in vitro showed a linear relationship. In conclusion, we have characterized the PK profile of PAL-353, demonstrated that it has favorable PK properties for transdermal administration for CUD, and provided preliminary evidence of the capacity of rodent data to predict human skin flux.

Nonhuman animal models of substance use disorders: Translational value and utility to basic science

BACKGROUND

The National Institute on Drug Abuse (NIDA) recently released a Request for Information (RFI) soliciting comments on nonhuman animal models of substance use disorders (SUD).

METHODS

A literature review was performed to address the four topics outlined in the RFI and one topic inspired by the RFI: (1) animal models that best recapitulate SUD, (2) animal models that best balance the trade-offs between resources and ecological validity, (3) animal models whose translational value are frequently misrepresented or overrepresented by the scientific community, (4) aspects of SUD that are not currently being modeled in animals, and (5) animal models that are optimal for examining the basic mechanisms by which drugs produce their abuse-related effects.

RESULTS

Models that employ response-contingent drug administration, use complex schedules of reinforcement, measure behaviors that mimic the distinguishing features of SUD, and use animals that are phylogenetically similar to humans have the greatest translational value. Models that produce stable and reproducible baselines of behavior, lessen the number of uncontrolled variables, and minimize the influence of extraneous factors are best at examining basic mechanisms contributing to drug reward and reinforcement.

CONCLUSIONS

Nonhuman animal models of SUD have undergone significant refinements to increase their utility for basic science and translational value for SUD. The existing literature describes numerous examples of how these models may best be utilized to answer mechanistic questions of drug reward and identify potential therapeutic interventions for SUD. Progress in the field could be accelerated by further collaborations between researchers using animals versus humans.

The acute toxic and neurotoxic effects of 3,4-methylenedioxymethamphetamine are more pronounced in adolescent than adult mice

3,4-methylenedioxymethamphetamine (MDMA) recently achieved breakthrough status from the Food and Drug Administration (FDA) for post-traumatic stress disorder (PTSD). However, evidence indicates that exposure to toxic doses of MDMA can lead to long-lasting dysregulation of brain monoaminergic neurotransmitters, primarily from studies conducted in young adult rodents. To date, there is a paucity of data on whether toxic doses of MDMA can differentially affect neurotransmitter systems in adolescents and mature adults, which is an important question as adolescents and adults may be differentially vulnerable to MDMA abuse. In the current study, adolescent (6-7 weeks of age) and mature adult (16-18 weeks of age) male, Swiss-Webster mice were exposed to MDMA (20 mg/kg) using a binge-like dosing regimen (4 administrations spaced every 2 h). Acute lethality, acute hyperthermia, and acute decreases in body weight following MDMA administration were more pronounced in adolescent than adult mice. Likewise, acute loss of striatal dopamine neurochemistry was also exacerbated in adolescents, as determined by high-pressure liquid chromatography coupled to electrochemical detection. Exposure to MDMA induced greater turnover of dopamine into its major metabolite dihydroxyphenylacetic acid (DOPAC) in adolescents, but not in adults, suggesting a novel mechanism through which adolescents may show increased vulnerability to the acute toxic and neurotoxic effects of MDMA, or conversely that mature adults show greater protection. These data caution that MDMA exposure in adolescence may be particularly dangerous and that the therapeutic window for MDMA may differ between adolescents and mature adults.

The synthetic cathinone psychostimulant α-PPP antagonizes serotonin 5-HT2A receptors: In vitro and in vivo evidence

Synthetic cathinones (SCs) are β-keto analogs of amphetamines. Like amphetamines, SCs target monoamine transporters; however, unusual neuropsychiatric symptoms have been associated with abuse of some SCs, suggesting SCs might possess additional pharmacological properties. We performed radioligand competition binding assays to assess the affinities of nine SCs at human 5-HT_2A receptors (5-HT_2A R) and muscarinic M₁ receptors (M₁ R) transiently expressed in HEK293 cells. None of the SCs exhibited affinity at M₁ R (minimal displacement of [~K_d ] [³ H]scopolamine up to 10 μM). However, two SCs, α-pyrrolidinopropiophenone (α-PPP) and 4-methyl-α-PPP, had low μM K_i values at 5-HT_2A R. In 5-HT_2A R-phosphoinositide hydrolysis assays, α-PPP and 4-methyl-α-PPP displayed inverse agonist activity. We further assessed the 5-HT_2A R functional activity of α-PPP, and observed it competitively antagonized 5-HT_2A R signaling stimulated by the 5-HT₂ R agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI; K_b  = 851 nM). To assess in vivo 5-HT_2A R activity, we examined the effects of α-PPP on the DOI-elicited head-twitch response (HTR) in mice. α-PPP dose-dependently blocked the HTR with maximal suppression at 10 mg/kg (P < 0.0001), which is a moderate dose used in studies investigating psychostimulant properties of α-PPP. To corroborate a 5-HT_2A R mechanism, we also tested 3,4-methylenedioxy-α-PPP (MDPPP) and 3-bromomethcathinone (3-BMC), SCs that we observed had 5-HT_2A R K_i s > 10 μM. Neither MDPPP nor 3-BMC, at 10 mg/kg doses, attenuated the DOI HTR. Our results suggest α-PPP has antagonist interactions at 5-HT_2A R in vitro that may translate at physiologically-relevant doses in vivo. Considering 5-HT_2A R antagonism has been shown to mitigate effects of psychostimulants, this property may contribute to α-PPPs unpopularity compared to other monoamine transporter inhibitors.

A review of nonhuman primate models of early life stress and adolescent drug abuse

Adolescence represents a developmental stage in which initiation of drug use typically occurs and is marked by dynamic neurobiological changes. These changes present a sensitive window during which perturbations to normative development lead to alterations in brain circuits critical for stress and emotional regulation as well as reward processing, potentially resulting in an increased susceptibility to psychopathologies. The occurrence of early life stress (ELS) is related to a greater risk for the development of substance use disorders (SUD) during adolescence. Studies using nonhuman primates (NHP) are ideally suited to examine how ELS may alter the development of neurobiological systems modulating the reinforcing effects of drugs, given their remarkable neurobiological, behavioral, and developmental homologies to humans. This review examines NHP models of ELS that have been used to characterize its effects on sensitivity to drug reinforcement, and proposes future directions using NHP models of ELS and drug abuse in an effort to develop more targeted intervention and prevention strategies for at risk clinical populations.

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ELS has long-lasting neurobiological and behavioral consequences.

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ELS is a major risk factor for the initiation of adolescent drug use.

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Sex differences are apparent in the consequences of ELS, including drug use.

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Nonhuman primate models of ELS are critical for understanding ELS effects on neurobiology and risk for adolescent drug use.

Utility of Nonhuman Primates in Substance Use Disorders Research

Substance use disorders (i.e., drug addiction) constitute a global and insidious public health issue. Preclinical biomedical research has been invaluable in elucidating the environmental, biological, and pharmacological determinants of drug abuse and in the process of developing innovative pharmacological and behavioral treatment strategies. For more than 70 years, nonhuman primates have been utilized as research subjects in biomedical research related to drug addiction. There are already several excellent published reviews highlighting species differences in both pharmacodynamics and pharmacokinetics between rodents and nonhuman primates in preclinical substance abuse research. Therefore, the aim of this review is to highlight three advantages of nonhuman primates as preclinical substance abuse research subjects. First, nonhuman primates offer technical advantages in experimental design compared to other laboratory animals that afford unique opportunities to promote preclinical-to-clinical translational research. Second, these technical advantages, coupled with the relatively long lifespan of nonhuman primates, allows for pairing longitudinal drug self-administration studies and noninvasive imaging technologies to elucidate the biological consequences of chronic drug exposure. Lastly, nonhuman primates offer advantages in the patterns of intravenous drug self-administration that have potential theoretical implications for both the neurobiological mechanisms of substance use disorder etiology and in the drug development process of pharmacotherapies for substance use disorders. We conclude with potential future research directions in which nonhuman primates would provide unique and valuable insights into the abuse of and addiction to novel psychoactive substances.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

Long-Term Cognitive Functioning in Single-Dose Total-Body Gamma-Irradiated Rhesus Monkeys ( Macaca mulatta )

In this study, the effects of a potentially lethal radiation exposure on the brain for long-term cognitive sequelae were investigated using Rhesus macaques ( Macaca mulatta ) adopted from other facilities after analysis of acute radiation response via the Centers for Medical Countermeasures against Radiation (CMCR) network. Fifty-nine animals were given the opportunity to participate in cognitive cage-side testing. The animals that received single-dose gamma irradiation were significantly less likely to engage in cognitive testing than the controls, suggesting that irradiated animals may have differences in cognitive ability. Five irradiated (6.75-8.05 Gy) and three naïve control animals self-selected, were extensively trained and administered a simple visual discrimination with reversal (SVD+R) task 2-3 times per week for 11-18 months. Each session consisted of 30 trials in which the animals were required to choose the correct visual stimulus for a food reward. After the initial presentation, the stimulus that signaled the presence of food was twice reversed once the animal reached criterion (90% accuracy across four consecutive sessions). While the limited sample size precluded definitive statistical analysis, irradiated animals took longer to reach the criterion subsequent to reversal than did control animals, suggesting a relative deficiency in cognitive flexibility. These results provide preliminary data supporting the potential use of a nonhuman primate model to study radiation-induced, late-delayed cognitive deficits.

A diffusion-tensor-based white matter atlas for rhesus macaques

Atlases of key white matter (WM) structures in humans are widely available, and are very useful for region of interest (ROI)-based analyses of WM properties. There are histology-based atlases of cortical areas in the rhesus macaque, but none currently of specific WM structures. Since ROI-based analysis of WM pathways is also useful in studies using rhesus diffusion tensor imaging (DTI) data, we have here created an atlas based on a publicly available DTI-based template of young rhesus macaques. The atlas was constructed to mimic the structure of an existing human atlas that is widely used, making results translatable between species. Parcellations were carefully hand-drawn on a principle-direction color-coded fractional anisotropy image of the population template. The resulting atlas can be used as a reference to which registration of individual rhesus data can be performed for the purpose of white-matter parcellation. Alternatively, specific ROIs from the atlas may be warped into individual space to be used in ROI-based group analyses. This atlas will be made publicly available so that it may be used as a resource for DTI studies of rhesus macaques.

Quantification of dopamine transporter density with [18F]FECNT PET in healthy humans

INTRODUCTION

Fluorine-18 labeled 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)nortropane ([(18)F]FECNT) binds reversibly to the dopamine transporter (DAT) with high selectivity. [(18)F]FECNT has been used extensively in the quantification of DAT occupancy in non-human primate brain and can distinguish between Parkinson's and healthy controls in humans. The purpose of this work was to develop a compartment model to characterize the kinetics of [(18)F]FECNT for quantification of DAT density in healthy human brain.

METHODS

Twelve healthy volunteers underwent 180 min dynamic [(18)F]FECNT PET imaging including sampling of arterial blood. Regional time-activity curves were extracted from the caudate, putamen and midbrain including a reference region placed in the cerebellum. Binding potential, BPND, was calculated for all regions using kinetic parameters estimated from compartmental and Logan graphical model fits to the time-activity data. Simulations were performed to determine whether the compartment model could reliably fit time-activity data over a range of BPND values.

RESULTS

The kinetics of [(18)F]FECNT were well-described by the reversible 2-tissue arterial input and full reference tissue compartment models. Calculated binding potentials in the caudate, putamen and midbrain were in good agreement between the arterial input model, reference tissue model and the Logan graphical model. The distribution volume in the cerebellum did not reach a plateau over the duration of the study, which may be a result of non-specific binding in the cerebellum. Simulations that included non-specific binding show that the reference and arterial input models are able to estimate BPND for DAT densities well below that observed in normal volunteers.

CONCLUSION

The kinetics of [(18)F]FECNT in human brain are well-described by arterial input and reference tissue compartment models. Measured and simulated data show that BPND calculated with reference tissue model is proportional to BPND calculated from the arterial input model.

Serotonin 2A receptors differentially contribute to abuse-related effects of cocaine and cocaine-induced nigrostriatal and mesolimbic dopamine overflow in nonhuman primates

Two of the most commonly used procedures to study the abuse-related effects of drugs in laboratory animals are intravenous drug self-administration and reinstatement of extinguished behavior previously maintained by drug delivery. Intravenous self-administration is widely accepted to model ongoing drug-taking behavior, whereas reinstatement procedures are accepted to model relapse to drug taking following abstinence. Previous studies indicate that 5-HT2A receptor antagonists attenuate the reinstatement of cocaine-maintained behavior but not cocaine self-administration in rodents. Although the abuse-related effects of cocaine have been closely linked to brain dopamine systems, no previous study has determined whether this dissociation is related to differential regulation of dopamine neurotransmission. To elucidate the neuropharmacological and neuroanatomical mechanisms underlying this phenomenon, we evaluated the effects of the selective 5-HT2A receptor antagonist M100907 on intravenous cocaine self-administration and drug- and cue-primed reinstatement in rhesus macaques (Macaca mulatta). In separate subjects, we evaluated the role of 5-HT2A receptors in cocaine-induced dopamine overflow in the nucleus accumbens (n = 4) and the caudate nucleus (n = 5) using in vivo microdialysis. Consistent with previous studies, M100907 (0.3 mg/kg, i.m.) significantly attenuated drug- and cue-induced reinstatement but had no significant effects on cocaine self-administration across a range of maintenance doses. Importantly, M100907 (0.3 mg/kg, i.m.) attenuated cocaine-induced (1.0 mg/kg, i.v.) dopamine overflow in the caudate nucleus but not in the nucleus accumbens. These data suggest that important abuse-related effects of cocaine are mediated by distinct striatal dopamine projection pathways.

Contributions of neuroimaging to understanding sex differences in cocaine abuse

A consistent observation in drug abuse research is that males and females show differences in their response to drugs of abuse. In order to understand the neurobiology underlying cocaine abuse and effective treatments, it is important to consider the role of sex differences. Sex hormones have been investigated in both behavioral and molecular studies, but further evidence addressing drug abuse and dependence in both sexes would expand our knowledge of sex differences in response to drugs of abuse. Neuroimaging is a powerful tool that can offer insight into the biological bases of these differences and meet the challenges of directly examining drug-induced changes in brain function. As such, neuroimaging has drawn much interest in recent years. Specifically, positron emission tomography (PET), single photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI) technology have emerged as effective noninvasive approaches for human and animal models. Studies have revealed sex-specific changes in patterns of brain activity in response to acute cocaine injection and after prolonged cocaine use. SPECT and PET studies have demonstrated changes in the dopamine transporter but are less clear on other components of the dopaminergic system. This review highlights contributions of neuroimaging toward understanding the role of sex differences in the drug abuse field, specifically regarding cocaine, and identifies relevant questions that neuroimaging can effectively address.

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.

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.