Paul W. Czoty

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity—yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research. Am. J. Primatol. 76:801–827, 2014.

Relationship Between Social Rank and Cortisol and Testosterone Concentrations in Male Cynomolgus Monkeys (Macaca fascicularis)

In nonhuman primate social groups, biological differences related to social status have proven useful for investigating the mechanisms of sensitivity to various disease states. Physiological and neurobiological differences between dominant and subordinate monkeys have been interpreted in the context of chronic social stress. The present experiments were designed to investigate the relationships between basal cortisol and testosterone concentrations and the establishment and maintenance of the social hierarchy in male cynomolgus monkeys. Cortisol concentrations were measured at baseline and following suppression with dexamethasone (DEX) and subsequent administration of adrenocorticotrophic hormone (ACTH) while monkeys were individually housed (n = 20) and after 3 months of social housing (n = 4/group), by which time dominance hierarchies had stabilised. Cortisol was also measured during the initial 3 days of social housing. Neither pre‐social housing hormone concentrations, nor hypothalamic‐pituitary‐adrenal (HPA) axis sensitivity predicted eventual social rank. During initial social housing, cortisol concentrations were significantly higher in monkeys that eventually became subordinate; this effect dissipated within 3 days. During the 12 weeks of social housing, aggressive and submissive behaviours were observed consistently, forming the basis for assignment of social ranks. At this time, basal testosterone and cortisol concentrations were significantly higher in dominant monkeys and, after DEX suppression, cortisol release in response to a challenge injection of ACTH was significantly greater in subordinates. These results indicate that basal cortisol and testosterone concentrations and HPA axis function are state variables that differentially reflect position in the dominance hierarchy, rather than trait variables that predict future social status.

Effect of Menstrual Cycle Phase on Dopamine D2 Receptor Availability in Female Cynomolgus Monkeys

Sex differences have been reported in a variety of affective and neurodegenerative disorders that involve dysfunctional dopamine (DA) neurotransmission. In addition, there is evidence for differences in sensitivity to the abuse-related effects of psychostimulants across the menstrual cycle which may result from effects of ovarian hormones on DA function. The goal of the present study was to extend previous work examining menstrual cycle-related changes in DA D2 receptor availability in humans to drug-naive female cynomolgus monkeys (n=7) using the selective D2-like receptor ligand [18F]fluoroclebopride (FCP) and a high-resolution microPET P4 scanner. Menstrual cycle phase was characterized by daily vaginal swabs and measurements of serum progesterone levels. PET studies were conducted once during the luteal phase and once during the follicular phase. Regions of interest in the caudate nucleus, putamen, and cerebellum were defined on coregistered MRIs. Distribution volumes were calculated for FCP in each structure and the distribution volume ratio (DVR) for both brain regions relative to the cerebellum was used as a measure of D2 receptor availability. FCP DVRs were significantly higher in the luteal phase compared to the follicular phase in both the caudate nucleus (11.7% difference, p=0.02) and putamen (11.6% difference, p=0.03). These findings extend earlier work in humans and suggest that changes in DA receptor availability may be involved in the variation in symptoms of various neuropsychiatric disorders across the menstrual cycle, including differences in sensitivity to the abuse-related effects of stimulants.

Positron emission tomography imaging studies of dopamine receptors in primate models of addiction

Animal models have provided valuable information related to trait and state variables associated with vulnerability to drug addiction. Our brain imaging studies in monkeys have implicated D2 receptors in cocaine addiction. For example, an inverse relationship between D2 receptor availability and rates of cocaine self-administration has been documented. Moreover, environmental variables, such as those associated with formation of the social hierarchy, can impact receptor availability and sensitivity to the abuse-related effects of cocaine. Similarly, both D2 receptor availability and cocaine self-administration can be altered by chronic drug administration and fluctuations in hormone levels. In addition, cocaine self-administration can be altered in an orderly fashion by presentation of an acute stressor, such as acting as an intruder into an unfamiliar social group, which can shift the cocaine dose–response curve to the left in subordinate monkeys and to the right in dominant animals, suggesting an interaction between social variables and acute stressors. Conversely, irrespective of social rank, acute environmental enrichment, such as increasing the size of the living space, shifts the cocaine dose–response curve to the right. These findings highlight a pervasive influence of the environment in modifying the reinforcing effects of cocaine and strongly implicate brain D2 receptors.

Cocaine-induced changes in extracellular dopamine determined by microdialysis in awake squirrel monkeys

Rationale: The behavioral effects of cocaine have been linked to brain dopamine systems. Extending the findings to neurochemical studies in the squirrel monkey would enhance our understanding of the behavioral pharmacology of cocaine in nonhuman primates. Objectives: The present studies characterized the effects of cocaine and the selective dopamine uptake inhibitor GBR 12909 on extracellular dopamine in the caudate nucleus of awake squirrel monkeys through microdialysis experiments. Methods: Guide cannulae were implanted in the caudate nucleus of four monkeys using a stereotaxic apparatus and coordinates obtained from a standard squirrel monkey brain atlas. Accurate probe placement was confirmed in all subjects with magnetic resonance imaging. Results: Collectively, the results support the feasibility of a repeated-measures design. Stability of tissue integrity after repeated probe insertion was supported by measurement of consistent basal levels of dopamine and its metabolites across several experiments, observation of potassium-induced dopamine release and absence of significant glial proliferation as assessed by GFAP (glial fibrillary acidic protein) immunochemistry. Moreover, peak drug effects and time-course of action were similar when multiple probes were positioned in the same anatomical site over several experiments. Cocaine (1.0 mg/kg i.m.) and GBR 12909 (3.0 mg/kg i.m.) elevated extracellular dopamine to approximately 300% of basal levels, but GBR 12909 produced a slower, more sustained elevation than cocaine. Conclusions: The results validate the use of microdialysis in awake primates using repeated sampling of the same anatomical site and demonstrate orderly changes in extracellular dopamine following administration of dopamine uptake inhibitors.

Effects of Cocaine and MDMA Self-Administration on Serotonin Transporter Availability in Monkeys

Although serotonin (5-HT) can interact with dopamine (DA) systems to modulate the subjective and reinforcing effects of psychostimulants such as cocaine and 3,4-methyldioxymethamphetamine (MDMA, ecstasy), the long-term effects of exposure to psychostimulants on brain 5-HT systems are not well characterized. The present study assessed 5-HT transporter (SERT) availability using positron emission tomography (PET) in rhesus monkeys with the SERT-specific radioligand [11C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB). SERT availability was assessed in regions of interest including the caudate nucleus, putamen, anterior cingulate cortex, and cerebellum. [11C]DASB distribution volume ratios (DVRs) were calculated using the cerebellum as the reference region. DVRs were calculated in control monkeys and in cocaine or MDMA self-administering monkeys approximately 24 h after the last self-administration (SA) session. SERT availability did not differ between monkeys with a history of MDMA SA and control monkeys in any region examined. In contrast, monkeys with a history of cocaine SA showed significantly higher levels of SERT availability in the caudate nucleus and putamen compared to control subjects. These results suggest that chronic SA of cocaine, but not MDMA, leads to alterations in serotonergic function in brain areas relevant to drug abuse. The higher level of SERT availability in cocaine-experienced monkeys may lead to a reduced inhibitory tone of 5-HT on the DA system, which may explain, in part, differences in the abuse liability between cocaine and MDMA.

Social Dominance in Female Monkeys: Dopamine Receptor Function and Cocaine Reinforcement

BACKGROUND Brain imaging and behavioral studies suggest an inverse relationship between dopamine (DA) D2/D3 receptors and vulnerability to cocaine abuse, although most research has used males. For example, male monkeys that become dominant in a social group have significant elevations in D2/D3 receptor availability and are less vulnerable to cocaine reinforcement. METHODS DA D2/D3 receptor availability was assessed in female cynomolgus monkeys (n = 16) with positron emission tomography (PET) while they were individually housed, 3 months after stable social hierarchies had formed, and again when individually housed. In addition, PET was used to examine changes in dopamine transporter (DAT) availability after social hierarchy formation. After imaging studies were complete, monkeys received implantation with indwelling intravenous catheters and self-administered cocaine (.001-.1 mg/kg/injection) under a fixed-ratio 30 schedule of reinforcement. Acquisition of cocaine reinforcement occurred when response rates were significantly higher than when saline was self-administered. RESULTS Neither DAT nor D2/D3 receptor availability in the caudate nucleus and putamen was predictive of social rank, but both significantly changed after formation of social hierarchies. DA D2/D3 receptor availability significantly increased in females that became dominant, whereas DAT availability decreased in subordinate females. Dominant female monkeys acquired cocaine reinforcement at significantly lower doses than subordinate monkeys. CONCLUSIONS The relationship between D2/D3 receptor availability and vulnerability to cocaine reinforcement seems, on the basis of these findings, opposite in females and males. These data indicate that the social environment profoundly affects the DA system but does so in ways that have different functional consequences for females than for males.

Pharmacological interactions between serotonin and dopamine on behavior in the squirrel monkey

Abstract The behavioral effects of GBR 12909, a selective dopamine uptake inhibitor, were determined in squirrel monkeys trained to respond under a fixed-interval (FI) schedule of stimulus termination and a second-order schedule of IV drug self-administration. Intermediate doses of GBR 12909 increased FI response rate markedly, and the highest dose decreased response rate below control values. The 5HT uptake inhibitors, alaproclate and fluoxetine, and the 5HT agonist, quipazine, attenuated the behavioral-stimulant effects of GBR 12909, whereas the 5HT2A/2C antagonist, ritanserin, enhanced the behavioral-stimulant effects of the lowest dose. GBR 12909 reliably maintained self-administration, and ritanserin increased response rate maintained by the highest dose. The dopamine agonist, quinpirole, increased FI response rate in only one of three subjects, and ritanserin enhanced the behavioral-stimulant effects of quinpirole in that subject. The dopamine agonist, apomorphine, only decreased FI response rate, and ritanserin did not alter its behavioral effects. The pharmacological profile of GBR 12909 administered alone and in combination with selective 5HT drugs in the present study was similar to that obtained previously with cocaine, further demonstrating that 5HT can reliably modulate the behavioral effects of psychomotor stimulants with prominent dopaminergic actions.

Prolonged Attenuation of the Reinforcing Strength of Cocaine by Chronic d-Amphetamine in Rhesus Monkeys

Chronic treatment with the indirect dopamine agonist d-amphetamine can reduce cocaine use in clinical trials and, in preclinical studies in laboratory animals, attenuates daily cocaine self-administration. The present study extended previous results to conditions designed to reflect a more clinically relevant experience of cocaine exposure and d-amphetamine treatment. Each morning, monkeys pressed a lever to receive food pellets under a 50-response fixed-ratio schedule of reinforcement. After determining a dose-response curve for cocaine (0.003–0.56 mg/kg per injection, i.v.) under a progressive-ratio (PR) schedule of reinforcement in the evening, cocaine self-administration sessions were suspended and d-amphetamine (0.01–0.056 mg/kg/h, i.v.) was administered continuously for at least 24 days, except during cocaine self-administration sessions, which were conducted using the PR schedule once every 8 days. When a persistent decrease in self-administration was observed, the cocaine dose-effect curve was redetermined. Cocaine- and food-maintained responding were also examined after discontinuation of d-amphetamine. Although individual differences in sensitivity were observed, d-amphetamine produced selective, qualitatively similar decreases in the reinforcing strength of cocaine in all monkeys that persisted at least 4 weeks. Moreover, cocaine dose-effect curves were shifted downward and/or to the right. For 2 weeks following discontinuation of d-amphetamine treatment, the reinforcing strength of cocaine varied within and across individuals, however, on the whole no increased sensitivity was apparent. These data provide further support for the use of agonist medications for cocaine abuse, and extend the conditions under which such treatment is successful to those that incorporate clinically relevant patterns of cocaine use and drug treatment.

Identifying Medication Targets for Psychostimulant Addiction: Unraveling the Dopamine D3 Receptor Hypothesis

The dopamine D3 receptor (D3R) is a target for developing medications to treat substance use disorders. D3R-selective compounds with high affinity and varying efficacies have been discovered, providing critical research tools for cell-based studies that have been translated to in vivo models of drug abuse. D3R antagonists and partial agonists have shown especially promising results in rodent models of relapse-like behavior, including stress-, drug-, and cue-induced reinstatement of drug seeking. However, to date, translation to human studies has been limited. Herein, we present an overview and illustrate some of the pitfalls and challenges of developing novel D3R-selective compounds toward clinical utility, especially for treatment of cocaine abuse. Future research and development of D3R-selective antagonists and partial agonists for substance abuse remains critically important but will also require further evaluation and development of translational animal models to determine the best time in the addiction cycle to target D3Rs for optimal therapeutic efficacy.