PET imaging of dopamine transporters and D2/D3 receptors in female monkeys: effects of chronic cocaine self-administration.

Brain imaging studies using positron emission tomography (PET) have shown that long-term cocaine use is associated with lower levels of dopamine (DA) D2/D3 receptors (D2/D3R); less consistent are the effects on DA transporter (DAT) availability. However, most studies have been conducted in male subjects (humans, monkeys, rodents). In this study, we used PET imaging in nine drug-naïve female cynomolgus monkeys to determine if baseline measures of DAT, with [18F]FECNT, and D2/D3R availability, with [11C]raclopride, in the caudate nucleus, putamen and ventral striatum were associated with rates of cocaine self-administration and if these measures changed during long-term (~13 months) cocaine self-administration and following time-off (3-9 months) from cocaine. Cocaine (0.2 mg/kg/injection) and 1.0 g food pellets were available under a multiple fixed-interval (FI) 3-min schedule of reinforcement. In contrast to what has been observed in male monkeys, baseline D2/D3R availability was positively correlated with rates of cocaine self-administration only during the first week of exposure; DAT availability did not correlate with cocaine self-administration. D2/D3R availability decreased ~20% following cumulative intakes of 100 and 1000 mg/kg cocaine; DAT availability did not significantly change. These reductions in D2/D3R availability did not recover over 9 months of time-off from cocaine. To determine if these reductions were reversible, three monkeys were implanted with osmotic pumps that delivered raclopride for 30 days. We found that chronic treatment with the D2/D3R antagonist raclopride increased D2/D3R availability in the ventral striatum but not in the other regions when compared to baseline levels. Over 13 months of self-administration, tolerance did not develop to the rate-decreasing effects of self-administered cocaine on food-reinforced responding, but number of injections and cocaine intake significantly increased over the 13 months. These data extend previous findings to female monkeys and suggest sex differences in the relationship between D2/D3R availability related to vulnerability and long-term cocaine use.

Prolonged exposure to cocaine self-administration results in a continued progression of alterations in functional activity in a nonhuman primate model.

Background: Studies of nonhuman primates with exposures of up to 100 days of cocaine self-administration (SA) have provided evidence that the central effects of cocaine progress over time. These durations of cocaine exposure, however, may be insufficient to capture the extent of the neurobiological alterations observed in cocaine users, many of whom use the drug for years. The goal of the present study was to determine whether 1.5 years of cocaine SA would result in further progression of alterations in functional brain activity. Methods: Adult male rhesus monkeys were exposed to 300 sessions of high-dose cocaine SA over 1.5 years. Following the final session rates of local cerebral glucose utilization (LCGU) were assessed with the 2-[14C]-deoxyglucose method and compared to rates of LCGU in control monkeys who responded for food reinforcement. In addition, LCGU in these animals was compared to a previously published group of monkeys that had self-administered cocaine or food for 100 sessions over a 4-5 month period. Results: Compared to 100 days of exposure, 300 days of cocaine SA further reduced LCGU in the post-commissural striatum and produced reductions in areas unaffected by the shorter duration of exposure, such as the hypothalamus, all of the amygdala, and large expanses of cortex. Conclusions: These findings demonstrate a clear progression of the impact of cocaine on functional activity with increasing durations of drug experience and have important implications for the development of potential strategies for the treatment of cocaine use disorder.

Association of dopamine D2-like and D3 receptor function with initial sensitivity to cocaine reinforcement in male rhesus monkeys.

Identifying neurobiological characteristics that predict the development of cocaine use disorder would be of great value in prevention efforts. Because of their importance in mediating the abuse-related effects of cocaine, brain dopamine receptors are logical candidates for investigation. We analyzed data from two recently published studies that characterized availability of dopamine D2-like receptors (D2R) with [11C]raclopride PET imaging and dopamine D3 receptor (D3R) sensitivity with quinpirole-induced yawning in cocaine-naïve rhesus monkeys who subsequently acquired cocaine self-administration and completed a cocaine self-administration dose-effect curve. The present analysis compared D2R availability in several brain areas and characteristics of quinpirole-induced yawning, both acquired when monkeys were drug-naïve, with measures of initial sensitivity to cocaine. D2R availability in the caudate nucleus was negatively correlated with the ED50 of the cocaine self-administration curve, although the significance of this relationship was driven by an outlier and was not present after the outlier was removed. No other significant associations were observed between D2R availability in any examined brain region and measures of sensitivity to cocaine reinforcement. However, there was a significant negative correlation between D3R sensitivity, represented by the ED50 of the quinpirole-induced yawning curve, and the dose at which monkeys acquired cocaine self-administration. We also report no change from baseline D2R availability when a second PET scan was conducted after completion of the dose-effect curves. These data suggest the utility of D3R sensitivity, but not D2R availability, as a biomarker for vulnerability and resilience to cocaine. The well-established relationships between dopamine receptors and cocaine reinforcement in cocaine-experienced humans and animals may require extensive cocaine exposure.

Residual deficits in functional brain activity after chronic cocaine self-administration in rhesus monkeys.

Previous studies in humans and in animals have shown dramatic effects of cocaine on measures of brain function that persist into abstinence. The purpose of this study was to examine the neurobiological consequences of abstinence from cocaine, using a model that removes the potential confound of cocaine cues. Adult male rhesus monkeys self-administered cocaine (0.3 mg/kg/injection; N = 8) during daily sessions or served as food-reinforcement controls (N = 4). Two times per week, monkeys were placed in a neutral environment and presented with a cartoon video for ~30 min, sometimes pre- and sometimes post-operant session, but no reinforcement was presented during the video. After ~100 sessions and when the cocaine groups had self-administered 900 mg/kg cocaine, the final experimental condition was a terminal 2-[14C]-deoxyglucose procedure, which occurred in the neutral (cartoon video) environment; for half of the monkeys in each group, this occurred after 1 day of abstinence and for the others after 30 days of abstinence. Rates of local cerebral glucose metabolism were measured in 57 brain regions. Global rates of cerebral metabolism were significantly lower in animals 1 day and 30 days post-cocaine self-administration when compared to those of food-reinforced controls. Effects were larger in 30- vs. 1-day cocaine abstinence, especially in prefrontal, parietal and cingulate cortex, as well as dorsal striatum and thalamus. Because these measures were obtained from monkeys while in a neutral environment, the deficits in glucose utilization can be attributed to the consequences of cocaine exposure and not to effects of conditioned stimuli associated with cocaine.

PET imaging of kappa opioid receptors and receptor expression quantified in neuron-derived extracellular vesicles in socially housed female and male cynomolgus macaques.

Recent positron emission tomography (PET) studies of kappa opioid receptors (KOR) in humans reported significant relationships between KOR availability and social status, as well as cocaine choice. In monkey models, social status influences physiology, receptor pharmacology and behavior; these variables have been associated vulnerability to cocaine abuse. The present study utilized PET imaging to examine KOR availability in socially housed, cocaine-naïve female and male monkeys, and peripheral measures of KORs with neuron-derived extracellular vesicles (NDE). KOR availability was assessed in dominant and subordinate female and male cynomolgus macaques (N = 4/rank/sex), using PET imaging with the KOR selective agonist [11C]EKAP. In addition, NDE from the plasma of socially housed monkeys (N = 13/sex; N = 6-7/rank) were isolated by immunocapture method and analyzed for OPRK1 protein expression by ELISA. We found significant interactions between sex and social rank in KOR availability across 12 of 15 brain regions. This was driven by female data, in which KOR availability was significantly higher in subordinate monkeys compared with dominant monkeys; the opposite relationship was observed among males, but not statistically significant. No sex or rank differences were observed for NDE OPRK1 concentrations. In summary, the relationship between brain KOR availability and social rank was different in female and male monkeys. This was particularly true in female monkeys. We hypothesize that lower [11C]EKAP binding potentials were due to higher concentrations of circulating dynorphin, which is consistent with greater vulnerability in dominant compared with subordinate females. These findings suggest that the KOR is an important target for understanding the neurobiology associated with vulnerability to abused drugs and sex differences, and detectable in peripheral circulation.

Effects of chronic cocaine and ethanol self-administration on brain dopamine receptors in a rhesus monkey model of polysubstance abuse.

Most individuals with cocaine use disorder also use alcohol; however, little is known about the behavioural and pharmacological mechanisms that promote co-abuse. For example, although studies in humans and animals have documented that chronic use of either alcohol or cocaine alone decreases D2-like receptor (D2R) availability, effects of co-abuse of these substances on dopamine receptor function have not been characterized. These studies examined the effects of long-term cocaine self-administration in 12 male rhesus monkeys who also consumed either ethanol or an ethanol-free solution each day (n = 6 per group). Specifically, all monkeys self-administered cocaine (0.1 mg/kg per injection) 5 days per week in the morning. In the afternoon, six monkeys consumed 2.0 g/kg ethanol over 1 h to model binge drinking and six monkeys drank an ethanol-free solution. Assessment of D2R availability using positron emission tomography (PET) and [11 C]raclopride occurred when monkeys were drug-naïve and again when monkeys had self-administered approximately 400-mg/kg cocaine. D3 R function was assessed at the same time points by determining the potency of the D3 R-preferring agonist quinpirole to elicit yawns. Chronic cocaine self-administration decreased D2R availability in subregions of the basal ganglia in control monkeys, but not those that also drank ethanol. In contrast, D3 R sensitivity increased significantly after chronic cocaine self-administration in ethanol-drinking monkeys but not controls. These results suggest that co-use of ethanol substantially changes the effects of chronic cocaine self-administration on dopamine receptors, specifically implicating D3 R as a target for medications in these individuals.

Mass Spectrometry-Based Proteome Profiling of Extracellular Vesicles Derived from the Cerebrospinal Fluid of Adult Rhesus Monkeys Exposed to Cocaine throughout Gestation.

Cocaine use disorder has been reported to cause transgenerational effects. However, due to the lack of standardized biomarkers, the effects of cocaine use during pregnancy on postnatal development and long-term neurobiological and behavioral outcomes have not been investigated thoroughly. Therefore, in this study, we examined extracellular vesicles (EVs) in adult (~12 years old) female and male rhesus monkeys prenatally exposed to cocaine (n = 11) and controls (n = 9). EVs were isolated from the cerebrospinal fluid (CSF) and characterized for the surface expression of specific tetraspanins, concentration (particles/mL), size distribution, and cargo proteins by mass spectrometry (MS). Transmission electron microscopy following immunogold labeling for tetraspanins (CD63, CD9, and CD81) confirmed the successful isolation of EVs. Nanoparticle tracking analyses showed that the majority of the particles were <200 nm in size, suggesting an enrichment for small EVs (sEV). Interestingly, the prenatally cocaine-exposed group showed ~54% less EV concentration in CSF compared to the control group. For each group, MS analyses identified a number of proteins loaded in CSF-EVs, many of which are commonly listed in the ExoCarta database. Ingenuity pathway analysis (IPA) demonstrated the association of cargo EV proteins with canonical pathways, diseases and disorders, upstream regulators, and top enriched network. Lastly, significantly altered proteins between groups were similarly characterized by IPA, suggesting that prenatal cocaine exposure could be potentially associated with long-term neuroinflammation and risk for neurodegenerative diseases. Overall, these results indicate that CSF-EVs could potentially serve as biomarkers to assess the transgenerational adverse effects due to prenatal cocaine exposure.

PET Imaging of [11C]MPC-6827, a Microtubule-Based Radiotracer in Non-Human Primate Brains.

Dysregulation of microtubules is commonly associated with several psychiatric and neurological disorders, including addiction and Alzheimer's disease. Imaging of microtubules in vivo using positron emission tomography (PET) could provide valuable information on their role in the development of disease pathogenesis and aid in improving therapeutic regimens. We developed [11C]MPC-6827, the first brain-penetrating PET radiotracer to image microtubules in vivo in the mouse brain. The aim of the present study was to assess the reproducibility of [11C]MPC-6827 PET imaging in non-human primate brains. Two dynamic 0-120 min PET/CT imaging scans were performed in each of four healthy male cynomolgus monkeys approximately one week apart. Time activity curves (TACs) and standard uptake values (SUVs) were determined for whole brains and specific regions of the brains and compared between the "test" and "retest" data. [11C]MPC-6827 showed excellent brain uptake with good pharmacokinetics in non-human primate brains, with significant correlation between the test and retest scan data (r = 0.77, p = 0.023). These initial evaluations demonstrate the high translational potential of [11C]MPC-6827 to image microtubules in the brain in vivo in monkey models of neurological and psychiatric diseases.

Regional elevations in microglial activation and cerebral glucose utilization in frontal white matter tracts of rhesus monkeys following prolonged cocaine self-administration.

It has been shown that exposure to cocaine can result in neuroinflammatory responses. Microglia, the resident CNS immune cells, undergo a transition to an activated state when challenged. In rodents, and possibly humans, cocaine exposure activates microglia. The goal of this study was to assess the extent and magnitude of microglial activation in rhesus monkeys with an extensive history of cocaine self-administration. Male rhesus monkeys (N = 4/group) were trained to respond on a fixed-interval 3-min schedule of food or 0.3 mg/kg/injection cocaine presentation (30 reinforcers/session) for 300 sessions. At the end of the final session, monkeys were administered 2-[14C]deoxyglucose intravenously and 45 min later euthanized. Brain sections were used for autoradiographic assessments of glucose utilization and for microglia activation with [3H]PK11195, a marker for the microglial 18-kDa translocator protein. There were no group differences in gray matter [3H]PK11195 binding, while binding was significantly greater in cocaine self-administration animals as compared to food controls in 8 of the 11 white matter tracts measured at the striatal level. Binding did not differ from control at other levels. There were also significant increases in white matter local cerebral glucose utilization at the striatal level, which were positively correlated with [3H]PK11195 binding. The present findings demonstrate an elevation in [3H]PK11195 binding in forebrain white matter tracts of nonhuman primates with a prolonged history of cocaine self-administration. These elevations were also associated with greater cerebral metabolic rates. These data suggest that white matter deficits may contribute to behavioral, motivational, and cognitive impairments observed in cocaine abusers.

Chronic Δ9-THC in Rhesus Monkeys: Effects on Cognitive Performance and Dopamine D2/D3 Receptor Availability.

Cannabis-related impairments to cognitive function may represent novel therapeutic targets for cannabis-use disorder, although the nature, persistence, and reversibility of such deficits remain unclear. Adult male rhesus monkeys (N = 6) responded in the morning on tasks designed to assess different cognitive domains using the Cambridge Neuropsychological Test Automated Battery (CANTAB) touchscreens followed by responding maintained under a fixed-ratio (FR) 10 schedule of food presentation in different operant chambers. First, the acute effects of Δ9-tetrahydrocannabinol (THC; 0.01-0.56 mg/kg, i.v.) on cognitive performance, FR responding, and body temperature were determined. Next, THC (1.0-2.0 mg/kg, s.c.) was administered daily after FR 10 sessions for 12 weeks, during which the residual effects of THC (i.e., 22 hours after administration) on cognition were examined and the acute effects of THC were redetermined. In a subgroup of monkeys, dopamine D2/D3 receptor availability was assessed after 4 weeks of chronic THC exposure and compared with drug-naive controls using positron emission tomography and [11C]-raclopride (N = 4/group). Acute THC pretreatments dose-dependently decreased FR responding and body temperature, and impairment to cognitive performance was task specific. During chronic treatment, THC produced persistent residual impairment only to working memory; tolerance differentially developed to acute cognitive impairments. There was recovery from residual cognitive impairments to working memory within 2 weeks of abstinence. Compared with controls, D2/D3 receptor availability was not altered during chronic THC treatment. In conclusion, THC-induced disruptions in cognition were task-specific, as was tolerance development, and not related to changes in D2/D3 receptor availability. Intervention strategies for cannabis-use disorder that enhance working memory performance may facilitate positive treatment outcomes.

Neural Correlates of Exposure to Cocaine Cues in Rhesus Monkeys: Modulation by the Dopamine Transporter.

BACKGROUND: A major goal of treatments for cocaine addiction is to reduce relapse-associated cravings, which are typically induced by environmental stimuli associated with cocaine use and related to changes in dopamine neurotransmission. METHODS: The present study used an animal model of cocaine seeking to determine functional consequences of cue exposure using fluorodeoxyglucose positron emission tomography and to relate findings to juvenile levels of dopamine transporter and D2-like receptor availabilities determined before any drug exposure. Adult male rhesus monkeys (N = 11) self-administered cocaine (0.2 mg/kg per injection) under a second-order schedule of reinforcement, in which responding was maintained by conditioned reinforcers. Positron emission tomography scans assessing glucose utilization, a marker of functional activation, were conducted during cocaine-cue responding and food-reinforced responding in a context where cocaine was never available. RESULTS: Compared with the noncocaine condition, we found significant functional activation in the medial prefrontal cortex, anterior cingulate, precuneus region of the parietal cortex, and striatum-findings similar to those reported in humans who abuse cocaine. Furthermore, these functional activations in the prefrontal, cingulate, and parietal cortex measured during cocaine-cue responding were significantly correlated with juvenile measures of dopamine transporter availability, whereas no significant relationship with prior D2-like receptor availability was observed in any brain region. CONCLUSIONS: The similarity between the present findings and findings in humans who use cocaine supports the use of this model for examination of factors that affect the development and intensity of cue-induced drug seeking and provides evidence for potential biomarkers for the evaluation of potential treatments (behavioral and pharmacologic) for cocaine abuse.

Evaluation of the Reinforcing Effect of Quetiapine, Alone and in Combination with Cocaine, in Rhesus Monkeys.

There are several case reports of nonmedicinal quetiapine abuse, yet there are very limited preclinical studies investigating quetiapine self-administration. The goal of this study was to investigate the reinforcing effects of quetiapine alone and in combination with intravenous cocaine in monkeys. In experiment 1, cocaine-experienced female monkeys (N = 4) responded under a fixed-ratio (FR) 30 schedule of food reinforcement (1.0-g banana-flavored pellets), and when responding was stable, quetiapine (0.003-0.1 mg/kg per injection) or saline was substituted for a minimum of five sessions; there was a return to food-maintained responding between doses. Next, monkeys were treated with quetiapine (25 mg, by mouth, twice a day) for approximately 30 days, and then the quetiapine self-administration dose-response curve was redetermined. In experiment 2, male monkeys (N = 6) self-administered cocaine under a concurrent FR schedule with food reinforcement (three food pellets) as the alternative to cocaine (0.003-0.3 mg/kg per injection) presentation. Once choice responding was stable, the effects of adding quetiapine (0.03 or 0.1 mg/kg per injection) to the cocaine solution were examined. In experiment 1, quetiapine did not function as a reinforcer, and chronic quetiapine treatment did not alter these effects. In experiment 2, cocaine choice increased in a dose-dependent fashion. The addition of quetiapine to cocaine resulted in increases in low-dose cocaine choice and number of cocaine injections in four monkeys, while not affecting high-dose cocaine preference. Thus, although quetiapine alone does not have abuse potential, there was evidence of enhancement of the reinforcing potency of cocaine. These results suggest that the use of quetiapine in cocaine-addicted patients should be monitored.

Preclinical laboratory assessments of predictors of social rank in female cynomolgus monkeys.

Physiological and behavioral differences between dominant and subordinate monkeys have been useful in preclinical models investigating numerous disease states. In captivity, it has been inferred that subordinate monkeys live in a context of chronic social stress and may be at risk for a variety of dysfunctions; however, the factors that influence eventual rank are not entirely known. The goal of the present study was to first evaluate several phenotypic characteristics as potential trait markers for eventual social rank and then to determine the consequences of social hierarchy on these measures (i.e., state markers). Baseline estradiol, progesterone, cortisol and testosterone concentrations were obtained from 16 pair-housed female cynomolgus monkeys before and after introduction into new social groups (n = 4/group). Furthermore, effects of the initial week of social rank establishment on outcome measures of cognitive performance and homecage activity were examined. Baseline body weight and mean serum estradiol concentrations were the only statistically significant predictors of eventual rank, with future subordinate monkeys weighing less and having higher estradiol concentrations. During initial hierarchy establishment, future subordinate monkeys had increased morning and afternoon cortisol concentrations, increased locomotor activity and impaired cognitive performance on a working memory task. After 3 months of social housing, subordinate monkeys had blunted circulating estradiol and progesterone concentrations. These findings demonstrate differential effects on gonadal hormones and cortisol as a function of social context in normally cycling female monkeys. Furthermore, disruptions in cognitive performance were associated with subordinate status, suggesting strong face validity of this model to the study of factors related to the etiology and treatment of human diseases associated with chronic stress. Am. J. Primatol. 78:402-417, 2016. © 2015 Wiley Periodicals, Inc.

Functional consequences of cocaine re-exposure after discontinuation of cocaine availability.

Cocaine users exhibit a wide range of behavioral impairments accompanied by brain structural, neurochemical and functional abnormalities. Metabolic mapping studies in cocaine users and animal models have shown extensive functional alterations throughout the striatum, limbic system, and cortex. Few studies, however, have evaluated the persistence of these effects following cessation of cocaine availability. The purpose of this study, therefore, was to assess the functional effects of re-exposure to cocaine in nonhuman primates after the discontinuation of cocaine self-administration for 30 or 90 days, using the quantitative autoradiographic 2-[14C]deoxyglucose (2DG) method. Rhesus monkeys self-administered cocaine (fixed interval 3-min schedule, 30 infusions per session, 0.3 mg/kg/infusion) for 100 sessions followed by 30 (n=4) or 90 days (n=3) during which experimental sessions were not conducted. Food-reinforced control animals (n=5) underwent identical schedules of reinforcement. Animals were then re-exposed to cocaine or food for one final session and the 2DG method applied immediately after session completion. Compared to controls, re-exposure to cocaine after 30 or 90 day drug-free periods resulted in lower rates of glucose utilization in ventral and dorsal striatum, prefrontal and temporal cortex, limbic system, thalamus, and midbrain. These data demonstrate that vulnerability to the effects of cocaine persists for as long as 90 days after cessation of drug use. While there was some evidence for recovery (fewer brain areas were affected by cocaine re-exposure at 90 days as compared to 30 days), this was not uniform across regions, thus suggesting that recovery occurs at different rates in different brain systems.

Further characterization of quinpirole-elicited yawning as a model of dopamine D3 receptor activation in male and female monkeys.

The dopamine (DA) D3 receptor (D3R) has been associated with impulsivity, pathologic gambling, and drug addiction, making it a potential target for pharmacotherapy development. Positron emission tomography studies using the D3R-preferring radioligand [(11)C]PHNO ([(11)C](+)-propyl-hexahydro-naphtho-oxazin) have shown higher binding potentials in drug abusers compared with control subjects. Preclinical studies have examined D3R receptor activation using the DA agonist quinpirole and the unconditioned behavior of yawning. However, the relationship between quinpirole-elicited yawning and D3R receptor availability has not been determined. In Experiment 1, eight drug-naive male rhesus monkeys were scanned with [(11)C]PHNO, and the ability of quinpirole (0.01-0.3 mg/kg i.m.) to elicit yawning was examined. Significant positive (globus pallidus) and negative (caudate nucleus, putamen, ventral pallidum, and hippocampus) relationships between D3R receptor availability and quinpirole-induced yawns were noted. Experiment 2 replicated earlier findings that a history of cocaine self-administration (n = 11) did not affect quinpirole-induced yawning and extended this to examine monkeys (n = 3) with a history of methamphetamine (MA) self-administration and found that monkeys with experience self-administering MA showed greater potency and significantly higher quinpirole-elicited yawning compared with controls. Finally, quinpirole-elicited yawning was studied in drug-naive female monkeys (n = 6) and compared with drug-naive male monkeys (n = 8). Sex differences were noted, with quinpirole being more potent and eliciting significantly more yawns in males compared with females. Taken together these findings support the use of quinpirole-elicited yawning as a behavioral tool for examining D3R activation in monkeys and that both drug history and sex may influence individual sensitivity to the behavioral effects of D3R compounds.

Nonhuman primate models of social behavior and cocaine abuse.

RATIONALE: Although cocaine is often abused in social situations, very few animal studies examine the effects of cocaine in the context of social behavior. OBJECTIVES: This review highlights studies investigating the behavioral effects of cocaine in the context of social housing conditions using nonhuman primates. In addition, this review presents recent findings examining the effects of self-administering cocaine on social behavior and the effects of manipulations hypothesized to be stressful or enriching on the interactions between cocaine reinforcement and social rank. The following dependent variables are examined: (1) cocaine-induced changes in social behavior and (2) cocaine self-administration in cynomolgus monkeys of varying social ranks. The independent variables examined include several environmental and pharmacological manipulations. CONCLUSIONS: The studies reviewed here indicate that several variables can differentially affect cocaine self-administration when studied in a social context, rather than in individually housed animals. These variables include the social rank and sex of the individual, drug history, the nature of the "fear"-inducing manipulation, and the reliability of cortisol as an appropriate measure of "stress." While the inclusion of socially housed animals necessitates larger sample sizes, animal models incorporating social behavior are more homologous to the human condition and should be implemented when possible.

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.

Effects of varenicline on the reinforcing and discriminative stimulus effects of cocaine in rhesus monkeys.

Varenicline is a low-efficacy, α4ß2* subtype-selective nicotinic acetylcholine receptor (nAChR) agonist that has shown success in smoking cessation and promise in preclinical assessments relating to other drugs of abuse. The primary goal of the present study was to examine the effects of varenicline on cocaine self-administration and cocaine discrimination and compare these effects with those of the nAChR agonist nicotine and antagonist mecamylamine. One limitation of agonist treatments is the potential for abuse. Thus, a second goal was to examine the abuse potential of varenicline in rhesus monkeys. In the first experiment, rhesus monkeys (n = 3) were trained to self-administer cocaine (saline, 0.01-0.56 mg/kg) under a progressive-ratio schedule of reinforcement; monkeys also earned all of their food by responding on another lever under a fixed-ratio 50 schedule of reinforcement. Chronic administration of varenicline (0.01-0.56 mg/kg p.o., salt) potentiated the reinforcing effects of cocaine, whereas mecamylamine (0.3-1.7 mg/kg p.o, i.m., i.v., salt) had no significant effects on cocaine self-administration up to doses that disrupted food-maintained responding. Neither varenicline (0.01-0.17 mg/kg, salt) nor nicotine (0.01-0.1 mg/kg, base) functioned as reinforcers when substituted for cocaine. Finally, in monkeys trained to discriminate self-administered 0.3 mg/kg cocaine, varenicline (0.1-0.3 mg/kg i.v.) did not substitute for cocaine but, along with mecamylamine (0.3-1.7 mg/kg i.v.) and nicotine (0.03-0.1 mg/kg i.v.), potentiated the discriminative stimulus effects of cocaine. These results suggest that varenicline has low abuse liability in monkey models of cocaine abuse, but would not be an effective medication for cocaine addiction.

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 [(18)F]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.

Long-term cocaine self-administration under fixed-ratio and second-order schedules in monkeys.

RATIONALE: Studies in rodents have demonstrated that increased access to cocaine can result in increases in drug intake per unit time. OBJECTIVES: The present studies characterized long-term changes in cocaine self-administration associated with quantitatively and qualitatively different conditions of cocaine availability in monkeys. MATERIALS AND METHODS: Separate groups of rhesus monkeys (n = 6/group) self-administered cocaine (0.2 mg/kg per injection) under a fixed ratio (FR) 30 schedule for 3 h twice daily for two consecutive days each week for 1 year, or responded under a second-order FR 10 (fixed interval 3-min:S) schedule of 0.2 mg/kg per injection cocaine during daily sessions. After 18 weeks, probe sessions were conducted once per week, in which responding was maintained under a fixed interval (FI) 30-min schedule in the presence of distinct stimuli. RESULTS: Weekly cocaine intakes under the FR schedule were stable in three subjects, but increased progressively in three monkeys over 1 year. In contrast, response rates under the second-order schedule were low and stable over time. Responding under the FI 30-min schedule was higher for monkeys in the FR group and pattern of responding was not indicative of FI performance, perhaps due to experimental history. CONCLUSIONS: These data suggest that increases in cocaine intake can be observed under ratio schedules in monkeys. The use of an FI 30-min "probe" to assess changes in "drug seeking" appeared to be influenced by experimental history. These data may aid in the development of behavioral models of cocaine abuse, which focus on the compulsive nature of drug taking.