3,4-Methylenedioxymethamphetamine facilitates fear extinction learning.

Acutely administered 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') has been proposed to have long-term positive effects on post-traumatic stress disorder (PTSD) symptoms when combined with psychotherapy. No preclinical data support a mechanistic basis for these claims. Given the persistent nature of psychotherapeutic gains facilitated by MDMA, we hypothesized that MDMA improves fear extinction learning, a key process in exposure-based therapies for PTSD. In these experiments, mice were first exposed to cued fear conditioning and treated with drug vehicle or MDMA before extinction training 2 days later. MDMA was administered systemically and also directly targeted to brain structures known to contribute to extinction. In addition to behavioral measures of extinction, changes in mRNA levels of brain-derived neurotrophic factor (Bdnf) and Fos were measured after MDMA treatment and extinction. MDMA (7.8 mg kg(-1)) persistently and robustly enhanced long-term extinction when administered before extinction training. MDMA increased the expression of Fos in the amygdala and medial prefrontal cortex (mPFC), whereas increases in Bdnf expression were observed only in the amygdala after extinction training. Extinction enhancements were recapitulated when MDMA (1 µg) was infused directly into the basolateral complex of the amygdala (BLA), and enhancement was abolished when BDNF signaling was inhibited before extinction. These findings suggest that MDMA enhances fear memory extinction through a BDNF-dependent mechanism, and that MDMA may be a useful adjunct to exposure-based therapies for PTSD and other anxiety disorders characterized by altered fear learning.

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.

A thermostable bacterial cocaine esterase rapidly eliminates cocaine from brain in nonhuman primates.

A long-acting, thermostable bacterial cocaine esterase (CocE) has been identified that rapidly degrades cocaine with a K(M) of 1.33+0.085 µM. In vivo evaluation of CocE has shown protection against convulsant and lethal effects of cocaine in rodents, confirming the therapeutic potential of CocE against cocaine overdose. However, the current study is the first to evaluate the effects of CocE on cocaine brain levels. Positron emission tomogrpahy neuroimaging of [(11)C]cocaine was used to evaluate the time course of cocaine elimination from brain in the presence and absence of CocE in nonhuman primates. Systemic administration of CocE eliminated cocaine from the rhesus-monkey brain approximately three times faster than control conditions via peripheral actions through attenuating the input function from blood plasma. The efficiency of this process is sufficient to alleviate or prevent adverse central nervous system effects induced by cocaine. Although the present study used tracer doses of cocaine to access brain clearance, these findings further support the development of CocE for the treatment of acute cocaine toxicity.

Relationship between dose, drug levels, and D2 receptor occupancy for the atypical antipsychotics risperidone and paliperidone.

Blockade of D2 family dopamine receptors (D2Rs) is a fundamental property of antipsychotics, and the degree of striatal D2R occupancy has been related to antipsychotic and motor effects of these drugs. Recent studies suggest the D2R occupancy of antipsychotics may differ in extrastriatal regions compared with the dorsal striatum. We studied this issue in macaque monkeys by using a within-subjects design. [(18)F]fallypride positron emission tomography scans were obtained on four different doses of risperidone and paliperidone (the 9-OH metabolite of risperidone) and compared with multiple off-drug scans in each animal. The half-life of the two drugs in these monkeys was determined to be between 3 and 4 h, and drug was administered by a constant infusion through an intragastric catheter. The D2R occupancy of antipsychotic was determined in the caudate, putamen, ventral striatum, and four prefrontal and temporal cortical regions and was related to serum and cerebrospinal fluid drug levels. Repeated 2-week treatment with risperidone or paliperidone did not produce lasting changes in D2R binding potential in any region examined. As expected, D2R binding potential was highest in the caudate and putamen and was approximately one-third that level in the ventral striatum and 2% of that level in the cortical regions. We found dose-dependent D2R occupancy for both risperidone and paliperidone in both basal ganglia and cortical regions of interest. We could not find evidence of regional variation in D2R occupancy of either drug. Comparison of D2R occupancy and serum drug levels supports a target of 40 to 80 ng/ml active drug for these two atypical antipsychotics.

The neuropharmacology of prolactin secretion elicited by 3,4-methylenedioxymethamphetamine ("ecstasy"): a concurrent microdialysis and plasma analysis study.

3,4-methylenedioxymethamphetamine (MDMA) is a substituted phenethylamine that is widely abused as the street drug "ecstasy". Racemic MDMA (S,R(+/-)-MDMA) and its stereoisomers elicit complex spectrums of psychobiological, neurochemical, and hormonal effects. In this regard, recent findings demonstrated that S,R(+/-)-MDMA and its stereoisomer R(-)-MDMA elicit increases in striatal extracellular serotonin levels and plasma levels of the hormone prolactin in rhesus monkeys. In the present mechanistic study, we evaluated the role of the serotonin transporter and the 5-HT(2A) receptor in S,R(+/-)-MDMA- and R(-)-MDMA-elicited prolactin secretion in rhesus monkeys through concurrent microdialysis and plasma analysis determinations and drug interaction experiments. Concurrent neurochemical and hormone determinations showed a strong positive temporal correlation between serotonin release and prolactin secretion. Consistent with their distinct mechanisms of action and previous studies showing that the serotonin transporter inhibitor fluoxetine attenuates the behavioral and neurochemical effects of S,R(+/-)-MDMA, pretreatment with fluoxetine attenuated serotonin release elicited by either S,R(+/-)-MDMA or R(-)-MDMA. As hypothesized, at a dose that had no significant effects on circulating prolactin levels when administered alone, fluoxetine also attenuated prolactin secretion elicited by S,R(+/-)-MDMA. In contrast, combined pretreatment with both fluoxetine and the selective 5-HT(2A) receptor antagonist M100907 was required to attenuate prolactin secretion elicited by R(-)-MDMA, suggesting that this stereoisomer of S,R(+/-)-MDMA elicits prolactin secretion through both serotonin release and direct agonism of 5-HT(2A) receptors. Accordingly, these findings inform our understanding of the neuropharmacology of both S,R(+/-)-MDMA and R(-)-MDMA and the regulation of prolactin secretion.

Pharmacokinetics of fluoxetine in rhesus macaques following multiple routes of administration.

BACKGROUND/AIMS: Fluoxetine (Prozac) is a selective serotonin reuptake inhibitor currently used to treat depression and mood disorders. It has been widely studied clinically and preclinically, yet there is limited knowledge of its pharmacokinetics in nonhuman primates. METHODS: The present study characterized the pharmacokinetics of fluoxetine and its active metabolite norfluoxetine in rhesus macaques following both acute (1, 3, 5.6 and 10 mg/kg) and chronic doses (5.6 and 10 mg/kg/day) via different routes of administration (intravenous, subcutaneous, intramuscular, and oral). Blood samples were collected at multiple time points following administration and analyzed using mass spectrometry. RESULTS: Fluoxetine had a half-life of 11-16 h and norfluoxetine had a half-life of 21-29 h. Potentially functionally significant serum concentrations of norfluoxetine were present at 24 h even after a single administration of fluoxetine. Similar to observations in humans under steady state conditions, norfluoxetine accounted for the greater percentage of active drug in the blood stream. CONCLUSION: A daily dose of 10 mg/kg administered orally maintained serum concentrations in the human clinical range over the course of 6 weeks. Given the long half-lives of fluoxetine and norfluoxetine observed in this study, precautions should be taken when designing preclinical studies to prevent accumulation of drug serum concentrations.

Endocrine and neurochemical effects of 3,4-methylenedioxymethamphetamine and its stereoisomers in rhesus monkeys.

3,4-Methylenedioxymethamphetamine (MDMA) is an amphetamine derivative that elicits complex biological effects in humans. One plausible mechanism for this phenomenon is that racemic MDMA is composed of two stereoisomers that exhibit qualitatively different pharmacological effects. In support of this, studies have shown that R(-)-MDMA tends to have hallucinogen-like effects, whereas S(+)-MDMA tends to have psychomotor stimulant-like effects. However, relatively little is known about whether these stereoisomers engender different endocrine and neurochemical effects. In the present study, the endocrine and neurochemical effects of each stereoisomer and the racemate were assessed in four rhesus monkeys after intravenous delivery at doses (1-3 mg/kg) that approximated voluntary self-administration by rhesus monkeys and human recreational users. Specifically, fluorescence-based enzyme-linked immunosorbent assay was used to assess plasma prolactin concentrations, and in vivo microdialysis was used to assess extracellular dopamine and serotonin concentrations in the dorsal striatum. R(-)-MDMA, but not S(+)-MDMA, significantly increased plasma prolactin levels and the effects of S,R(+/-)-MDMA were intermediate to each of its component stereoisomers. Although S(+)-MDMA did not alter prolactin levels, it did significantly increase extracellular serotonin concentrations. In addition, S(+)-MDMA, but not R(-)-MDMA, significantly increased dopamine concentrations. Furthermore, as in the prolactin experiment, the effects of the racemate were intermediate to each of the stereoisomers. These studies demonstrate the stereoisomers of MDMA engender qualitatively different endocrine and neurochemical effects, strengthening the inference that differences in these stereoisomers might be the mechanism producing the complex biological effects of the racemic mixture of MDMA in humans.

Integrative proteomic analysis of the nucleus accumbens in rhesus monkeys following cocaine self-administration.

The reinforcing effects and long-term consequences of cocaine self-administration have been associated with brain regions of the mesolimbic dopamine pathway, namely the nucleus accumbens (NAc). Studies of cocaine-induced biochemical adaptations in rodent models have advanced our knowledge; however, unbiased detailed assessments of intracellular alterations in the primate brain are scarce, yet essential, to develop a comprehensive understanding of cocaine addiction. To this end, two-dimensional difference in gel electrophoresis (2D-DIGE) was used to compare changes in cytosolic protein abundance in the NAc between rhesus monkeys self-administering cocaine and controls. Following image normalization, spots with significantly differential image intensities (P<0.05) were identified, excised, trypsin digested and analyzed by matrix-assisted laser-desorption ionization time-of-flight time-of-flight (MALDI-TOF-TOF). In total, 1098 spots were subjected to statistical analysis with 22 spots found to be differentially abundant of which 18 proteins were positively identified by mass spectrometry. In addition, approximately 1000 protein spots were constitutively expressed of which 21 proteins were positively identified by mass spectrometry. Increased levels of proteins in the cocaine-exposed monkeys include glial fibrillary acidic protein, syntaxin-binding protein 3, protein kinase C isoform, adenylate kinase isoenzyme 5 and mitochondrial-related proteins, whereas decreased levels of proteins included beta-soluble N-ethylmaleimide-sensitive factor attachment protein and neural and non-neural enolase. Using a complimentary proteomics approach, the differential expression of phosphorylated proteins in the cytosolic fraction of these subjects was examined. Two-dimensional gel electrophoresis (2DGE) was followed by gel staining with Pro-Q Diamond phosphoprotein gel stain, enabling differentiation of approximately 150 phosphoprotein spots between the groups. Following excision and trypsin digestions, MALDI-TOF-TOF was used to confirm the identity of 15 cocaine-altered phosphoproteins. Significant increased levels were detected for gamma-aminobutyric acid type A receptor-associated protein 1, 14-3-3 gamma-protein, glutathione S-transferase and brain-type aldolase, whereas significant decreases were observed for beta-actin, Rab GDP-dissociation inhibitor, guanine deaminase, peroxiredoxin 2 isoform b and several mitochondrial proteins. Results from these studies indicate coordinated dysregulation of proteins related to cell structure, signaling, metabolism and mitochondrial function. These data extend and compliment previous studies of cocaine-induced biochemical alterations in human postmortem brain tissue, using an animal model that closely recapitulates the human condition and provide new insight into the molecular basis of the disease and potential targets for pharmacotherapeutic intervention.

Discriminative stimulus effects of psychostimulants and hallucinogens in S(+)-3,4-methylenedioxymethamphetamine (MDMA) and R(-)-MDMA trained mice.

3,4-Methylenedioxymethamphetamine (MDMA) is a substituted phenethylamine more commonly known as the drug of abuse "ecstasy." The acute and persistent neurochemical effects of MDMA in the mice are distinct from those in other species. MDMA shares biological effects with both amphetamine-type stimulants and mescaline-type hallucinogens, which may be attributable to distinct effects of its two enantiomers, both of which are active in vivo. In this regard, among the substituted phenethylamines, R(-)-enantiomers tend to have hallucinogen-like effects, whereas S(+)-enantiomers tend to have stimulant-like effects. In the present study, mice were trained to discriminate S(+)- or R(-)-MDMA from vehicle. Drug substitution tests were then undertaken with the structurally similar phenethylamine dopamine/norepinephrine releaser S(+)-amphetamine, the structurally dissimilar tropane nonselective monoamine reuptake inhibitor cocaine, the structurally similar phenethylamine 5-hydroxytryptamine (5-HT)(2A) agonist 2,5-dimethoxy-4-(n)-propylthiophenethylamine (2C-T-7), and the structurally dissimilar mixed action tryptamine 5-HT(2A) agonist/monoamine reuptake inhibitor N,N-dipropyltryptamine (DPT). S(+)-amphetamine fully substituted in the S(+)-MDMA-treated animals but did not substitute for the R(-)-MDMA cue. 2C-T-7 fully substituted in the R(-)-MDMA-trained animals but did not substitute for the S(+)-MDMA cue. Cocaine and DPT substituted for both training drugs, but whereas cocaine was more potent in S(+)-MDMA-trained mice, DPT was more potent in R(-)-MDMA-trained mice. These data suggest that qualitative differences in the discriminative stimulus effects of each stereoisomer of MDMA exist in mice and further our understanding of the complex nature of the interoceptive effects of MDMA.

Effects of the monoamine uptake inhibitors RTI-112 and RTI-113 on cocaine- and food-maintained responding in rhesus monkeys.

Cocaine blocks uptake of the monoamines dopamine, serotonin and norepinephrine, and monoamine uptake inhibitors constitute one class of drugs under consideration as candidate "agonist" medications for the treatment of cocaine abuse and dependence. The pharmacological selectivity of monoamine uptake inhibitors to block uptake of dopamine, serotonin and norepinephrine is one factor that may influence the efficacy and/or safety of these compounds as drug abuse treatment medications. To address this issue, the present study compared the effects of 7-day treatment with a non-selective monoamine uptake inhibitor (RTI-112) and a dopamine-selective uptake inhibitor (RTI-113) on cocaine- and food-maintained responding in rhesus monkeys. Monkeys (N=3) were trained to respond for cocaine injections (0.01 mg/kg/inj) and food pellets under a second-order schedule [FR2(VR16:S)] during alternating daily components of cocaine and food availability. Both RTI-112 (0.0032-0.01 mg/kg/hr) and RTI-113 (0.01-0.056 mg/kg/h) produced dose-dependent, sustained and nearly complete elimination of cocaine self-administration. However, for both drugs, the potency to reduce cocaine self-administration was similar to the potency to reduce food-maintained responding. These findings do not support the hypothesis that pharmacological selectivity to block dopamine uptake is associated with behavioral selectivity to decrease cocaine- vs. food-maintained responding in rhesus monkeys.

Self-administration of cocaine and the cocaine analog RTI-113: relationship to dopamine transporter occupancy determined by PET neuroimaging in rhesus monkeys.

Dopaminergic mechanisms are thought to play a central role in the reinforcing effects of cocaine. The present study examined the reinforcing effects of 3beta-(4-chlorophenyl)tropane-2beta-carboxylic acid phenyl ester (RTI-113), a long-acting, selective, high-affinity dopamine uptake inhibitor. Additionally, the effects of RTI-113 pretreatment on cocaine self-administration were determined. Monkeys were trained to respond under a second-order schedule for intravenous cocaine administration (0.10 or 0.17 mg/kg/infusion). When responding was stable, cocaine (0.0030-1.0 mg/kg/infusion) and RTI-113 (0.010-0.30 mg/kg/infusion) were substituted for the cocaine training dose. Cocaine and RTI-113 were equipotent for their reinforcing effects. However, cocaine maintained higher response rates in two of the three monkeys tested. When administered as a pretreatment, RTI-113 (0.10-0.30 mg/kg) dose-dependently reduced responding maintained by two doses of cocaine. Drug effects on behavior were related to dopamine transporter (DAT) occupancy in monkey striatum during neuroimaging with positron emission tomography. DAT occupancy was determined by displacement of 8-(2-[(18)F]fluroethyl)2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (FECNT). DAT occupancy was between 65-76% and 94-99% for doses of cocaine and RTI-113 that maintained maximum response rates, respectively. DAT occupancy did not differ markedly across RTI-113 pretreatment doses and ranged between 72-84%. The results suggest that the pharmacokinetic profile of RTI-113 (i.e., long-acting) may influence its ability to maintain self-administration, and therefore its abuse liability. Additionally, high DAT occupancy is required for RTI-113 to reduce cocaine-maintained responding.

The dopamine transporter and cocaine medication development: drug self-administration in nonhuman primates.

Despite intensive medication development efforts, no effective pharmacotherapy for cocaine abuse has demonstrated efficacy for long-term use. Given the obvious importance of the dopamine transporter in the addictive properties of cocaine, the development and use of compounds that target the dopamine transporter represents a reasonable approach for the pharmacological treatment of cocaine abuse. The therapeutic approach of replacement or substitute agonist medication has been successful, as shown with methadone maintenance for heroin dependence and nicotine replacement for tobacco use. A number of preclinical studies with dopamine transporter inhibitors provide evidence that substitute agonists may be used effectively to reduce cocaine use. Nonhuman primate models of drug self-administration provide a rigorous, systematic approach to characterize medication effectiveness in subjects with a documented history of drug use. Several cocaine analogs and other dopamine transporter inhibitors, including analogs of GBR 12909 and WIN 35,065-2, have been shown to reduce cocaine self-administration in nonhuman primates. A possible limitation to the use of selective dopamine transporter inhibitors as medications is their potential for abuse liability given their demonstrated reinforcing effects in nonhuman primates. However, limited reinforcing properties in the context of treatment programs may be advantageous, contributing to improved patient compliance and enhanced medication effectiveness. Moreover, pharmacokinetic properties that result in slow onset and long duration of action may enhance their effectiveness to reduce cocaine use while limiting their abuse liability.

Fetal development in rhesus monkeys exposed prenatally to cocaine.

Using a timed-breeding protocol, one group of female rhesus monkeys was implanted subcutaneously with osmotic minipumps containing 0.3 mg/kg/h cocaine (N=18) or saline (N=18) from day 24 postconception through gestation. Another group received cocaine (N=12) or saline (N=8) from conception through day 42 of gestation. Mean levels of cocaine in maternal serum were approximately 150 ng/ml during pregnancy. A total of 56 pregnancies were documented in 42 adult monkeys, and 39 pregnancies completed full-term. Maternal food consumption and body weight increased during pregnancy, and there were no significant differences among experimental groups. Although both groups with a history of cocaine exposure had lower survival rates compared to pair-fed controls, of the fetuses that survived, fetal heart rate, fetal biparietal diameter, and mean gestational length were in the normal range for all experimental groups. Similarly, body weight, biparietal diameter, body length, and modified Apgar scores at birth did not differ significantly among experimental groups. The results indicate that surviving fetuses exhibited normal growth.

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

Nonhuman primates offer a unique resource in neuroimaging research, providing the opportunity to manipulate appropriate biological and behavioral variables under well-controlled experimental conditions in an animal model that is closely related to humans, both functionally and neuroanatomically. The present report describes the development and standardization of PET neuroimaging protocols in conscious rhesus monkeys and their application to characterize the acute effects of cocaine on cerebral blood flow. Specific attention was devoted to the development of an effective and comfortable head restraint device to be used in the imaging of conscious monkeys. The restraint device was designed to attach to a standard primate chair to facilitate frequent immobilization. Subjects received extensive behavioral training prior to neuroimaging in order to ensure their comfort and minimize potential stress associated with the imaging protocols. Functional changes in cerebral blood flow were characterized in three subjects with the positron-emitting tracer 15O water following acute i.v. administration of cocaine. Regions of interest were defined on MRI scans with a high degree of accuracy. Cocaine caused pronounced increases in cerebral blood flow at 5 min postinjection that diminished markedly within 25 min. The results document the feasibility to conduct PET neuroimaging studies of cerebral blood flow in conscious nonhuman primates. Extension of the methodology to include brain activation during behavioral studies could contribute significantly to the growing discipline of behavioral neuroscience.

Behavioral effects of AMI-193, a 5-HT(2A)- and dopamine D(2)-receptor antagonist, in the squirrel monkey.

8-[3-(4-Fluorophenoxy) propyl]-1-phenyl-1,3,8-triazaspiro[4, 5]decan-4-one (AMI-193) was developed as a 5-HT(2A)-selective antagonist with in vivo activity suitable for behavioral studies. However, AMI-193 is a potent dopamine D(2)-receptor antagonist with low nanomolar affinity. Accordingly, D(2)-actions may contribute to its behavioral pharmacology. In the present study, the effects of AMI-193 on operant behavior were characterized in squirrel monkeys. In subjects trained under a fixed-interval (FI) schedule of stimulus termination, AMI-193 (0.003-0.01 mg/kg) dose-dependently decreased response rate. When administered in combination with cocaine (0.03-3. 0 mg/kg) or the selective dopamine uptake inhibitor, GBR 12909 (0. 03-3.0 mg/kg), the rate-decreasing effects of AMI-193 were reversed by both dopamine indirect agonists. In drug-discrimination experiments, AMI-193 (0.003 and 0.01 mg/kg) attenuated the discriminative-stimulus effects of cocaine. AMI-193 (0.003 and 0.01 mg/kg) also reduced response rate under a second-order schedule of i. v. self-administration of cocaine (0.1 mg/infusion). The profile of behavioral effects and drug interactions observed in the present study, in conjunction with the relatively high affinity of AMI-193 for dopamine D(2) receptors, suggests that its D(2)-antagonist effects play a prominent role in the behavioral pharmacology of AMI-193.

Comparative behavioral pharmacology of cocaine and the selective dopamine uptake inhibitor RTI-113 in the squirrel monkey.

The behavioral effects of 3beta-(4-chlorophenyl)tropane-2beta-carboxylic acid phenyl ester hydrochloride (RTI-113; 0.03-1.0 mg/kg), a selective dopamine uptake inhibitor, were compared with those of cocaine (0.03-3.0 mg/kg) and 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (GBR 12909; 0.03-3.0 mg/kg) in squirrel monkeys. Intermediate doses of each drug produced significant increases in response rate maintained by a fixed-interval (FI) 300-s schedule of stimulus termination, but RTI-113 was less effective than cocaine or GBR 12909. The order of potency for increasing response rate was RTI-113 >/= cocaine > GBR 12909. In drug time course determinations, RTI-113 and GBR 12909 had longer durations of action than cocaine. RTI-113 substituted completely for cocaine in subjects trained to discriminate cocaine and saline under a two-lever drug-discrimination procedure maintained by food delivery. RTI-113 also reliably maintained self-administration behavior in subjects trained under a second-order FI 900-s schedule of i.v. cocaine delivery. Pretreatment with RTI-113 significantly decreased responding for cocaine at the highest pretreatment dose, but RTI-113 had similar effects on responding maintained by a second-order FI 900-s schedule of stimulus termination. The results indicate that the behavioral pharmacology of RTI-113 is similar to that of cocaine, further implicating a prominent role for dopamine uptake inhibition in the behavioral effects of cocaine. Its longer duration of action in conjunction with less pronounced behavioral-stimulant effects are desirable properties for a substitute pharmacotherapy for cocaine abuse. RTI-113 effectively decreased cocaine self-administration behavior, although its direct rate-altering effects may have contributed to the interactions obtained.

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 chronic caffeine administration on respiration and schedule-controlled behavior in rhesus monkeys.

The effects of chronic caffeine administration on ventilation and schedule-controlled behavior were studied in 12 adult rhesus monkeys. In seated subjects prepared with a head plethysmograph, ventilation was measured during exposure to air (normocapnia) and to elevated levels of CO2 (3%, 4% and 5%) mixed in air (hypercapnia). Acute administration of caffeine (10.0-30.0 mg/kg i.m.) produced marked, dose-dependent increases in ventilation during conditions of normocapnia and hypercapnia. However, daily administration of caffeine (10.0 mg/kg i.m.) for 8 consecutive days resulted in tolerance to its respiratory-stimulant effects that was surmountable with higher doses. Caffeine-tolerant subjects also were cross-tolerant to theophylline, an active metabolite of caffeine, and to rolipram and Ro 20-1724, selective phosphodiesterase inhibitors. When chronic administration was terminated and the acute effects of caffeine were redetermined, sensitivity returned to levels obtained before chronic administration within 9 days. Drug effects on behavior were studied in monkeys trained to respond under a fixed-interval schedule of stimulus termination. Acute administration of caffeine (1.0-30.0 mg/kg i.m.) produced significant rate-increasing effects on fixed-interval responding, but chronic administration resulted in tolerance that was insurmountable, such that no dose increased responding above control rates. Although the time course for development and loss of tolerance to the behavioral effects of caffeine corresponded closely with respiration, cross-tolerance did not extend to the behavioral effects of rolipram. Chronic caffeine administration had little effect on caffeine metabolism or clearance, which indicated that caffeine tolerance was pharmacodynamic. The results suggest that different neurochemical mechanisms mediate the effects of caffeine on respiration and behavior, and that inhibition of type IV phosphodiesterase plays a prominent role in caffeine-induced respiratory stimulation.

Serotonergic modulation of the discriminative-stimulus effects of cocaine in squirrel monkeys.

In order to investigate the potential modulatory role of serotonin on the discriminative-stimulus effects of cocaine, two groups of squirrel monkeys were trained to discriminate 0.3 mg/kg or 1.0 mg/kg cocaine and saline under a two-lever drug-discrimination procedure. Substitution of a range of cocaine doses (0.03-1.7 mg/kg) occasioned orderly, dose-dependent increases in cocaine-lever responding. When administered alone, the non-selective serotonin direct agonist, quipazine, also occasioned increases in cocaine-lever responding which were more pronounced in subjects trained with the lower cocaine dose. When quipazine was administered in combination with cocaine, there was an increase in cocaine-lever responding, indicating an additive effect. The serotonin uptake inhibitor, fluoxetine, occasioned saline-lever responding when administered alone. However, in combination with cocaine, fluoxetine enhanced the discriminative effects of cocaine in subjects trained at the lower cocaine dose. The 5-HT2-selective antagonists, ketanserin and ritanserin, did not occasion cocaine-lever responding when administered alone. In combination with cocaine, ketanserin attenuated the discriminative effects of cocaine in most subjects, and ritanserin attenuated the discriminative effects of cocaine in subjects trained at the higher dose. These results indicate that the discriminative-stimulus effects of cocaine may be increased by direct- and indirect-acting serotonin agonists and attenuated by serotonin antagonists in squirrel monkeys.