Positron emission tomography (PET) imaging of nicotine-induced dopamine release in squirrel monkeys using [18F]Fallypride.

BACKGROUND: Nicotine, the principal psychoactive tobacco constituent, is thought to produce its reinforcing effects via actions within the mesolimbic dopamine (DA) system. The objective of the current study was to examine the effect of nicotine on DA D2/D3 receptor availability in the nonhuman primate brain with the use of the radioligand [18F]fallypride and positron emission tomography (PET). METHODS: Ten adult male squirrel monkeys were used in the current study. Each subject underwent two PET scans, one with an injection (IV) of saline and subsequently one with an injection of nicotine (0.032mg/kg). The DA D2/D3 antagonist, [18F]fallypride, was delivered IV at the beginning of each scan, and nicotine or saline was delivered at 45min into the scan. Regions of interest (ROI) were drawn on specific brain regions and these were used to quantify standard uptake values (SUVs). The SUV is defined as the average concentration of radioactivity in the ROI x body weight/injected dose. Using the cerebellum as a reference region, SUV ratios (SUVROI/SUVcerebellum) were calculated to compare saline and nicotine effects in each ROI. RESULTS: Two-way repeated ANOVA revealed a significant decrease of SUV ratios in both striatal and extrastriatal regions following an injection of nicotine during the PET scans. CONCLUSIONS: Like other drugs of abuse, these results indicate that nicotine administration may produce DA release, as suggested by the decrease in [18F]fallypride signal in striatal regions. These findings from a nonhuman primate model provide further evidence that the mesolimbic DA system is affected by the use of products that contain nicotine.

An intravenous self-administration procedure for assessing the reinforcing effects of hallucinogens in nonhuman primates.

INTRODUCTION: Self-administration procedures are the gold standard for investigating the reinforcing effects of drugs. The notable exception to good correspondence between laboratory self-administration studies and human drug taking behavior has historically been the classic hallucinogens. METHOD: The present study used a well-established daily access procedure, followed by a novel intermittent access procedure, to investigate the reinforcing effects of LSD in baboons. RESULTS: Rates of self-injection in the daily access procedure were minimal. One baboon self-administered 0.001mg/kg and a second baboon self-administered 0.0032mg/kg above vehicle levels, though rates of self-injection were clearly low and neither of the two remaining baboons self-administered any LSD dose tested in the daily access procedure. Rates of self-injection using an intermittent access procedure with discriminative stimuli resulted in two doses of LSD being self-administered above vehicle levels in two of three baboons tested (0.01 and 0.032mg/kg in one baboon; 0.0032 and 0.01mg/kg in a second). In addition, the number of self-injections at these doses was higher (range=3-6 injections) in the intermittent access procedure than in the daily access procedure (range=1-2 injections). DISCUSSION: The present study is the first to demonstrate LSD self-administration in a laboratory animal, and though the results are limited, they indicate intermittent access procedures with discriminative stimuli may provide a reliable and valid method for investigating the reinforcing effects of IV self-administered hallucinogens in laboratory animals. The usefulness of such procedures should be further evaluated in a larger number of subjects.

The Reinforcing Effects of Nicotine in Humans and Nonhuman Primates: A Review of Intravenous Self-Administration Evidence and Future Directions for Research.

INTRODUCTION: Cigarette smoking is largely driven by the reinforcing properties of nicotine. Intravenous (IV) self-administration procedures are the gold standard for investigating the reinforcing effects of psychoactive drugs. The goal of this review was to examine the results of published investigations of the reinforcing effects of nicotine measured using IV self-administration procedures in humans and nonhuman primates. RESULTS: The body of literature using nonhuman primate subjects indicates nicotine functions as a positive reinforcer when available for self-administration via IV catheters. However, it can also be difficult to establish IV nicotine self-administration in nonhuman primates and sometimes supplemental strategies have been required (e.g., priming injections or food deprivation) before subjects acquire the behavior. Although the body of literature using human subjects is limited, the evidence indicates nicotine functions as a reinforcer via the IV route of administration in adult cigarette smokers. Rates of nicotine self-injection can be variable across subjects and responding is sometimes inconsistent across sessions in both humans and nonhuman primates. CONCLUSIONS: The Family Smoking Prevention and Tobacco Control Act, enacted in 2009, gave the Food and Drug Administration regulatory authority over the manufacture, marketing, and distribution of tobacco products. Research examining the threshold reinforcing doses for initiation and maintenance of nicotine self-administration, comparisons of the reinforcing effects of nicotine in adolescent versus adult subjects, investigations of gender differences in the reinforcing effects of nicotine, and studies of the abuse liability of non-nicotine tobacco product constituents and their ability to alter the reinforcing effects of nicotine will inform potential tobacco regulatory actions.

Effects of adolescent treatment with nicotine, harmane, or norharmane in male Sprague-Dawley rats.

The initiation of tobacco use occurs most often in adolescence and may be especially detrimental as the adolescent brain is undergoing substantial development. In addition to nicotine, there are over 9000 other compounds present in tobacco products, including the ß-carbolines harmane and norharmane. The present study aimed to determine the long-term effects of adolescent exposure to nicotine (NIC), harmane (HAR), or norharmane (NOR) on locomotor activity, learning and memory, anxiety-like behavior, motor coordination, and monoamine/metabolite concentrations in the striatum and nucleus accumbens of male Sprague-Dawley rats. Beginning on postnatal day (PND) 27 and continuing through PND 55, subjects received twice daily intraperitoneal injections of 1ml/kg saline (CON), 0.5mg NIC/kg, 0.5mg HAR/kg, or 0.5mg NOR/kg. Body weight, food, and water intake were measured daily (PNDs 27-96). Locomotor activity was assessed on PND 40 or 41, PND 55, and PNDs 81 and 82. Other behaviors (anxiety-like behavior, motor coordination, and spatial learning and memory) were assessed at least 25 days after drug exposure ended (PNDs 80-91). On PND 97, subjects were decapitated and the striatum and nucleus accumbens were dissected and frozen for analysis. NIC treatment significantly decreased food intake, but did not alter locomotor activity during or after treatment. HAR and NOR treatment, however, caused significant open field hypoactivity. Motor coordination, water maze performance, and concentrations of monoamines and metabolites in the striatum and nucleus accumbens were unaltered by any drug treatment. These results indicate a long-lasting effect on activity levels from adolescent HAR or NOR treatment; however, there were few long-lasting NIC effects. Given the paucity of data describing effects of HAR or NOR exposure, these data should encourage additional studies of these tobacco constituents as well as constituent combination studies.

Behavioral effects and pharmacokinetics of (±)-3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) after intragastric administration to baboons.

(±)-3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy") is a popular drug of abuse. We aimed to characterize the behavioral effects of intragastric MDMA in a species closely related to humans and to relate behavioral effects to plasma MDMA and metabolite concentrations. Single doses of MDMA (0.32-7.8 mg/kg) were administered via an intragastric catheter to adult male baboons (N = 4). Effects of MDMA on food-maintained responding were assessed over a 20-hour period, whereas untrained behaviors and fine-motor coordination were characterized every 30 minutes until 3 hours postadministration. Levels of MDMA and metabolites in plasma were measured in the same animals (n = 3) after dosing on a separate occasion. MDMA decreased food-maintained responding over the 20-hour period, and systematic behavioral observations revealed increased frequency of bruxism as the dose of MDMA was increased. Drug blood level determinations showed no MDMA after the lower doses of MDMA tested (0.32-1.0 mg/kg) and modest levels after higher MDMA doses (3.2-7.8 mg/kg). High levels of 3,4-dihydroxymethamphetamine (HHMA) were detected after all doses of MDMA, suggesting extensive first-pass metabolism of MDMA in the baboon. The present results demonstrate that MDMA administered via an intragastric catheter produced behavioral effects that have also been reported in humans. Similar to humans, blood levels of MDMA after oral administration may not be predictive of the behavioral effects of MDMA. Metabolites, particularly HHMA, may play a significant role in the behavioral effects of MDMA.

Physical dependence on gamma-hydroxybutrate (GHB) prodrug 1,4-butanediol (1,4-BD): time course and severity of withdrawal in baboons.

BACKGROUND: 1,4-Butanediol (1,4-BD) is a gamma-hydroxybutyrate (GHB) pro-drug, with multiple commercial uses, and a drug of abuse. Although there are case reports of a withdrawal syndrome following 1,4-BD use, no studies have evaluated the physical dependence potential of 1,4-BD and characterized the time course of withdrawal. METHODS: Vehicle and then 1,4-BD were administered continuously 24 h/day via intragastric catheters in male baboons (Papio anubis, n=3). Dosing was initiated at 100 mg/kg and increased by 100mg/kg/day to 400mg/kg. After a stabilization period, doses of 500 and then 600 mg/kg/day were each maintained for 3-4 weeks. Plasma levels of 1,4-BD and GHB were determined for each dose condition. Physical dependence was assessed via administration of a GABA-B antagonist (precipitated withdrawal test) during administration of the 600 mg/kg dose and via abrupt termination of chronic 1,4-BD administration (spontaneous withdrawal test). Outcome measures included the number of food pellets earned, performance on a fine-motor task, observed behaviors, and plasma levels of GHB and 1,4-BD. RESULTS: Following maintenance of 1,4-BD 600 mg/kg for 3 weeks, the number of food pellets earned was significantly decreased. At the end of chronic 1,4-BD dosing, the levels of GHB in plasma ranged from 1290 to 2300 µmol/L and levels of 1,4-BD in plasma ranged from 13.1 to 37.9 µmol/L. Signs of physical dependence were observed following precipitated and spontaneous withdrawal tests. Seizures were not observed. CONCLUSIONS: These data indicate chronic 1,4-BD produced physical dependence in baboons and the withdrawal syndrome can be characterized as mild to intermediate.

Self-administration of gamma-hydroxybutyric acid (GHB) precursors gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD) in baboons.

RATIONALE: Gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD) are gamma-hydroxybutyrate (GHB) pro-drugs and drugs of abuse. OBJECTIVE: Given the reports of abuse, and the ease at which GBL and 1,4-BD may be obtained, we investigated the reinforcing effects of GBL (n = 5) and 1,4-BD (n = 4) in baboons using IV self-administration procedures. METHODS: Sessions ran 24 h/day. Each injection was contingent upon completion of a fixed number (120 or 160) of lever responses. A 3-h timeout period followed each injection, limiting the total number of injections to eight per day. Self-administration was first established with cocaine (0.32 mg/kg/injection). GBL (10-130.0 mg/kg/injection), 1,4-BD (10-100 mg/kg/injection), or vehicle was substituted for cocaine for at least 15 days. Food pellets were available ad libitum 24 h/day and were contingent upon completion of ten lever responses. RESULTS: GBL (32-100 mg/kg/injection) maintained significantly greater numbers of injections when compared to vehicle in four of five baboons, and the mean rates of injection were high (more than six per day) in three baboons and moderate in the fourth baboon (four to six per day). 1,4-BD (78-130 mg/kg/injection) maintained significantly greater numbers of injections when compared to vehicle in only two out of four baboons, and mean rates were moderate to high in both baboons. Self-injection of these doses of GBL and 1,4-BD generally inhibited food-maintained responding. CONCLUSIONS: GBL and 1,4-BD have abuse liability. Given that GBL and 1,4-BD are self-administered, are easier to obtain than GHB, and are detected in seized samples, additional legal control measures of these GHB pro-drugs may be needed.

Metabolism and disposition of 3,4-methylenedioxymethamphetamine ("ecstasy") in baboons after oral administration: comparison with humans reveals marked differences.

The baboon is potentially an attractive animal for modeling 3,4-methylenedioxymethamphetamine (MDMA) effects in humans. Baboons self-administer MDMA, are susceptible to MDMA neurotoxicity, and are suitable for positron emission tomography, the method most often used to probe for MDMA neurotoxicity in humans. Because pharmacokinetic equivalence is a key feature of a good predictive animal model, we compared the pharmacokinetics of MDMA in baboons and humans. Baboons were trained to orally consume MDMA. Then, pharmacokinetic profiles of MDMA and its major metabolites were determined after various oral MDMA doses using the same analytical method recently used to perform similar studies in humans. Results indicate that MDMA pharmacokinetics after oral ingestion differ markedly between baboons and humans. Baboons had little or no MDMA in their plasma but had high plasma concentrations of 3,4-dihydroxymethamphetamine (HHMA), pointing to much more extensive first-pass metabolism of MDMA in baboons than in humans. Other less prominent differences included less O-methylation of HHMA to 4-hydroxy-3-methoxymethamphetamine, greater N-demethylation of MDMA to 3,4-methylenedioxyamphetamine, and a shorter half-life of HHMA in the baboon. To our knowledge, this is the first study to characterize MDMA metabolism and disposition in the baboon. Differences in MDMA pharmacokinetics between baboons and humans suggest that the baboon may not be ideal for modeling human MDMA exposure. However, the unusually rapid conversion of MDMA to HHMA in the baboon may render this animal uniquely useful for clarifying the relative role of the parent compound (MDMA) versus metabolites (particularly HHMA) in the biological actions of MDMA.

Intravenous self-administration of γ-hydroxybutyrate (GHB) in baboons.

BACKGROUND: Abuse of gamma-hydroxybutyrate (GHB) poses a public health concern. In previous studies, intravenous (IV) self-administration of GHB doses up to 10 mg/kg was not maintained in non-human primates under limited-access conditions, which was inconsistent with the usual good correspondence between drugs abused by humans and those self-injected by laboratory animals. METHODS: Self-administration of GHB was studied in 10 baboons using procedures standard for our laboratory to assess drug abuse liability. Each self-injection depended on completion of 120 or 160 lever responses. Sessions ran continuously; a 3-h timeout limited the number of injections per 24h to 8. Self-injection was established at 6-8 injections/day with cocaine (0.32 mg/kg/injection) prior to substitution of each GHB dose (3.2-178 mg/kg/injection) or vehicle for 15 days. Food pellets were available 24h/day. RESULTS: GHB maintained significantly greater numbers of injections when compared to vehicle in 6 of the 9 baboons that completed GHB evaluations that included 32 mg/kg/injection or higher. The baboons that self-administered GHB at high rates were ones for which GHB was the first drug each had tested under the 24-h/day cocaine baseline procedure. Self-injection of the highest doses of GHB decreased food-maintained responding. CONCLUSIONS: High-dose GHB can function as a reinforcer in non-human primates under 24-h access, but self-administration history may be important. The findings are consistent with the demonstrated abuse liability of GHB in humans, and remove GHB as an exception to the typical good correspondence between those drugs abused by humans and those self-administered by nonhuman primates.

Dissociation of alcohol-seeking and consumption under a chained schedule of oral alcohol reinforcement in baboons.

BACKGROUND: Initiation and maintenance of compulsive alcohol drinking involves a sequence of behaviors which occur in the presence of environmental cues. Animal models using chained schedules of alcohol reinforcement may be useful for examining the complex interactions between cues and alcohol-seeking and -consumption. METHODS: Four baboons self-administered alcohol under a 3-component chained schedule of reinforcement; distinct cues were presented in the context of different behavioral contingencies associated with gaining access to 4% w/v alcohol (alcohol-seeking) and concluding with alcohol self-administration. First, the response strength of alcohol-related seeking responses was evaluated using a between-sessions progressive ratio (PR) procedure in which the response requirement to initiate the final contingency and gain access to the daily supply of alcohol was increased each session. The highest response requirement completed that resulted in alcohol access was defined as the breaking point (BP). Second, water was substituted for alcohol and PR procedures were repeated. The effects of increasing the "seeking" response requirement on subsequent alcohol or water consumption were also determined. RESULTS: When alcohol was available, operant responses to gain access to and self-administer alcohol were maintained. When water was substituted for alcohol, alcohol-related cues continued to maintain alcohol-seeking responses. However, higher BPs, higher rates of self-administration and higher volumes of intake occurred when alcohol was available compared with water. Increasing the response requirement to gain access to alcohol did not reduce alcohol consumption (total alcohol intake). CONCLUSIONS: These results show that alcohol-related cues maintained alcohol-seeking even after a prolonged period of only water availability. Cue-maintained alcohol-seeking behavior can be dissociated from subsequent alcohol consumption.

The value of nonhuman primates in drug abuse research.

The use of nonhuman primates (NHP) is invaluable for drug abuse research. The laboratory animals most closely related to humans are NHP. The phylogeny, anatomy, physiology, neurochemistry, and behavior of NHP are more similar to humans than other laboratory species. There is now an extensive body of literature documenting the neuroanatomical, neurochemical, and neuropharmacological similarities between NHP and humans and the differences between NHP and other laboratory species in dopamine, norepinephrine, serotonin, opioid, and gamma aminobutyric acid systems. Comprehensive studies comparing pharmacokinetics in humans, monkeys, dogs, and rats have shown that data in monkeys are the most predictive of human pharmacokinetic parameters. The long life span and extended adolescent period for NHP permits intensive, long-term investigations and the use of within-subject experimental designs similar to those used in human laboratory studies. Within-subject designs require fewer subjects than standard between-group designs and permit the careful evaluation of individual differences. NHP have been used extensively in drug abuse research for over 40 years and have provided useful information on the behavioral processes associated with drug abuse and addiction as well as drug abuse liability in humans. This review focuses on important species differences between rodents and NHP and on the value of NHP in bridging the gap between rodents and humans to enhance the ability to generalize preclinical findings to human drug abuse.

Environmental cues, alcohol seeking, and consumption in baboons: effects of response requirement and duration of alcohol abstinence.

BACKGROUND: Environmental stimuli (cues) that have been paired with alcohol drinking may evoke classically conditioned states that in turn influence alcohol consumption and relapse to heavy drinking. Animal models using chained schedules of alcohol reinforcement may be useful for examining such complex interactions. METHODS: Alcohol drinking was established in 4 baboons. A sequence of lights and tones was presented during daily 3-hour sessions. First, cues were presented alone and no programmed contingencies were in effect. Second, cues were paired with 3 linked components consisting of different behavioral contingencies leading to and concluding with access to alcohol for self-administration in the last component (i.e., a chained schedule of alcohol reinforcement). Third, the effects of withholding alcohol access (i.e., forced abstinence) and increasing the number of lever responses required per drink were evaluated. RESULTS: Cues paired with a chained schedule of alcohol reinforcement engendered behaviors that brought baboons into contact with alcohol-related cues and occasioned operant responding that facilitated access to alcohol (alcohol seeking) during components that preceded alcohol access. Increasing the response requirement for each drink decreased the number of drinks and volume of alcohol consumed, but did not alter alcohol seeking. On the first session after 14 days of alcohol abstinence, latency to complete the operant requirement that produced alcohol access was decreased while both alcohol self-administration and volume of alcohol consumed were increased. CONCLUSIONS: Alcohol self-administration and consumption were sensitive to increases in response requirement and duration of alcohol abstinence, while seeking was only enhanced by duration of alcohol abstinence. This animal model may be useful to further examine the interactions between environmental cues and behaviors associated with seeking and consumption of alcohol and to evaluate the efficacy of potential alcohol treatment drugs on these behaviors.

Chronic intragastric administration of gamma-butyrolactone produces physical dependence in baboons.

RATIONALE: Abuse of gamma-hydroxybutyrate (GHB) and its precursors is a public health concern. Gamma-butyrolactone (GBL) is found in commercially available products and, when ingested, is metabolized to GHB. OBJECTIVE: The goal was to evaluate the physical dependence potential and behavioral effects of GBL. METHODS: Vehicle and then GBL were administered continuously (24 h per da y) in baboons (Papio anubis, n=5) via intragastric catheters. GBL dosing was initiated at 100 mg/kg/day and then progressively increased stepwise by increments of 100 mg/kg to a final dose of 600 mg/kg. The number of food pellets earned, fine-motor task performance, and observed behaviors were used as dependent measures. Precipitated withdrawal was evaluated after administration of GABA-B and benzodiazepine receptor antagonists during chronic GBL dosing (400-600 mg/kg). Spontaneous withdrawal was evaluated after discontinuation of chronic GBL 600 mg/kg. Blood GHB levels were determined during chronic dosing of each GBL dose by isotope dilution assay. RESULTS: Chronic GBL dose-dependently decreased food-maintained behavior, disrupted performance on the fine-motor task, and produced signs of sedation and muscle relaxation. The GABA-B antagonist SGS742 [56 mg/kg, intramuscular (IM)] precipitated a withdrawal syndrome, whereas the benzodiazepine antagonist flumazenil (5 mg/kg, IM) produced little or no effect. Signs of physical dependence were also demonstrated when chronic GBL dosing was discontinued. Analysis of plasma indicated GBL was metabolized to GHB; levels were 825 to 1,690 micromol l(-1) GHB and 2,430 to 3,785 micromol l(-1) GHB after week 1 of 400 and 600 mg/kg/day, respectively. CONCLUSIONS: These data indicate that, like GHB, chronic GBL dosing produced physical dependence that likely involved the GABA-B receptor.

Metabolism of gamma-hydroxybutyrate to d-2-hydroxyglutarate in mammals: further evidence for d-2-hydroxyglutarate transhydrogenase.

gamma-Hydroxybutyratic acid (GHB), and its prodrugs 4-butyrolactone and 1,4-butanediol, represent expanding drugs of abuse, although GHB is also used therapeutically to treat narcolepsy and alcoholism. Thus, the pathway by which GHB is metabolized is of importance. The goal of the current study was to examine GHB metabolism in mice with targeted ablation of the GABA degradative enzyme succinic semialdehyde dehydrogenase (SSADH(-/-) mice), in whom GHB persistently accumulates, and in baboons intragastrically administered with GHB immediately and persistently. Three hypotheses concerning GHB metabolism were tested: (1) degradation via mitochondrial fatty acid beta-oxidation; (2) conversion to 4,5-dihydroxyhexanoic acid (a putative condensation product of the GHB derivative succinic semialdehyde); and (3) conversion to d-2-hydroxyglutaric acid (d-2-HG) catalyzed by d-2-hydroxyglutarate transhydrogenase (a reaction previously documented only in rat). Both d-2-HG and 4,5-dihydroxyhexanoic acid were significantly increased in neural and nonneural tissue extracts derived from SSADH(-/-) mice. In vitro studies demonstrated the ability of 4,5-dihydroxyhexanoic acid to displace the GHB receptor ligand NCS-382 (IC(50) = 38 micromol/L), although not affecting GABA(B) receptor binding. Blood and urine derived from baboons administered with GHB also accumulated d-2-HG, but not 4,5-dihydroxyhexanoic acid. Our results indicate that d-2-HG is a prominent GHB metabolite and provide further evidence for the existence of d-2-hydroxyglutarate transhydrogenase in different mammalian species.

Involvement of gamma-hydroxybutyrate (GHB) and GABA-B receptors in the acute behavioral effects of GHB in baboons.

RATIONALE: Gamma-hydroxybutyrate (GHB) is used for the treatment of narcolepsy, but it is also a drug of abuse. The behavioral pharmacology of GHB is not well defined. OBJECTIVES: The current study was conducted to characterize the behavioral effects of a range of GHB doses in baboons (N=4) and to evaluate whether a GABA-B receptor antagonist and a GHB receptor antagonist would block a behaviorally active dose of GHB. METHODS: In the first experiment, GHB (32-420 mg/kg) or vehicle was administered via an intragastric catheter. Sixty min after dosing, subjects were presented with a fine-motor task and observed. Food pellets were available under a fixed-ratio schedule of reinforcement 20-h/day. In the second experiment, the GABA-B antagonist CGP36742 (10-56 mg/kg), the putative GHB antagonist NCS-382 (0.1-10 mg/kg), or vehicle were administered alone and then in combination with GHB (320 mg/kg). RESULTS: GHB dose-dependently decreased the number of food pellets earned. Performance in the motor task was also impaired and accompanied by signs of sedation and gastrointestinal discomfort. Pretreatment with CGP36742 antagonized GHB-induced suppression of food-maintained behavior and performance on the fine-motor task. Signs of abdominal discomfort, ataxia, and muscle relaxation produced by GHB were also reduced by pretreatment with CGP36742. In contrast, pretreatment with NCS-382 sometimes restored performance in the fine-motor task and increased food-maintained behavior, but the effect was variable across doses and baboons. Some doses of NCS-382 appeared to exacerbate ataxia and gastrointestinal discomfort produced by GHB in some subjects. CONCLUSIONS: These data indicate that while GABA-B receptors play a significant role in mediating the behavioral effects of GHB in baboon, the role of GHB receptors is less clear.

Spontaneous and precipitated withdrawal after chronic intragastric administration of gamma-hydroxybutyrate (GHB) in baboons.

RATIONALE: gamma-Hydroxybuyrate (GHB) is a current drug of abuse that may produce physical dependence. OBJECTIVES: The present study characterized the behavioral effects of chronic GHB in baboons (n = 4), and evaluated whether signs of withdrawal occurred (1) after administration of the GABA-B antagonist CGP36742 during chronic GHB administration (precipitated withdrawal) and (2) following discontinuation of chronic GHB administration (spontaneous withdrawal). METHODS: Water (vehicle) and then GHB was continuously infused via intragastric (IG) catheters. GHB administration was initiated at 350 mg/kg per day, and the dose was increased by 100 mg/kg over 4 days to 750 mg/kg per day. Food pellets were available 20 h/day under a fixed ratio (FR5 or 10) schedule of reinforcement. Observation sessions and a 2-min fine motor task were conducted during vehicle and GHB administration. CGP36742 (32 and 56 mg/kg, IM) was administered during vehicle and chronic GHB administration. After a total of 32-36 days GHB administration was abruptly discontinued. Blood samples were collected during all interventions and analyzed for GHB content. RESULTS: Chronic GHB decreased food-maintained behavior, disrupted performance of the fine motor task, and produced ataxia, muscle relaxation, tremors and jerks. At the end of GHB administration, plasma levels of GHB ranged from 486 to 2080 micromol/L. Administration of CGP36742 during chronic GHB administration produced increases in aggression, self-directed behaviors, vomit/retch, tremors and/or jerks, which is consistent with a precipitated withdrawal syndrome. Similar signs were observed when GHB administration was discontinued. Seizures were not observed. CONCLUSIONS: These data indicate that chronic GHB administration produced physical dependence and that activation of the GABA-B receptor may be important for GHB physical dependence.

Serotonergic-dopaminergic mediation of MDMA's discriminative stimulus effects in a three-choice discrimination.

(+/-)3,4-Methylenedioxymethamphetamine (MDMA; "Ecstasy") is a common drug of abuse that is often described as both a psychostimulant and a hallucinogen. Two-choice drug discriminations (i.e. drug vs. nondrug) in nonhumans comparing the discriminative stimulus properties of MDMA to psychostimulants or hallucinogens have produced somewhat inconsistent findings. The relative contribution of serotonergic versus dopaminergic actions to MDMA's discriminative stimulus effects may depend on the training stimulus conditions employed. We have previously demonstrated that rats can learn to discriminate the effects of MDMA and D-amphetamine in a three-choice drug discrimination procedure, and that LSD produced nearly complete substitution for MDMA under these conditions, and fenfluramine fully substituted for MDMA. In the present study, 12 rats were trained to discriminate LSD (0.08 mg/kg) and MDMA (1.5 mg/kg) from saline in a three-choice drug discrimination procedure under a fixed-ratio (FR) 10 schedule of food reinforcement. D-Amphetamine produced only partial substitution for MDMA while fenfluramine produced complete stimulus generalization. Low doses of D-amphetamine and fenfluramine produced greater stimulus generalization when administered in combination than when given alone. The serontonin(2) antagonist MDL-100,907 only partially blocked the MDMA cue, but completely antagonized LSD discrimination. The dopamine antagonist haloperidol also failed to block MDMA discrimination. These results indicate that 5-HT release is a salient feature to MDMA's discriminative stimulus effects but that MDMA produces a compound discriminative stimulus.