Naloxone benzylhydrazone is a µ-selective opioid antagonist without kappa agonist effects in rhesus monkeys

Naloxone benzoylhydrazone (NalBzoH) has been suggested to be a selective opioid agonist, exerting its effects through the kappa(3) subtype of opioid receptor. In the present experiments NalBzoH was studied for its discriminative stimulus, analgesic, and respiratory effects in rhesus monkeys. Up to the largest doses administered (1.0-3.2mg/kg), NalBzoH failed to substitute for the discriminative stimulus effects of the kappa agonist ethylketocyclazocine or those of the μ agonist alfentanil. However, in morphine-treated (3.2mg/kg/day) monkeys NalBzoH substituted completely for the discriminative stimulus effects of the opioid antagonist naltrexone. NalBzoH antagonized the discriminative stimulus effects of alfentanil in morphine-treated subjects and, at larger doses, antagonized the discriminative stimulus effects of ethylketocyclazocine. NalBzoH did not produce analgesic effects and had only modest effects on respiration. However, NalBzoH antagonized the analgesic effects as well as the respiratory-depressant effects of alfentanil; at doses 10-15 times larger than those that antagonized alfentanil, NalBzoH also antagonized analgesic effects of the kappa agonist U-50,488. For both μ and kappa agonists, NalBzoH was slightly more potent in antagonizing discriminative stimulus effects as compared to analgesic effects. Thus, NalBzoH is a μ-selective opioid antagonist in rhesus monkeys and is equipotent to naloxone in antagonizing alfentanil. While demonstrating μ-selective antagonism, the current study fails to provide support for the proposed kappa agonist actions of NalBzoH.     

Chronic Δ⁹-tetrahydrocannabinol treatment in rhesus monkeys: differential tolerance and cross-tolerance among cannabinoids

BACKGROUND AND PURPOSE

The extent to which behavioural effects vary as a function of CB₁ receptor agonist efficacy is not clear. These studies tested the hypothesis that cannabinoid tolerance and cross-tolerance depend upon the CB₁ agonist efficacy of drugs to which tolerance/cross-tolerance develops.

EXPERIMENTAL APPROACH

Sensitivity to cannabinoids, including the cannabinoid antagonist rimonabant, low efficacy agonist Δ⁹-tetrahydrocannabinol (Δ⁹-THC), and high efficacy agonists CP 55940 and WIN 55212-2, was determined before and after chronic Δ⁹-THC treatment in rhesus monkeys. Two measures of behavioural effect were assessed: effects of drugs to decrease fixed ratio responding for food presentation and stimulus-shock termination and discriminative stimulus effects in monkeys discriminating Δ⁹-THC (0.1 mg·kg⁻¹, i.v.).

KEY RESULTS

Δ⁹-THC decreased responding for both food presentation and stimulus-shock termination; these effects were antagonized by the CB₁ antagonist rimonabant. Chronic Δ⁹-THC (1 mg·kg⁻¹ per 12 h, s.c.) resulted in tolerance to the rate-decreasing effects of Δ⁹-THC and cross-tolerance to CP 55940 and WIN 55212-2; however, cross-tolerance was less than tolerance. Chronic Δ⁹-THC increased sensitivity to rimonabant without changing sensitivity to the non-cannabinoids midazolam and ketamine. In monkeys discriminating Δ⁹-THC (0.1 mg·kg⁻¹, i.v.), both CP 55940 and WIN 55212-2 produced high levels of drug-lever responding. Chronic Δ⁹-THC (1 mg·kg⁻¹ per day, s.c.) decreased sensitivity to Δ⁹-THC without producing cross-tolerance to CP 55940 or WIN 55212-2.

CONCLUSIONS AND IMPLICATIONS

In Δ⁹-THC-treated monkeys, the magnitude of tolerance and cross-tolerance to other CB₁ receptor agonists varied inversely with agonist efficacy, suggesting that CB₁ agonist efficacy is an important determinant of behavioural effects.     

The effects of Δ9-tetrahydrocannabinol alone and in combination with cannabidiol on fixed-interval performance in rhesus monkeys

It has been reported that cannabidiol (CBD) antagonizes the effects of ⁹-tetrahydrocannabinol (THC) on operant behavior in rats and pigeons. We have replicated this finding with rhesus monkeys. Four rhesus monkeys were trained to lever press on a fixedinterval 5-min schedule of food presentation with a 1-min limited hold and 1-min time out between successive intervals. The effects of 0.3 and 1.0 mg/kg THC alone were determined three times during the experiment; before the CBD-THC interaction, after the CBD-THC interaction and once with the CBD vehicle. A dose of 30 mg/kg CBD, which alone resulted in a 24% reduction in responding, completely antagonized the response rate reduction produced by 0.3 mg/kg THC. The effects of THC revealed a rate-dependent effect that did not conform to the log-linear rate-dependency plots described for most other drugs.This research was reported at the FASEB Meeting in Atlantic City, NJ in 1978 [Fed. Proc. 37: 739 (Abs.) 1978]     

Comparison of naltrexone, 6α-naltrexol, and 6β-naltrexol in morphine-dependent and in nondependent rhesus monkeys

Rationale Some opioid receptor ligands that appear to be neutral antagonists can have inverse agonist activity under conditions of increased constitutive activity (e.g., agonist treatment). Objectives This study compared the opioid receptor antagonist naltrexone and its metabolites 6α-naltrexol and 6β-naltrexol in nondependent and morphine-dependent monkeys to see whether their potencies varied according to drug treatment and, presumably, to differences in constitutive activity of μ opioid receptors. Results In monkeys (n = 4) receiving 3.2 mg/kg per day of morphine and discriminating 0.0178 mg/kg naltrexone, naltrexone and each metabolite increased responding on the naltrexone lever in a dose-related manner with naltrexone being 8- and 71-fold more potent than 6α- and 6β-naltrexol, respectively. After 27 h of no-morphine treatment, monkeys responded on the naltrexone lever, and this effect was reversed by morphine. Naltrexone and each metabolite prevented morphine reversal of naltrexone-lever responding, and their rank order potency was the same as their substitution for naltrexone; however, the potency between naltrexone and each metabolite was slightly greater in morphine-dependent as compared to morphine-deprived monkeys. In a separate group (n = 3) of nondependent monkeys discriminating 1.78 mg/kg of morphine, all three compounds antagonized morphine with the same potency as in the reversal study (morphine-dependent monkeys), with Schild analyses showing no difference in apparent affinities (pA ₂) between nondependent and morphine-dependent monkeys. Conclusion Naltrexone and 6α- and 6β-naltrexol have qualitatively similar effects, and their potencies do not vary markedly with opioid treatment, suggesting that under these conditions, they do not vary with regard to inverse agonism.     

The repeated administration of rhIL-12 for 14 weeks in rhesus monkeys: A toxicity assessment

Interleukin-12 (IL-12) is known to exert antitumor immune effects by promoting the activation and proliferation of T cells and NK cells within the immune system. However, clinical trials have observed systemic toxicity associated with the administration of IL-12. This has shelved development plans for its use as a cancer therapeutic drug. Therefore, it is critical that we perform a systematic evaluation of the toxicity and safety of repeated IL-12 administration. In this study, we conducted a comprehensive evaluation of the toxicity and safety of repeated rhIL-12 (recombinant human interleukin-12) administration in rhesus monkeys by assessing its effects on the immune system, organ function, and vital signs. Rhesus monkeys were subcutaneously injected with 0.5, 2.5, and 12.5 μg/kg of rhIL-12 for up to for 14 consecutive weeks. The low dose exhibited no signs of toxicity, whereas animals receiving higher doses displayed symptoms such as loose stools, reduced activity, anemia, and elevated liver function indicators (AST and TBIL). Following three administrations of 12.5 μg/kg, high dosing was adjusted to 7.5 μg/kg due to manifestations of symptoms like loose stools, decreased activity, and huddling in the cage. Furthermore, rhesus monkeys exhibited marked immunogenic responses to recombinant human interleukin-12 (rhIL-12). However, based on overall study findings, the No Observed Adverse Effect Level (NOAEL) for the subcutaneous injection of rhIL-12, when repeatedly administered for 3 months in rhesus monkeys, was considered to be 0.5 μg/kg. The Highest Non-Severely Toxic Dose (HNSTD) was considered to be 7.5 μg/kg.     

Modification of ethanol’s reinforcing effectiveness in rhesus monkeys by cocaine,flunitrazepam, or gamma-hydroxybutyrate

Background Although ethanol is frequently used in combination with other psychoactive drugs, the behavioral and pharmacological reasons for this form of polydrug abuse have not been well described. Materials and methods Rhesus monkeys with indwelling intravenous catheters produced intravenous injections of ethanol (50, 100, or 200 mg/kg/inj), flunitrazepam (0.001–0.03 mg/kg/inj), cocaine (0.01 or 0.03 mg/kg/inj), or combinations of ethanol and these drugs or gammahydroxybutyrate (GHB) (1.0 or 3.2 mg/kg/inj) by lever pressing according to a fixed-ratio schedule. The response requirement for each drug or drug combination was increased across sessions (10, 32, 100, 320, or 1,000). The dependent variables were rates of responding maintained by the drug or drug combination and the elasticity of drug demand when consumption was expressed as a function of price. Results Elasticity (P _max) values for each drug varied among the monkeys but retained the same rank order for the monkeys, suggesting a fundamental difference in the animals’ apparent sensitivities to the reinforcing effects of the drugs. Combining ethanol with the other drugs did not increase their reinforcing effectiveness. GHB (ineffective in previous studies) did not modify ethanol’s reinforcing effects; demand functions for the combination of ethanol and flunitrazepam were slightly less elastic than for ethanol alone, but no different from that for flunitrazepam alone; adding ethanol to cocaine detracted from the reinforcing effectiveness of cocaine. Conclusions The hypothesis that use of ethanol in combination with sedative and stimulant drugs is due to an ability of ethanol to enhance the reinforcing effects of these drugs is not supported.     

The nicotinic α7 receptor agonist GTS-21 improves cognitive performance in ketamine impaired rhesus monkeys

The cognitive deficits associated with schizophrenia are recognized as a core component of the disorder, yet there remain no available therapeutics to treat these symptoms of the disease. As a result, there is a need for establishing predictive preclinical models to identify the therapeutic potential of novel compounds. In the present study, rhesus monkeys were trained in the object retrieval-detour task, which is dependent on the prefrontal cortex, a brain region implicated in the cognitive deficits associated with schizophrenia. The NMDA receptor antagonist ketamine significantly impaired performance without affecting measures of motor or visuospatial abilities. Pre-treatment with the nicotinic α7 agonist GTS-21 (0.03?mg/kg) significantly attenuated the ketamine-induced impairment, consistent with reports from clinical trials suggesting that nicotinic α7 receptor agonism has pro-cognitive potential in clinical populations. In contrast, pretreatment with the acetylcholinesterase inhibitor donepezil failed to reverse the ketamine-induced impairment, consistent with studies showing a lack of pro-cognitive effects in patients with schizophrenia. These data suggest that the ketamine-impaired object retrieval-detour task could provide a model with improved predictive validity for drug development, and confirm the need for additional efforts in back-translation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.     

Self-administration of methohexital,midazolam and ethanol: effects on the pituitary–adrenal axis in rhesus monkeys

Rationale There is disagreement in the literature with respect to how drugs of abuse affect the functioning of the hypothalamic–pituitary–adrenal (HPA) axis, and whether these changes in endocrine function may be related to the rewarding effects of these drugs.Objectives To determine whether reinforcing drugs with different mechanisms of action affect HPA axis function at doses at which they serve as reinforcers.Methods Seven monkeys (6 male) were randomly assigned to self-administer methohexital—a barbiturate (n=4), midazolam—a benzodiazepine (n=3), or ethanol (n=5). Each monkey had a surgically implanted indwelling venous catheter, and was trained to respond on a fixed ratio of 30 lever presses to receive an injection of drug or saline. Blood samples were obtained before, during, and after the self-administration sessions for the measurement of ACTH and cortisol by radioimmunoassay.Results Although methohexital, midazolam, and ethanol all maintained self-administration behavior across a range of doses, they differed in their effects on ACTH and cortisol. Ethanol inhibited ACTH and cortisol secretion. Methohexital and midazolam both tended to decrease ACTH and cortisol at large doses, and increase these hormones at small doses, but the HPA effects of neither drug differed significantly from when saline was available.Conclusions The neutral overall effect of methohexital and midazolam on HPA activity is consistent with other monkey and human studies, whereas the inhibitory effect of self-administered ethanol in the monkey contrasts with both the rat and human literature. The data in this study suggest that a change in HPA axis activity is not a requirement for drug-reinforced behavior in monkeys.Results from this paper were first presented at the annual meeting of ISPNE, Quebec City, Canada, in August 2001.     

Local administration of Δ9-tetrahydrocannabinol attenuates capsaicin-induced thermal nociception in rhesus monkeys: a peripheral cannabinoid action

  Rationale: Cannabinoids can reduce nociceptive responses by acting on peripheral cannabinoid receptors in rodents. Objectives: The study was conducted to evaluate the hypothesis that local administration of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) can attenuate capsaicin-induced nociception in rhesus monkeys. Methods: Capsaicin (100 μg) was applied locally in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, in normally innocuous 46°C water. Δ⁹-THC (10–320 μg) was coadministered with capsaicin in the tail to assess local antinociceptive effects. In addition, a local antagonism study was performed to confirm the selectivity of Δ⁹-THC action. Results:Δ⁹-THC dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by small doses (10–100 μg) of the cannabinoid CB₁ antagonist, SR141716A, applied in the tail. However, 100 μg SR141716A injected subcutaneously in the back did not antagonize local Δ⁹-THC. Conclusions: These results indicate that the site of action of locally applied Δ⁹-THC is in the tail. It provides functional evidence that activation of peripheral cannabinoid CB₁ receptors can attenuate capsaicin-induced thermal nociception in non-human primates and suggests a new approach for cannabinoids in pain management. Received: 1 December 1998 / Final version: 13 January 1999     

Self-administration of fentanyl, cocaine and ketamine: effects on the pituitary–adrenal axis in rhesus monkeys

Rationale Drugs of abuse can affect the functioning of the hypothalamic–pituitary–adrenal (HPA) axis. Acute administration of drugs that serve as reinforcers have been observed to stimulate the rat HPA axis, leading to the suggestion that these stimulatory effects may contribute to the development of drug-maintained behaviors.Objectives To determine whether reinforcing drugs that are dissimilar with respect to their mechanisms of action have similar effects on HPA axis activity at doses that are self-administered. Rhesus monkeys were randomly assigned to self-administer the -opioid agonist fentanyl, the psychomotor stimulant cocaine, or the NMDA antagonist ketamine.Methods Each monkey was trained to press a lever in order to receive an intravenous injection of drug or saline. Blood samples were obtained before, during, and after the self-administration sessions and assayed for ACTH and cortisol by radioimmunoassay.Results Fentanyl, cocaine, and ketamine were each self-administered across a range of doses. However, the three drugs differed in their effects on ACTH and cortisol. Cocaine stimulated ACTH and cortisol secretion, a finding that is consistent with previous rat and primate studies. Self-administration of both fentanyl and ketamine inhibited HPA axis activity. HPA inhibition by fentanyl is consistent with other monkey and human studies, and contrasts with the stimulatory effects of -opioids in rodents. The inhibitory effect of ketamine on ACTH and cortisol secretion contrasts with findings in the few primate studies that have evaluated NMDA antagonists. Neither fentanyl nor cocaine, at doses that maintained maximum rates of responding, produced significant changes in ACTH and cortisol levels.Conclusions There appears to be little commonality between different classes of abused drugs and their effects on the HPA axis, which calls into question the necessity for HPA axis stimulation in the reinforcement of drug-maintained behavior.Results from this paper were first presented at the annual meeting of CPDD, Scottsdale, Ariz., USA, in June 2001.     

Apparent affinity estimates of rimonabant in combination with anandamide and chemical analogs of anandamide in rhesus monkeys discriminating Δ9-tetrahydrocannabinol

Rationale  Anandamide and Δ⁹-tetrahydrocannabinol (Δ⁹-THC) sometimes produce different discriminative stimulus effects and, therefore, appear to differ in their mechanism of action. In order to understand the widespread use of cannabis and the therapeutic potential of cannabinoids, mechanisms responsible for behavioral effects need to be identified. Objective  Drug discrimination was used to compare the mechanism of action of Δ⁹-THC, anandamide, and two structural analogs of anandamide in rhesus monkeys. Materials and methods  Monkeys discriminated Δ⁹-THC (0.1 mg/kg i.v.) from vehicle. Δ⁹-THC, anandamide, methanandamide, and arachidonylcyclopropylamide (ACPA) were administered i.v. alone and in combination with at least one dose of rimonabant. Schild analysis and single-dose apparent affinity estimates were used to estimate the potency of rimonabant as an antagonist of each cannabinoid; these values were compared to examine whether the same receptors mediated discriminative stimulus effects. Results  Δ⁹-THC, ACPA, methanandamide, and anandamide produced greater than 96% of responses on the Δ⁹-THC lever. The ED₅₀ values were 0.024 mg/kg for Δ⁹-THC, 0.14 mg/kg for ACPA, 0.28 mg/kg for methanandamide, and 1.7 mg/kg for anandamide. The duration of action of Δ⁹-THC was 4–6 h and longer than the duration of action ACPA, methanandamide, and anandamide (i.e., each less than 50 min). Rimonabant surmountably antagonized the discriminative stimulus effects of each agonist, and the apparent affinity estimates (pA ₂ and pK _B values) were 6.24–6.83. Conclusions  Rimonabant can produce surmountable antagonism of the behavioral effects of not only Δ⁹-THC but also anandamide, methanandamide, and ACPA, and the interactions appear simple, competitive, and reversible. These cannabinoid agonists act at the same receptors to produce discriminative stimulus effects.     

Reinforcing effects of methylenedioxy amphetamine congeners in rhesus monkeys: are intravenous self-administration experiments relevant to MDMA neurotoxicity?

Rationale Many animal models relevant to the persistent effects of drugs of abuse necessitate the application of interspecies dose scaling procedures to approximate drug administration regimens in humans, but drug self-administration procedures differ in that they allow animal subjects to control their own drug intake.Objectives This report reviews the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA), its enantiomers, and several structural analogs in rhesus monkeys, paying particular attention to the pharmacological mechanisms of such reinforcing effects, the development of structure activity relationships among these compounds, the stability of MDMA self-administration behavior over time, and the persistent effects of self-administered MDMA on monoamines.Results The methylenedioxy amphetamine congeners MDMA, 3,4-methylenedioxyamphetamine, N-ethyl-3,4-methylenedioxyamphetamine, and N-methyl-1-(3,4-methylenedioxyphenyl)-2-butanamine function as reinforcers in rhesus monkeys, maintaining self-administration behavior greater than that engendered by contingent saline but less than that engendered by traditional psychostimulants. These findings are remarkable as structurally distinct serotonergic hallucinogen-like drugs do not maintain reliable self-administration in laboratory animals. During prolonged MDMA self-administration, MDMA-maintained responding progressively weakens, and MDMA eventually fails to maintain significant self-administration. The neurochemical correlates of this effect have not yet been identified.Conclusions Procedures in which MDMA and related compounds are self-administered can be established in rhesus monkeys. These techniques can be used to engender contingent MDMA exposure without resorting to controversial methods of interspecies dose scaling. As such, further application of self-administration methods may provide important new insights into the persistent effects of MDMA on brain and behavior in nonhuman primates.     

3’- and 4’-chloro-substituted analogs of benztropine: intravenous self-administration and in vitro radioligand binding studies in rhesus monkeys

Rationale: The reinforcing effects of many psychomotor stimulants have been related to increased dopaminergic neurotransmission. Drugs that block dopamine (DA) uptake have generally been found to function as positive reinforcers. Benztropine (BZT) and several of its halogenated analogs have previously been characterized as potent DA-uptake inhibitors with behavioral profiles that indicate diminished psychomotor stimulant effects relative to cocaine. Objectives: The present experiments were designed to examine, in rhesus monkeys, the reinforcing effects of the DA-uptake inhibitor BZT and two chloro-analogs 3’-C1-BZT and 4’-Cl-BZT, and to compare self-administration and binding profiles. Methods: Four rhesus monkeys self-administered cocaine i.v. under a fixed-ratio 10 (FR10) schedule until stable responding was established. Saline, and various doses of cocaine, BZT, and the BZT analogs were then made available for self-administration. Binding of these compounds to monoaminergic and cholinergic sites in monkey brain were determined using standard radioligand binding techniques. Results: Self-administration was maintained by both 3’-C1-BZT and 4’-Cl-BZT, but not by BZT. Results suggested that 3’-C1-BZT and 4’-Cl-BZT were weak positive reinforcers. BZT and analogs bound DA transporters (DAT) with affinities higher than that of cocaine and had affinity for muscarinic binding sites. Conclusions: Surprisingly, high affinity at DATs was associated with weak or no reinforcing effects. The mechanism(s) that may underlie this dissociation between DAT actions and reinforcing effects remains to be established. These data support the proposal that a lead for the discovery of a pharmacotherapeutic agent for cocaine abuse may come from this group of compounds. Received: 24 May 1999 / Final version: 28 July 1999     

Systemic effects of E-2078, a stabilized dynorphin A(1–8) analog, in rhesus monkeys

  Rationale: E-2078 ([N-methyl-Tyr¹, N-methyl-Arg⁷, d-Leu⁸] dynorphin A(1–8) ethylamide) is a dynorphin A(1–8) analog with a reduced tendency to be biotransformed, when compared to the unmodified opioid peptide. E-2078 has been found to produce κ-opioid agonist effects in vivo in rodents. Objective: In the present studies, we investigated whether systemically administered E-2078 could produce κ-agonist effects in rhesus monkeys, in tests of antinociception, diuresis and ethylketocyclazocine (EKC) discrimination. Methods: E-2078 (0.32–18 mg/kg, SC, IM or IV) was tested in the warm water (50°, 55°C) tail withdrawal assay of thermal antinociception. The diuretic effects of E-2078 (0.056– 1.8 mg/kg, SC) were also compared to those of the κ-agonist, U69,593 (0.01–0.32 mg/kg, SC). Lastly, the effects of E-2078 (0.1–3.2 mg/kg, SC or IV) were studied in rhesus monkeys trained to discriminate EKC (0.0056 mg/kg SC) from vehicle, in a food-reinforced operant procedure. Results: E-2078 did not produce thermal antinociception in rhesus monkeys following SC or IM administration, up to the largest doses presently studied (i.e., 18 and 10 mg/kg, respectively). E-2078 caused thermal antinociception by the IV route, but this effect was not apparently mediated by κ- or μ-opioid receptors, as shown by its insensitivity to quadazocine (1 mg/kg) pretreatment. However, SC E-2078 caused diuresis, and this effect was blocked by quadazocine pretreatment, consistent with mediation by κ-opioid receptors. E-2078 generalized in EKC-discriminating monkeys, but only after the largest dose (3.2 mg/kg), and only following IV administration. Conclusions: The present studies suggest that systemically administered E-2078 can produce some κ-receptor mediated effects in rhesus monkeys, but its profile of action is not identical to non-peptidic κ-agonists following all routes of administration, or across all experimental situations. Received: 5 August 1998 / Final version: 6 November 1998     

Discriminative stimulus effects of the cannabinoid CB1 antagonist SR 141716A in rhesus monkeys pretreated with Δ9-tetrahydrocannabinol

Rationale Drug discrimination can be used to examine tolerance and dependence in agonist-treated animals by establishing an appropriate antagonist as a discriminative stimulus.Objective Establish intravenous SR 141716A as a discriminative stimulus in four rhesus monkeys pretreated with a relatively small dose of Δ⁹-tetrahydrocannabinol (Δ⁹-THC).Methods Rhesus monkeys received i.v. Δ⁹-THC (0.32 mg/kg) and discriminated i.v. SR 141716A (1 mg/kg) from vehicle while responding under a fixed ratio (FR) 5 schedule of stimulus-shock termination.Results The discriminative stimulus effects of SR 141716A were dose-dependent (ED₅₀=0.33 mg/kg) and were mimicked by the CB₁ antagonist AM 251 (ED₅₀=0.98 mg/kg), but not by a benzodiazepine (midazolam) or an N-methyl-D-aspartate antagonist (ketamine). An additional dose (0.32 mg/kg in addition to 0.32 mg/kg administered before the session) of Δ⁹-THC shifted the SR 141716A dose–effect curve 3-fold rightward. Omitting Δ⁹-THC before test sessions resulted in responding on the SR 141716A lever that was attenuated by subsequent administration of Δ⁹-THC (ED₅₀=0.13 mg/kg), CP 55940 (ED₅₀=0.013 mg/kg), and WIN 55212-2 (ED₅₀=0.35 mg/kg); midazolam and ketamine did not attenuate responding on the SR 141716A lever. SR 141716A (1 mg/kg) shifted the Δ⁹-THC and CP 55940 dose–effect curves 3.4-fold rightward; the WIN 55212-2 dose–effect curve was not significantly modified by a dose of 1 mg/kg of SR 141716A.Conclusions SR 141716A can be established as a discriminative stimulus in animals pretreated with Δ⁹-THC, and this assay is selective for cannabinoid activity. Differential antagonism of cannabinoids by SR 141716A might indicate that the mechanism of action of WIN 55212-2 is not identical to other cannabinoids. This study demonstrates that, under the appropriate conditions, drug discrimination has utility for examining cannabinoid dependence and withdrawal.     

Reinforcement by orally delivered methadone, cocaine, and methadone-cocaine combinations in rhesus monkeys: are the combinations better reinforcers?

RATIONALE: Polydrug abuse is a problem that has been infrequently examined. In the present study, drug self-administration procedures were used to investigate the reinforcing effects of drug combinations. OBJECTIVES: To determine the absolute and relative response rates maintained by orally delivered methadone, cocaine, and their combinations under sequential and concurrent access. Choice between drug combinations containing different concentrations of cocaine was also determined. METHODS: Oral intake of methadone, cocaine, and their combinations was studied with rhesus monkeys during daily 3-h sessions. Lip contact (the operant response) was reinforced by delivery of liquid contingent upon completion of a fixed-ratio schedule. In one series, the drugs and drug combinations were studied sequentially with the water vehicle concurrently available. In the next series, the drugs and drug combinations were concurrently available. In the third series, pairs of drug combinations containing different concentrations of cocaine were also concurrently available. RESULTS: Methadone, cocaine and their combinations functioned as reinforcers. Under sequential access, response rates for the drug combinations and the component drugs were often similar. However, under concurrent access, response rates for the drug combinations were greater than response rates for the component drugs at the highest FR size for each condition. Also, drug combinations containing higher cocaine concentrations were preferred to combinations containing lower cocaine concentrations. CONCLUSIONS: Combinations of methadone and cocaine have relatively greater reinforcing effects than the component drugs, and these greater reinforcing effects are better detected with concurrent measures than with sequential measures.     

Quadazocine decreases responding reinforced by ethanol, sucrose, and phencyclidine fluid deliveries in rhesus monkeys: comparison to naltrexone’s effects

  Rationale: The endogenous opioid system may mediate the reinforcing effects of ethanol as well as sweet-tasting solutions. For example, opioid antagonists, such as naltrexone, reduce ethanol- and sucrose-reinforced responding in rhesus monkeys. If these effects are due to blockade of the μ-receptor, then an opioid antagonist such as quadazocine with a receptor selectivity profile similar to that of naltrexone should reduce responding at doses correlated with its μ-selectivity. Objectives: To determine whether quadazocine would reduce responding for ethanol and sucrose at μ-selective doses, and whether quadazocine and naltrexone would reduce responding for a bitter-tasting drug solution such as phencyclidine. Methods: Rhesus monkeys were given access to ethanol, sucrose, or phencyclidine concurrently with water. Prior to the drinking sessions, quadazocine (0.032–3.2 mg/kg) or saline was injected intramuscularly. During the phencyclidine experiment, naltrexone (0.1 and 0.32 mg/kg) was also tested. Results: The highest quadazocine doses (1 and 3.2 mg/kg) reduced ethanol and sucrose fluid deliveries without affecting the concurrently available water. Quadazocine reduced the fluid deliveries of both phencyclidine and water when concurrently available. Naltrexone reduced only phencyclidine fluid deliveries. Conclusions: The opioid antagonist effect on oral-reinforced responding is not selective for ethanol or sweet-tasting solutions; responding for phencyclidine was reduced as well. Quadazocine and NTX may reduce responding by blocking the μ-receptor because the relative potency of these antagonists to reduce oral self-administration was similar to their relative potency to produce withdrawal in morphine-dependent monkeys. However, water responding was low in these experiments, and thus we cannot rule out rate-dependent effects of the antagonists. Received: 24 June 1998 / Accepted: 18 February 1999     

The discriminative stimulus effects of midazolam are resistant to modulation by morphine,amphetamine, dizocilpine,and γ-butyrolactone in rhesus monkeys

Rationale  

Although abuse of benzodiazepines alone is uncommon, it is high in polydrug abusers, including those who primarily use opioids or stimulants.     

The negative GABAA modulator methyl β-carboline-3-carboxylate attenuates the behavioral effects of the positive GABAA modulators triazolam and pregnanolone in rhesus monkeys

RATIONALE: Many of the effects of benzodiazepines (BZs), barbiturates, and neuroactive steroids are mediated by the gamma-aminobutyric acid (GABA)(A) receptor complex. OBJECTIVES: This study tested the hypothesis that negative GABA(A) modulators attenuate the behavioral effects of different positive GABA(A) modulators that vary in their site of action on the receptor complex. METHODS: Rhesus monkeys responding under a multiple fixed ratio (FR:FR) schedule of food presentation and stimulus-shock termination received GABA(A) modulators under cumulative dosing procedures. RESULTS: The BZ site negative GABA(A) modulator methyl beta-carboline-3-carboxylate (beta-CCM), and not the BZ site neutral modulator flumazenil, decreased FR responding under the multiple schedule. FR responding was also decreased by positive modulators, including the BZ triazolam, the neuroactive steroid pregnanolone, and the barbiturate pentobarbital in that order of potency. beta-CCM, and not flumazenil, antagonized pregnanolone, suggesting that pregnanolone increased GABA-mediated chloride flux at a non-BZ site. beta-CCM antagonized triazolam with the slope of the Schild plot for beta-CCM and triazolam (food component) conforming to unity and yielding a pA2 value of 6.44. The effects of pentobarbital were not altered by beta-CCM, suggesting that barbiturates might act at a population of GABA(A) receptors different from those where neuroactive steroids and BZs act, or that barbiturate site positive GABA(A) modulators are not amenable to modulation by negative modulators. CONCLUSIONS: These results confirm a competitive interaction between beta-CCM and triazolam, and further demonstrate that the effects of neuroactive steroids on FR responding are attenuated by a BZ site negative GABA(A) modulator. Negative GABA(A) modulators might prove especially useful for characterizing important differences among positive GABA(A) modulators that act through different sites on the receptor complex.