Schedule Of Stimulus-shock Termination Research Articles

Nicotine-like discriminative stimulus effects of acetylcholinesterase inhibitors and a muscarinic receptor agonist in Rhesus monkeys

Acetylcholinesterase (AChE) inhibitors and positive allosteric nicotinic acetylcholine receptor (nAChR) modulators are potential pharmacotherapies for nicotine dependence. Because some smoking cessation aids (e.g. varenicline) appear to work by mimicking the effects of nicotine, we used drug discrimination to examine whether AChE inhibitors and nAChR allosteric modulators mimic the effects of nicotine. Rhesus monkeys discriminated 1.78 mg/kg of nicotine s.c. under an FR5 schedule of stimulus-shock termination. Nicotine and the AChE inhibitors donepezil and galantamine dose-dependently increased responding on the nicotine-appropriate lever with ED50 values of 0.35, 0.22, and 0.77 mg/kg, respectively. Donepezil (0.56 mg/kg) produced nicotine-like effects for at least 6 h, whereas the duration of action of galantamine (1.78 mg/kg) was less than 3 h. The positive allosteric nAChR modulator PNU-120596 (up to 10 mg/kg) and midazolam (up to 1.0 mg/kg) produced no more than 22% nicotine-lever responding. Oxotremorine, a muscarinic acetylcholine receptor agonist that was used to explore the extent to which muscarinic receptor agonism might contribute to the effects of AChE inhibitors, produced 94% nicotine-lever responding (ED50 value 0.013 mg/kg). The muscarinic antagonist atropine significantly antagonized the effects of both oxotremorine and nicotine; however, the dose of atropine antagonizing oxotremorine was smaller than the dose required to antagonize nicotine. Collectively, these results suggest that AChE inhibitors can mimic the effects of nicotine by indirectly stimulating both nicotinic and muscarinic receptors. Inasmuch as some smoking cessation aids work by exerting nicotine-like effects, the current results are consistent with the potential use of AChE inhibitors as novel smoking cessation aids.

Rapid nicotine tolerance and cross-tolerance to varenicline in rhesus monkeys: Drug discrimination.

Acute tolerance to effects of nicotine plays an important role in nicotine dependence, but the mechanism underlying these effects is unclear. Drug discrimination was used in the current study to examine the impact of nicotine pretreatment on sensitivity to the discriminative stimulus effects of nicotine and the FDA-approved smoking cessation pharmacotherapy varenicline. Rhesus monkeys (n = 4) discriminated 0.032 mg/kg nicotine base iv from saline under an FR5 schedule of stimulus-shock termination. Both nicotine and varenicline increased drug-appropriate responding; ED50 values (95% confidence limits) were 0.0087 [0.0025, 0.030] and 0.028 [0.0096, 0.082] mg/kg, respectively. Additional pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced tolerance to its discriminative stimulus effects and the magnitude of this effect was related to the number of pretreatment injections administered. Two pretreatment injections of the training dose of nicotine (0.032 mg/kg, iv) produced a 5.4-fold rightward shift in the nicotine dose-response function and a sevenfold rightward shift in the varenicline dose-response function. The duration of tolerance under these conditions was less than 60 min. These results demonstrate that tolerance to the discriminative stimulus effects of nicotine can be produced by acute nicotine exposure. Acute cross-tolerance from nicotine to varenicline is consistent with similar actions at nAChRs, and suggests that conditions resulting in acute nicotine tolerance could impact sensitivity to other nAChR agonists. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Enhanced discriminative stimulus effects of Δ9-THC in the presence of cannabidiol and 8-OH-DPAT in rhesus monkeys

BackgroundCannabidiol, a therapeutic with potential serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A receptor agonist activity, is the second most prevalent cannabinoid in Cannabis after Δ9-THC. The extent to which cannabidiol modifies the effects of Δ9-THC has not been firmly established, especially with respect to abuse-related effects in rhesus monkeys where previously antagonistic interactions have been reported for some behavioral outcomes. MethodsCannabidiol and the 5-HT1A receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) were tested in two separate discrimination assays in rhesus monkeys. One group (n=6) discriminated Δ9-tetrahydrocannabinol (Δ9-THC; 0.1mg/kg i.v.); a second group (n=6) discriminated the cannabinoid antagonist rimonabant (1mg/kg i.v.) while receiving Δ9-THC daily (1mg/kg/12hs.c.). Responding was maintained under a fixed ratio 5 schedule of stimulus-shock termination. ResultsBoth training drugs dose-dependently increased the percentage of responses on the respective drug-associated levers. Cannabidiol (up to 17.8mg/kg) and 8-OH-DPAT (up to 0.178mg/kg) did not substitute for either training drug; however, both significantly increased the potency of Δ9-THC to produce discriminative stimulus effects. Moreover, 8-OH-DPAT significantly attenuated the discriminative stimulus effects of rimonabant, whereas cannabidiol did not modify the rimonabant discriminative stimulus. ConclusionsThese results, which are consistent with cannabidiol lacking CB1 receptor agonist or antagonist activity in vivo, demonstrate enhancement of the effects of Δ9-THC by cannabidiol, albeit at cannabidiol amounts larger than those in Cannabis or cannabidiol-based therapeutics (nabiximols). In addition to showing that cannabidiol and a 5-HT1A receptor agonist have overlapping behavioral effects, the current results suggest that 5-HT1A agonism enhances the CB1 receptor-mediated effects of Δ9-THC.

Characterization of a Nicotine Discriminative Stimulus in Rhesus Monkeys

In general, the discriminative stimulus effects of drugs are examined following s.c. or i.m. administration; this may not be predictive of the effects of drugs that are abused by other means. Nicotine is primarily inhaled in the form of cigarette smoke. Here, two drug discrimination assays were used to compare the effects of nicotine administered i.v. or s.c. in rhesus monkeys responding under an FR5 schedule of stimulus shock termination. In monkeys discriminating 1.78 mg/kg nicotine s.c. (n=5), the ED50 of nicotine was 0.41 mg/kg. In monkeys discriminating 0.032 mg/kg nicotine i.v. (n=4), the ED50 of nicotine was 0.0067 mg/kg. In 3 of these monkeys, a dose of 3.2 mg/kg nicotine s.c. was required to substitute for the i.v. discriminative stimulus; up to 5.6 mg/kg nicotine did not substitute in the fourth monkey. The nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine antagonized the discriminative stimulus effects of nicotine in both discriminations; 1 mg/kg mecamylamine produced rightward shifts of the nicotine dose‐effect function of 3.2‐fold in the s.c. discrimination and a minimum 15‐fold in the i.v. discrimination. A dose of 3.2 mg/kg of the β2 subtype‐selective nAChR antagonist dihydro‐β‐erythroidine (DHβE) did not significantly antagonize the discriminative stimulus effects of nicotine in the s.c. discrimination. However, when combined with the training dose (0.032 mg/kg) of nicotine in the i.v. discrimination, DHβE dose‐dependently decreased discriminative stimulus effects (ED50=0.17 mg/kg). These results indicate there is a critical difference in the involvement of the β2 subtype of nAChR between the two discriminations that cannot be explained by the difference in training dose. Supported by USPHS Grant DA25267.

The discriminative stimulus effects of mecamylamine in nicotine-treated and untreated rhesus monkeys.

The extent to which chronic nicotine treatment can alter the effects of the nicotinic acetylcholine receptor antagonist mecamylamine, and whether those effects can be attenuated by nicotine have not been clearly established in the literature. Here, the discriminative stimulus effects of mecamylamine were compared between one group of rhesus monkeys receiving a continuous infusion of nicotine base (5.6 mg/kg/day subcutaneously) and another group of monkeys not receiving nicotine treatment. Both groups responded under a fixed ratio 5 schedule of stimulus-shock termination. Stimulus control was obtained at doses of 1.78 mg/kg mecamylamine in monkeys receiving continuous nicotine and 5.6 mg/kg mecamylamine in monkeys not receiving continuous nicotine treatment. Nicotine did not attenuate the discriminative stimulus effects of mecamylamine in either group. Discontinuation of continuous nicotine produced responding on the mecamylamine lever within 24 h in some but not all monkeys. This may indicate a qualitative difference in the discriminative stimulus effects of mecamylamine between groups, perhaps reflecting antagonism of nicotine and nicotine withdrawal in monkeys receiving continuous nicotine. The failure of nicotine to reverse the effects of mecamylamine is consistent with a noncompetitive interaction at nicotinic acetylcholine receptors and indicates that mecamylamine-induced withdrawal cannot be readily modified by nicotine.

Quantitative pharmacological analyses of the interaction between flumazenil and midazolam in monkeys discriminating midazolam: Determination of the functional half life of flumazenil

The duration of action of a drug is commonly estimated using plasma concentration, which is not always practical to obtain or an accurate estimate of functional half life. For example, flumazenil is used clinically to reverse the effects of benzodiazepines like midazolam; however, its elimination can be altered by other drugs, including some benzodiazepines, thereby altering its half life. This study used Schild analyses to characterize antagonism of midazolam by flumazenil and determine the functional half life of flumazenil. Four monkeys discriminated 0.178mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination; flumazenil was given at various times before determination of a midazolam dose–effect curve. There was a time-related decrease in the magnitude of shift of the midazolam dose–effect curve as the interval between flumazenil and midazolam increased. The potency of flumazenil, estimated by apparent pA2 values (95% CI), was 7.30 (7.12, 7.49), 7.17 (7.03, 7.31), 6.91 (6.72, 7.10) and 6.80 (6.67, 6.92) at 15, 30, 60 and 120min after flumazenil administration, respectively. The functional half life of flumazenil, derived from potency estimates, was 57±13min. Thus, increasing the interval between flumazenil and midazolam causes orderly decreases in flumazenil potency; however, across a broad range of conditions, the qualitative nature of the interaction does not change, as indicated by slopes of Schild plots at all time points that are not different from unity. Differences in potency of flumazenil are therefore due to elimination of flumazenil and not due to pharmacodynamic changes over time.

Acute tolerance to chlordiazepoxide qualitatively changes the interaction between flumazenil and pregnanolone and not the interaction between flumazenil and midazolam in rhesus monkeys discriminating midazolam

Benzodiazepines and neuroactive steroids act at distinct binding sites on γ-aminobutyric acidA (GABAA) receptors where they positively modulate GABA, resulting in similar acute behavioral effects. Tolerance to benzodiazepines can develop with repeated treatment; however, cross tolerance to neuroactive steroids does not develop, perhaps due to conformational changes in benzodiazepine, and not neuroactive steroid, binding sites. Three monkeys discriminated 0.178mg/kg midazolam while responding under a fixed-ratio 10 schedule of stimulus-shock termination. On separate occasions, dose-effect curves for midazolam and pregnanolone were determined when monkeys had not received chlordiazepoxide and when they received 10mg/kg chlordiazepoxide 46hours earlier; for some tests, flumazenil was given before determination of dose-effect curves. Midazolam and pregnanolone produced ≥80% midazolam-lever responding. When administered 46h before sessions, chlordiazepoxide did not produce pregnanolone-lever responding; under those treatment conditions, midazolam dose-effect curves were shifted 2.8-fold rightward and pregnanolone dose-effect curves were not changed. Flumazenil antagonized midazolam; Schild (linear) analyses yielded slopes that were not different from unity and pA2 values of 7.46 when monkeys had not received chlordiazepoxide and 7.44 when they received chlordiazepoxide 46h earlier. Flumazenil did not alter the effects of pregnanolone in chlordiazepoxide-treated monkeys. Thus, interactions between flumazenil and midazolam were not qualitatively or quantitatively changed in monkeys acutely tolerant to chlordiazepoxide, suggesting that mechanisms other than alterations of benzodiazepine binding sites account for the development of acute tolerance.

Interactions between dopamine transporter and cannabinoid receptor ligands in rhesus monkeys

Δ(9)-tetrahydrocannabinol (Δ(9)-THC) modifies dopamine efflux. However, the extent to which cannabinoid and dopamine drugs modify each other's behavioral effects has not been fully established. This study examined dopamine releasers and/or transport inhibitors alone and in combination with cannabinoids in two drug discrimination assays. Experimentally and pharmacologically experienced rhesus monkeys (n = 5) discriminated Δ(9)-THC (0.1 mg/kg i.v.) from vehicle while responding under a fixed ratio 5 schedule of stimulus-shock termination. A separate group (n = 6) of monkeys responded under the same schedule, received daily Δ(9)-THC (1 mg/kg/12 h s.c.), and discriminated the cannabinoid antagonist rimonabant (1 mg/kg i.v.), i.e., cannabinoid withdrawal, from vehicle. A sign of withdrawal sign (head shaking) was examined in monkeys receiving Δ(9)-THC daily. Rimonabant antagonized the Δ(9)-THC discriminative stimulus and a dose of Δ(9)-THC greater than the daily treatment attenuated the rimonabant discriminative stimulus. In monkeys discriminating Δ(9)-THC, the dopamine transporter ligands cocaine, amphetamine, bupropion, RTI 113, and RTI 177 produced a maximum of 2% responding on the drug lever and blocked the discriminative stimulus effects of Δ(9)-THC. In Δ(9)-THC treated monkeys discriminating rimonabant, the dopamine transporter ligands partially substituted for and increased the potency of rimonabant to produce discriminative stimulus effects. The dopamine antagonist haloperidol enhanced the Δ(9)-THC discriminative stimulus without significantly modifying the rimonabant discriminative stimulus. Imipramine and desipramine, which have low affinity for dopamine transporters, were less effective in modifying either the Δ(9)-THC or rimonabant discriminations. The dopamine transporter ligands and haloperidol attenuated head shaking, whereas imipramine and desipramine did not. Dopamine release and/or inhibition of dopamine transport blocks detection of Δ(9)-THC and is potentially the mechanism by which some therapeutics (e.g., bupropion) reduce the subjective effects of marijuana and enhance the subjective effects of marijuana withdrawal.

Eating high-fat chow increases the sensitivity of rats to quinpirole-induced discriminative stimulus effects and yawning

Discriminative stimulus effects of direct acting dopamine receptor agonists (e.g. quinpirole) appear to be mediated by D3 receptors in free-feeding rats. Free access to high-fat chow increases sensitivity to quinpirole-induced yawning, and this study examined whether eating high-fat chow increases sensitivity to the discriminative stimulus effects of quinpirole. Five rats discriminated between 0.032 mg/kg quinpirole and vehicle while responding under a continuous reinforcement schedule of stimulus shock termination. When rats had free access to high-fat chow (discrimination training was suspended), the quinpirole discrimination dose-response curve shifted leftward, possibly indicating enhanced sensitivity at D3 receptors. In the same rats, both the ascending (mediated by D3 receptors) and descending (mediated by D2 receptors) limbs of the dose-response curve for quinpirole-induced yawning shifted leftward. When rats had free access to a standard chow (discrimination training was suspended), the quinpirole discrimination and yawning dose-response curves did not change. Together with published data showing that the discriminative stimulus effects of quinpirole in free-feeding rats are mediated by D3 receptors and the insensitivity of this effect of quinpirole to food restriction (shown to increase sensitivity to D2 but not D3-mediated effects), these results suggest that the leftward shift of the discrimination dose-response curve when rats eat high-fat chow is likely because of enhanced sensitivity at D3 receptors. Thus, eating high-fat food enhances drug effects in a manner that might impact clinical effects of drugs or vulnerability to drug abuse.

Rimonabant-Induced Δ9-Tetrahydrocannabinol Withdrawal in Rhesus Monkeys: Discriminative Stimulus Effects and Other Withdrawal Signs

Marijuana-dependent individuals report using marijuana to alleviate withdrawal, suggesting that pharmacotherapy of marijuana withdrawal could promote abstinence. To identify potential pharmacotherapies for marijuana withdrawal, this study first characterized rimonabant-induced Δ⁹-tetrahydrocannabinol (Δ⁹-THC) withdrawal in rhesus monkeys by using drug discrimination and directly observable signs. Second, drugs were examined for their capacity to modify cannabinoid withdrawal. Monkeys receiving chronic Δ⁹-THC (1 mg/kg/12 h s.c.) discriminated the cannabinoid antagonist rimonabant (1 mg/kg i.v.) under a fixed ratio schedule of stimulus-shock termination. The discriminative stimulus effects of rimonabant were dose-dependent (ED₅₀ = 0.25 mg/kg) and accompanied by head shaking. In the absence of chronic Δ⁹-THC treatment (i.e., in nondependent monkeys), a larger dose (3.2 mg/kg) of rimonabant produced head shaking and tachycardia. Temporary discontinuation of Δ⁹-THC treatment resulted in increased responding on the rimonabant lever, head shaking, and activity during the dark cycle. The rimonabant discriminative stimulus was attenuated fully by Δ⁹-THC (at doses larger than mg/kg/12 h) and the cannabinoid agonist CP 55940 [5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol], and partially by the cannabinoid agonist WIN 55212-2 [(<i>R</i>)-(+)-[2, 3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] and the α₂-adrenergic agonist clonidine. In contrast, a benzodiazepine (diazepam) and monoamine agonist (cocaine) did not attenuate the rimonabant discriminative stimulus. Head shaking was attenuated by all test compounds. These results show that the discriminative stimulus effects of rimonabant in Δ⁹-THC-treated monkeys are a more pharmacologically selective measure of cannabinoid withdrawal than rimonabant-induced head shaking. These results suggest that cannabinoid and noncannabinoid (α₂-adrenergic) agonists are potentially useful therapeutics for marijuana dependence inasmuch as they attenuate the subjective experience of Δ⁹-THC withdrawal.

Differential Effects of Serotonin 5-HT1A Receptor Agonists on the Discriminative Stimulus Effects of the 5-HT2A Receptor Agonist 1-(2,5-Dimethoxy-4-methylphenyl)-2-aminopropane in Rats and Rhesus Monkeys

Although many drugs act by indirectly stimulating multiple receptors (e.g., reuptake inhibitors), relatively little is known about interactions between agonism at different receptors. This study compared the effect of serotonin (5-HT)(1A) receptor agonists with the discriminative stimulus effects of the 5-HT(2A) receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) in rats and rhesus monkeys. Eight rats discriminated 0.56 mg/kg i.p. DOM and responded under a fixed ratio (FR) 10 schedule of food presentation, whereas three rhesus monkeys discriminated 0.32 mg/kg s.c. DOM and responded under an FR 5 schedule of stimulus shock termination. DOM and the 5-HT(2A) receptor agonists 2,5-dimethoxy-4-n-propylthiophenethylamine (2C-T-7) and dipropyltryptamine (DPT), but not the 5-HT(1A) receptor agonists 8-hydroxy-2-(di-n-propylamino) tetralin hydrochloride (8-OH-DPAT) and 3-chloro-4-fluorophenyl-(4-fluoro-4-([(5-methyl-6-methylaminopyridin-2-ylmethyl) amino) methyl] piperidin-1-yl) methanone (F13714), occasioned responding on the DOM-associated lever in rats and monkeys. Both 8-OH-DPAT and F13714 attenuated the discriminative stimulus effects of DOM in monkeys but not in rats; these effects of 8-OH-DPAT and F13714 were prevented by the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (WAY 100635). DPT and 2C-T-7 enhanced the discriminative stimulus effects of DOM in rats and monkeys in an additive manner. Taken together, the results suggest that the DOM discriminative stimulus is pharmacologically similar and mediated by 5-HT(2A) receptors in rats and monkeys; however, the ability of 5-HT(1A) receptor agonists to modify the effects of DOM is markedly different between these species. These results indicate possible differences in the neurobiology of 5-HT systems that could be important for studying drugs that have multiple mechanisms of action (e.g., reuptake inhibitors that indirectly stimulate multiple receptors).

Discriminative Stimulus Effects of 1-(2,5-Dimethoxy-4-Methylphenyl)-2-Aminopropane in Rhesus Monkeys: Antagonism and Apparent pA2 Analyses

Discriminative stimulus effects of the serotonin (5-HT) receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) have been studied in rats and, more recently, in rhesus monkeys. This study examined DOM, 2,5-dimethoxy-4-(n)-propylthiophenethylamine (2C-T-7), and dipropyltryptamine hydrochloride (DPT) alone and in combination with three antagonists, MDL100907 [(+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol]], ketanserin [3-[2-[4-(4-fluorobenzoyl)piperidin-1-yl]ethyl]-1H-quinazoline-2,4-dione], and ritanserin [6-[2-[4-[bis(4-fluorophenyl)methylidene]piperidin-1-yl]ethyl]-7-methyl-[1,3]thiazolo[2,3-b]pyrimidin-5-one], to identify the 5-HT receptor subtype(s) that mediates the discriminative stimulus effects of these 5-HT receptor agonists. Four adult rhesus monkeys discriminated between 0.32 mg/kg s.c. DOM and vehicle while responding under a fixed ratio 5 schedule of stimulus shock termination. DOM, 2C-T-7, and DPT dose-dependently increased responding on the DOM-associated lever. MDL100907 (0.001-0.01 mg/kg), ketanserin (0.01-0.1 mg/kg), and ritanserin (0.01-0.1 mg/kg) each shifted the dose-response curves of DOM, 2C-T-7, and DPT rightward in a parallel manner. Schild analysis of each drug combination was consistent with a simple, competitive, and reversible interaction. Similar apparent affinity (pA(2)) values were obtained for MDL100907 in combination with DOM (8.61), 2C-T-7 (8.58), or DPT (8.50), for ketanserin with DOM (7.67), 2C-T-7 (7.75), or DPT (7.71), and for ritanserin with DOM (7.65), 2C-T-7 (7.75), or DPT (7.65). Potency of antagonists in this study was correlated with binding affinity at 5-HT(2A) receptors and not at 5-HT(2C) or alpha(1) adrenergic receptors. This study used Schild analysis to examine receptor mechanisms mediating the discriminative stimulus effects of hallucinogenic drugs acting at 5-HT receptors; results provide quantitative evidence for the predominant, if not exclusive, role of 5-HT(2A) receptors in the discriminative stimulus effects of DOM, 2C-T-7, and DPT in rhesus monkeys.

Apparent pA2 analysis of 5‐HT2A receptor antagonists in rhesus monkeys discriminating DOM

Discriminative stimulus effects of 1‐(2,5‐dimethoxy‐4‐methylphenyl)‐2‐aminopropane (DOM) have been well characterized in rats and less so in other species. This study used Schild analysis and a cumulative‐dosing procedure to systematically examine the receptor mechanisms that mediate the discriminative stimulus effects of DOM in rhesus monkeys. Four rhesus monkeys discriminated between 0.32 mg/kg (s.c.) DOM and vehicle while responding under an FR 5 schedule of stimulus shock termination. DOM and the 5‐HT1A/5‐HT2A receptor agonist dipropyltryptamine (DPT) increased responding on the DOM lever. M100907 (0.001–0.01 mg/kg), ketanserin (0.01–0.1 mg/kg) and ritanserin (0.01–0.1 mg/kg) shifted the dose‐response curves of DOM and DPT rightward in a parallel manner. Schild analyses were consistent with a simple, competitive interaction and yielded a pA2 value of 8.5 for M100907 with DOM or DPT; a pA2 value of 7.7 was obtained for ketanserin and for ritanserin with DOM or DPT. These results confirm the role of 5‐HT2A receptors in the discriminative stimulus effects of DOM and extend the use of Schild analysis to this discrimination procedure in monkeys, thereby providing the basis for future studies investigating the role of particular 5‐HT receptor subtypes in the behavioral effects of DOM and related drugs. CPF is supported by RCA DA17918.

Discriminative Stimulus Effects of 1-(2,5-Dimethoxy-4-methylphenyl)-2-aminopropane in Rhesus Monkeys

Discriminative stimulus effects of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) and related drugs have been studied extensively in rodents, although the generality of those findings across species is not known. The goals of this study were to see whether monkeys could discriminate DOM and to characterize the DOM discriminative stimulus by studying a variety of drugs, including those with hallucinogenic activity in humans. Four rhesus monkeys discriminated between 0.32 mg/kg s.c. DOM and vehicle after an average of 116 (range = 85-166) sessions while responding under a fixed ratio 5 schedule of stimulus shock termination. Increasing doses of DOM occasioned increased responding on the drug lever with the training dose occasioning DOM-lever responding for up to 2 h. The serotonin (5-HT)(2A/2C) receptor antagonists ritanserin and ketanserin, the 5-HT(2A) receptor antagonist (+)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL100907), and its (-)stereoisomer MDL100009 [(-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol], but not haloperidol, completely blocked the discriminative stimulus effects of DOM. Quipazine as well as several drugs with hallucinogenic activity in humans, including (+)lysergic acid diethylamide, (-)DOM, and 2,5-dimethoxy-4-(n)-propylthiophenethylamine (2C-T-7), occasioned DOM-lever responding. The kappa-opioid receptor agonists U-50488 and salvinorin A (a hallucinogen) did not exert DOM-like effects and neither did ketamine, phencyclidine, amphetamine, methamphetamine, cocaine, morphine, yohimbine, fenfluramine, 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), or (+/-)-2-(N-phenethyl-N-1'-propyl)amino-5-hydroxytetralin hydrochloride (N-0434). These data confirm in nonhuman primates a prominent role for 5-HT(2A) receptors in the discriminative stimulus effects of some drugs with hallucinogenic activity in humans. The failure of another drug with hallucinogenic activity (salvinorin A) to substitute for DOM indicates that different classes of hallucinogens exert qualitatively different discriminative stimulus effects in nonhumans.

Time-dependent decreases in apparent pA2 values for naltrexone studied in combination with morphine in rhesus monkeys

One assumption of Schild analyses, which have been used extensively to characterize opioids in vitro and in vivo, is that the concentration of antagonist is at equilibrium; violation of this assumption would yield slopes of Schild plots that deviate from unity. For in vivo studies, the concentration of antagonist changes as it is eliminated; however, if studies are conducted at a time when the antagonist is maximally effective, the assumption of equilibrium is not violated. The goal of these studies was to determine how increasing the delay between antagonist administration and determination of dose-effect curves alters results of Schild analyses. Monkeys received 3.2 mg/kg/day of morphine and discriminated naltrexone while responding under a fixed-ratio 5 schedule of stimulus-shock termination. In monkeys acutely deprived of morphine (27 h), naltrexone was administered 1, 2, 4, or 6 h before determination of morphine dose-effect curves. Morphine-deprived monkeys responded on the naltrexone-appropriate lever, and this effect was reversed by morphine. Naltrexone dose- and time-dependently antagonized morphine. Schild analyses yielded slopes that did not deviate from unity; however, as the delay between naltrexone administration and determination of morphine dose-effect curves increased, apparent pA(2) values decreased. The assumption of equilibrium of antagonist at receptor sites does not appear to be violated, regardless of when it is administered, and changes in naltrexone concentration as it is eliminated are reflected in orderly decreases in potency. These results further indicate the strength of Schild analyses for describing interactions between drugs and receptors in vivo.

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

Drug discrimination can be used to examine tolerance and dependence in agonist-treated animals by establishing an appropriate antagonist as a discriminative stimulus. Establish intravenous SR 141716A as a discriminative stimulus in four rhesus monkeys pretreated with a relatively small dose of Delta9-tetrahydrocannabinol (Delta9-THC). Rhesus monkeys received i.v. Delta9-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. 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 Delta9-THC shifted the SR 141716A dose-effect curve 3-fold rightward. Omitting Delta9-THC before test sessions resulted in responding on the SR 141716A lever that was attenuated by subsequent administration of Delta9-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 Delta9-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. SR 141716A can be established as a discriminative stimulus in animals pretreated with Delta9-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.

SR 141716A differentially attenuates the behavioral effects of ??9-THC in rhesus monkeys

The prototypic cannabinoid CB1 antagonist SR 141716A is one important pharmacologic tool for examining CB1 receptors that mediate the behavioral and physiologic effects of delta9-tetrahydrocannabinol (delta9-THC). This study examined the effects of SR 141716A on the rate-decreasing, hypothermic and discriminative stimulus effects of delta9-THC in rhesus monkeys. In monkeys (n=4) responding under a multiple fixed ratio (FR-10:FR-10) schedule of food presentation and stimulus-shock termination, the potency of i.m. delta9-THC to decrease responding in the food component (ED50=0.64 mg/kg) was threefold greater than its potency in the stimulus-shock termination component (ED50=2.14 mg/kg). In the same monkeys, hypothermia was induced by delta9-THC at a dose (e.g. 0.32 mg/kg) that did not alter responding in either schedule component; the maximum decrease was 2.1 degrees C at a dose of 3.2 mg/kg. A dose of 0.32 mg/kg of SR 141716A, significantly attenuated delta9-THC-induced hypothermia without attenuating the rate-decreasing effects of delta9-THC in either component of the multiple schedule. The largest dose of i.m. SR 141716A that was studied, 1.0 mg/kg, significantly decreased rectal temperature and responding in the food component but did not significantly decrease responding in the stimulus-shock termination component of the multiple schedule. In a separate group of monkeys (n=3) that discriminated i.v. delta9-THC (0.1 mg/kg) while responding under an FR-5 schedule of stimulus-shock termination, SR 141716A (0.32 and 1 mg/kg) significantly increased the ED50 of the delta9-THC by 2.3- and 3.7-fold, respectively. Collectively, these results demonstrate that the behavioral effects of delta9-THC are not equally attenuated by SR 141716A.

Different types of GABA A receptors may mediate the anticonflict and response rate-decreasing effects of zaleplon, zolpidem, and midazolam in squirrel monkeys

The role of different types of GABA(A) receptors in mediating anticonflict and response rate-decreasing effects of benzodiazepines in primate species is not known. To examine the behavioral effects of the benzodiazepine-site, GABA(A) agonists zolpidem, zaleplon, and midazolam in the presence of two antagonists, flumazenil and beta-carboline-3-carboxylate-t-butyl ester (beta-CCt) in squirrel monkeys. Two schedules of operant responding were used: (1) a multiple fixed-ratio (FR) schedule of food presentation involving punished and nonpunished behavior, and (2) an FR schedule of stimulus shock-termination. Midazolam (0.03-1.0 mg/kg), zolpidem (0.1-3.0 mg/kg), and zaleplon (0.1-3.0 mg/kg) increased rates of punished responding and decreased rates of nonpunished responding under the multiple schedule. Pretreatment with flumazenil (0.3-1.0 mg/kg) antagonized the anticonflict and response rate-decreasing effects of all three agonists. Pretreatment with beta-CCt (3-10 mg/kg) antagonized the anticonflict and rate-decreasing effects of midazolam, as well as the rate-decreasing effects of zolpidem and zaleplon. However, beta-CCt did not antagonize the anticonflict effects of zolpidem and zaleplon; instead, these effects of zolpidem and zaleplon were apparently enhanced in the presence of beta-CCt. Under the schedule of stimulus shock-termination, both flumazenil and beta-CCt antagonized zolpidem and zaleplon; however, the effects of beta-CCt were less consistent than the effects of flumazenil. In nonhuman primates, different types of GABAA receptors may mediate the anticonflict and the response rate-decreasing effects of the nonselective GABAA agonist midazolam and the selective GABAA1 agonists zolpidem and zaleplon.

Discriminative stimulus effects of flumazenil in untreated and in diazepam-treated rhesus monkeys.

Long-term use of benzodiazepine agonists can have adverse effects (e.g., development of dependence), thereby limiting their clinical usefulness. The goal of the current study was to examine the discriminative stimulus effects of flumazenil in untreated and diazepam-treated monkeys to determine whether this type of procedure could be used to examine benzodiazepine dependence. Flumazenil (0.32 mg/kg s.c.) was established as a discriminative stimulus in eight monkeys receiving 5.6 mg/kg/day of diazepam (p.o.); four responded under a fixed ratio (FR)5 schedule of stimulus-shock termination (SST) and four responded under a FR5 schedule of food presentation. For comparison, 1.0 mg/kg flumazenil (s.c.) was established as a discriminative stimulus in four untreated monkeys responding under a FR5 schedule of SST. Flumazenil dose-dependently increased responding on the flumazenil-appropriate lever in all monkeys. In diazepam-treated monkeys, Ro 15-4513, ethyl beta-carboline-3-carboxylate and bretazenil substituted for flumazenil with pentylenetetrazole substituting in some monkeys; other drugs failed to substitute for flumazenil. Acute administration of 10.0 mg/kg diazepam (s.c.) shifted the flumazenil dose-effect curve threefold to the right of the control dose-effect curve. Temporary suspension of diazepam treatment produced a time-related increase in flumazenil-lever responding that was reversed by diazepam. In untreated monkeys, midazolam substituted for flumazenil, with other drugs, including those with primary mechanisms of action at non-gamma-aminobutyric acid(A) receptors, substituting in some monkeys. Ro 15-4513 did not substitute in any untreated monkey. The flumazenil discriminative stimulus appears to be pharmacologically selective in treated monkeys with only negative and low efficacy positive modulators substituting for flumazenil; in contrast, a variety of drugs substitute for flumazenil in untreated monkeys. This apparent difference in selectivity suggests that diazepam treatment modifies the flumazenil discriminative stimulus perhaps due to the development of dependence.

Effects of a Novel Fentanyl Derivative on Drug Discrimination and Learning in Rhesus Monkeys

Three monkeys discriminated 1.78 mg/kg of mirfentanil while responding under a fixed-ratio 5 schedule of stimulus-shock termination. Two mirfentanil derivatives, OHM3295 and OHM10579, substituted for mirfentanil in all subjects. However, other drugs produced variable effects among monkeys; for example, μ and κ opioid agonists and clonidine substituted for mirfentanil on some occasions in two monkeys. Cocaine, amphetamine, and ketamine did not substitute in any subject. Opioid antagonists did not attenuate the effects of mirfentanil. In monkeys responding under a repeated acquisition and performance procedure, errors increased only during the acquisition phase at doses of mirfentanil that decreased response rates. Thus, unlike fentanyl, the discriminative stimulus effects of mirfentanil do not appear to be mediated exclusively through opioid receptors. Finally, mirfentanil does not appear to disrupt complex behavioral processes.