Discriminative stimulus properties of oxazepam in the pigeon

Discriminative Stimulus Properties of S(-)-Nicotine: "A Drug for All Seasons"

S(-)-Nicotine is the major pharmacologically active substance in tobacco and can function as an effective discriminative stimulus in both experimental animals and humans. In this model, subjects must detect and communicate the nicotine drug state versus the non-drug state. This review describes the usefulness of the procedure to study nicotine, presents a general overview of the model, and provides some relevant methodological details for the establishment of this drug as a stimulus. Once established, the (-)-nicotine stimulus can be characterized for dose response and time course effects. Moreover, tests can be conducted to determine the similarity of effects produced by test drugs to those produced by the training dose of nicotine. Such tests have shown that the stimulus effects of nicotine are stereoselective [S(-)-nicotine >R(+)-nicotine] and that other "natural" tobacco alkaloids and (-)-nicotine metabolites can produce (-)-nicotine-like effects, but these drugs are much less potent than (-)-nicotine. Stimulus antagonism tests with mecamylamine and DHβE (dihydro-β-erythroidine) indicate that the (-)-nicotine stimulus is mediated via α4β2 nicotinic acetylcholine receptors (nAChRs) in brain; dopamine systems also are likely involved. Individuals who try to cease their use of nicotine-based products are often unsuccessful. Bupropion (Zyban®) and varenicline (Chantix®) may be somewhat effective as anti-smoking medications because they probably produce stimulus effects that serve as suitable substitutes for (-)-nicotine in the individual who is motivated to quit smoking. Finally, it is proposed that future drug discrimination studies should apply the model to the issue of maintenance of abstinence from (-)-nicotine-based products.

Effects of several benzodiazepines, alone and in combination with flumazenil, in rhesus monkeys trained to discriminate pentobarbital from saline

The purpose of the present study was to further investigate the relationship between the DS effects of PB and those of benzodiazepines (BZs) and to begin to collect pharmacological information concerning receptor mechanisms involved in this behavioral effect of BZs. Rhesus monkeys (n = 3), trained to discriminate pentobarbital (PB; 10 mg/kg, IG) from saline under a discrete-trials shock avoidance procedure, were given IG diazepam (0.3-10 mg/kg), chlordiazepoxide (1.0-30 mg/kg), or etizolam (0.3-10 mg/kg) alone and in combination with flumazenil (0.01-1.7 mg/kg, IM). Flumazenil was administered 10 min prior to the administration of saline, PB or the BZs. All three BZs fully substituted for PB in all monkeys. Diazepam was the most potent with a mean ED50 of 0.81 mg/kg (SEM = 0.04) while chlordiazepoxide was the least potent (mean ED50 = 5.78 mg/kg, SEM = 1.22 mg/kg). The ED50 for etizolam was 1.22 mg/kg (SEM = 0.37 mg/kg). Pretreatment with flumazenil (0.01-1.0 mg/kg) resulted in a dose-related parallel shift to the right in the dose-response function for PB-appropriate responding in all monkeys for all three BZs. The mean (n = 3) pKB value with 0.1 mg/kg flumazenil was 6.51 (SEM = 0.42) for diazepam and 6.57 (SEM = 0.17) for chlordiazepoxide. This value could not be calculated for etizolam because only one monkey was tested with 0.1 mg/kg flumazenil. However, the mean pKB for etizolam considering all monkeys and all doses of flumazenil was 6.58 (SEM = 0.47). Apparent pA2 values for flumazenil with diazepam were 6.02 for one monkey and 7.11 for another. All three BZs tended to increase average latency to respond. Apparent pKB and pA2 analysis may prove useful for elucidating receptor mechanisms involved in the behavioral effects of BZs.

Comparison of the discriminative stimulus effects of midazolam after intracranial and peripheral administration in the rat

Rats were trained in a two-lever drug discrimination paradigm to discriminate midazolam (0.32 mg/kg, i.p. or 1.0 mg/kg, i.p.) from the no-drug condition. After completion of i.p. and s.c. midazolam generalization gradients (0.032-1.0 mg/kg), rats were surgically implanted with unilateral cannulae into the lateral ventricles. Intracerebroventricular (i.c.v.) doses of 1.1-44.2 micrograms midazolam were delivered to unrestrained rats. Midazolam produced dose-dependent increases in drug-appropriate responding by all three routes of administration, but was 2.4- to 4.3-fold more potent when given i.c.v. than when given s.c. or i.p. Midazolam, over the dose range tested, did not produce substantial decreases in response rate by any route of administration. The discriminative-stimulus effect of i.c.v. midazolam was blocked by peripherally administered flumazenil, and such antagonism was surmounted by a 2- to 5-fold increase in the i.c.v. midazolam dose. Taken together, these data suggest that the discriminative-stimulus effects of midazolam are mediated via central benzodiazepine (BZ) receptors.

Drug discrimination models in anxiety and depression

Drug discrimination is a technique for investigating the stimulus properties of centrally active drugs. Although many studies have employed animals to investigate the stimulus properties of substances used clinically for the treatment of anxiety and depression, it would be a mistake to consider the internal discriminative stimuli as being related specifically to the anxiolytic or antidepressant properties of these drugs. Rather drug cues are better considered as relating to the pharmacological action of classes of compounds. Thus, benzodiazepine cues generalize to other compounds acting at benzodiazepine receptors, but not to substances (anxiolytic or otherwise) acting at 5-HT1A receptors. Similarly, antidepressants with different pharmacological properties, for example the tricyclic imipramine, or the phenylaminoketone buproprion produce distinct, unrelated discriminative stimuli. For this reason, the limits of drug discrimination techniques for investigating novel anxiolytic or antidepressant drugs should be clearly recognized. Attempts to identify an anxiogenic discriminative stimulus using pentylenetetrazole have also been misguided. In this technique it has proven difficult to separate unequivocally the pharmacological proconvulsant effects of the drug from the psychological construct anxiety. Nevertheless, drug discrimination remains a valuable technique for investigating pharmacological interactions in animals and man.

Midazolam induces amnesia in a simple, one-trial, maze-learning task in young chicks

We report a simple, one-trial, learning paradigm which we have developed for use in young chicks. Chicks were separated from their brood mates and placed in a small isolation chamber. A 'T' corridor, or maze, connected the isolation chamber to the brood space, allowing the chick to escape isolation stress and rejoin the brood. When the chick successfully negotiated the corridor, the latency to perform this task was recorded. On a subsequent trial, any improvement in the speed of performance was reasoned to reflect the chick's memory of the task. Undrugged chicks always showed significant improvement in task latency if they were replaced in the maze 3 hours after a successful escape, suggesting that they had remembered the task. Chicks given midazolam (0.1 or 0.3 mg/kg, IP), a benzodiazepine, before the first escape, showed no improvement on their second escape. Improved performance was seen, however, if a second injection of midazolam was given before the second escape, suggesting a state-dependent effect.

Differential generalization to pentobarbital in rats trained to discriminate lorazepam, chlordiazepoxide, diazepam, or triazolam

In drug discrimination studies benzodiazepine-trained animals have typically responded on the drug lever when tested with barbiturates. In a recent study, greater specificity appeared to be shown when lorazepam was used as a training drug. The generality and limits of this finding were explored in the present set of experiments. The asymmetrical cross-generalization found in lorazepam- and pentobarbital-trained baboons was replicated in rats and was shown not to be a function of either lorazepam (0.1., 0.32, or 1.0 mg/kg) or pentobarbital (10 or 25 mg/kg) training dose (i.e., pentobarbital-trained rats responded on the drug lever in tests with lorazepam, but lorazepam-trained rats did not show comparable pentobarbital generalization). In the next experiment, groups of rats were trained to discriminate chlordiazepoxide (10 mg/kg), triazolam (0.1 mg/kg), or diazepam (1.0 mg/kg). Generalization to both lorazepam and pentobarbital was shown by these rats. Finally after daily pentobarbital administration, lorazepam-trained rats made a sufficient number of responses after high pentobarbital doses to permit extension of the range of pentobarbital doses tested. Pentobarbital generalization increased, but still did not occur in all rats and was unreliable in successive tests in the same rats. These results suggest less homogeneity in the discriminative stimulus effects of "depressant drugs" than generally has been recognized.