Buspirone is differentiated from diazepam in humans using a three-response drug discrimination procedure

The Discriminative Stimulus Properties of Drugs Used to Treat Depression and Anxiety

Drug discrimination is a powerful tool for evaluating the stimulus effects of psychoactive drugs and for linking these effects to pharmacological mechanisms. This chapter reviews the primary findings from drug discrimination studies of antidepressant and anxiolytic drugs, including novel pharmacological mechanisms. The stimulus properties revealed from these animal studies largely correspond to the receptor affinities of antidepressant and anxiolytic drugs, indicating that subjective effects may correspond to either therapeutic or side effects of these medications. We discuss drug discrimination findings concerning adjunctive medications and novel pharmacologic strategies in antidepressant and anxiolytic research. Future directions for drug discrimination work include an urgent need to explore the subjective effects of medications in animal models, to better understand shifts in stimulus sensitivity during prolonged treatments, and to further characterize stimulus effects in female subjects. We conclude that drug discrimination is an informative preclinical procedure that reveals the interoceptive effects of pharmacological mechanisms as they relate to behaviors that are not captured in other preclinical models.

Short-term and long-term effects of diazepam on the memory for discrimination and generalization of scopolamine

Benzodiazepines are among the most widely prescribed and misused psychopharmaceutical drugs. Although they are well-tolerated, they are also capable of producing amnestic effects similar to those observed after pharmacological or organic cholinergic dysfunction. To date, the effect of benzodiazepine diazepam on the memory for discrimination of anticholinergic drugs has not been reported. The aim of the present study was to analyze the immediate and long-term effects of diazepam on a drug discrimination task with scopolamine. Male Wistar rats were trained to discriminate between scopolamine and saline administration using a two-lever discrimination task. Once discrimination was acquired, the subjects were divided into three independent groups, (1) control, (2) diazepam, and (3) diazepam chronic administration (10 days). Subsequently, generalization curves for scopolamine were obtained. Additionally, the diazepam and control groups were revaluated after 90 days without having been given any other treatment. The results showed that diazepam produced a significant reduction in the generalization gradient for scopolamine, indicating an impairment of discrimination. The negative effect of diazepam persisted even 90 days after drug had been administered. Meanwhile, the previous administration of diazepam for 10 days totally abated the generalization curve and the general performance of the subjects. The results suggest that diazepam affects memory for the stimulus discrimination of anticholinergic drugs and does so persistently, which could be an important consideration during the treatment of amnesic patients with benzodiazepines.

Clinical neuropharmacology of drugs of abuse: a comparison of drug-discrimination and subject-report measures

Advances in molecular pharmacology and behavioral science have helped elucidate the structure and function of the central nervous system and its relationship to behavior and has sparked the development of pharmacological agents that have increasingly selective and potent effects with fewer adverse side effects. The sensitivity and predictive validity of the two most commonly used methodologies for assessing the neuropharmacological effects of centrally active drugs, subject report of drug effects and drug discrimination, were examined. The sensitivity of the measures was comparable across stimulant, sedative, and opioid drugs. Results with drug-discrimination methodologies were generally consistent with hypothesized neuropharmacological mechanisms across all drug classes, whereas subject reports conformed under more limited testing conditions. Firm conclusions regarding the relative utility of drug-discrimination and subject-report measures for clinical studies of neuropharmacological mechanisms are limited by the small number of studies in which the two methodologies have been tested using identical pharmacological pretreatment manipulations.

The anxiolytic CRF(1) antagonist DMP696 fails to function as a discriminative stimulus and does not substitute for chlordiazepoxide in rats

RATIONALE

Compounds with a mechanism of action different from benzodiazepines may retain the anxiolytic effects of benzodiazepines with fewer side effects. CRF(1) antagonists have anxiolytic-like effects but may have different discriminative stimulus (DS) effects compared with benzodiazepines.

OBJECTIVE

The present study evaluated the similarity of DS effects of a CRF(1) antagonist DMP696 to the benzodiazepine chlordiazepoxide and the ability of DMP696 to produce DS effects on its own using drug discrimination procedures, as well as its anxiolytic-like effects after acute or chronic administration.

METHODS

Rats were trained to discriminate chlordiazepoxide (5.0 mg/kg, IP, 30 min prior to session) from vehicle under a fixed-ratio 10 schedule of food reinforcement and drug- or vehicle-lever selection following administration of DMP696 was determined. The effects of DMP696 on latency to exit a dark chamber (defensive withdrawal model of anxiety) were used as an index of anxiolytic-like activity.

RESULTS

In chlordiazepoxide-trained rats, DMP696 (1.0-100 mg/kg, PO) resulted in most of the animals selecting the vehicle lever, as did another anxiolytic, the 5-HT(1A) partial agonist buspirone (0.3-10 mg/kg, IP). DMP696 reduced exit latency in defensive withdrawal at 10 mg/kg administered either acutely or chronically for 14 days. Thus, the doses of DMP696 studied in drug discrimination were up to 10-fold higher than those active in the anxiety model. In addition, DMP696 (10-60 mg/kg, PO) could not be established as a DS under the conditions used in this study. In a subsequent study, chlordiazepoxide was established as a DS in these same animals.

CONCLUSIONS

Lack of substitution of DMP696 for the chlordiazepoxide DS in rats and its inability to acquire DS properties suggest that the DS effects of DMP696 differ from those of benzodiazepines.

Effects of alprazolam, caffeine, and zolpidem in humans trained to discriminate triazolam from placebo

This study compared the discriminative stimulus effects of zolpidem, a nonbenzodiazepine hypnotic, to benzodiazepines. Eight participants learned to discriminate triazolam (0.35 mg/70 kg) from placebo. The discriminative stimulus effects, self-reported subjective effects, and performance effects of triazolam (0.05-0.35 mg/70 kg), alprazolam (0.25-1.75 mg/70 kg), zolpidem (2.5-35 mg/70 kg) and caffeine (75-525 mg/70 kg) were assessed under two-response and novel-response drug discrimination procedures. Under the two-response procedure, triazolam, alprazolam and zolpidem fully substituted for triazolam and caffeine did not. Under the novel-response procedure, triazolam and alprazolam substituted for triazolam and zolpidem partially substituted for triazolam. Zolpidem, but not triazolam or alprazolam, also produced some novel responding. Caffeine produced both placebo-appropriate and novel responding. The self-reported effects of triazolam, alprazolam and zolpidem were similar. Overall, zolpidem produced similar, but not identical, effects as the benzodiazepines.

The novel-response procedure in humans

The novel-response drug discrimination procedure is one of several three-choice procedures developed to address interpretational difficulties that can occur under standard two-response procedures. The novel-response procedure is unique among three-choice discrimination procedures by using instructions, rather than explicit training procedures. With the novel-response procedure, participants are trained under a standard two-response (drug vs. placebo) discrimination, and then instructed that in the presence of a drug stimulus unlike either of the training drugs, responses should be made on the novel-response alternative. Several studies have assessed the utility of the novel-response procedure by comparing effects under a standard two-response and the novel-response procedure in participants trained to discriminate triazolam from placebo. Results indicate that the novel-response procedure can increase the selectivity of both placebo- and drug-appropriate responding, and in this way, allows for finer distinctions to be made among sedatives than a standard two-response procedure.