Discriminative stimulus effects and antipunishment effects of drugs measured during the same session

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.

Use of an analog task to study effects of diazepam on taste perception, consummatory behaviors, and risk taking in a social context

In the present study, the authors examined the effects of diazepam (Valium) on various physical, social, and emotional variables within the framework of a task designed to test risk-taking behavior. Participants received either 10 mg of diazepam or a placebo. After the participants tasted strong-flavored liquids, they were able to engage in a risky behavior (i.e., drinking from a confederate's "used" water bottle). Half the participants received additional verbal pressure to drink from the bottle. The authors expected that diazepam would increase health-risk behavior, but the results were inconsistent with that prediction. The second goal of the experiment was to explore diazepam's effects on arousal, mood, social anxiety, and taste. Diazepam users exhibited differences in taste perception and social anxiety, which might in part explain the health-risk results.

Effects of drug discrimination history on anti-punishment properties of chlordiazepoxide in rats

Several studies have indicated that acquiring discriminative stimulus control for a certain anxiolytic drug influences its subsequent anti-conflict properties. To further elaborate on the question whether drug discrimination procedures affect behaviour in a conflict paradigm, a classical two-lever drug discrimination procedure was combined with an operant conflict procedure within the same animals. To this extent, rats were trained to discriminate the anxiolytic chlordiazepoxide (CDP, 30 mg/kg, po) from saline (SAL), and subsequently punished responding periods were introduced within the same session. In addition to the rats that were trained to discriminate CDP from vehicle, a group of rats was trained on a random relationship between CDP and the rewarded lever. CDP and alprazolam completely substituted for CDP, whereas mianserin did not. Responding during punished components in a session was increased by CDP and alprazolam, but not by mianserin in rats that were trained to discriminate CDP from vehicle and in randomly trained rats. The data indicate that rats can be reliably trained and tested in drug discrimination and conflict procedures within a single session and that CDP's discriminative stimulus does not alter its anti-conflict effects.

Pharmacophore/receptor models for GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) via a comprehensive ligand-mapping approach

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

The effects of drug discrimination history on drug discrimination and on punished and unpunished responding

Pigeons trained to discriminate methamphetamine from saline after a history of training to discriminate pentobarbital from saline responded on the drug key after both pentobarbital and methamphetamine, but the association of these drugs by reinforcing their discriminative stimulus responses on the same key did not influence their effects on either punished or unpunished responding. Similarly, pigeons trained to discriminate morphine from saline after a history of discrimination of buspirone from saline, responded on the drug key after both buspirone and morphine, but the association of their discriminative stimulus responses did not influence their effects on either punished or unpunished responding. Whether the effects of these drugs as discriminative stimuli, or their effects on punished and unpunished responding were studied first in the session did not influence the effects of these drugs. Low doses of methamphetamine or pentobarbital did not produce responding on the drug key in birds trained to discriminate higher doses of both drugs, but combinations of these doses did produce responding on the drug key. However, these combinations of low doses of methamphetamine with pentobarbital did not increase the rate-increasing effects of pentobarbital on punished responding.