Effects of benzodiazepine receptor ligands and ethanol in rats trained to discriminate pregnanolone

Abstract

Although GABA_A receptor positive modulators share many behavioral effects, subtle differences have been detected among their discriminative stimulus effects. The purpose of the present study was to determine the extent of shared discriminative stimulus effects of pregnanolone with various benzodiazepine receptor ligands and with ethanol. Naive male Sprague–Dawley rats were trained to discriminate the endogenous neuroactive steroid pregnanolone (5.6 or 8.0 mg/kg) from vehicle. The benzodiazepine receptor agonists, triazolam and lorazepam, the benzodiazepine receptor partial agonist, bretazenil, the benzodiazepine1 (BZ1) receptor subtype selective agonists, zolpidem and zaleplon and ethanol were tested. Triazolam, lorazepam and bretazenil substituted for pregnanolone. Lorazepam, but not triazolam or bretazenil, decreased response rates at the highest dose tested. Zaleplon completely substituted for pregnanolone with no effect on response rates. Zolpidem substituted for pregnanolone only at a dose that severely disrupted response rates. Ethanol partially substituted for pregnanolone and decreased response rates. The results are consistent with GABA_A receptor mediation of the discriminative stimulus effects of pregnanolone. The effects on response rates suggest subtle differentiation among the GABA_A receptor-mediated cues.

Introduction

It has been demonstrated previously that the endogenous neuroactive steroid pregnanolone can function as a discriminative stimulus [23]. Positive modulation of GABA_A receptors is thought to mediate the discriminative stimulus effects of pregnanolone. Further, neuroactive steroids have been shown to share discriminative stimulus effects with other positive modulators of the GABA_A receptor complex [2], [10], [12], [23]. For example, in rats trained to discriminate diazepam or pentobarbital from its vehicle, the neuroactive steroids 3α,21-dihydroxy-5α-pregnan-20-one (3α,5α-THDOC; allotetrahydrodeoxycorticosterone) and 3α-hydroxy-5α-pregnan-20-one (3α,5α-P; allopregnanolone) exhibited full substitution for the training drug [2]. Similarly, pentobarbital generalized to 3α,21-dihydroxy-3β-trifluoromethyl-5β-pregnan-20-one (Co 8-7071), a synthetic neuroactive steroid in monkeys trained to discriminate pentobarbital from saline [15]. Additionally, allopregnanolone substituted for ethanol in monkeys [12]; allotetrahydrodeoxycorticosterone and 3α,21-dihydroxy-5β-pregnan-20-one (3α,5β-THDOC; tetrahydrodeoxycorticosterone) have been shown to substitute for ethanol in rats [1], [8].

However, subtle differences among the discriminative stimuli of various GABA_A receptor positive modulators are well documented. For example, the benzodiazepine diazepam and the barbiturate pentobarbital exhibit cross-generalization and diazepam and another benzodiazepine lorazepam exhibit cross-generalization, but pentobarbital and lorazepam exhibit asymmetrical cross-generalization with pentobarbital generalizing to lorazepam and lorazepam failing to generalize to pentobarbital [4]. Thus, although there appears to be some crossover, the discriminative stimulus effects of benzodiazepines and barbiturates are not identical. Differences among the discriminative stimulus effects of benzodiazepine ligands have been reported as well. Bretazenil, a partial agonist at benzodiazepine receptors, completely substitutes for the full benzodiazepine agonist chlordiazepoxide as a discriminative stimulus, whereas bretazenil attenuates the discriminative stimulus effects of the benzodiazepine1 (BZ1) receptor subtype selective agonist zolpidem [16]. Further differences among GABA_A receptor positive modulators have been revealed using three-choice procedures. For example, humans were able to acquire a discrimination between zolpidem and the nonselective benzodiazepine agonist triazolam in a three-choice procedure (zolpidem vs. triazolam vs. placebo; [14]).

The purpose of the present experiment was to determine the extent similarities and differences among the discriminative stimulus effects of pregnanolone compared to various benzodiazepine receptor ligands and to ethanol. The nonselective benzodiazepine full agonists, triazolam and lorazepam and the benzodiazepine partial agonist bretazenil were tested. In addition, the BZ1 receptor subtype selective full agonists zolpidem and zaleplon were evaluated.