Negative GABA_A modulators attenuate the discriminative stimulus effects of benzodiazepines and the neuroactive steroid pregnanolone in rhesus monkeys

Rationale: Negative GABA_A modulators (i.e., inverse agonists) might be useful for identifying mechanisms at the GABA_A receptor complex that mediate the effects of positive GABA_A modulators, especially those for which there are no available competitive antagonists. Objective: Drug discrimination was used to examine antagonism of a 5-beta neuroactive steroid (pregnanolone) and a benzodiazepine (midazolam) by several negative GABA_A modulators in rhesus monkeys. Methods: One group of monkeys (n=5) received 5.6 mg kg^-1 day^-1 of diazepam (p.o.) and discriminated the benzodiazepine antagonist flumazenil (0.1 or 0.32 mg/kg s.c.); another group of monkeys (n=5) discriminated the benzodiazepine midazolam (0.32 mg/kg s.c.). Results: In diazepam-treated monkeys, negative GABA_A modulators with increasing efficacy, including Ro 15-4513, ethyl β-carboline-3-carboxylate (β-CCE), methyl β-carboline-3- carboxylate (β-CCM) and methyl-6,7-dimethoxyl-4-ethyl-β-carboline-3- carboxylate (DMCM), substituted for flumazenil. In monkeys discriminating midazolam, pregnanolone occasioned high levels of midazolam-lever responding, and these effects were attenuated by β-CCE and β-CCM, but not by flumazenil, Ro 15-4513, or DMCM. The midazolam discriminative stimulus also was attenuated by β-CCM and DMCM; Schild analysis was consistent with a simple competitive interaction between midazolam and β-CCM but not between midazolam and DMCM. Conclusions: Negative modulators are qualitatively similar to neutral modulators in diazepam-treated animals; however, interactions between negative modulators and midazolam suggest that different receptors mediate the effects of some (DMCM) and not other (β-CCM) negative modulators. Negative modulators at benzodiazepine sites exert efficacy-dependent antagonism of positive modulators at neuroactive steroid sites. Without competitive antagonists at neuroactive steroid or barbiturate sites, negative modulators could prove useful for examining the mechanism of action of different classes of positive GABA_A modulator.