Comparing the discriminative stimuli produced by either the neuroactive steroid pregnanolone or the benzodiazepine midazolam in rats

Abstract

Rationale-Neuroactive steroids might be therapeutic alternatives for benzodiazepines because they have similar anxiolytic, sedative and anticonvulsant effects, and their actions at different modulatory sites on GABA A receptors might confer differences in adverse effects.Objectives-This study used drug discrimination to compare discriminative stimuli produced by positive GABA A modulators that vary in their site of action on GABA A receptors.Methods-Two groups of rats discriminated either 3.2 mg/kg of pregnanolone or 0.56 mg/kg of midazolam from vehicle while responding under a fixed ratio 10 schedule of food presentation.Results-Pregnanolone, midazolam and flunitrazepam produced ≥80% drug-lever responding in both groups; each drug was more potent in rats discriminating pregnanolone. Pentobarbital produced ≥80% drug-lever responding in all rats discriminating pregnanolone and in 1/3 of the rats discriminating midazolam with larger doses decreasing response rates to <20% of control. Morphine and ketamine produced predominantly saline-lever responding in both groups. Flumazenil antagonized midazolam and flunitrazepam in both groups; slopes of Schild plots were not different from unity and pA 2 values for flumazenil ranged from 5.86 to 6.09. Flumazenil did not attenuate the discriminative stimulus effects of pregnanolone.Conclusions-The midazolam and pregnanolone discriminative stimuli were qualitatively similar, although the effects of pentobarbital were not identical in the two groups. Although acute effects of midazolam and pregnanolone are similar, suggesting that neuroactive steroids might retain the therapeutic effects of benzodiazepines, differences emerge during chronic treatment, indicating that neuroactive steroids might produce fewer adverse effects than benzodiazepines. Keywordsregnanolone; midazolam; flumazenil; drug discrimination; Schild analyses; rats γ-Aminobutyric acid A (GABA A ) receptors are important therapeutic targets, and drugs that act at benzodiazepine binding sites on GABA A receptors are used clinically. Despite their effectiveness and large margin of safety, there are adverse effects associated with the clinical use of benzodiazepines, including the development of tolerance and dependence (Lader 2008; Cloos and Ferreira 2008). Treatment might be improved by developing drugs that retain the therapeutic effects of benzodiazepines while reducing their adverse effects. One strategy for identifying potential replacements for benzodiazepines is to target different Given the differences between benzodiazepines and neuroactive steroids, their acute behavioral effects might also not be identical. One procedure in which differences in the acute effects of benzodiazepines and neuroactive steroids have been observed is drug discrimination. Drugs acting at benzodiazepine, barbiturate or neuroactive steroid binding sites have been established as discriminative stimuli, and generally, they share discriminative stimulus effects (e.g., de la Garza and Johanson 1987;Ator et al. 1993;McMahon et a...

Using drug combinations to assess potential contributions of non-GABAA receptors in the discriminative stimulus effects of the neuroactive steroid pregnanolone in rats

“…Both kinds of studies were used previously to demonstrate similarities in the role of GABA A receptors in the discriminative stimulus effects of pregnanolone and midazolam. In substitution studies, positive GABA A modulators produced drug-lever responding and drugs with actions at other receptors produced vehicle-lever responding in rats discriminating either pregnanolone from vehicle or midazolam from vehicle (Bai and Gerak 2011), demonstrating the importance of GABA A receptors in the discriminative stimulus effects of pregnanolone and midazolam. Drug combination studies further support the role of GABA A receptors in these effects.…”

“…These training doses were selected because previous studies indicated that the potency of pregnanolone and midazolam to produce drug-lever responding would be similar across the two groups with these training doses (Bai and Gerak 2011). Sessions were divided into 15-min cycles and there could be up to 8 cycles in a session.…”

“…Drugs acting at any of these distinct sites on GABA A receptors can be established as discriminative stimuli (e.g., de la Garza and Johanson 1987; Ator et al 1993; Engel et al 2001; Bai and Gerak 2011). Regardless of which training drug is used to establish the discrimination, positive GABA A modulators generally produce drug-lever responding, although some exceptions have been reported.…”

“…That GABA A receptors have a predominant role in the discriminative stimulus effects of pregnanolone has been established (Bai and Gerak 2011; Eppolito et al 2012; Gerak and France 2013); however, a role of other receptors has not been clearly determined. Drugs with actions at 5-HT 3 and NMDA receptors have been studied previously in subjects discriminating pregnanolone, and the results have been inconsistent with 5-HT 3 receptor agonists and NMDA receptor antagonists producing pregnanolone-lever responding in some studies (Engel et al 2001) and not in others (Shannon et al 2005; Eppolito et al 2012).…”

“…Two separate groups of rats were used with one group discriminating pregnanolone from vehicle and the second group discriminating midazolam from vehicle; experimental conditions were identical in the two groups except for the training drug. Because the midazolam discriminative stimulus is pharmacologically selective with only positive GABA A modulators producing effects similar to those of the training drug (Lelas et al 1999; Bai and Gerak 2011), drugs acting at other receptors would not be expected to produce midazolam-lever responding or to enhance the effects of either midazolam or pregnanolone (i.e., shift dose-effect curves for positive modulators to the left) in rats discriminating midazolam. In contrast, actions of neuroactive steroids at 5-HT 3 or NMDA receptors might contribute to their discriminative stimulus effects.…”