Differential interactions engendered by benzodiazepine and neuroactive steroid combinations on schedule-controlled responding in rats

Benzodiazepines are positive allosteric modulators of the GABAA receptor and are prescribed as anxiolytics, hypnotics, and anticonvulsants. While these drugs clearly have clinical value, their use is associated with unwanted side effects such as sedation and motor impairment. Neuroactive steroids are endogenous modulators of GABAA receptors and recent evidence has shown that combinations of the triazolo-benzodiazepine triazolam and the endogenous neuroactive steroid pregnanolone can produce both supra-additive anxiolytic effects and infra-additive reinforcing effects. In the present study, we investigated these same combinations as well as combinations of two clinically-relevant drugs from different chemical classes, the 1, 4 substituted (7-nitro) benzodiazepine clonazepam and the synthetic neuroactive steroid ganaxolone, in rats trained under a 10-response, fixed ratio (FR) schedule of food reinforcement. All four drugs induced a significant and dose-dependent suppression of food-maintained responding. From the dose-response functions, ED50s (i.e., the doses that engendered 50% of the maximum rate-decreasing effect) were generated for each drug. Dose-response functions for combinations of triazolam/pregnanolone, clonazepam/ganaxolone, triazolam/ganaxolone, and clonazepam/pregnanolone were then determined. Isobolographic analysis of the rate-decreasing effects of these combinations revealed that the potencies of the triazolam/pregnanolone combinations were supra-additive while the clonazepam/ganaxolone combinations were additive or infra-additive in relation to predicted values based on dose-additive effects. Furthermore, mixtures of clonazepam/pregnanolone were supra-additive while triazolam/ganaxolone combinations were additive, infra-additive and supra-additive. These results suggest that the ability of benzodiazepine and neuroactive steroid combinations to attenuate rates of food-maintained responding depends critically on both the constituent drugs and the dose of drug in the mixtures.