Naloxone potentiates the anxiolytic but not the amnestic action of chlordiazepoxide in C57BL/6 mice

This study aimed to investigate the effects of the opiate antagonist naloxone (10 mg/kg) on the action of a subeffective dose of chlordiazepoxide. C57BL/6 mice were tested for either anxiety, using an elevated plus maze, emotional memory in a passive avoidance paradigm or spatial memory in a radial maze. In the elevated plus maze, chlordiazepoxide 2.5 mg/kg was ineffective pep se but, when combined,vith naloxone, it increased the proportion of open-arm entries, as did a higher dose of chlordiazepoxide (5 mg/kg). Closed-arm entries were increased,vith chlordiazepoxide (2.5 mg/kg) injected alone but naloxone, alone or combined,vith chlordiazepoxide, did not modify this measure. In the passive avoidance test, chlordiazepoxide 2.5 mg/kg decreased the latency to enter a compartment associated,vith an electric foot-shock, while a dose of 1.25 mg/kg, alone or in combination with naloxone, was ineffective. Finally, in the radial arm maze, chlordiazepoxide (5 mg/kg) induced amnesia. However, a dose of 2.5 mg/kg, alone or,vith naloxone, had no effect on learning. Naloxone elicited no intrinsic action in any of these behavioural models. It can be concluded that naloxone potentiates the anxiolytic but not the amnestic action of chlordiazepoxide. Behav Pharmacol 1998; 9:691-698 (C) 1998 Lippincott Williams & Wilkins.