Naloxone blocks the effects of chlordiazepoxide on acquisition but not performance of differential reinforcement of low rates of response (DRL).

The opiate antagonist naloxone can completely or partially reverse the effects of the benzodiazepines on appetitive behaviours and conflict tasks involving electric shook. If naloxone changes the anxiolytic action of the benzodiazepines it should theoretically, be effective in tasks employing nonreward as well as those employing shock. We tested naloxone and chlordiazepoxide on acquisition and performance of a nonreward task, DRL. With both continuous administration during acquisition of DRL, and intermittent administration during stable performance, chlordiazepoxide (5 mg/kg IP) increased burst responding and shifted the peak of the inter-response time (IRT) distribution curve to shorter IRTs. Naloxone (3 mg/kg IP) blocked the effects of chlordiazepoxide on acquisition of DRL. Naloxone (3 mg/kg and 10 mg/kg IP) did not change the effects of chlordiazepoxide on well-learned performance of the DRL schedule. These results show that endogenous opiates could mediate some but not all of the actions of the benzodiazepines. They also suggest that state-dependent and "truly anxiolytic" effects of the benzodiazepines (McNaughton 1985) may have different pharmacological substrates.