Anxiolytic-like action of melatonin on acquisition but not performance of DRL.

The behavioural effects of melatonin have been attributed to a general reduction in motor activity; interference with memory fixation; a decrease in emotionality; or an anxiolytic action. The present experiments compared the effects of melatonin with an anxiolytic benzodiazepine, chlordiazepoxide (Librium), on a schedule of differential reinforcement of low rates of response (DRL) increasing 'burst' responding and premature responding. No doses of melatonin tested (0.03-8.1 mg/kg, IP) affected performance of well-learned DRL. Both low (0.03 mg/kg) and high (1.0 mg/kg) doses of melatonin impaired acquisition of DRL in a similar manner to chlordiazepoxide (5.0 mg/kg) and to much the same extent. Chlordiazepoxide had its usual effects on both acquisition and performance of DRL. These results show that melatonin shares a subset of the effects of chlordiazepoxide. The nature of the effects favours an 'anxiolytic' hypothesis of melatonin action rather than the other hypotheses so far proposed.