Acute Effects of Naloxone and Naltrexone, but Lack of Delayed Effects, on Exploratory Behavior in the Rat

Exploratory head-poke responses, locomotor activity, and rearing were studied in five groups of rats (n = 8 per group) during two 30-min sessions. The opiate antagonists naloxone and naltrexone (0.5 and 2 mg/kg) were administered once, 30 min before the first session. Delayed drug effects were studied during the second session, 24 h after the first session. During the initial 10 min of the first session naloxone and naltrexone decreased the number of head-poke responses without reducing locomotor activity and rearing. Both drugs decreased the number of head-poke responses, locomotor activity, and rearing during the remainder of this session. Equivalent doses of naloxone and naltrexone produced similar effects. The duration of a head-poke response increased during the session. Drug effects on head-poke duration were not observed. Delayed drug effects on behavior during the second session were not obtained. The observation that both naloxone and naltrexone were effective in the present procedure suggests that the effects of naloxone and naltrexone reported here are a function of opiate antagonist properties of these drugs. The data suggest further that the extent to which opiate receptor blockade results in a specific reduction of exploratory behavior may be partly dependent upon the length of the test session.