Daily biorhythmicity influences, homing behavior, psychopharmacological responsiveness, learning, and retention of suckling rats.

A total of 385 suckling rats were used to investigate the effects of 24-hr biorhythmicity on performance. Nest seeking (homing) in 7-day-old pups and its facilitation by d-amphetamine varied across time of day. Analysis of initial choice revealed that the pups' ability to detect and position themselves in the direction of the nest did not vary across time of day. Homing behavior was not an artifact of random activity level but was related to the pups' motor performance which varied with a daily rhythm. Circadian rhythmic influences on the acquisition and retention of an amphetamine-induced odor aversion were shown by manipulating time of day of conditioning and subsequent retention testing. The time of day effect on conditioning was attributed, in part, to daily rhythmic changes in the effectiveness of the unconditional stimulus. Retention was optimal in pups tested at the time of day corresponding to that of their original training. Time of day may serve as a contextual stimulus that is important for retention in immature as well as adult animals. An appreciation of these 24-hr rhythms is warranted for an accurate assessment of the behavioral competence of developing animals.