Endogenous corticosterone alterations and appetitive learning in the rat

Abstract

Two experiments were conducted to determine the pattern of daily variation in plasma corticosterone in the rat, and to assess the relationship between high and low endogenous levels of plasma corticosterone and appetitive maze learning. Plasma corticosterone concentrations were found to vary along a 24-hour time scale with a significant rise 3 hours prior to lights off, resulting in a peak at dark onset. The peak was followed by a sharp decline, and a return to basal levels. The second experiment demonstrated a time of day factor involved in appetitive maze learning. Rats tested in the afternoon acquired the task sooner and demonstrated superior performance in terms of time taken to traverse the maze and in errors made, when compared to rats tested in the morning. High and low corticosterone treatments were induced by manipulating feeding schedules. These groups showed little difference in acquisition or performance during afternoon test conditions. In morning test conditions, the low corticosterone group ran faster times and made fewer errors than the high corticosterone group. High endogenous corticosterone levels may interfere with performance on appetitive tasks. It is suggested that the time of day differences observed in goal-directed behavior may relate to the sensitivity of the central nervous system to peripheral hormonal influences or to daily rhythms in brain transmitter levels, possibly serotonin.