[Diurnal variation in passive avoidance response and serum corticosterone in rats]

A possible relationship between diurnal variation in performance of step-through passive avoidance response (PAR) and that in adrenocortical response was investigated. In Wistar-Imamichi strain rats, retention latency of PAR and serum corticosterone (CORT) level immediately after retention test were measured as a function of time of day. Basal CORT level was found to be low during early phase of light period, and thereafter to elevate toward dark period (Exp. 1). Eighty animals were divided into eight groups, which consisted of a combination of two diurnal times of training/testing (light phase = 14:00 and dark phase = 2:00) and four shock intensities for training (0, 0.5, 0.75, 1.0 mA) (Exp. 2). The latency of PAR was found to be longer in the light than in the dark phase, and CORT markedly increased only in the light phase in the groups with 0.75 mA. In addition, PAR was conducted at four times of day (8:00, 14:00, 20:00, 2:00) with 0.75 mA shock (Exp. 3). It was confirmed that the diurnal variation of PAR latency was associated with that the CORT elevation, with the maximum at 14:00 and the minimum at 2:00. These results indicate that diurnal variation in performance of PAR has a close relationship with that in emotional or stress response to aversive situation.