Circadian dependence of corticosterone release to light exposure in the rat.

Previous studies have demonstrated a positive correlation between glucocorticoid levels and circadian reentrainment time following a shift in the light:dark (LD) cycle. We conducted a series of experiments to examine the circadian dependence of the corticosterone (CORT) response to light. Exp. 1 measured CORT release in rats exposed to light at six timepoints. Light presented during the subjective night increased CORT (p<0.05), while light presented during the subjective day did not. In Exp. 2, we documented the time course of the CORT response to light in entrained animals. Rats exposed to light at zeitgeber time (ZT) 18 had a maximal increase in CORT levels following 60 min of stimulus presentation (p<0.05). There was also an increase in adrenocorticotropic hormone following 15 min of light at ZT18 (p<0.05). In an effort to elucidate the effect of changes in the LD cycle on the circadian profile of CORT, Exp. 3 followed the CORT rhythm (in cerebrospinal fluid) of rats prior to and following a shift in the LD cycle. The CORT nadir was elevated following a 6 h photic advance (p<0.05), as was the mean CORT concentration during the peak phase (p<0.05). Most components of the circadian CORT rhythm, however, failed to show an immediate shift towards the change in the light cycle. Together, these data support the hypothesis that a photic phase-shift results in elevated CORT levels, while the rhythm of CORT secretion is robust against changes in the photic environment.