Evidence for a Free-Running Pituitary-Adrenal Circadian Rhythm in Constant Light-Treated Adult Rats

Abstract

To determine whether the circadian rhythm in pituitary-adrenal function persists in adult rats exposed to constant light, 24-hr patterns of plasma corticoster-one concentrations were compared in individual constant light (LL) and light-dark (LD) (lights on 0600-18000 hr) maintained adult female rats. Nonstress plasma corti-costerone concentrations were determined in serial blood samples obtained from a tail vein of each rat at 4-hr intervals for a 44-hr period 15 weeks after initiation of constant light treatment. Plasma corticosterone of eight of nine rats kept in LD had a 24-hr periodicity with peaks entrained to the onset of dark. Five of the nine LL rats demonstrated an unentrained circadian periodicity in plasma corticosterone. The other four LL rats had apparently arrhythmic fluctuations. These findings suggest that in adult rats the plasma corticosterone rhythm is not dependent on an LD environment but that constant light does have a disruptive effect on the rhythmicity of this hormone system.