Diurnal rhythm of total and free concentrations of serum corticosterone in the rat.

Daily variations in serum concentrations of total and free corticosterone and corticosteroid-binding globulin (CBG) were determined at 4 h intervals in male rats maintained on a 12:12 h light/dark cycle (lights on at 05.00 h). Total corticosterone concentrations exhibited a classical rhythm with peak values occurring at the onset of dark (17.00 h). Serum CBG concentrations displayed daily fluctuations but these appeared to be non-rhythmic in nature. The % free corticosterone exhibited a diurnal rhythm with peak values occurring during the dark phase. When absolute concentrations of free corticosterone were calculated, the profile could be superimposed on that of total corticosterone with the zenith at the onset of the dark phase (17.00 h) and the nadir coinciding with the onset of light (05.00 h). These data suggest that the primary determinant of the rhythm in free corticosterone concentrations is the daily rhythm in total corticosterone.