ULTRADIAN RHYTHM OF BASAL CORTICOSTERONE RELEASE IN THE FEMALE RAT - DYNAMIC INTERACTION WITH THE RESPONSE TO ACUTE STRESS

The present study investigated the dynamic regulation of the hy pothal amo-pituitary-adrenal axis and its significance to acute stress respon siveness in the female rat. An automated, frequent blood-sampling tech nique allowed the circadian rhythm of corticosterone to be resolved in to a series of pulses. These were equally distributed (mean interval, 50.9 +/- 3.7 min) throughout the 24-h cycle, but their magnitude varie d significantly, being higher between 1800 - 2200h(137 +/- 9 ng/ml) th an between 0600-1000 h (75 +/- 17 ng/ml). This pattern of release indi cates continuous, but variable, activity of the axis throughout the da y. The pulsatile ultradian rhythm suggested alternate periods of secre tion and inhibition, which were found to have a profound effect on the corticosterone responses to acute stress. Noise stress (10 min, 114 d ecibels) evoked a transient increase in corticosterone, which reached a maximum (377 +/- 87 ng/ml) 20 min after onset. However, within this group (n = 26) the response varied depending on the underlying basal a ctivity. When stress coincided with a rising (secretory) phase of a pu lse, corticosterone concentrations rose to 602 +/- 150% of mean basal concentrations (P < 0.001). In contrast, when stress coincided with a falling (nonsecretory) phase of a pulse, a significantly smaller respo nse, no greater than a basal pulse,was evoked. Thus, the alternate per iods of secretion and inhibition generating basal hypothalamo-pituitar y-adrenal activity are an important determinant of responses to acute stress.