Ultradian Rhythm of Basal Corticosterone Release in the Female Rat: Dynamic Interaction with the Response to Acute Stress

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Abstract
The present study investigated the dynamic regulation of the hy- pothalamo-pituitary-adrenal axis and its significance to acute stress responsiveness in the female rat. An automated, frequent blood-sam- pling technique allowed the circadian rhythm of corticosterone to be resolved into a series of pulses. These were equally distributed (mean interval, 50.9 6 3.7 min) throughout the 24-h cycle, but their mag- nitude varied significantly, being higher between 1800 -2200 h (137 6 9 ng/ml) than between 0600 -1000 h (75 6 17 ng/ml). This pattern of release indicates continuous, but variable, activity of the axis throughout the day. The pulsatile ultradian rhythm suggested alter- nate periods of secretion and inhibition, which were found to have a profound effect on the corticosterone responses to acute stress. Noise stress (10 min, 114 decibels) evoked a transient increase in cortico- sterone, which reached a maximum (377 6 87 ng/ml) 20 min after onset. However, within this group (n 5 26) the response varied de- pending on the underlying basal activity. When stress coincided with a rising (secretory) phase of a pulse, corticosterone concentrations rose to 602 6 150% of mean basal concentrations (P , 0.001). In contrast, when stress coincided with a falling (nonsecretory) phase of a pulse, a significantly smaller response, no greater than a basal pulse, was evoked. Thus, the alternate periods of secretion and in- hibition generating basal hypothalamo-pituitary-adrenal activity are an important determinant of responses to acute stress. (Endocrinol- ogy 139: 443- 450, 1998)

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