SUPRACHIASMATIC NUCLEUS LESION INCREASES CORTICOSTERONE SECRETION

The diurnal rhythm of corticosteroid secretion is controlled by the su prachiasmatic nucleus (SCN). In rats, plasma corticosteroid levels ris e just before the onset of the activity period during the dark phase. Our previous results indicated that vasopressin as a neurotransmitter from the SCN inhibited corticosteroid secretion in the area of the par aventricular/dorsomedial nucleus of the hypothalamus. We hypothesized that during the day the SCN may serve as an inhibitory system for cort icosteroid secretion. To investigate this possibility, intact and SCN- lesioned animals were exposed to mild stress in the morning and evenin g and their plasma corticosteroid levels were monitored. The results i ndicate that SCN-lesioned animals have higher morning corticosteroid l evels and respond both in the morning and evening with higher corticos teroid levels after stress than do intact control animals. We conclude , therefore, that these results indicate an inhibitory role of the SCN on corticosteroid secretion. The apparent discrepancy with the report ed stimulatory role of the SCN on adrenocorticotropic hormone secretio n is discussed.