The diurnal expression of genes encoding vasopressin and vasoactive intestinal peptide within the rat suprachiasmatic nucleus is influenced by circulating glucocorticoids.
The mammalian suprachiasmatic nucleus (SCN) is the endogenous pacemaker generating the diurnal rhythm of the stress hormones ACTH and glucocorticoid secretion. In the present study, we have employed male rats entrained to a 12:12 h (light:dark) photoperiod to investigate the effects of chronic and acute administration of exogenous glucocorticoids upon the diurnal expression of vasopressin and vasoactive intestinal peptide (VIP) mRNA in the SCN by semiquantitative in situ hybridization histochemistry. Chronic administration of exogenous glucocorticoids significantly enhanced vasopressin mRNA expression only at zeitgeber time (ZT) 5, while the otherwise rhythmic expression of vasopressin mRNA was unaffected at ZT11, ZT17 and ZT23. In contrast, the same treatment abolished the rhythmic expression of VIP mRNA resulting in constantly elevated mRNA levels. In adrenalectomized rats given an overnight supplement of dexamethasone in their drinking water, the expression of both vasopressin and VIP mRNA in the SCN was elevated the following morning at ZT6 when compared to adrenalectomised rats kept on 0.9% saline. These results suggest that glucocorticoids influence the expression of vasopressin during a narrow window of time in the diurnal cycle coinciding with the time where entrainment of the circadian pacemaker with non-photic cues is possible. Constantly elevated levels of glucocorticoids may also interfere with the suprachiasmatic expression of VIP mRNA which is thought to be driven by photic cues.