THE DIURNAL EXPRESSION OF GENES ENCODING VASOPRESSIN AND VASOACTIVE-INTESTINAL-PEPTIDE WITHIN THE RAT SUPRACHIASMATIC NUCLEUS IS INFLUENCEDBY CIRCULATING GLUCOCORTICOIDS

The mammalian suprachiasmatic nucleus (SCN) is the endogenous pacemake r generating the diurnal rhythm of the stress hormones ACTH and glucoc orticoid secretion. In the present study, we have employed male rats e ntrained to a 12:12 h (light:dark) photoperiod To investigate the effe cts of chronic and acute administration of exogenous glucocorticoids u pon the diurnal expression of vasopressin and vasoactive intestinal pe ptide (VIP) mRNA in the SCN by semiquantitative in situ hybridization histochemistry. Chronic administration of exogenous glucocorticoids si gnificantly enhanced vasopressin mRNA expression only at zeitgeber tim e (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 sup plement of dexamethasone in their drinking water, the expression of bo th vasopressin and VIP mRNA in the SCN was elevated the following morn ing at ZT6 when compared to adrenalectomised rats kept on 0.9% saline. These results suggest that glucocorticoids influence the expression o f vasopressin during a narrow window of time in the diurnal cycle coin ciding with the time where entrainment of the circadian pacemaker with non-photic cues is possible. Constantly elevated levels of glucocorti coids may also interfere with the suprachiasmatic expression of VIP mR NA which is thought to be driven by photic cues.