Involvement of the suprachiasmatic nucleus in diurnal ACTH and corticosterone responsiveness to stress

We explored the contribution of the suprachiasmatic nucleus (SCN) in ACTH a nd corticosterone (CORT) diurnal responsiveness of the rat to restraint str ess applied either in the morning (AM) or in the evening (PM). Ablation of the SCN caused the diurnal rhythmicity of the CORT response to disappear bu t had no effects on AM vs. PM differences in the ACTH response. Stress-resp onse curves in SCN-lesioned rats that had prestress levels of CORT either i n the AM range or in the PM range, when compared with those obtained for AM and PM controls, showed that the SCN differentially regulates the stress r esponse depending on the underlying secretory activity of the adrenal corte x. When basal CORT secretion is at its lowest, the SCN inhibits CORT respon siveness to stress by controlling pituitary corticotrophs; but when it is a t its highest, it has a permissive action that will bypass the hypophysis a nd reach the adrenals to adjust the response of the gland to ACTH.