Anatomical and functional demonstration of a multisynaptic suprachiasmaticnucleus adrenal (cortex) pathway

In view of mounting evidence that the suprachiasmatic nucleus (SCN) is dire ctly involved in the setting of sensitivity of the adrenal cortex to ACTH, the present study investigated possible anatomical and functional connectio ns between SCN and adrenal. Transneuronal virus tracing from the adrenal re vealed first order labelling in neurons in the intermedio-lateral column of the spinal cord that were shown to receive an input from oxytocin fibres a nd subsequently second-order labelling in neurons of the autonomic division of the paraventricular nucleus. The latter neurons were shown to receive a n input from vasopressin or vasoactive intestinal peptide (VIP) containing SCN efferents. The true character of this SCN input to second-order neurons was also demonstrated by the fact that third-order labelling was present w ithin the SCN, vasopressin or VIP neurons. The functional presence of the S CN-adrenal connection was demonstrated by a light-induced fast decrease in plasma corticosterone that could not be attributed to a decrease in ACTH. U sing intact and SCN-lesioned animals, the immediate decrease in plasma cort icosterone was only observed in intact animals and only at the beginning of the dark period. This fast decrease of corticosterone was accompanied by c onstant basal levels of blood adrenaline and noradrenaline, and is proposed to be due to a direct inhibition of the neuronal output to the adrenal cor tex by light-mediated activation of SCN neurons. As a consequence, it is pr oposed that the SCN utilizes neuronal pathways to spread its time of the da y message, not only to the pineal, but also to other organs, including the adrenal, utilizing the autonomic nervous system.