Projections from the parvocellular vasopressin‐ and neurophysin‐containing neurons of the suprachiasmatic nucleus

It is concluded that vasopressin and neurophysin from parvocellular SCN neurons are not released into the bloodstream, but may affect neurons in the projection areas described.

Abstract

Abstract The efferent projections of the parvocellular vasopressin‐ and neurophysin‐containing neurons of the suprachiasmatic nucleus (SCN) have been investigated in the mouse, rat and guinea pig, using specific antisera to vasopressin and neurophysin in the unlabelled antibody‐enzyme immunoperoxidase method. In all three species, parvocellular perikarya containing vasopressin and neurophysin were found in the SCN, primarily in the medial, dorsal and rostral part of the nucleus. The fine‐caliber vasopressin‐ and neurophysin‐containing fibers arising from SCN neurons can easily be distinguished from the large‐caliber vasopressin and neurophysin fibers of magnocellular neurons which pass from the supraoptic and paraventricular nuclei primarily to the neural lobe of the pituitary, but also to extrahypothalamic sites. Fine‐caliber fibers leave the SCN in various directions to form several pathways. The major projections run (1) rostrodorsally to the lateral septum, and (2) dorsally to the medial dorsal thalamus and lateral habenula. Smaller projections course rostrally to the nucleus of the diagonal tract (of Broca), and dorsocaudally to the posterior hypothalamus and interpeduncular nucleus. In addition, many fibers of the dorsal thalamic projection continue beyond the lateral habenulae through the central grey of the mesencephalon to the area of the nucleus of the solitary tract. In all target areas, fibers were observed only in distinct portions of the nuclei and were not distributed to the whole nucleus. None of the fine‐caliber fibers from SCN neurons project to, or even towards, the median eminence and neural lobe. No oxytocin‐positive SCN neurons or fine‐caliber fibers could be found. In Brattleboro rats, with hereditary vasopressin deficiency, no vasopressin‐ or neurophysin‐positive SCN neurons or fine‐caliber fibers could be found. In contrast to vasopressin and neurophysin fibers from magnocellular neurons of the supraoptic and paraventricular nuclei, fine‐caliber vasopressin and neurophysin fibers from SCN neurons do not terminate at capillaries. A large number of these fibers make axo‐somatic contacts with neurons in the projection areas. It is concluded that vasopressin and neurophysin from parvocellular SCN neurons are not released into the bloodstream, but may affect neurons in the projection areas described.