Suprachiasmatic nucleus in the mouse: retinal innervation, intrinsic organization and efferent projections

The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker of t he mammalian circadian timing system. The SCN is composed of two anatomical ly and functionally distinct subdivisions, designated core and shell, which can be distinguished on the basis of their chemo architecture and connecti ons in the rat. In the present study, we examine the intrinsic organization and the afferent and efferent connections of the mouse SCN using immunocyt ochemistry and ocular injections of cholera toxin. Neurons of the SCN shell contain GABA, calbindin (CALB), arginine vasopressin (AVP), angiotensin H (AH) and met-enkephalin (mENK), and receive input from galanin (GAL) and va soactive intestinal polypeptide (VIP) immunoreactive fibers. Neurons of the SCN core synthesize GABA, CALB, VIP, calretinin (CALR), gastrin releasing peptide (GRP), and neurotensin (NT), and receive input from the retina and from fibers that contain neuropeptide Y (NPY) and 5-hydroxytryptamine (5HT) . Fibers projecting from SCN neurons that are immunoreactive for AVP and VI P exhibit a characteristic morphology, and project to the lateral septum, a series of medial hypothalamic areas extending from the preoptic to the pos terior hypothalamic area and to the paraventricular thalamic nucleus. The o rganization of the mouse SCN, and its connections, are similar to that in o ther mammalian species. (C) 2001 Elsevier Science B.V. All rights reserved.