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Abstract
The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker of the mammalian
circadian timing system. The SCN is composed of two anatomically and functionally
distinct subdivisions, designated core and shell, which can be distinguished on the
basis of their chemoarchitecture and connections in the rat. In the present study,
we examine the intrinsic organization and the afferent and efferent connections of
the mouse SCN using immunocytochemistry and ocular injections of cholera toxin. Neurons
of the SCN shell contain GABA, calbindin (CALB), arginine vasopressin (AVP), angiotensin
II (AII) and met-enkephalin (mENK), and receive input from galanin (GAL) and vasoactive
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 VIP exhibit a characteristic morphology, and project to
the lateral septum, a series of medial hypothalamic areas extending from the preoptic
to the posterior hypothalamic area and to the paraventricular thalamic nucleus. The
organization of the mouse SCN, and its connections, are similar to that in other mammalian
species.