Fig. 2.

Schematic diagram of the ventral core/dorsal shell conceptualization of the organization of the suprachiasmatic nucleus illustrating the anatomical and functional heterogeneity of the nucleus. Major afferent pathways including the retinohypothalamic tract (RHT), geniculohypothalamic tract (GHT), and a projection from the raphe nucleus (Raphe) terminate primarily in the ventral core, although RHT terminals can also be found in the dorsal shell. Neurons in the ventral core contain GABA and a variety of neuropeptides including vasoactive intestinal peptide (VIP) and gastrin releasing peptide (GRP), all of which are frequently colocalized in the same neuron. Some neurons in the ventral core display endogenous rhythmicity (~) while others do not (−). In the dorsal shell, more neurons display endogenous rhythmicity. Neurons in the dorsal shell contain GABA and neuropeptides including arginine-vasopressin (AVP), all of which are frequently colocalized in the same neuron. Neurons in the dorsal shell and ventral core can communicate via GABA and probably other neurochemical signals.