Calbindin-D 28K -immunoreactive cells are tightly packed within a discrete region of the caudal aspect of the suprachiasmatic nuclei of hamsters. These cells receive direct retinal input and are Fos-positive in response to a light pulse. Knowledge of their afferent and efferent connections is necessary to understand suprachiasmatic nucleus organization. The first aim of the present study is to identify interconnections between calbindin and other peptidergic cells of the suprachiasmatic nuclei, using epi-and confocal microscopy and intra-suprachiasmatic nucleus tract tracing. The results indicate that essentially all calbindin cells receive numerous appositions from vasoactive intestinal polypeptide (VIP), neuropeptide Y and serotonin fibers and that most receive appositions from gastrin releasing peptide (GRP) and cholecystokinin (CCK) fibers. Reciprocal connections are seen from VIP, GRP and CCK cells but surprisingly, not from dorsomedial vasopressin cells. Injection of biotinylated dextran amine into the suprachiasmatic nucleus indicates that the ventrolateral suprachiasmatic nucleus projects to the entire nucleus, while the dorsal and medial regions of the suprachiasmatic nucleus project densely to most of the nucleus, except to the calbindin region. Analysis of colocalization of the peptides in the calbindin cell region shows that 91% of the substance P cells, 42% of the GRP cells and 60% of the VIP cells in the calbindin subnucleus coexpress calbindin-D 28K .Our results reveal a highly specialized topographical organization of connections among suprachiasmatic nucleus cells. Keywords suprachiasmatic nucleus; calbindin; vasopressin; confocal microscopy; tract tracingThe suprachiasmatic nuclei (SCN) are the locus of the brain clock regulating circadian rhythms in mammals (reviewed in Silver and Moore, 1998). In rodents, these paired hypothalamic nuclei are each composed of about 8000 neurons that are heterogeneous in their content of neuronal antigens (van den Pol, 1980;van den Pol and Tsujimoto, 1985). Some of the neuropeptides that have been identified to date in the rodent SCN are vasopressin (AVP), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine, gastrin releasing peptide (GRP), sub-stance P (SP), cholecystokinin (CCK) and somatostatin. We have reported that a subregion in the caudal SCN is characterized by the (Silver et al., 1996). These cells are densely packed, receive direct retinal synaptic input (Bryant et al., 2000) and are responsive to photic stimuli, with about 80% of the cells showing Fos-like immunoreactivity (ir) following a light pulse given during subjective night (Silver et al., 1996). The foregoing evidence indicates these CalB cells are on the input pathway for photic stimuli reaching the SCN.The function of this subregion of the SCN has been examined in hamsters. Animals with lesions that destroyed the CalB subnucleus, but spared other compartments of the SCN, lost their locomotor activity rhythm. Animals with partial bilateral SCN lesions sparing part ...

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