Isoforms of the NRG-1 gene have been shown to be important in the expression of specific ion channels and receptors, as well as in the maintenance of synapses in the peripheral nervous system (Buonanno & Fischbach, 2001). One of the many splice variants, CRD-NRG (a.k.a. TYPE III NRG-1), appears to be prominently expressed in select regions of the CNS (see Ramirez et al, SFN abstracts 2002). We have used in situ hybridization and immunohistochemical techniques to assess the distribution of the CRD-NRG expression in the input and target neurons of the SCN. CRD-NRG is highly expressed in retinal ganglion cells and in the paraventricular nucleus of the hypothalamus (PVN), as well as in the SCN per se. Immunohistochemical analysis with probes to the proposed external and cytoplasmic domains of the A forms of NRG-1 reveals punctate staining surrounding subsets of SCN neurons. In particular, these include cells in the retino-recipient area of the SCN, as defined by the presence of vasoactive intestinal polypeptid...