VIP binding sites in adult rat hypothalamus: nuclear distribution and daily variations

Vasoactive intestinal peptide (VIP) neurons are present in the suprachiasmatic nucleus (SCN) and project to various hypothalamic nuclei. Several results suggest that hypothalamic VIP is involved in the mediation of synchronizing information. [125I]Vasoactive intestinal peptide binding has been reported to be especially high in the hypothalamus. We show here by radioautography by dipping that [125I]VIP binding is high and homogeneously distributed in the SCN. The correspondence between VIP binding and terminals densities supports the hypothesis that VIP acts through synapses in the SCN. Moderate and fairly homogeneous VIP binding is also found in the paraventricular nucleus (PaVN), whereas VIP terminals are reported to be restricted to the sub-paraventricular area. This suggests that VIP could diffuse and thus function as a paracrine neuromodulator in the PaVN. [125I]Vasoactive intestinal peptide binding is stable along the nycthemer in the suprachiasmatic, peri-, and paraventricular nuclei as well as in all other areas investigated.