Diurnal rhythm of neuropeptide Y-like immunoreactivity in the suprachiasmatic, arcuate and paraventricular nuclei and other hypothalamic sites.

The diurnal rhythm of neuropeptide Y (NPY)-like immunoreactivity was examined in 9 discrete hypothalamic sites of rats maintained on a 12:12 h light/dark cycle. Significant bimodal rhythms of NPY concentration were detected in the suprachiasmatic and arcuate nuclei, with significant peaks just prior to onset of the nocturnal period and also at onset of the light period. In the parvocellular division of the paraventricular nucleus, a unimodal NPY peak was observed prior to dark onset. No diurnal rhythm was seen in the magnocellular division of the paraventricular nucleus, nor in 5 other hypothalamic areas examined.