Neuropeptide Y receptor distribution and regulation in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus).

The suprachiasmatic nucleus (SCN) receives a direct photic projection from the retina, the retinohypothalamic tract (RHT), and an indirect photic projection from the intergeniculate leaflet of the thalamus, the geniculohypothalamic tract (GHT). The primary neurochemical signal in the GHT appears to be neuropeptide Y (NPY), and several lines of evidence indicate that NPY may be involved in determining the response of the SCN to light. The purpose of the present study was (i) to localize NPY binding sites in the hamster SCN and to compare the distribution of these binding sites with the terminal field of the RHT and (ii) to determine if SCN levels of NPY binding change during the day. RHT fibers, defined using the carbocyanine dye DiI, were localized primarily within the ventrolateral region of the SCN. The distribution of NPY receptors, as visualized by 125I-peptide YY (PYY) binding, overlapped the projection field of the RHT. Specific binding of 125I-PYY was significantly greater in the ventrolateral SCN than in the medial SCN. To determine whether NPY binding changes during the day, the levels of 125I-PYY in the SCN were determined 4 h before the onset of darkness, 1 h before the onset of darkness and 2 h after the onset of darkness in hamsters housed in a 14:10 light-dark cycle. The levels of binding at 4 and 1 h before dark onset were significantly lower than 2 h after the onset of darkness. In contrast, no significant differences were observed in 125I-PYY binding over these same sampling intervals in either the medial preoptic area or the lateral hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)