HYPOTHALAMIC NEUROPEPTIDE-Y AND ITS GENE-EXPRESSION - RELATION TO LIGHT DARK CYCLE AND CIRCULATING CORTICOSTERONE/

The hypothalamic neuropeptide Y (NPY) system, along with levels of cir culating corticosterone (CORT), were examined in rats at different tim es across the light/dark cycle. Tissue samples were taken from the med iobasal hypothalamus (MBH), which contains the primary hypothalamic NP Y cell group of the arcuate nucleus (ARC), and the mediodorsal (MDH) h ypothalamus, which contains the paraventricular and dorsomedial nuclei that receive a dense NPY innervation from the ARC. In these dissectio ns, measurements of NPY mRNA and peptide levels were taken using a sol ution hybridization/nuclease protection assay procedure and radioimmun oassay. The results demonstrate that (i) NPY mRNA levels in the MBH, b ut not MDH, vary significantly in relation to the light/dark cycle, sh owing a sharp rise 4-6 h before dark onset, sustained high levels over the next 3-4 h and then, a sharp decline 1 h before dark onset; (ii) this rise in NPY mRNA in the MBH before dark onset, while associated w ith stable levels of MBH NPY during this time, is followed 2-4 h later , around dark onset, by a rise in NPY peptide levels of the MDH simult aneous to a decrease in NPY levels of the MBH; (iii) levels of circula ting CORT shift dramatically across the light-dark cycle, exhibiting a n increase from basal levels (< 0.3 mu g/dl) to 5 mu g/dl approximatel y 4 h before dark onset, a further rise that peaks at 26 mu g/dl aroun d dark onset, and then a significant decline to 16 mu g/dl at 2 h afte r dark onset; and (iv) there exists a positive relationship between CO RT and NPY mRNA or peptide levels in the MBH during the 4-6 h before d ark onset, while in the MDH, a positive relationship between this ster oid and NPY peptide levels is obtained at dark onset. It is proposed t hat these rhythms, involving a predark rise in CORT and NPY gene expre ssion leading to a peak in CORT and peptide levels at dark onset, are active in stimulating feeding behavior, particularly carbohydrate inge stion, which predominates at that time. (C) 1994 Academic Press, Inc.