Diurnal changes in paraventricular hypothalamic alpha(1) and alpha(2)-adrenoceptors and food intake in rats

The prominent feeding rhythm evident in rats may reflect circadian variatio n in activity of feeding-relevant adrenoceptors within the hypothalamic par aventricular nucleus (PVN). In the present study, separate groups of rats w ere sacrificed at six time points (ZT0, ZT4, ZT8, ZT12, ZT16, ZT20) over a diurnal cycle. Food intakes were recorded during the 4-h period prior to sa crifice in each group. Brain sections were incubated with either an alpha(1 )-adrenoceptor ligand (H-3)-prazosin [(H-3)-PRZ] or an alpha(2)-adrenocepto r ligand (H-3) para-aminoclonidine [(H-3)-PAC] prior to autoradiography ana lyses. Binding of (H-3)-PRZ within the PVN varied as a function of the diur nal cycle, with significantly greater binding evident during the light phas e of ZT0 (first 4 h of the light phase) and at ZT4, compared to nadir bindi ng during the dark phase at ZT16 (first 4 h of the dark phase). Binding of (H-3)-PAC within the PVN also varied as a function of the diurnal cycle, wi th significantly greater binding evident during the first 8 h of the dark p hase (ZT16 and ZT20) than during the light phase. Food intake and alpha(1)- adrenergic binding were inversely related across the diurnal cycle. These r esults support the hypothesis that PVN adrenergic systems may be organized in an antagonistic fashion so as to modulate feeding in the rat. (C) 1999 E lsevier Science Inc.