Diurnal changes in paraventricular hypothalamic alpha1 and alpha2-adrenoceptors and food intake in rats

The prominent feeding rhythm evident in rats may reflect circadian variation in activity of feeding-relevant adrenoceptors within the hypothalamic paraventricular nucleus (PVN). In the present study, separate groups of rats were 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 sacrifice in each group. Brain sections were incubated with either an alpha1-adrenoceptor ligand (3H)-prazosin [(3H)-PRZ] or an alpha2-adrenoceptor ligand (3H) para-aminoclonidine [(3H)-PAC] prior to autoradiography analyses. Binding of (3H)-PRZ within the PVN varied as a function of the diurnal cycle, with significantly greater binding evident during the light phase of ZT0 (first 4 h of the light phase) and at ZT4, compared to nadir binding during the dark phase at ZT16 (first 4 h of the dark phase). Binding of (3H)-PAC within the PVN also varied as a function of the diurnal cycle, with significantly greater binding evident during the first 8 h of the dark phase (ZT16 and ZT20) than during the light phase. Food intake and alpha1-adrenergic binding were inversely related across the diurnal cycle. These results support the hypothesis that PVN adrenergic systems may be organized in an antagonistic fashion so as to modulate feeding in the rat.