Hypothalamic norepinephrine may modulate normal eating behavior through activation of alpha 2-noradrenergic receptors, localized in the paraventricular nucleus (PVN). We investigated whether these receptors, which stimulate food ingestion, may in turn be altered by the nutritional state of the organism. Thus the impact of food deprivation, on the specific binding of [3H]-p-aminoclonidine ([3H]PAC) to alpha 2-noradrenergic receptors in discrete hypothalamic areas, was examined in rats. The results of our first experiment revealed that 48 hr food deprivation reduced (by 50%) the maximum number of binding sites (Bmax) of the high affinity component of [3H]PAC binding to alpha 2 receptors. This effect occurred exclusively in the medial hypothalamus (which includes the PVN), without any change in the affinity (Kd) of these receptors. A smaller decline was seen in the low affinity binding sites of the medial hypothalamus, whereas no changes were observed in the density or affinity of the high and low affinity alpha 2 receptor sites in the lateral hypothalamus or frontal cortex. The alpha 1-noradrenergic receptor sites, as defined by [3H]prazosin and [3H]WB-4101 binding, were also unaffected in the different brain areas by 48 hr food deprivation. An additional analysis of alpha 2 receptors in discrete hypothalamic nuclei demonstrated that the deprivation-induced decline in alpha 2-receptor binding: occurred specifically in the PVN; was apparent after as little as 3 hr food deprivation; and occurred only when this brief deprivation fell at the onset of the dark cycle, as opposed to at the end of the dark cycle.(ABSTRACT TRUNCATED AT 250 WORDS)