[The role of central noradrenergic neurons in the hypothalamic-pituitary-adrenocortical axis of rats: with special reference to feeding cycle and biological rhythm].

It is well established that plasma corticosterone levels in rats increase just prior to the meal (prefeeding peak), when animals are subjected to restricted feeding, in which daily food and water supply is restricted to a few hours at a fixed time of day. A self-sustained oscillatory mechanism is suggested underlying the prefeeding hormone peak, but its mechanism is different in origin from that involved in the light-entrainable circadian rhythm; the former dose not depend on the suprachiasmatic nucleus (SCN) but the latter dose. The purpose of the present study is to elucidate roles of the central noradrenergic system in the prefeeding corticosterone peak under restricted feeding. Male rats were bilaterally injected with 6-hydroxydopamine (6-OHDA) into the ventral noradrenergic ascending bundle (VNAB) and hypothalamic nuclei, the SCN, the paraventricular nucleus (PVN), the ventromedial hypothalamic nucleus, the lateral hypothalamic nucleus and the median eminence to examine effects of the catecholamine depletion on the circadian corticosterone rhythm under ad libitum feeding condition and the prefeeding corticosterone peak under restricted feeding. Blood samples were obtained from individual rats successively and plasma corticosterone was determined by competitive protein binding assay. Noradrenaline contents were measured in the individual hypothalamic nuclei by high performance liquid chromatograph to attest the effects of 6-OHDA injection. The prefeeding corticosterone peak was suppressed by 6-OHDA injected into the PVN and VNAB, but was not by 6-OHDA into the other brain regions. On the other hand, the circadian rhythm of plasma corticosterone was not affected by 6-OHDA injection into the PVN. These findings suggested that the noradrenergic projection to the PVN is responsible for the manifestation of the prefeeding corticosterone peak, but is not for the circadian rhythm of plasma corticosterone.