Roles of paraventricular catecholamines in feeding-associated corticosterone rhythm in rats

Effects of local destruction of the brain catecholaminergic neurons were examined on the light- and feeding-associated circadian rhythms in plasma corticosterone in rats. 6-Hydroxydopamine (6-OHDA), a selective and long-lasting neurotoxin of the catecholaminergic neurons, was microinjected into the following discrete areas of the brain: the paraventricular nucleus (PVN), median eminence (ME), suprachiasmatic nucleus (SCN), ventromedial hypothalamic nucleus (VMH), lateral hypothalamic nucleus (LH), and the ascending bundle of noradrenergic neurons (NAB). And the feeding-associated as well as the light-associated circadian rhythms in plasma corticosterone were determined. The light-associated circadian rhythm was assayed under a 24-h light-dark cycle with free access to food, whereas the feeding-associated circadian rhythm was measured under restricted daily feeding in which rats had free access to food at a fixed time of day. 6-OHDA reduced the norepinephrine concentrations in respective regions to 10-30% of the control value, except for the LH. The light-associated circadian rhythm was not affected by 6-OHDA into the SCN or PVN. By contrast, 6-OHDA into the PVN or ventral NAB suppressed the feeding-associated circadian peak. 6-OHDA into the VMH and LH showed some effects on plasma corticosterone level but not on the feeding-associated circadian rhythm. 6-OHDA had no systematic effect on plasma corticosterone level when injected into the SCN, ME, and dorsal NAB. These findings indicate that the catecholaminergic neurons projecting to the PVN are involved in the feeding-associated but not in the light-associated circadian rhythms.

[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.