Prefeeding increase in paraventricular NE release is regulated by a feeding-associated rhythm in rats

Extracellular norepinephrine (NE) in the vicinity of the paraventricular nucleus was continuously measured over 24 h by means of in vivo microdialysis in rats under restricted daily feeding and ad libitum feeding. A 24-h rhythm in plasma corticosterone was monitored in separate rats under identical conditions. Under ad libitum feeding, the paraventricular NE showed a 24-h rhythm with lower levels in the light period and higher levels in the dark period. The temporal pattern was not affected by food deprivation for 3 days. The circadian peak of paraventricular NE lagged slightly behind that of plasma corticosterone under ad libitum feeding. On the other hand, when rats were fed for 2 h at a fixed time of day for 3 wk, the NE rhythm was changed and showed a major peak just before daily meals. To examine the nature of the prefeeding NE peak, a 24-hour variation of paraventricular NE was measured in rats under food deprivation that had been subjected to restricted daily feeding for 3 wk and subsequently to ad libitum feeding for 7 days. The paraventricular NE was high around the time when meal had been supplied under restricted daily feeding and low in the dark period. Similar changes were observed in plasma corticosterone. The findings indicate that the prefeeding increase in the paraventricular NE release is regulated by a feeding-associated circadian rhythm manifest in rats under restricted daily feeding.