The interaction of vasopressin and the photic oscillator in circadian rhythms

Telemetered body temperature (BT), heart rate (HR), and activity (AC) data were collected in vasopressin-containing Long-Evans (LE) and vasopressin-deficient Brattleboro (DI) rats. The rats were exposed to a 12/12 h light/dark cycle under three conditions: 1) ad lib feeding throughout the 24-h cycle, 2) two scheduled-feeding periods during the diurnal component of the light/dark cycle, and 3) two scheduled-feeding periods during the nocturnal component of the light/dark cycle. With ad lib feeding, natural nocturnal cycles of BT, HR, and AC were maintained in both strains. Marked changes were observed under the condition of scheduled feeding during the diurnal component of the light/dark cycle. In DI animals the influence of the photic oscillator was lost and BT, HR, and AC shifted from nocturnal to diurnal patterns. Circadian rhythms in DI animals were now synchronized by the nonphotic zeitgeber of scheduled food presentation. On the other hand, LE animals lost a well-defined circadian rhythmicity resulting from adherence to the photic oscillator, while at the same time being influenced by the nonphotic oscillator. Under the condition of scheduled feeding during the nocturnal component of the light/dark cycle, the circadian rhythms were similar in DI and LE rats. Results show that vasopressin has a significant interaction with the photic oscillator, which is obvious only when the photic and nonphotic oscillators are uncoupled. In addition, the results demonstrate that the strength of the photic oscillator is decreased or that the effect of this oscillator is masked or lost in DI rats compared to LE rats.