Food-restriction interacts with vasopressin modulation of activity

The effects of peripheral injection of various doses of lysine-vasopressin (LVP), administered 30 min before a 5-min session in a hole-board apparatus, were compared as a function of food restriction. Comparison of performance for various indices of general activity clearly showed that the food-restricted rats were more active and exhibited less photophobia than normally fed ones. The differences between the two groups were maintained in a second session 24 hours later. There was no sign of behavioral habituation to the apparatus among restricted animals. Different doses (0.2, 1, 2 micrograms of LVP) affected food-restricted animals differently from the rats fed ad lib. Only the highest dose reduced activity in both groups. A posttest injection of the smallest dose (0.2 micrograms) had an opposite effect on the activity in the hole-board, measured 24 hours after the injection. A second experiment showed that plasma and adrenal corticosterone were higher in deprived rats. The administration of 0.2 micrograms of LVP was followed by an increase in corticosterone. In food-restricted rats this increase was bigger and was still observed 24 hours after the injection. There is an interaction between feeding conditions and LVP injections which affects both the internal hormonal state and spontaneous reactivity to environment. These findings are of relevance to the effect of vasopressin on behavior.