Changes in feeding behavior and plasma vasopressin concentration during water deprivation in goats

To further characterize the suppression of feeding that normally accompanies water deprivation and to test whether vasopressin contributes to this hypophagia, food intake, meal patterns and plasma vasopressin concentrations were measured during 24 h or 72 h of water deprivation in pygmy goats. The effects of exogenous vasopressin and of a V1-receptor antagonist on feeding during water deprivation were also assessed. The hypophagia during water deprivation was primarily due to decreases in meal size. The plasma vasopressin concentration increased about 2.5-fold and 10-fold after 24 and 72 h of water deprivation, respectively. Plasma osmolality also increased (measured only after 72 h of water deprivation). Intraperitoneally (ip) injected vasopressin (1.5 micrograms/kg b. wt.) that previously reduced food intake in goats with ad lib, access to water (Meyer et al., 1989), failed to affect cumulative food intake in water deprived goats, but led to a transient increase in meal size. The V1-receptor antagonist (2.5 micrograms/kg b. wt., ip) did not affect cumulative food intake or meal patterns either. These findings indicate that endogenous vasopressin is not crucially involved in the hypophagia during water deprivation. The results are in line with the hypothesis that an abnormal prandial increase in the osmolality of the ruminal fluid is a major contributor to the hypophagia during water deprivation.