Do peripheral and cerebroventricular injections of angiotensin II act at the same site? Studies on additivity of drinking.

The urge of rats to drink following SC injections of angiotensin II (AII) far outlasts the relatively short half life of this hormone in the circulation. When water was withheld for 1 hr following SC administration of AII, water intake was similar to that of rats administered AII and given immediate access to water. Saralasin, the AII receptor antagonist, blocked this delayed drinking response, suggesting that AII receptor occupancy is critical to the appearance of the delayed drink. In contrast, the urge to drink following cerebroventricular (IVT) injection decayed within 30 min. Intravenous (IV) infusion of AII for 1 hr caused drinking during the infusion but, when water was withheld until the infusion had ended, the urge to drink decayed within 15-30 min. Additional studies tested the additivity of the drinking response to two separate doses of AII when administered peripherally and IVT. The combined administration of AII SC and IVT failed to produce clear signs of additivity over a large range of interstimulus intervals. In contrast, the combined administration of IV and IVT AII were clearly additive with respect to drinking.