Physiological role of brain angiotensin III in drinking behavior in the rat

We examined the physiologic role of endogenous brain angiotensin III (AIII), an active degradative product of angiotensin II, in drinking behavior. Adult, male spontaneously hypertensive (SH) and Wistar-Kyoto normotensive (WKY) rats that were instrumented with an intracerebroventricular (i.c.v.) cannula connected to an osmotic minipump for chronic infusion were used. 7-day i.c.v. infusion of the specific AIII antagonist, Ile⁷-AIII (10 or 100 pmol/min), resulted in no significant alteration in daily (24 h), diurnal (8:00 a.m.8:00 p.m.) or nocturnal (8:00 p.m.-8:00 a.m.) basal water intake in both SH and WKY rats. Similar results were obtained with i.c.v. infusion of the amino-peptidase inhibitor, bestatin (150 or 300 pmol/min), given alone or simultaneously with Ile⁷-AIII (10 pmol/min). Rats that were water-deprived for the first 3 days of 7-day infusion of Ile⁷-AIII consumed significantly less water during the first 2 h after water became available. Furthermore, the accumulated water intake during the first 24 h was appreciably greater in SH than WKY rats. We interpret these results to suggest that the endogenous brain AIII may not be tonically involved in fluid homeostasis. Instead, it must be activated under conditions of dehydration, such as water deprivation, particularly in the SHRs, to initiate drinking behavior.