Central angiotensin II induces thirst-related responses in an amphibian.

Angiotensin II (A-II), a potent inducer of thirst-related behavior in many vertebrate species, was injected into the third ventricle of the brain of the spadefoot toad, Scaphiopus couchii. Following injection of 10 ng A-II the animals demonstrated a significant increase in water absorption response (WR) behavior, in which toads press their ventral skin to a moist surface and absorb water by osmosis. This increase in the frequency of WR behavior was positively correlated with an increase in water gain during a 2-hr period indicating that centrally injected A-II stimulates water intake by this amphibian species. We have previously demonstrated that WR behavior is also induced by intraperitoneal (i.p.) injections of A-II in several anuran species, including S. couchii. Thus, amphibians, like other vertebrates, demonstrate an increase in water intake in response to either centrally administered or circulating A-II. A second series of experiments was conducted to determine whether the above response to A-II might be secondary to increases in the circulating levels of aldosterone (ALDO) or antidiuretic hormone because the release of both of these hormones has been shown by others to be stimulated by A-II. Scaphiopus couchii injected i.p. with either ALDO or arginine vasotocin in dosages of 1, 10, and 100 micrograms/100 g body weight showed no increase in WR behavior relative to toads injected with saline alone. These results suggest that A-II acts directly on the brain of S. couchii to induce WR behavior.