Pregastric and postabsorptive inhibitory effects of water on angiotensin-induced Fos in rat brain.

Intraventricularly (ICV)-injected Ang II activates cells in several forebrain structures of rats, including SFO, OVLT, PVN and SON, and produces a spectrum of physiological and behavioral responses. It has been reported that the Fos-like immunoreactivity (FLI; a marker of cell activation) that is induced in PVN and SON by exogenous Ang II is prevented by allowing rats to ingest water after injection. The present study tests whether this inhibitory effect of water is due to either preabsorptive (oral) or postingestive factors, or both. Rats received cerebroventricular injection of Ang II (100 ng) and were killed 1 h later. One group received no water after the injection, another group drank water, a third group drank water but had it drain from a gastric fistula (sham drinking), and a fourth group received an intragastric infusion of water. In confirmation of a previous study, rats allowed to drink water after Ang II showed an almost complete absence of FLI in SON and PVN, compared with those not allowed access to water. Both sham drinking and intragastrically-infused rats showed less FLI in SON and PVN than nondrinkers. The reduction was greater in the infused group than the sham drinkers, but was not as great as in those allowed to drink water and absorb it. This suggests that both orally-mediated as well as postabsorptive factors can inhibit Ang II-induced FLI by impinging upon cells in the SON and PVN. In contrast, Ang II-induced FLI in OVLT and SFO was not affected by any of the oral or gastric water conditions.