Effects of SFO lesion or captopril on drinking induced by intragastric hypertonic saline

This study examined the hypothesis that the subfornical organ (SFO), a circumventricular organ with both osmosensitive elements and dipsogenic receptors for circulating angiotensin (ANG) II, is important for the water drinking response that follows an intragastric (ig) load of hypertonic NaCl. A 2-ml saline load was administered ig at 300, 900, or 1200 mOsm/kg to rats with sham lesions or lesions of the SFO, and intake was measured periodically for 2 h. Hypertonic loads caused sham-lesioned rats, but not SFO-lesioned rats, to drink earlier in the test or to drink more water than did the isotonic load. Inhibition of ANG II synthesis in unoperated rats with 100 mg/kg of captopril reduced water intake only during the initial 15 min after a gavage of 1200 mOsm/kg saline. Loads of 900 and 1200 mOsm/kg both increased plasma osmolality and sodium concentration by 15 min after gavage without greatly affecting hematocrit or plasma protein concentration. Thus, the SFO is important for the osmotically-induced water drinking response after acute ig administration of hypertonic saline. With the possible exception of the first 15 min, this drinking response is independent of the peripheral synthesis of ANG II.