Losartan blocks drinking and cFos expression induced by central ornithine vasotocin in rats

We previously reported that an intracerebroventricular (icv) injection of the oxytocin receptor antagonist ornithine vasotocin (OVT) caused water and saline intakes, a pressor response, and Fos-like immunoreactivity (Fos-IR) in the median preoptic nucleus of the rat brain. In the present report, rats receiving an icv injection of isotonic saline vehicle followed by an icv injection of 10 μg of OVT 20 min later drank 5.5 ± 1.1 ml of total water and saline intake in 60 min after the OVT; rats receiving 10 μg of losartan before the OVT drank only 0.9 ± 0.3 ml of total fluid. In a separate study, rats were treated as above except that they were not allowed to drink and were perfused for analysis of Fos-IR in the median preoptic nucleus at 90 min. Fos-IR in the dorsal part of the median preoptic nucleus was significantly suppressed from 2.69 ± 0.57 cells per 10,000 square μm in vehicle-treated rats to 0.89 ± 0.20 in losartan-treated rats. Losartan alone had no effect on Fos-IR. Losartan did not reduce intake of saccharin in a dessert test. This suggests that the OVT-induced drinking may result from an activation or disinhibition of angiotensin type AT1 receptors in the median preoptic nucleus.