Expression of Fos in the SON and PVN of rats pretreated with l-NNA or vehicle, followed by intraperitoneal injection of either isotonic saline or 2 or 4 ml/kg 1.5m NaCl (A). Note that a significant increase in the number of Fos-positive neurons was observed in the SON of l-NNA-pretreated rats compared with vehicle-pretreated rats after intraperitoneal administration of the higher dose of hypertonic saline. Data are means ± SEM; numbers of rats areabove bars; *p < 0.05 compared with controls. B–E, Photomicrographs of Fos expression 90 min after intraperitoneal injection of 4 ml/kg 1.5 m NaCl in the PVN (B, C) and SON (D, E) in l-NNA-pretreated rats (C, E) and vehicle-pretreated controls (B, D). Mp, Medial parvocellular; Pm, posterior magnocellular;OC, optic chiasm; V3, third ventricle. Scale bars, 100 μm.