Action of angiotensin converting enzyme inhibitors in rat brain: interaction with isoproterenol assessed by Fos immunocytochemistry.

Low and high doses of angiotensin converting enzyme (ACE) inhibitors have been shown either to enhance or suppress, respectively, the water intake of rats induced by acute administration of isoproterenol. In order to assess the role and sites of action of angiotensin II (Ang II) in this dual action of ACE inhibitors, rats were administered either low or high doses of enalapril or captopril, followed by isoproterenol, and were sacrificed 1 h later for determination of Fos-like immunoreactivity (FLI) in brain. Isoproterenol induced strong FLI in the magnocellular paraventricular (PVN) and supraoptic (SON) nuclei, and moderate staining along the structures of the rostral wall of the lamina terminalis (LT). Low doses of ACE inhibitors either had no effect or slightly increased FLI along the LT following isoproterenol. Enalapril reduced FLI in some other regions, including the parvocellular PVN. In contrast, high doses of ACE inhibitors abolished FLI along the LT, and reduced FLI in the PVN and SON. Captopril, but not enalapril, induced some FLI in the LT, SON and PVN. The data are discussed in terms of access of ACE inhibitors to the brain, and interactions with structures involved in Ang-related water intake.