Localization of Central Angiotensin II Receptors with [125I]-sar1, ile8-Angiotensin II: Periventricular Sites of the Anterior Third Ventricle

Abstract

The radiolabeled angiotensin II (ANG II) antagonist, [¹²⁵I]-sar¹, ile⁸-ANG II, was used to study brain ANG II receptors by both homogenate binding and in vitro autoradiography. In homogenate preparations of the hypothalamus, thalamus, septum and midbrain (HTSM), [¹²⁵I]-sar¹ ile⁸-ANG II bound to a single class (Hill slope 0.84 ± 0.05) of high affinity binding sites (K_D 0.42 ± 0.03 nM, B_max 5.98 ± fmol/mg protein). Competition for the [¹²⁵I]-sar¹ ile⁸-ANG II binding site in HTSM membranes demonstrated a rank order potency characteristic of binding to the ANG II receptor, with the unlabeled antagonist being slightly more potent than ANG II (Ki 0.22 ± 0.03 vs. 0.95 ± 0.06 nM, respectively). Brain slices from the region of the rostral third ventricle were incubated with 0.5 nM [¹²⁵I]-sar¹, ile⁸-ANG II in the presence or absence of 1 µM ANG II and exposed to LKB Ultrofilm. Autoradiographic images of [¹²⁵I]-sar’, ile⁸-ANG II binding revealed that structures situated within the anterior wall of the third ventricle, i.e. the lamina terminalis, were heavily labeled; including the subfornical organ, median preoptic nucleus and organum vasculosum laminae terminalis. These results show the utility of [¹²⁵I]-sar¹, ile⁸-ANG II as a probe to study brain ANG II receptors and provides pharmacological evidence for the rostral third ventricle as a possible site for central ANG II actions.