Regulatory Peptides, 44 (1993) 119-130 119 © 1993 Elsevier Science Publishers B.V. All rights reserved 0167-0115/93/$06.00 REGPEP 01297 Distribution of angiotensin II receptor subtypes in the rabbit brain G. Peter Aldred, Siew Yeen Chai, Keifu Song, Jialong Zhuo, Duncan P. MacGregor and Frederick A.O. Mendelsohn University of Melbourne, Department of Medicine, Austin Hospital, Heidelberg (Australia) (Received 20 November 1992; accepted 17 December 1992) Key words: Renin-angiotensin system; In vitro autoradiography; Losartan; Circumventricular organ Summary We have determined the distribution of angiotensin II receptor subtypes in rabbit brain using in vitro auto- radiography. AT1 receptors were found in very high concentrations in the forebrain circumventricular organs - the subfornical organ, organum vasculosum of the lamina terminalis, and the median eminence as observed in other mammals. However, there was very little labeling in the area postrema. In the paraventricular nucleus, median preoptic nucleus, supraoptic nucleus there were high levels of predominantly AT 1 receptors. High densities of AT l receptors were also found in the nucleus of the solitary tract and the rostral and caudal ven- trolateral medulla. All of these regions have putative roles in the regulation of blood pressure and fluid and electrolyte balance. In the rabbit brain there is less AT 2 receptor binding than the rat, with most AT z binding found in the mo- lecular layer of the cerebellum and in the septohypothaiamic nucleus. In the subthalamic nucleus, the me- diodorsai and ventroposterior nuclei of the thalamus, locus coeruleus and inferior olivary nuclei, areas con- taining mostly AT 2 receptors in the rat, no binding was detected in the rabbit except in the locus coeruleus which contains moderate levels of AT 1 receptors. Taken in conjunction with our previous results in the rat and human brains, these results reveal that AT 1 receptors predominate in rostral forebrain, hypothalamus and autonomic control centers of the medulla ob- longata in all three species. However, the distribution and density of AT 2 bearing sites in regions such as the septum, thalamus subthalamic nuclei, locus coeruleus, cerebellum and inferior olivary nuclei show marked species differences. Correspondence to: G.P. Aldred, University of Melbourne, Department of Medicine, Austin Hospital, Heidelberg, Victoria, 3084, Australia.