BRAIN RESEARCH ELSEVIER Brain Research 705 (1995) 24-30 Research report Stimulation of angiotensin II AT 1 receptors in rat median eminence increases phosphoinositide hydrolysis Alicia M. Seltzer, Stefan Zorad, Juan M. Saavedra * Section on Pharmacology, Laboratory of Clinical Science, National Institute of Mental Health, National Institutes of Health, 10 Center Drive, Bldg. 10, Room 2D-45, Bethesda, MD 20892, USA Accepted 15 August 1995 Abstract The aim of our study was to determine the second messenger systems for angiotensin II in the rat median eminence. Angiotensin II AT 1 receptors are highly expressed in the median eminence and binding is selectively inhibited by the guanine nucleotide GTPyS, indicating possible coupling to G-proteins. In male rats, angiotensin II increased phosphatidylinositol hydrolysis about 45% over basal values, with an ECs0 of about 2.7 nM. This effect was antagonized by 10 /xM losartan, the selective AT~ antagonist, but not by the AT 2 competitor PD 123319. Conversely, angiotensin II, 1 /xM, did not alter basal or forskolin-stimulated cAMP production, and failed to influence cGMP production. These results support a role for angiotensin II, through stimulation of AT 1 receptors and increased phosphatidylinositol hydrolysis, in the median eminence. Angiotensin II increased the phosphatidylinositol hydrolysis not only in male rats but also in ovariectomized rats, with or without estrogen-progesterone replacement. However, angiotensin II (up to 1 /xM) failed to increase the phosphatidylinositoi hydrolysis in randomly selected intact female rats. Estrogen treatment did not alter the number or affinity of median eminence AT 1 receptors in ovariectomized rats. The increase in phosphatidylinositol hydrolysis resulting from stimulation of median eminence AT 1 receptors appears to be sexually dimorphic, but hormonal manipulations failed to point to a role for reproductive hormones in this phenomenon. Keywords: Angiotensin receptor; Median eminence; Second messenger system; Quantitative autoradiography 1. Introduction Angiotensin II (Ang II) receptors have been classified, based on their sensitivity to selective competitors, into AT~ and AT 2 subtypes. AT~ receptors have the highest affinity for losartan; AT 2 receptors have the highest affinity for CGP 42112 and PD 123177 [1,24]. Both AT 1 and AT 2 receptors are present not only in peripheral tissues but also in the brain [13]. As it is the case in the periphery [1,24] stimulation of brain AT 1 receptors is necessary to produce the well known actions of Ang II, regulation of drinking and increase in blood pressure [13]. Central regulation of hormone secretion by Ang II has also been proposed to be mediated by stimula- tion of brain AT~ receptors, because the hypothalamic areas related to anterior and posterior pituitary control, and * Corresponding author. Fax: (1) (301) 402-0337. 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0006-8993(95)01100-5 the anterior pituitary, express only this receptor subtype [25,261. In addition to hypothalamic nuclei and the anterior pituitary, AT 1 receptors are highly expressed in the rat median eminence, a regulatory center for pituitary releas- ing factors [13,26], and this indicates the possibility of multilevel regulation of pituitary hormone secretion by AT~ receptor stimulation [13]. In the median eminence, there are large numbers of Ang II-containing fibers, local- ized to its external layer [9]. Thus, median eminence AT 1 receptors could be stimulated by blood-borne Ang II, or by locally formed peptide, and Ang II could act as a neuro- transmitter or neuromodulator of hormone release in this structure. In the anterior pituitary, AT~ receptor stimulation is coupled to phosphatidylinositol hydrolysis, inhibition of adenylyl cyclase, and arachidonic acid release [2,13]. In- formation on second messenger systems for brain AT~ receptors is more limited and sometimes contradictory [3,231.