J. s&mid Biochem. Vol. 20, No. 4A, pp. 841-847, 1984 Printed in Great Britain. All rights reserved 0022-4731/84 $3.00 + 0.00 Copyright 0 1984 Pergamon Press Ltd zyxwvut ESTROGEN-LIKE EFFECTS OF NORETHISTERONE ON THE HYPOTHALAMIC PITUITARY UNIT OF OVARIECTOMIZED RATS* FERNANDO LARREAP. OCTAVIO MOCTEZUMA~ and GREGORIO BREZ-PALACIOS Department of Reproductive Biology, Instituto National de la Nutrici6n Salvador ZubirLn, Mexico City, Mexico (Received 8 Augusr 1983) Summary-The acute and chronic effects of norethisterone (NET) upon gonadotropin dynamics were studied in long-term ovariectomized adult Wistar rats. Time course studies showed that plasma LH declined within 1 h following a single NET SC injection and remained low for the next 4 h duration of the experiment. Pre-treatment with tamoxifen prevented the acute anti-gonadotropic effect of NET and GnRH administration to NET treated rats resulted in an abolishment of the pituitary responsiveness in terms of LH. Chronic administration of NET but not progesterone to castrated animals induced a significant decrease of plasma and pituitary LH with a concomitant replenishment of hypothalamic GnRH in an identical fashion to that observed with chronic administration of 17fi-estradiol. Furthermore, administration of NET and 17g-estradiol but not progesterone diminished the number of GnRH pituitary binding sites to levels found prior castration. These results indicate that the effects of this 19-nor synthetic progestin upon gonadotropin modulation in the long-term gonadectomized rat do not resemble those of progesterone and suggest that NET is acting through different mechanisms. The data are consistent with an estrogen-like mode of action INTRODUCTION Since the pioneering studies of Moore and Price in 1932 [l] considerable evidence exists regarding the relationship between gonadal steroids and gonado- tropins. Steroid mediated gonadotropin control might be exerted in the hypothalamus on the secre- tion of gonadotropin-releasing hormone (GnRH) or at pituitary level by modifying its responsiveness to endogenous GnRH [2,3-51. Removal of endogenous steroids by castration leads to elevated levels of plasma and pituitary LH and FSH, and adminis- tration of gonadal steroids results in an inhibition of both gonadotropins [6]. It is well known that es- trogens exert a variety of effects on the central nervous system including regulation of gonadotropin and prolactin secretion and the activation of sexual behavior [7,8]. Many of these actions are believed to involve the binding of estrogens to specific estrogen-receptor molecules. Therefore experimental *This work was supported in part by grants from the Population Council (New York), The WHO Special Programme of Research, Development and Research Training in Human Reproduction (Geneva, Switzer- land), and The National Council of Science and Tech- nology of Mexico (CONACYT). tAddress for correspondence: Department of Reproductive Biology, National Institute of Nutrition, Salvador ZubirLn, Calle de Quiroga No. 15, 14000 Mtxico, D.F. SPost-Doctoral Research Fellow in Reproductive Biology supported by S.S.A. Grant. §The following trivial names and abbreviations were used: Norethisterone (NET): 4-estren-17a-ethynil-17p-ol- 3-one, 178 estradiol: 1,3,5 (IO)-estratrien-3, 178-dial, Progesterone: 4-pregnen-3,20-dione, Gonadotropin- releasing hormone (GnRH). conditions which interfere with estrogen action such as in the long-term ovariectomized rat can prevent estrogen mediated neural events including the in- duction of cytoplasmic progestin receptors [9-l 11. Indeed it is well recognized that estrogens and an- drogens have an inhibitory effect upon both gonado- tropins while progesterone alone has negligible if any effects on LH and FSH in gonadectomized female rats [12]. Regarding this there are a number of syn- thetic steroids having progesterone-like effects such as norethisterone (NET), that exhibit anti- gonadotropic activity in the castrated rat [13]. Thus suggesting that NET a synthetic progestin can be recognized by intracellular steroid binding sites different from those of progesterone. The purpose of this study was to investigate the mechanisms by which NET exerts its effects at neuroendocrine level, particularly in its ability to inhibit gonadotropins in the long-term castrated female rat. MATERIAL AND METHODS zyxwvutsrqponmlkj Hormones Non-radioactive steroidss were purchased from Sigma Chemical Co. (St Louis, MO) and their chemical purity was established by paper chromatog- raphy and recrystallization. The GnRH agonist analog (D-Ser(tBu)6) des-Gly’O-GnRH-N-ethylamide (GnRH-A) used for iodination and as a standard in the radioreceptor assay was kindly provided by Dr Andrew V. Schally (V. A. Hospital, New Orleans, LA) and natural GnRH was donated by Hoechst de Mexico, S.A. All solvents and reagents were of analytical grade. 841