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Sex Steroids Are Involved in the Regulation of Gonadotropin-Releasing Hormone and Dopamine D2 Receptors in Female Tilapia Pituitary¹

Department of Animal Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, Hebrew University, Rehovot 76100, Israel

ABSTRACT

Althoughmolecular mechanisms underlying steroid effects on GnRH and dopamine receptors are well documented in mammals, little is known in fish. Herein, we describe the expression of pituitary GnRH and dopamine receptors relative to gonadotropin expression and release. We exposed female tilapia to graded doses of estradiol or 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) in vitro, and of estradiol in vivo, and determined mRNA levels ofgnrhr1,gnrhr3,drd2,lhb, andfshbby real-time PCR. We also determined gonadotropin levels using specific ELISAs. Exposure to low doses of estradiol caused increasedgnrhr3mRNA levels in vivo and in vitro, probably related to positive feedback on FSH release. Increasing concentrations of estradiol resulted in increaseddrd2mRNA levels in vivo and in vitro, inhibition of LH and FSH release, and inhibition oflhbmRNA levelsin vivo, possibly related to negative feedback. At high doses of estradiol, FSH release increased in preparation for a new generation of follicles. Exposure to nanomolar doses of DHP resulted in increaseddrd2mRNA levels, probably related to negative feedback on LH release. A decrease indrd2levels at the micromolar range of DHP (concomitant with increasedgnrhr3andfshbmRNA levels) may be related to the recruitment of a new generation of oocytes. Exposure to DHP also resulted in increasedlhbmRNA levels toward final oocyte maturation. Salmon GnRH analog (sGnRHa) increased mRNA levels ofgnrh1andgnrh3; when combined with DHP, sGnRHa synergistically increased expression ofgnrh3only. These results emphasize the role of sex steroids on positive and negative feedbacks controlling the reproductive cycle.

dopamine, estradiol, gonadotropin-releasing hormone receptor, neuroendocrinology, pituitary

¹ Supported by grant 775/01 from the Israel Science Foundation.

² Correspondence: Berta Levavi-Sivan, Department of Animal Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, Hebrew University, P.O. Box 12, Rehovot 76100, Israel. FAX: 972 8 946 5763; sivan{at}agri.huji.ac.il