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2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) Stimulates Gonadotropin Secretion in the Immature Female Sprague-Dawley Rat Through a Pentobarbital- and Estradiol-Sensitive Mechanism but Does Not Alter Gonadotropin-Releasing Hormone (GnRH) Secretion by Immortalized GnRH Neurons In Vitro¹

Center for Reproductive Sciences³ Departments of Molecular and Integrative Physiology⁴ and Toxicology, Pharmacology, and Experimental Therapeutics,⁵ University of Kansas Medical Center, Kansas City, Kansas 66160 Physiology and Biophysics,⁶ University of Colorado School of Medicine, Denver, Colorado 80220

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces aberrant release of gonadotropins, FSH, and LH and blocks ovulation during induced ovarian follicular development in rats by an unknown mechanism. In the current study, TCDD (0, 8, or 32 µg/kg orally) was administered to immature female Sprague-Dawley rats, and synchronous follicular development was induced 24 h later with equine chorionic gonadotropin (eCG, 5 IU s.c.). Both doses of TCDD induced a significant premature increase in serum FSH and LH (P < 0.05) at 12 h post-eCG. This premature gonadotropin surge was facilitated by the administration of a long-acting estradiol (estradiol cypionate, 0.01, 0.1, and 0.5 mg/kg s.c.), whereas the progesterone and cortisol receptor antagonist RU486 (0, 1, and 10 mg/kg s.c.) potentiated the premature release of FSH and LH following TCDD as well. Pentobarbital (32 mg/kg i.p.) administered at 6 or 9 h, but not 0 h, post-eCG ablated the ability of TCDD to stimulate the release of FSH and LH in vivo. TCDD had no significant effect on GnRH accumulation in vitro from immortalized GnRH neuronal (GT1–7) cells and failed to alter the cell number. Transfection of these cells with a rat GnRH promoter-reporter construct revealed no significant acute effect of TCDD on GnRH promoter activity. Aryl hydrocarbon receptor mRNA was not detected in the GT1–7 cells by reverse transcription polymerase chain reaction. TCDD appears to stimulate premature gonadotropin release in the gonadotropin-primed immature rat by interacting with an estradiol- and pentobarbital-sensitive neural signal for GnRH release but not by acting upon the GnRH neuron directly.

¹ Supported by grants from the Mellon Foundation (B.K.P.) as well as the National Institutes of Health (NIH 5T3HD07455 and NIH 1F32ES05880, B.K.P.).

² Correspondence: Brian K. Petroff, Center for Reproductive Sciences, Department of Molecular and Integrative Physiology, University of Kansas Medical Center, 3901 Rainbow Blvd., Kansas City, KS 66160. FAX: 913 588 7180; bpetroff{at}kumc.edu

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