Gonadotropin-Releasing Hormone Regulation of Gonadotropin Subunit Gene Expression in Female Rats: Actions on Follicle-Stimulating Hormone b Messenger Ribonucleic Acid (mRNA) Involve Differential Expression of Pituitary Activin (b-B) and Follistatin mRNAs*

GnRH is the primary stimulus in the regulation of gonadotropin subunit mRNA expression. Additionally, local (pituitary) production of activin and follistatin appear to modulate the expression of FSH b mRNA. The current studies aimed to determine whether GnRH regulation of pituitary activin (b-B) and follistatin mRNAs could play a role in the differential actions of GnRH pulse pattern on gonadotropin mRNA expression in female rats. In response to altered GnRH pulse amplitude, the expression of FSH b and follistatin mRNAs followed an inverse pattern. Only high dose GnRH increased expression of follistatin whereas, in contrast, b-B and FSH b expression were increased following lower doses of GnRH. To determine whether increased follistatin mRNA expression was correlated with FSH b mRNA responses, we examined their temporal relationship following high dose GnRH. Both FSH b and follistatin mRNAs were increased within 2 h and remained increased through 6 h. However, by 12 h FSH b mRNA levels returned to values seen in controls, suggesting that increased follistatin requires 6–12 h to reduce FSH b mRNA. In response to altered GnRH pulse frequency, FSH b expression was increased at all pulse intervals (8–240 min) examined. Rapid GnRH pulse frequencies (8-min intervals) increased follistatin expression, whereas b-B mRNA was only increased after 30-min pulse intervals, which also resulted in maximal FSH b mRNA concentrations. These results suggest that changes in pituitary activin (b-B) and follistatin mRNA expression may be important components of gonadotrope responses to pulsatile GnRH, and potentially imply that GnRH stimulation of activin and follistatin peptide production provides regulatory control over the production of FSH. (Endocrinology 140: 903–908, 1999) I IS WELL recognized that changes in the profile of GnRH secretion are essential for the dynamic pattern of gonadotropin (LH and FSH) synthesis and secretion. Following gonadectomy, during sexual maturation, and during the estrous cycle in rats, GnRH release and gonadotrope function are altered in well characterized patterns (for review, see Ref. 1). Additionally, studies of exogenous GnRH have supported the central role of GnRH in reproduction. In both primates and rodents, a pulsatile GnRH is needed to maintain gonadotrope secretory responses. Conversely continuous GnRH results in desensitization and loss of GnRH receptors and LH secretion (2–4). Extremes of GnRH secretory pattern are likely of physiologic significance as increasing pulsatile release (amplitude and frequency) characterize the proestrus gonadotropin surge, whereas desensitization has been postulated, at least in part, to be responsible for the subsequent decline in LH secretion later on proestrus evening (5–7). Data such as these support the notion that an understanding of gonadotrope responses to a GnRH signal is essential toward understanding reproductive physiology. To date, the majority of in vivo studies in which GnRHdeficient animal models were used to assess responses to exogenous GnRH pulse patterns used the castrate/testosteronereplaced adult male rat model (8–12). In these animals, a pulsatile GnRH signal is required to increase the hormone specific LH b and FSH b mRNAs, whereas the common a-subunit mRNA is increased by either a pulsatile or continuous pattern. Changes in pulse amplitude selectively regulate LH b, whereas a and FSH b increase in response to a wide range of pulse doses. In contrast, pulse frequency differentially regulates both b subunit mRNAs with faster frequencies, favoring only LH b and a subunit mRNAs, whereas slower frequencies increase only FSH b gene expression. Notably, gonadotropin synthesis and secretion as well as gonadal hormonal responses are relatively stable in the adult male rat, whereas dynamic changes are well recognized in female rats (as noted above). Hence, considerable effort has been given toward the development of a GnRH-deficient female rat model. We have recently reported that pulsatile GnRH increases a, LH b and FSH b gene expression in the ovariectomized (OVX)/phenoxybenzamine-treated (PBZ)/ testosterone-replaced adult female rat (13). It was the priReceived April 28, 1998. Address all correspondence and requests for reprints to: Dr. Alan C. Dalkin, University of Virginia Health Sciences Center, Division of Endocrinology, Department of Internal Medicine, 5041 MR4 Building, Lane Road, Charlottesville, Virginia 22908. E-mail [email protected]. * Supported by USPHS Grant HD-11489 (to J.C.M.) and the Reproductive Center Grant HD-28934. 0013-7227/99/$03.00/0 Vol. 140, No. 2 Endocrinology Printed in U.S.A. Copyright © 1999 by The Endocrine Society