Gonadotropin-releasing hormone increases the amount of messenger ribonucleic acid for gonadotropins in ovariectomized ewes after hypothalamic-pituitary disconnection

Abstract

To investigate the role of GnRH in regulating the synthesis and secretion of gonadotropins, GnRH (250 ng/6 min every other hour for 7 days) or saline was administered to ovariectomized (OVX) ewes after hypothalamic-pituitary disconnection (HPD). Blood samples were collected from all HPD ewes on the day before and the day after HPD and on days 1 and 7 of GnRH or saline. At the end of day 7, anterior pituitary glands were removed for analysis of hormone, receptor, and mRNA content. The amount of mRNA for gonadotropins was lower (P < 0.05) in saline-treated HPD ewes than in GnRHtreated HPD or OVX ewes. Administration of GnRH restored the amount of mRNA for FSHβ and α-subunits to levels similar (P > 0.05) to those measured in OVX ewes. The amount of mRNA for LH0 was higher (P < 0.05) in GnRH-treated HPD ewes than in saline-treated HPD ewes, but lower (P < 0.05) than that in OVX ewes. The pituitary content of LH and FSH was lower (P < 0.05) in saline-treated HPD ewes than in OVX ewes. Administration of GnRH to HPD ewes maintained the pituitary content of LH, but not FSH, compared to the pituitary gonadotropin content in OVX ewes. There were no differences (P > 0.05) in the amount of mRNA for GH or PRL or the pituitary content of these hormones among treatments. The number of hypophyseal receptors for GnRH was reduced in saline-treated HPD ewes (P < 0.05) compared to that in OVX ewes and GnRH-treated HPD ewes. The number of hypophyseal receptors for 17β-estradiol was lower (P < 0.05) in GnRH- and saline-treated HPD ewes than in OVX ewes. Serum LH concentrations were lower (P < 0.05) after HPD than before HPD, but were restored to normal (P > 0.05) by GnRH replacement. Serum concentrations of FSH were lower (P < 0.05) after HPD and were not affected by GnRH replacement. Serum PRL concentrations in all ewes were higher (P < 0.05) after HPD than before HPD. Serum GH concentrations in all ewes were similar (P > 0.05) before and after HPD. Since synthesis and secretion of GH and PRL were not diminished after HPD, it was considered that the pituitary gland remained viable and functioned independently of hypothalamic input in OVX ewes after HPD. It is concluded that GnRH stimulates the production of mRNAs for gonadotropins. GnRH also stimulates LH secretion, but FSH secretion occurs independently of GnRH administration in OVX ewes after HPD. © 1988 by The Endocrine Society.