Regulation of Anterior Pituitary Gonadotrophin Subunit mRNA Levels by Gonadotrophin‐Releasing Hormone in the Ewe

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Abstract
Levels of mRNA for the common a subunit and for the beta subunits of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in the pituitary glands of ovariectomized hypothalamo-pituitary disconnected ewes. A control group (n = 7) received 250 ng pulses of gonadotrophin-releasing hormone (GnRH) each hour for one week. To examine the effects of changing GnRH pulse amplitude four sheep were given 250 ng pulses of GnRH for one week and then 25 ng pulses for one week. Plasma LH and FSH concentrations were lowered by reducing the GnRH pulse amplitude but pituitary levels of mRNA for a subunit were increased. Levels of mRNA for FSHbeta and LHbeta were similar with 25 ng and 250 ng pulses of GnRH. To examine the importance of pulsatile versus continuous GnRH inputs, a group of sheep was given a constant infusion of 250 ng/h GnRH for one week. Compared to sheep given 250 ng pulses of GnRH the mRNA levels for LHbeta and FSHbeta were lower in sheep given a constant infusion of GnRH; levels of a subunit mRNA were similar in the two groups. To examine the short-term effects of removing GnRH inputs, ovariectomized, hypothalamo-pituitary disconnected ewes that had been receiving 250 ng pulses of GnRH each hour were deprived of GnRH for 6 h (n = 4) or 30 h prior to slaughter; levels of mRNA for the three subunits were similar to control values in both of these groups. These studies show that wide variation in GnRH pulse amplitude has little effect on mRNA levels for the gonadotrophin subunits but message levels are affected by the mode of GnRH input (constant versus pulsatile). The maintenance of gonadotrophin subunit mRNA levels for at least 30 h after GnRH deprivation suggests that these mRNA species have a long half-life or that transcription continues after GnRH withdrawal.

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