Divergent responses of gonadotropin subunit messenger RNAs to continuous versus pulsatile gonadotropin-releasing hormone in vitro

Episodic GnRH input is necessary for the maintenance of LH and FSH secretion. In the current study we have assessed the requirement of a pulsatile GnRH signal for the regulation of gonadotropin α-and β-subunit gene expression. Using a dispersed rat pituitary perifusion system, GnRH (10 mvi) was administered as a continuous infusion vs. hourly pulses. Secretion of free α-subunit, LH, and FSH were monitored over 5-min intervals for the entire 12-h treatment period before the responses of α, LHβ, and FSHβ mRNAs were assessed. Basal release of all three glycoproteins declined slowly over 6-8 h before reaching a plateau. The cells were responsive to each pulse of GnRH, but continuous GnRH elicited only a brief episode of free α-subunit, LH, and FSH release, followed by a return to unstimulated levels. Despite the similar patterns of secretion, differences were observed in the responses of gonadotropin mRNAs to the two modes of GnRH. α mRNA increased in response to continuous (1.6-fold) or pulsatile (1.7-fold) GnRH. FSHβ mRNA was suppressed to 48% of the control value after continuous GnRH, but was stimulated over 4-fold by the pulses. LHβ mRNA was unresponsive to either treatment paradigm. We conclude that in vitro 1) α mRNA levels are increased in response to GnRH independent of the mode of stimulation; 2) under the conditions studied, LHβ mRNA levels are unresponsive to either mode of GnRH input; and 3) the response of FSHβ mRNA to GnRH is highly dependent on the mode of administration, with levels depressed in response to continuous GnRH, but stimulated by pulsatile GnRH. We further observe a divergence in the secretory response compared to levels of mRNA, suggesting that distinct signaling pathways may be involved in the regulation of these two cellular events.