From basal luteinizing hormone (LH) concentrations to the pre-ovulatory LH surge: Titration of the physiological effect of gonadotrophin surge-inhibiting attenuating factor

Pulsatile luteinizing hormone (LH) release is thought to be controlled by the functional antagonism between gonadatrophin-releasing hormone (GnRH) and the putative ovarian factor gonadotrophin surge-inhibiting/attenuating factor (GnSIF/AF). Our experiments were designed to titrate the input of endogenous basal and follicle stimulating hormone (FSH)-stimulated GnSIF/AF in this system. The effects were studied of five consecutive pulses of GnRH administration (10-250 pmol/pulse/kg rat), 1 h apart, on the self-priming effect in pro-oestrus phenobarbital-blocked rats injected with FSH or saline. In response to the two high pulses of GnRH, a clear GnRH self-priming was demonstrated. However, pulses of 25 pmol GnRH induced only a minor self-priming effect and the LH blood concentrations were within the physiological range. Pulses of 10 pmol GnRH had no effect. FSH suppressed the unprimed LH response and delayed GnRH self-priming by similar to 1 h. Increasing the GnRH pulse frequency of the 25 pmol GnRH pulses from one to three per hour induced pre-ovulatory-like LH surges. The effect was facilitated by ovariectomy to eliminate GnSIF/AF. This, the results are consistent with relative inputs of GnRH and GnSIF/AF being responsible for the changes in LH concentration during the ovarian cycle, including the generation of the mid-cycle LH surge.