On the dynamics between gonadotrophin surge-inhibiting factor and gonadotrophin releasing hormone (GnRH): role of self-priming and desensitization in the luteinizing hormone response to GnRH after follicle stimulating hormone treatment.

The effects were studied of follicle stimulating hormone (FSH)-induced production of gonadotrophin surge-inhibiting factor (GnSIF) on three phases of the pituitary responsiveness to gonadotrophin releasing hormone (GnRH): the unprimed, primed and desensitized phases. Rats were injected with FSH on two occasions during the oestrous cycle. Spontaneous luteinizing hormone (LH) surges were measured as well as GnRH-induced LH surges on the day of pro-oestrus during infusions with 100-4000 pmol GnRH/rat/10 h, in phenobarbital blocked rats. The spontaneous LH surges were attenuated or completely inhibited by the FSH treatment. FSH suppresses and prolongs the unprimed LH response and delays GnRH self-priming, especially during infusions with low concentrations of GnRH. This treatment does not affect the total LH response (area under curve) to the highest concentrations of GnRH and after ovariectomy. On the other hand, this response is suppressed during infusions with the lower concentrations of GnRH. Hence, FSH, via GnSIF, delays maximal priming of the LH response to GnRH, whereas the suppression of LH release is a consequence of the GnRH-induced progressed state of desensitization. The inconsistent effects of FSH on the mid-cycle LH surges are explained as a result of the interaction between the relative strengths of GnRH and GnSIF.