Modulation of sensitivity to cervicovaginal stimulation during the estrous cycle: Evidence for an extrapituitary action of LH-RH

Abstract

Single stimulations of the vagina and cervix were performed between proestrus and the first day of diestrus with a stimulator designed to grade the intravaginal penetration of a rod. The percent incidence of pseudopregnancy after this stimulation was exponentially related to the extent of intravaginal penetration and was also affected by the stage of the cycle at which the stimulation was performed. At 10.00 h on proestrus, an exponential increase in the incidence of pseudopregnancy was observed with shallow penetrations, while an exponential decrease was found when deeper penetrations were applied. Such negative exponential correlation had disappeared at 22.00 h on proestrus. At that time, also, some responses were elicited by very shallow penetrations (17 mm) and all the animals responded to penetrations of 20 mm or more. Sensitivity to cervicovaginal stimulation at 10.00 h on estrus was lower than that at 22.00 h on proestrus and it was even lower at 10.00 h on the first day of diestrus. The response to 18 mm of penetration was studied every 3 h between 10.00 h on proestrus and 10.00 h on estrus, and then every 12 h until 10.00 h on the first day of diestrus. This stimulation was usually ineffective to induce pseudopregnancy, except for a brief period encompassing the night between proestrus and estrus, when a peak in the incidence of responses was reached. This peak sensitivity could be advanced following the s.c. administration of 250 and 500 ng of LH-RH at 11.00 h on proestrus. Other doses were ineffective. The peptide (500 ng) was unable to induce pseudopregnancy in rats that received no cervicovaginal stimulation. Also, the number of ova shed on that cycle was not affected by this treatment. However, several doses of LH, FSH or their combinations failed to mimic the facilitation of the response to cervicovaginal stimulation by LH-RH.