A study of the sensory requirements for eliciting the pseudopregnancy response

Stimulations of the vagina and cervix were performed with a stimulator designed to grade intravaginal penetration of a rod. Single cervicovaginal stimulations performed in estrus or in diestrus II elicited immediate or delayed luteal responses, respectively. The probabilities of both types of response increased as a function of the strength (penetration) and duration of the stimulus. Immediate responses were significantly more probable than delayed responses. The other observed parameters of the response (plasma progesterone and diestrus length) were not related to stimulus magnitude. The probabilities of immediate or delayed responses were related to rod penetration by power functions, implying an exponential transformation of the received stimulus. An exponential transformation of rod penetration into pressure on the cervix, which occurs at the vaginal level, seems not to contribute substantially to the final exponential transformation. The relationship between stimulus duration and response probability could not be defined mathematically. No further increase in probability of response was caused by prolonging a deep penetration (24 mm) beyond 5 s (immediate responses) and 15 s (delayed responses). The novel stimuli from an unusual procedure to immobilize the rat may interfere with the response to cervicovaginal stimulation. However, the extravaginal stimuli the female receives during coitus did not seem to facilitate or inhibit the response to a deep (24 mm) penetration of the rod. The results establish some quantitative relationships between stimulus and response parameters and indicate the importance of cervicovaginal stimuli, among the multiple sensory stimulation of mating, to induce the luteal response.