Sensory Mechanisms Involved in the Induction of Pseudopregnancy by Progesterone: Increased Sensitivity to Stimulation of the Pudendal Sensory Field*

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Abstract
The sensory mechanisms that participate in the induction of pseudopregnancy after a single injection of progesterone were investigated. Unless otherwise indicated, rats were kept in group cages and vaginal smears were taken daily. Progesterone evoked pseudopregnancy in a dose-dependent manner when administered to proestrous or estrous rats that received no cervicovaginal stimulation. The probability of pseudopregnancy after progesterone was higher one estrus. Cervicovaginal stimulation of proestrous rats that received 5 mg progesterone 10 h before was performed with a rod with a sliding stop attached to regulate its intravaginal penetration. Progesterone facilitated responsiveness to this stimulus, although the amount injected was not significantly effective in increasing the incidence of pseudopregnancy in nonstimulated rats. However, the mere application of the stop of the stimulator on the perineal skin was followed by a significantly higher incidence of pseudopregnancy in progesterone-injected rats than in their vehicle-injected controls, which suggested an action of the steroid on perineal sensitivity. Accordingly, the pseudopregnancy-evoking effect of progesterone was clearly inhibited by refraining from taking vaginal smears for 5 days after steroid injection on estrus. No further inhibition was observed after isolating the animals in single rat cages. Daily finger stimulation of the perineal skin of nonsmeared rats restored to a normal level response to progesterone. Furthermore, this response was severely impaired by transecting the pudendal nerves before the injection. Pseudopregnancy is induced in progesterone-treated rats through sensory stimulation of the pudendal receptive field and apparently the pudendal nerve may subserve as a secondary afferent system to elicit the pseudopregnancy response. The possibility progesterone also acts on other afferent systems including the main afferent system constituted by the pelvic nerve is discussed.