Neuroendocrinology of coitally and noncoitally induced pseudopregnancy

In this paper, I have outlined our research perspective on coitally and noncoitally induced PSP. Coitally induced PSP requires a minimal threshold of vaginal stimulation to establish the repetitive nocturnal PRL surges which are essential for converting the corpora lutea from a nonfunctional to a functional state. The induction of PSP requires the activation of the adrenergic system and simultaneous inhibition of the cholinergic system. Reduction of neural activity in the medial preoptic area occurs at the time of coital stimulation, and is believed to be associated with the initiation of the nocturnal PRL surges. Apparently, cervical stimulation acts by inhibiting the nocturnal PRL surge inhibitory neurons at this brain site, thereby instituting PSP. It is proposed that the specific mating pattern of cervical stimulation activates a "mnemonic system" which retains information from vaginocervical stimulation permitting the expression of the repeated nightly PRL surges. In our view, each pregnancy originates as a direct continuation of the initial "hypothalamic PSP" triggered by the neural signal of mating. The mechanisms involved in the activation of the "mnemonic system" and the hypothalamically pseudopregnant state remain to be elucidated. Pseudopregnancy can be initiated by a number of socioenvironmental factors as well, such as cohabitation in all-female groups in mice or concaveation with foster pups in virgin rats. However, unlike coitally induced PSP which activates the "mnemonic system," noncoitally induced pseudopregnancies lack a "mnemonic system," and therefore require the continued presence of the initiating stimulus until progesterone levels become sufficiently elevated to provide positive feedback on PRL secretion.