Mating-induced neuroendocrine responses during pseudopregnancy in the female mouse

Pseudopregnancy (PSP) is a neuroendocrine reflex triggered by vaginocervical stimulation similar to the neuroendocrine response of early pregnancy and is characterised by short-term neural activity, resulting in long-term neuroendocrine responses that cause repeated release of pituitary prolactin (PRL) over many days. PSP is a useful model to study how somatosensory input is transduced in the brain into neuroendocrine responses, and has been extensively characterised in rats. With increasing use of mice as an experimental model, however, and to allow use of transgenic mice to investigate mechanisms of this sensory response, it is important to characterise the principal neuroendocrine response of pseudopregnancy in this species. The present study aimed to examine the induction and neuroendocrine responses of PSP in mice using vasectomised stud males, to investigate mating-induced changes in vaginal cytology, uterine growth, and PRL secretion, and to map certain aspects of somatosensory transduction by assessing the neural activity marker FOS. Unlike the induction of pseudopregnancy in rats, which can be induced simply by multiple intromissions from a male or artificial mechanical stimulation of the cervix, PSP induction in mice required the receipt of an ejaculation from a male. In mice that received PSP-inducible mating stimuli, FOS expression was observed in a slightly different range of brain regions than has been observed in rats, with increases in the bed nucleus of the stria terminalis, medial preoptic area, and ventromedial hypothalamus, but not in limbic areas examined. Moreover, PSP mice expressed a single diurnal PRL surge on day 6 of PSP. Thus, the data demonstrate important species differences in the neuroendocrine mechanisms activated in response to a mating stimulus in mice compared with rats. A clear understanding of the species-specific response will be required in interpreting research into the reproductive biology of this species.