Changes in immunostaining for oxytocin in the forebrain of the female rat during late pregnancy, parturition and early lactation.

Serial brain sections of female rats at late pregnancy, parturition or early lactation were immunostained for oxytocin. Immunoreactive perikarya were visible in the magnocellular nuclei in all experimental animals as well as in ovariectomized, nulliparous controls. During late pregnancy and at parturition additional immunostaining appeared in groups of perivascular neurons in the preoptic region, the lateral subcommissural nucleus, the perifornical region and scattered throughout the ventral portion of the hypothalamus. Immunostaining of almost all of these perivascular neurons disappeared by day two postpartum, while another population of oxytocin neurons, without association with blood vessels, appeared in these brain regions after parturition. Immunostaining of processes from oxytocinergic neurons in the periventricular nucleus increased markedly near parturition. Many of these processes projected toward the third ventricle. Oxytocinergic neuronal systems that are activated in late pregnancy and early postpartum may contribute to several physiological changes associated with parturition and lactation including the onset of maternal behavior.