OXYTOCIN AND VASOPRESSIN IMMUNOREACTIVITY IN RABBIT HYPOTHALAMUS DURING ESTRUS, LATE PREGNANCY, AND POSTPARTUM

Mother rabbits construct an elaborate maternal nest before parturition and display a single, brief, daily nursing bout throughout lactation. These features present a unique model for investigating the relevance of changes in neuroendocrine secretion associated with pregnancy and parturition for the regulation of maternal behavior. In the present st udy we analyzed changes in the location, somal size, and number of oxy tocin (OT)- and arginine vasopressin (AVP)-immunoreactive (IR) neurons in the hypothalamus of rabbits in estrus, late pregnancy (day 29), an d postpartum day 1. From estrus to late pregnancy, the number of OT-IR neurons increased in the scattered cell groups located in the lateral hypothalamic area (LHA), but not in the magnocellular nuclei, i.e., p araventricular nucleus (PVN) and supraoptic nucleus (SON). On postpart um day 1 the increase in the number of OT-IR neurons was sustained in the LHA and became apparent also in the main body of the PVN, in which the number of OT-IR neurons doubled. Increases in the somal size of O T-IR cells were seen in all three nuclei only on postpartum day 1. No OT-IR cells were found in the suprachiasmatic nucleus (SCN). From late pregnancy and into postpartum day 1 increases in the somal size of AV P-IR neurons were detected in the PVN, SON, and LHA but not in the SCN . The number of AVP-IR neurons increased between late pregnancy and po stpartum day 1 in the SON only. The changes observed in OT and AVP exp ression in specific hypothalamic nuclei may be related to specific som atic and behavioral events occurring around the time of parturition, e .g., nest-building, maintenance of homeothermy, elevation of blood vol ume, and nursing in mother rabbits.