Ontogeny of oxytocin-like immunoreactivity in the Brazilian opossum brain

The neuropeptide oxytocin (OT) has been shown to function as a neurotransmitter and/or neuromodulator in addition to its hormonal function in the periphery in the adult central nervous system (CNS). Previously, we have studied the postnatal neurogenesis of the paraventricular and supraoptic nuclei and ontogeny of arginine vasopressin-like immunoreactivity in the Brazilian opossum brain, Monodelphis domestica. In this study, we have described the ontogeny of oxytocin-like immunoreactivity (OT-IR) in the opossum brain. As a marsupial, opossum pups are in an extremely immature state, with neurogenesis and morphogenesis continuing into the second week of postnatal life. Thus, opossum pups are a good model for developmental studies. In the adult opossum brain, OT-IR was localized in regions as reported for the adult rat and other species, except for a few differences. These findings suggest similar functional roles for OT in the adult opossum brain as in other mammals. Unlike the prenatal expression of arginine vasopressin, OT-IR was first detected in the forming median eminence on day 1 of postnatal life (1 PN). Between 3 and 5 PN, OT-IR was present in the hypothalamic supraoptic and paraventricular nuclei and posterior pituitary. At this time, neurogenesis of these nuclei is not completed. By 10 to 15 PN, OT-IR was seen in several brain areas, and begins to resemble that of the adult between 45 and 60 PN. These results indicate that the time course of appearance of the OTnergic system does not directly parallel the early expression of the vasopressinergic system. However, the expression of OT-IR in the opossum brain before neurogenesis and morphogenesis is completed suggests a potential role for OT in developmental events. Similar to arginine vasopressin, oxytocin may also be involved in the regulation of autonomic functions that are essential for the opossum's adaptation to an ex utero environment. Future studies utilizing experimental manipulations of the OTnergic system will help determine the significance of this peptide in the neonatal opossum.