ONTOGENY OF ARGININE VASOPRESSIN-LIKE IMMUNOREACTIVITY IN THE BRAZILIAN OPOSSUM BRAIN

The neuropeptide arginine vasopressin is involved in many centrally me diated functions and brain development. In this study, we have examine d the ontogeny of arginine vasopressin-like immunoreactivity (AVP-IR) in the Brazilian opossum (Monodelphis domestica) brain to further unde rstand the involvement of AW in the forming central nervous system. Mo nodelphis is a small pouchless marsupial and its pups are born in an e xtremely immature state before neurogenesis is completed. In the adult brain, cell bodies containing AW-IR were found in several nuclear gro ups and areas, and immunoreactive fibers were found to be widely distr ibuted throughout the brain. The distribution of AVP-IR in the adult o possum brain generally resembled that reported for other species inclu ding the rat, however, some differences in localization of immunoreact ive cells were observed. In the developing opossum brain, AVP-IR was f irst seen in the mesencephalon and diencephalon between embryonic days 12 and 13. Subsequently, a distinct group of AVP immunoreactive cells was present in the forming supraoptic nucleus on day 1 of postnatal l ife (1 PN) and at 3 PN in the paraventricular nucleus. Between 1 and 3 PN, a few cells transiently expressed AVP-IR in the forming thalamus and tegmental area. At these ages a few immunoreactive fibers were als o detected in the forming cerebellum. These fibers were not seen at la ter ages in these areas. By 5 PN, an increased expression of AVP-IR wa s seen in the forming supraoptic and paraventricular hypothalamic nucl ei, median eminence, and posterior pituitary. At 7 PN, immunoreactive cells and fibers were seen in several forebrain areas. The distributio n pattern of AVP-IR became adult-like by 60 PN. A sex difference in th e amount of AVP-IR in the lateral septum was also observed in the opos sum brain at 60 PN. This difference persisted in the adult brain. Due to the early presence of AVP-IR in the Monodelphis brain before neurog enesis and morphogenesis is completed, we suggest that AVP may be invo lved in morphogenesis of the central nervous system. In addition, AVP may have a significant physiological function in regard to homeostasis before the forebrain contributes to these control mechanisms. Further studies, including physiological and developmental manipulations, wil l define the significance of the early presence of AVP during the diff erentiation and maturation of the central nervous system in Monodelphi s.