AUTORADIOGRAPHIC LOCALIZATION OF ARGININE-VASOPRESSIN BINDING-SITES IN THE BRAIN SF ADULT AND DEVELOPING BRAZILIAN OPOSSUMS

We are utilizing the Brazilian short-tailed opossum, Monodelphis domes tica, to study the development of the vasopressinergic system. Earlier studies demonstrated that arginine vasopressin-like immunoreactivity was present very early in the Brazilian opossum brain, suggesting a ro le for vasopressin in the developing central nervous system of mammals . In this study, we have utilized [H-3]arginine vasopressin autoradiog raphy to describe the distribution of arginine vasopressin binding sit es in adult and developing Brazilian opossum brains. In general, argin ine vasopressin binding patterns in adult opossum brains resembled tho se of other species. However, we found very few labelled areas in neon atal Brazilian opossum brains. At birth, only the ventral tegmental ar ea and the nucleus of the solitary tract were labelled. Binding was no t evident in the forebrain until 25 days of postnatal age. The anterio r pituitary was heavily labelled from birth onward, but binding in the brain itself remained at low levels until 35 days post natal. Heavy b inding was observed in only a few areas of the brain in adults, includ ing the dorsal part of the lateral septal nucleus, the suprachiasmatic nucleus, the dorsal and median raphe, the nucleus of the solitary tra ct, and the caudal part of the spinal trigeminal nucleus. Surprisingly , arginine vasopressin binding sites in the Brazilian opossum appeared much later than arginine vasopressin immunoreactivity and, in many ca ses, after neurogenesis was complete. These findings suggest that the arginine vasopressin binding sites are not playing a developmental rol e in opossums, although the peptide is present at an early age.