Appearance and transient expression of vasopressin and oxytocin receptors in the rat brain.

Binding sites for AVP and for OT were studied by in vitro autoradiography in sections from the brain of rat fetuses, neonates and infants; their distribution was compared to that of the brain of adults. Specific binding sites were first detected in the vagal complex for OT and in the reticular formation for AVP at E14 and E16 respectively. In the perinatal period, other areas become labeled. Approximately one week after birth, a "stable" pattern of distribution is established for AVP binding sites, and a different "stable" pattern obtained for OT binding sites. For both types of sites and in many areas, the density of labeling increases during the next two weeks to reach adult levels, whereas labeling decreases concomitantly in other areas of the brain. The distribution of AVP binding sites is of the adult pattern by the time of weaning. In contrast, the adult pattern of distribution of OT binding sites is only established after puberty, when new OT receptors appear in some regions of the hypothalamus and basal forebrain. "Transient" binding sites for AVP and OT, i.e. sites located in areas which were labeled in neonates but not in weanlings, were shown to have the same ligand affinity than the binding sites present in the adult. Electrophysiological studies suggest that at least some of these "transient" binding sites represent authentic receptors and may be involved in neuronal signaling.