Ontogeny of opiate receptors in the rat medial preoptic area: Critical periods in regional development

Opiate receptor labeling was examined throughout the early postnatal period using autoradiography to localize and quantify [³H]naloxone binding to μ-type opiate receptors in the medial preoptic area (MPOA). This region begins to exhibit sexual dimorphism of volume and dendritic growth shortly after birth. A distinct concentration of opiate receptor labeling appears on postnatal day 3 in females: this labeling is directly associated with the sexually dimorphic nucleus of the preoptic area (SDN-POA). SDN-POA labeling becomes denser through postnatal day 10 in females and the densely labeled area increases in size to encompass and surround the SDN-POA. These changes in opiate receptor labeling occur only in females, since males show relatively uniform labeling across the region throughout the early postnatal period. The critical time of formation of dense MPOA opiate receptor labeling may be related to endogenous MPOA opioid function and to the critical period of dendritic growth of SDN-POA neurons. The timing of these critical periods and their focus in the SDN-POA are coincident. The possible role of MPOA opiate receptors in modulating growth of MPOA neurons is discussed.