Neurogenesis of the sexually dimorphic nucleus of the preoptic area in the rat

The sexually dimorphic nucleus of the preoptic area (SDN-POA) of Sprague-Dawley rats is larger in volume in the male and hormone-dependent early in postnatal life. In the present study, we compared for each sex the time course of neuroblast proliferation which forms SDN-POA or adjacent medial preoptic area (MPOA) neurons. Additionally, we investigated whether there is a temporal gradient of production of neurons in relation to their final position within the SDN-POA. On day 14, 15, 16, 17, or 18 postfertilization (pf) pregnant rats were given a single injection of ³H-thymidine (*thy). At 30 postnatal days of age the pups were sacrificed and brain sections were prepared and processed for autoradiography. Three sections of the SDN-POA and an adjacent area just lateral to it in the MPOA wer also analyzed. In the MPOA and the SDN-POA the percentage (%) of labeled neurons decreases as the day of injection of *thy approaches the end of gestation, but the time period in which neuroblast divisions occurred is markedly different for the SDN-POA as compared to that for the MPOA. DNA synthesis occurs as late as day 18pf for neurons which form the SDN-POA but ceases on day 16pf for those destined for the MPOA. There is a sex difference in neuronal production on both day 14 and 17pf for neurons destined for the SDN-POA. After injection on day 14pf the % labeled neurons is larger in the female than in the male but after injection on day 17pf this is reversed. There are also significant sex differences as well as a temporal gradient associated with the % labeled neurons in the SDN-POA in relation to their final anterior-posterior position. In addition, this study confirms our previous results which justify labeling the SDN-POA a nucleus, since neuronal density in this region in the male and female is significantly greater than that in the surrounding MPOA. These data illustrate that the specific neurons which comprise the SDN-POA in both the male and female are being produced as late as day 18pf, whereas neurons located in the MPOA but not in the SDN-POA have all been born by day 16pf. Neuroblast division which produces the neurons of the SDN-POA may begin earlier and terminate sooner in the female than in the male. These differences in neuronal production may partially account for the sexual dimorphism seen in the volume and neuronal number of the SDN-POA of the adult rat.