IDENTIFICATION OF A SEXUALLY DIMORPHIC NEURAL POPULATION IMMUNOREACTIVE FOR CALCITONIN GENE-RELATED PEPTIDE (CGRP) IN THE RAT MEDIAL PREOPTIC AREA

The medial preoptic area (MPOA) of the rat exhibits morphological sex differences and is implicated in sex-specific functioning and behaviour. Using immunocytochemistry, the distribution and numbers of cells containing calcitonin gene-related peptide (CGRP) were examined in the MPOA of adult male and female rats. In the intact female rat, CGRP-immunoreactive (-IR) cells were found in a continuum within the MPOA extending from the caudal aspects of the organum vasculosum of the lamina terminalis through the anteroventral periventricular nucleus (AVPv) to the region of the medial preoptic nucleus (MPN). An additional small group of CGRP-IR cells was noted at the level of the caudal MPN in the ventrolateral (VL) region, Compared with males, the AVPv and MPN regions of the female contained over 25-fold more CGRP-IR cells (P < 0.01). The VL region contained similar numbers of CGRP-IR cells in both sexes. Ovariectomy 1 month earlier, with or without subsequent 17-beta estradiol treatment, had no effect on the numbers or distribution of CGRP-IR cells in the MPOA. Gonadectomy of male rats resulted in a significant increase (P < 0.01) in the numbers of CGRP-IR cells in the AVPv and MPN regions. Subsequent administration of testosterone propionate for 1 week reduced (P < 0.05) numbers of CGRP-IR cells to levels observed in the intact male. Neurones containing CGRP in the VL group were not altered by gonadal steroid manipulation. This study shows that CGRP neurones in the AVPv/MPN region are sexually dimorphic. Further, these results suggest that part of the sex difference results from a sustained inhibitory action of testosterone on CGRP cell numbers in the adult male. It remains to be determined whether these marked sex differences in preoptic CGRP populations are related to sex-specific functioning of the MPOA.