INCREASED FOS EXPRESSION IN PREOPTIC CALCITONIN-GENE-RELATED PEPTIDE (CGRP) NEURONS FOLLOWING MATING BUT NOT THE LUTEINIZING-HORMONE SURGE IN FEMALE RATS

The functional relationship between sexually dimorphic neural populati ons and sex differences in reproductive functioning is unclear. The pr esent study has investigated the function of the sexually dimorphic, e strogen-receptive, calcitonin gene-related peptide (CGRP) neurones in the female preoptic area by examining patterns of Fos immunoreactivity within these cells in relation to the luteinizing hormone surge and l ordosis behaviour. In the first experiment, ovariectomized rats were t reated with estradiol alone or estradiol plus progesterone to induce t he luteinizing hormone surge. The percentage of CGRP neurones with Fos -positive nuclei was not different in estradiol alone (18 +/- 4%) and estradiol/progesterone-treated (24 +/- 3%) rats although the number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 2-fold (P < 0.01) in estrogen/progesterone-treated rats and 40 +/- 5% of luteinizing hormone-releasing hormone neurones were found to expre ss Fos in this group. In the second experiment, ovariectomized rats we re treated with estradiol and progesterone and either, mated with a si ngle male or placed in an empty cage, for 30 min. The number of Fos-im munoreactive cells in the medial preoptic nucleus was increased 4-fold in mated rats (P < 0.01) and the percentage of CGRP neurones with Fos -positive nuclei increased from 24 +/- 3% to 38 +/- 2% (P < 0.01) in m ated animals. No differences were detected in the number of luteinizin g hormone-releasing hormone neurones with Fos-positive nuclei in mated and non-mated animals. These results suggest that a sub-population of CGRP neurones in the medial preoptic nucleus may express Fos on a con stitutive basis in steroid-treated animals and that, while not altered in relation to the luteinizing hormone surge. Fos expression by these cells is increased following mating. Although the precise role of the se CGRP neurones has yet to be ascertained, the present experiments pr ovide direct evidence of a functional relationship between a specific sexually dimorphic neural population and a component of sexually diffe rentiated reproductive functioning.