SEX DIFFERENCE IN COEXPRESSION BY GALANIN NEURONS ACCOUNTS FOR SEXUALDIMORPHISM OF VASOPRESSIN IN THE BED NUCLEUS OF THE STRIA TERMINALIS

Vasopressin (VP) neurons in the bed nucleus of the stria terminalis (B NST) are steroid sensitive and sexually dimorphic. The number of VP me ssenger RNA. (mRNA)-expressing neurons is larger in male than in femal e rats. This initial observation suggested that sexual dimorphism resu lted from enhanced proliferation and/or survival of VP neurons after g onadal hormone exposure during the critical perinatal period. However, galanin (GAL) and VP mRNAs were recently reported to be coexpressed i n the BNST of adult male rats, and GAL gene expression, unlike VP gene expression, is not sexually dimorphic. These findings are consistent with the hypothesis that the sex difference in VP cell number in the B NST results from a sex difference in the number of GAL neurons dedicat ed to express the VP gene. To test this hypothesis, double in situ hyb ridization histochemistry was performed for GAL and VP mRNAs in the BN ST of adult male and female rats. For quantification, the posterior BN ST was divided into its two anatomical regions: medial (BSTM) and late ral (BSTL) divisions. Extending previous results for the whole BNST, t he number of GAL-expressing cells in either the BSTM or the BSTL was n ot sexually dimorphic. A significant sex difference was found in the n umber of GAL cells coexpressing VP in the BSTM (mean +/- SE, male, 124 +/- 8; female, 56 +/- 6; P less than or equal to 0.0001), but not in the BSTL (male, 80 +/- 9; female, 83 +/- 15). Accordingly, the number of cells expressing GAL mRNA only was significantly lower (P less than or equal to 0.002) in the BSTM of male (43 +/- 5) than in female (85 +/- 9) rats. Evidence is provided that the reduced incidence of coexpr ession of VP by GAL neurons in the BSTM of female rats may account for the reported sex difference in VP cell number in the entire BNST. The results suggest that gonadal hormones in the perinatal period may not influence the proliferation and/or survival of VP neurons in the BNST per se but influence, instead, the capacity of GAL neurons to synthes ize VP.