VASOPRESSIN AND GALANIN MESSENGER-RNAS COEXIST IN THE NUCLEUS OF THE HORIZONTAL DIAGONAL BAND - A NOVEL SITE OF VASOPRESSIN GENE-EXPRESSION

Vasopressin (VP) neurons have been identified in several brain regions where VP has been hypothesized to act as a neurotransmitter or neuromodulator. In many sites, VP is colocalized with the neuropeptide galanin (GAL). Here, using single in situ hybridization histochemistry, we have identified a novel group of neurons within the nucleus of the horizontal diagonal band of Broca (HDB) that express the VP gene and have assessed the distribution of these cells in adult male and female rats (90 days old, n = 7/group). VP mRNA-expressing neurons were scattered throughout the rostrocaudal extent of the HDB, and the number of VP neurons detected unilaterally ranged from 1 to 17 cells per 20 mu m section. Using double in situ hybridization histochemistry on alternate sections, we have assessed the number of cells expressing VP and/or GAL mRNA in the diagonal band and have determined the extent of their colocalization. Approximately 50% of all VP-expressing neurons in the HDB coexpressed GAL mRNA, and 33% of GAL-expressing neurons in this region coexpressed VP mRNA. No sex differences were detected in the number of neurons expressing either VP or GAL mRNA or in the incidence of coexpression of VP and GAL mRNAs in this region. VP neurons in the HDB exhibited a low level of expression, and cellular VP mRNA content did not differ between male and female rats. However, sex differences were present in the bed nucleus of the stria terminalis (BNST) of these same rats. Consistent with our previous findings, both the number of VP-expressing neurons (P less than or equal to 0.0004) and the cellular VP mRNA content (P less than or equal to 0.02) in the BNST were greater in male than in female rats. These results suggest that VP gene expression is differentially regulated by gonadal hormones in the HDB compared to the BNST. Because GAL was previously reported to colocalize with acetylcholine (ACh) and gonadotropin-releasing hormone (GnRH) in the diagonal band of Broca, our findings suggest that VP may also colocalize with these transmitters in this brain region. Our observations may provide an anatomical basis for the postulated actions of VP in functions involving ACh or GnRH pathways. (C) 1995 Wiley-Liss, Inc.