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 neurom odulator. In many sites, VP is colocalized with the neuropeptide galan in (GAL). Here, using single in situ hybridization histochemistry, we have identified a novel group of neurons within the nucleus of the hor izontal diagonal band of Broca (HDB) that express the VP gene and have assessed the distribution of these cells in adult male and female rat s (90 days old, n = 7/group). VP mRNA-expressing neurons were scattere d 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 se ction. Using double in situ hybridization histochemistry on alternate sections, we have assessed the number of cells expressing VP and/or GA L mRNA in the diagonal band and have determined the extent of their co localization. Approximately 50% of all VP-expressing neurons in the HD B coexpressed GAL mRNA, and 33% of GAL-expressing neurons in this regi on coexpressed VP mRNA. No sex differences were detected in the number of neurons expressing either VP or GAL mRNA or in the incidence of co expression of VP and GAL mRNAs in this region. VP neurons in the HDB e xhibited a low level of expression, and cellular VP mRNA content did n ot differ between male and female rats. However, sex differences were present in the bed nucleus of the stria terminalis (BNST) of these sam e rats. Consistent with our previous findings, both the number of VP-e xpressing 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 expres sion is differentially regulated by gonadal hormones in the HDB compar ed 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 colocali ze with these transmitters in this brain region. Our observations may provide an anatomical basis for the postulated actions of VP in functi ons involving ACh or GnRH pathways. (C) 1995 Wiley-Liss, Inc.