Perinatal development of galanin-immunoreactivity in the mouse brain

Vasopressinergic projections from the bed nucleus of the stria terminalis (BNST) are sexually dimorphic, with males having denser projections and more vasopressin cells than females in most species studied. This sex difference is established and maintained by steroid hormones. Since the BNST only begins expressing vasopressin on postnatal day 3 (P3) and then in males only, vasopressin cells are difficult to recognize at the time when steroid hormones have their organizing effect. However, vasopressin neurons in the BNST are a subset of a larger group of galanin (GAL)-expressing cells, which themselves show no detectable sex difference. Therefore, this study explores whether GAL can be used as a marker for the vasopressin cells by exploring the development of GAL expression. Immunocytochemistry was used to analyze GAL expression at embryonic days 15 (E15) and 19 (E19), and P3. On E15, scattered GAL-immunoreactive (GAL-ir) cells were found in the BNST but no GAL-ir fibers in projection areas of the BNST such as the lateral septum. On E19 and P3, cells as well as fibers were found in both sexes. At all ages, dense GAL-ir fiber bundles were found in the median eminence, presumably coming from the paraventricular and supraoptic nuclei. However, these nuclei showed only clear GAL-ir cell bodies on E19 and P3. In addition, dense GAL-ir innervation of the striatum and areas of the cerebral cortex was found at E19, a phenomenon that had disappeared by P3. These results are being confirmed by in situ hybridization to detect GAL mRNA. The results from this study indicate that GAL is present in the BNST before circulating hormones have their organizing effect, making it plausible to use GAL as a marker for the sexually dimorphic AVP neurons in the same region.