Cell-specific gene expression in oxytocin and vasopressin magnocellular neurons

The oxytocin (OT) and vasopressin (VP) expressing magnocellular neurons in the hypothalamic-neurohypophysial system (HNS) have been the most studied of all the neuroendocrine cell-types. Despite this, our understanding of the mechanisms that underly the cell-specific expression of the peptide genes in these neurons has remained obscure. Part of the reason for this may be related to the close apposition of the OT and VP genes in the chromosomal locus, the genes being separated by as little as 3.5 kb in the mouse, and their interactions which are critical for cell-specific expression of the genes. Recent studies using intact rat OT and VP constructs in transgenic mice, and rat and mouse VP genes with CAT inserts in exon III as reporters in transgenic rats and mice, respectively, have suggested the presence of cell-specific enhancer elements in the 3' downstream (intergenic region, IGR) region of the VP gene. Evidence in favor of this view is presented from transgenic mouse studies on the expression of mouse OT- and VP-CAT gene constructs. Oxytocin and vasopressin phenotypes in the magnocellular neuronal population have traditionally been assessed by either immunocytochemical or in situ hybridization histochemical methods leading to the view that these genes are never coexpressed. However, more sensitive methods show that most OT cells also express some VP mRNA, and most VP cells contain some OT mRNA. A third phenotype containing equivalent levels of both OT and VP mRNA can also be found under some conditions, thereby complicating our analysis of cell-specificity. A continuing problem hindering studies of the regulation of OT and VP gene expression in neurons, is the absence of an appropriate cell line to examine these issues. We have found that stationary slice-explant cultures allow for excellent preservation of highly differentiated magnocellular neurons in long-term culture, and that these cultures can be used for physiological and pharmacological studies and analysis of gene expression.