Dynamical study of tyrosine hydroxylase expression and its correlation with vasopressin turnover in the magnocellular neurons of the supraoptico-posthypophysial system under long-term salt loading of adult rats.

Using immunocytochemistry, in situ hybridization and image analysis, we attempted to compare the dynamical expression of tyrosine hydroxylase (TH) and vasopressin (VP) mRNAs and proteins in the magnocellular neurons of the supraoptic nucleus in rats drinking 2% NaCl for 1, 2 and 3 weeks. Three stages in the reaction of VPergic neurons have been distinguished. The initial stage (first week) showed a synchronous activation of TH and VP mRNAs and protein expression as well as an increased number of TH-immunoreactive neurons. The next stage (second week) was characterized by a further increase in the number of TH-immunoreactive neurons. The number of VPergic neurons also increased significantly. Although the TH and VP mRNAs levels fell during the second week of osmotic stimulation, the TH content increased significantly, and the VP content remained at the same level. During the last stage (third week), TH-immunoreactive neurons increased in number and were as numerous as VP-immunoreactive neurons in intact rats. These data suggest that, finally, all the VPergic neurons begin to synthesize TH. The concentrations of VP and TH mRNAs did not change during the third week of osmotic stimulation, while the VP and TH contents increased. Thus, our study shows that there is a correlation between TH expression and VP expression and suggests similar mechanisms for the regulation of VP and TH gene expression and synthesis during long-term osmotic stimulation.