Expression of tyrosine hydroxylase in vasopressinergic neurons of the supraoptic nucleus in rat ontogenesis and its modulation by noradrenergic afferents

An attempt is made to find out, at what stage of ontogenesis an expression of gene and synthesis of tyrosine hydroxylase (TH) is started, and whether noradrenergic afferents participate in regulation of these processes. The study is carried out on rats at the 21st embryonal day (E21), P3 and P13 with use of quantitative and semi-quantitative immunocytochemistry and hybridization in situ. Animals of all ages were subjected to a salt load, in some cases on the background of introduction of and α1-adrenoreceptor inhibitor, prazozine. According to the obtained data, the TH expression in SON neurons in response to the salt load begins at P3. The number of VP-ergic neurons expressing TH during the salt load is 3-fold reduced from P3 to P13. Taking into account that the innervation of VP-ergic SON neurons is realized for this period of development, we formulated a hypothesis that the TH expression is inhibited by noradrenergic afferents. According to the obtained data, TH is not expressed in osmotically stimulated VP-ergic neurons on the background of prazozine injection at E21; however, this combined effect results in increased TH expression at P3 and P13. At P13, i.e., in animals with a more developed afferent innervation, the amount of TH-immunoreactive neurons is three times lower, than at P3. Thus, in ontogenesis of rats, VP-ergic neurons begin to respond to osmotic stimulation by inclusion of the TH gene expression and its synthesis at the neonatal period, the both processes being under the inhibitory control of noradrenergic afferents mediated through α1-adrenoreceptors.