PHENOTYPIC CHARACTERISTICS OF EXPRESSED TYROSINE-HYDROXYLASE PROTEIN IN THE ADULT-RAT NUCLEUS-TRACTUS-SOLITARIUS - PLASTICITY REVEALED BY RU24722 TREATMENT

The phenotypic characteristics of expressed tyrosine hydroxylase prote in have been precisely analysed in the rat nucleus tractus solitarius, which contains the majority of A2 noradrenergic and C2 adrenergic neu rons of the medulla oblongata. This study was based upon quantitative analysis of immunohistochemical and immunoradioautographic staining of tyrosine hydroxylase protein in serial coronal sections. In control r ats, there were few tyrosine hydroxylase-expressing cell bodies which express less than 2% of the immunoradiolabeled tyrosine hydroxylase pr otein measured in the structure. These cell bodies were scattered thro ughout an extensive immunopositive neuropile, which precisely delimite d the topological space of the nucleus tractus solitarius quantitative ly reconstructed using a polar coordinate system. The quantification o f tyrosine hydroxylase tissue concentration from immunoradioautograms allowed us to subdivide the structure into two distinct regions. The p osterior region of the nucleus tractus solitarius, which mainly corres ponds to the A2 cell group, contains a relatively high tissue concentr ation of tyrosine hydroxylase protein (18.56 +/- 0.154 units per mg of tissue). The anterior region, which mainly corresponds to the C2 cell group, exhibits a; relatively low concentration (12.09 +/- 0.81) of t his protein. Three days after an intraperitoneal injection of RU24722, there was a strong increase (90 +/- 17%) in tyrosine hydroxylase prot ein content only in the anterior region of the nucleus tractus solitar ius. This increase was associated with a dramatic elevation (142 +/- 2 0%) in the number of tyrosine hydroxylase-expressing cell bodies. The additional cell bodies were mainly located inside the initial perikary a-containing area. Furthermore, in this region a large and preferentia l lateral extension in the mean topological space of the nucleus tract us solitarius as defined by tyrosine hydroxylase radiolabeling was als o quantified. These results suggested a particular plasticity in tyros ine hydroxylase protein expression in the anterior adrenergic region o f the rat nucleus tractus solitarius.