The phenotypic characteristics of expressed tyrosine hydroxylase protein have been precisely analysed in the rat nucleus tractus solitarius, which contains the majority of A2 noradrenergic and C2 adrenergic neurons 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 rats, there were few tyrosine hydroxylase-expressing cell bodies which express less than 2% of the immunoradiolabeled tyrosine hydroxylase protein measured in the structure. These cell bodies were scattered throughout an extensive immunopositive neuropile, which precisely delimited the topological space of the nucleus tractus solitarius quantitatively reconstructed using a polar coordinate system. The quantification of tyrosine hydroxylase tissue concentration from immunoradioautograms allowed us to subdivide the structure into two distinct regions. The posterior region of the nucleus tractus solitarius, which mainly corresponds to the A2 cell group, contains a relatively high tissue concentration 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 this protein. Three days after an intraperitoneal injection of RU24722, there was a strong increase (90 ± 17%) in tyrosine hydroxylase protein content only in the anterior region of the nucleus tractus solitarius. This increase was associated with a dramatic elevation (142 ± 20%) in the number of tyrosine hydroxylase-expressing cell bodies. The additional cell bodies were mainly located inside the initial perikarya-containing area. Furthermore, in this region a large and preferential lateral extension in the mean topological space of the nucleus tractus solitarius as defined by tyrosine hydroxylase radiolabeling was also quantified.

These results suggested a particular plasticity in tyrosine hydroxylase protein expression in the anterior adrenergic region of the rat nucleus tractus solitarius.