Catecholamine-synthesizing enzymes in the rat pituitary

Abstract

The catecholamine-containing nerve fibers of the rat pituitary were studied by immunohistochemical demonstration of the catecholamine-synthesizing enzymes tyrosine hydroxylase (TH), dopamine-β-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT). Immunohistochemical demonstration of TH confirms earlier catecholamine fluorescence histochemical studies showing a fine network of varicose fibers in both the intermediate and the neural lobe, with the most dense aggregation of fibers at the border between the lobes. DBH-immunoreactive fibers were much less in number, and confined to the neural lobe, where both vascular and parenchymal fibers were seen. With the antibody to PNMT bright staining was seen in all the glandular cells of the intermediate lobe, while the neural lobe was negative. No immunoreactive structures were observed in the anterior lobe. Functionally the study confirms the presence of an extensive dopaminergic innervation of the neurointermediate lobe, giving an anatomical basis for the tonic inhibitory action of dopamine on the intermediate lobe cells and for recent observations attributing dopamine a local regulatory function also in the neural lobe. In addition to vascular noradrenaline-containing fibers as described earlier the study shows parenchymal DBH-immunoreactive fibers in the neural lobe, suggesting a local role for noradrenaline in this lobe. The nature of the cellular PNMT-immunoreactivity in the intermediate lobe remains to be established. The cellular localization of the PNMT-immunoreactivity was distinctly different than that of the α-MSH-immunoreactivity within the intermediate lobe cells and reserpine treatment did not affect the PNMT-immunoreactivity, although it induced a heterogenous depletion of α-MSH and related peptides. Cross-reaction of the PNMT-antibody with the known secretory products of these cells thus seems unlikely. Biochemical studies are needed in order to show whether an actual PNMT activity is present.