Co-localization of tyrosine hydroxylase and phenylethanolamine-N-methyltransferase immunoreactivity in the rat retina: a re-examination using double labeling on semi-thin sections

The precise morphology and distribution of phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive cells has been observed and compared with the previously described tyrosine hydroxylase (TH)-immunoreactive cells, in the rat retina. The PNMT-positive cells are small amacrine neurons, located either in the inner nuclear layer or in the ganglion-cell layer. They send processes mainly to the middle sublayer and to a lesser extent to the outermost sublayer of the inner plexiform layer. They resemble the small TH-positive bouquet cells described previously. In order to ascertain the relationship between PNMT-positive cells and TH-positive bouquet cells, a double immunohistochemical labeling technique, using anti-TH and anti-PNMT antisera, has been developed on semi-thin sections. The result of these experiments clearly indicate that the populations of TH-positive cells and PNMT-positive (presumably epinephrinergic) cells are separated. The absence of TH enzyme in the PNMT-positive cells raises the question of the enzymatic activity of PNMT, which appears to be different from the classical pathway of catecholamine biosynthesis in the retina.