Phenylethanolamine N‐methyltransferase (PNMT)‐expressing horizontal cells in the rat retina: A study employing double‐label immunohistochemistry

Phenylethanolamine N‐methyltransferase (PNMT), the final enzyme in the catecholamine biosynthetic pathway that converts norepinephrine to epinephrine, has been detected in the retinas of various vertebrate species. The expression of PNMT has generally been thought to occur in amacrine cells of the ganglion cell and inner nuclear layers. By using immunohistochemical techniques, we have found a population of PNMT‐ and neurofilament‐positive neurons at the border of the inner nuclear layer and the outer plexiform layer in the rat retina. We have classified these cells as horizontal neurons based on their location adjacent to the outer plexiform layer, their morphology, and their expression of vimentin and neurofilaments.