Catecholaminergic horizontal and amacrine cells in the ferret retina

Enzymes involved in the synthesis of catecholamines were detected in amacrine and what appeared to be a specific class of horizontal cells in the ferret retina. Antisera directed against the enzymes tyrosine hydroxylase (TH), which converts tyrosine to DOPA, and phenylethanolamine N-methyltransferase (PNMT), which converts norepinephrine to epinephrine, were used with conventional immunohistochemical techniques. A population of perikarya located at the outer margin of the inner nuclear layer (INL) exhibited TH-like immunoreactivity. The cell bodies were 9-12 μm in diameter and gave rise to stout dendrites that tapered rapidly after emergence from the somata. The processes formed a planar array in the inner half of the outer plexiform layer (OPL) slightly external to the cells of origin. We could not detect any inwardly directed processes. A population of PNMT-positive cells was also observed in the outer tier of cells in the INL. These cells were very similar to those exhibiting TH immunoreactivity. An apparent difference between the 2 populations was that there were areas of intense, somewhat punctate PNMT immunoreactivity in the outer OPL. These were not observed in the TH-stained sections. Examination of horizontal sections showed that each TH-positive cell body gave rise to 4-5 major dendrites that branched to form a roughly circular dendritic field. In the periphery of the retina, an individual cell's dendrites encompassed an area up to 170 μm in diameter. The dendritic fields of cells near the center of the retina were substantially smaller. The PNMT-positive cells in horizontal sections appeared identical with respect to cell size and dendritic field characteristics. In the central retina, there were about 200 cells/mm² with substantial overlap of dendrites of neighboring cells. On the basis of somatic position, dendritic field size and pattern, and the apparent lack of an axon terminal, we suggest that these cells correspond to the A-type horizontal cells described in cat (Kolb, 1974; Boycott et al., 1978). As in other vertebrates, a prominent population of TH-positive amacrine cells was observed in the inner tiers of the INL. These cells arborized in the inner, middle, and outer laminae of the inner plexiform layer (IPL). The TH-positive amacrine cells were clearly distinguishable from the immunoreactive cells in the outer tier of the INL by their position, branching pattern, and intensity of staining.