An antiserum directed against tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, was used to study the pigeon retina. Labeled cells were observed in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Two populations of TH-immunoreactive neurons were observed in the INL. Some of these cells were 7−10 μ in diameter and gave rise to processes that arborized in three Layers of the inner plexiform layer (IPL). These cells appeared similar to the dopaminergic amacrine cells described previously (Marc, 1988). Other labeled cells in the INL were 12−20 μ in diameter and were recognizable as a previously described subpopulation of TH-immunoreactive displaced ganglion cells (Britto et al., 1988).
A population of labeled cells was observed in the GCL. Counts of these cells in two retinae revealed 5000 and 7000 cells, respectively. They ranged in size from 8−15 μ in diameter in the central retina and from 8−20 m in diameter in the peripheral retina. The density of labeled cells was highest in the central retina and red field and lowest in the retinal periphery. The difference in cell size and cell density as a function of eccentricity is characteristic of the total population of ganglion cells in the avian retina (Ehrlich, 1981; Hayes, 1982). Some of the TH-positive cells in the GCL could be classified as ganglion cells for two reasons: (1) The axons of many of the TH-positive cells in the GCL were TH-immunoreactive as well and could be followed to the optic nerve head. (2) The injection of rhodamine-labeled microspheres into the nucleus geniculatus lateralis, pars ventralis (GLv), resulted in the retrograde labeling of many of the TH-positive cells in the contralateral retina.