Dopaminergic neurons in the retina of Xenopus laevis: amacrine vs. interplexiform subtypes and relation to bipolar cells.

Presumed dopaminergic neurons were visualized in the retina of the clawed frog, Xenopus laevis, by anti-tyrosine hydroxylase (TH) immunoreactivity. The studied cells constitute a uniform population with perikarya at the junction of inner nuclear (INL) and inner plexiform (IPL) layers. Each cell body gives rise to 4-6 relatively stout processes (0.5-2.0 microns in diameter) which run for up to 1.2 mm in strata 4-5 of the IPL. These processes have a very asymmetric distribution in the horizontal plane of the retina. A dense plexus of TH fine fibers is distributed uniformly in stratum 1 of the IPL. TH cells are distributed evenly but sparsely (16-20 cells/mm2) across the retina. About 20% of the TH neurons emit 1-3 distally directed fine processes, the majority of which extend < 20 microns, which barely suffices to reach the outer plexiform layer (OPL). Other longer processes are typically unbranched; some reach the OPL, others run tangentially in the INL. The axon terminals of Golgi-impregnated bipolar cells are characterized according to the strata of the IPL in which they arborize. About 80% are confined either to strata 1-2 or 3-5, conforming to the 'off' and 'on' zones defined by Famiglietti and Kolb (1976). The remainder appear to end in both zones, some extending across the entire width of the IPL. EM examination showed that TH processes receive bipolar synaptic input in both distal and proximal portions of the IPL.