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

Abstract

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.

Wiring the retinal circuits activated by light during early development

“…The interplexiform ACs extend short processes towards the OPL and a neurite to the IPL, while the more classic ACs extend a process to the IPL [53]. In order to characterize the morphology of the TH + ACs, we electroporated a construct encoding GFP into the Stage 28 eye primordium [54], and used GFP to assess the morphology of TH+/GFP + cells at Stage 42 (Figure 5F).…”

“…3) A dopaminergic AC connects mHCs to mRGCs. Interplexiform dopaminergic cells are present in teleost and frog retinas [53,64-66], but absent in mammals, such as rabbits [67]. In teleost fish, these cells extend processes to both the IPL and OPL, connecting to HCs in the OPL [64,68].…”

Elevated dopamine concentration in light‐adapted zebrafish retinas is correlated with increased dopamine synthesis and metabolism

“…This morphology is characteristic of interplexiform cells observed in lower vertebrates (Dowling and Ehinger ; Kalloniatis and Marc ; Schutte and Witkovsky ; Witkovsky et al . ; Witkovsky ), including zebrafish (Yazulla and Studholme ). TH‐positive interplexiform cells (Mitrofanis et al .…”

What the Salamander Eye Has Been Telling the Vision Scientist’s Brain

“…They are all spiking cells, receive ON as well as OFF sustained excitation from bipolar cells, and release GABA or glycine (Yang and Yazulla, 1988a). Dopaminergic interplexiform cells, which have been found in other animals such as frog (Witkovsky et al, 1994), appear to be absent or extremely rare in the salamander retina (Watt et al, 1988).…”