Amacrine cells in the tiger salamander retina: morphology, physiology, and neurotransmitter identification

Amacrine cells of the vertebrate retina comprise multiple neurochemical types. Yet details of their electrophysiological and morphology properties as they relate to neurotransmitter content are limited. This issue of relating light responsiveness, dendritic projection, and neurotransmitter content has been addressed in the retinal slice preparation of the tiger salamander. Amacrine cells were whole-cell clamped and stained with Lucifer yellow (LY), then processed to determine their immunoreactivity (IR) to GABA, glycine, dopamine or tyrosine hydroxylase (TOH), and glucagon antisera. Widefield, ON-OFF amacrine cells were glycine-IR. The processes of these cells extended laterally in the inner plexiform layer (IPL) from 250-600 microns. They were either multistratified in the IPL or monostratified near the IPL midline. Three multistratified ON-OFF narrowfield glycine-IR cells also were found. Four types of ON amacrine cells were found to be GABA-IR; all types had their processes concentrated in the proximal IPL (sublamina b). Type I cells were narrowfield (approximately 100 microns) with a compact projection. Type II cells were widefield (220-300 microns) with a sparse projection. Type III cells had an asymmetrical projection and varicose processes. Type IV cells were pyriform and monostratified in sublamina b. One narrowfield ON-OFF amacrine cell, with processes broadly distributed in the middle of the IPL, was GABA-IR. This cell appeared similar to an ON-OFF cell that was glycine-IR and may comprise a type in which GABA and glycine colocalize. Another class of amacrine cell, with processes forming a major plexus along the distal border of the IPL and a lesser plexus in the proximal IPL, produced slow responses at light ON and OFF; these cells were dopamine/TOH-IR. A narrowfield class of transient ON-OFF amacrine cell, with processes ramifying throughout both sublaminae a and b of the IPL, were glucagon-IR; these cells appeared to be dye-coupled at the soma. We have shown that, with respect to GABA, glycine, dopamine, and glucagon, salamander amacrine cells fall into rather discrete groups on the basis of ramification patterns in the IPL and responses to photic stimulation. The physiological, structural, and neurochemical diversity of amacrine cells is indicative of multiple and complex roles in retinal processing.