Four types of amacrine in the cat retina that accumulate GABA

Abstract

Roughly one‐quarter of neurons in the amacrine cell layer accumulate exogenous γ‐aminobutyric acid (GABA). Some of these (8%) are interplexiform cells; the remainder are true amacrine cells. We partially reconstructed, from serial electron microscope autoradiograms, 25 GABA‐accumulating amacrines and distinguished four types based on cytoplasmic appearance, soma size and shape, and the form of primary and secondary processes. Type 1 had a large (609 ± 60 μm³), dark soma, and multiple, medium‐diameter (0.6 μm) processes splayed from the soma margins like the appendages from a crab. Type 2 had a medium (360 ± 40 μm³), helmet‐shaped, pale soma, and medium‐diameter (0.8 μm) processes that branched in sublamina α. Type 3 had a small (267 ± 44 μm³), dark, pyriform soma. The latter formed a single stout (3.0 μm) process that bifurcated in the middle of sublamina α. Type 4 had a very large, pale soma (860 μm³). This was pyriform, tapering into a stout (2.0 μm) process that descended into the middle of sublamina α where it emitted smaller tangential processes. It is to be expected that each of these amacrine cell types will have distinct functions in neurotransmitter retinal circuitry.