Four types of amacrine in the cat retina that accumulate GABA

Roughly one-quarter of neurons in the amacrine cell layer accumulate exogenous gamma-aminobutyric acid (GABA). Some of these (8%) are interplexiform cells; the remainder are true amacrine cells. We partially reconstructed, from serial electron microscopy 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 microns3), dark soma, and multiple, medium-diameter (0.6 microns) processes splayed from the soma margins like the appendages from a crab. Type 2 had a medium (360 +/- 40 microns3), helmet-shaped, pale soma, and medium-diameter (0.8 microns) processes that branched in sublamina alpha. Type 3 had a small (267 +/- 44 microns3), dark, pyriform soma. The latter formed a single stout (3.0 microns) process that bifurcated in the middle of sublamina alpha. Type 4 had a very large, pale soma (860 microns3). This was pyriform, tapering into a stout (2.0 microns) process that descended into the middle of sublamina alpha 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.