Distribution of GABA-immunoreactive amacrine cell synapses in the inner plexiform layer of macaque monkey retina

The distribution patterns of GABA immunoreactive (+) and immunonegative (-) amacrine cell synapses and profiles in the inner plexiform layer (IPL) were analyzed in three macaque monkey retinas using postembedding electron-microscopic (EM) immunogold cytochemistry. Synapses and profiles were counted at 5% intervals throughout the IPL depth in three EM montages (total area = 6509 microns 2), with 0% depth at the inner nuclear layer/IPL border. Nearly 70% of all amacrine synapses were GABA+, and they contacted all major classes of neurons that arborize in the IPL: bipolars (45%), ganglion cells (25%), and GABA+ (20%) and GABA- (10%) amacrines. A major relationship was seen between GABA+ amacrine processes and bipolar terminals: 76% of all amacrine-to-bipolar synapses were GABA+, and 82% of bipolar output dyads contained at least one GABA+ amacrine. GABA+ amacrine profiles (N = 2455) were concentrated in three wide bands at IPL depths of 0-25%, 40-60%, and 75-100%, corresponding to the dense bands seen with light-microscopic immunocytochemistry. In contrast, GABA+ amacrine synapses (N = 1081) were distributed evenly throughout the IPL depth, rather than being confined to the three dense bands. GABA- amacrine synapses (N = 516) were concentrated at 40% and 60% depths. Each category of amacrine output synapses had a characteristic pattern of stratification in the IPL. GABA+ amacrine-to-bipolar synapses occurred throughout the IPL but were most frequent at 20% and 80% IPL depths, where the dendrites of midget cone bipolars arborize (Polyak, 1941). In contrast, GABA+ amacrine-to-ganglion cell synapses were concentrated at 30% and 70% IPL depths, near the dendritic arborizations of parasol ganglion cells (Watanabe & Rodieck, 1989). GABA+ synapses onto bipolars and amacrines were also concentrated at the level of rod bipolar terminals. GABA+ amacrines must play significant but different roles in ON and OFF midget and parasol pathways as well as the rod pathway.