GABA-immunoreactive profiles provide synaptic input to the soma, axon hillock, and axon initial segment of ganglion cells in primate retina

GABA-immunoreactive displaced amacrines were previously shown to make synapses onto neuronal profiles in the ganglion cell layer (GCL) of macaque monkey retina [Koontz, Hendrickson and Ryan (1989) Visual Neuroscience, 2, 19-25]. These postsynaptic elements have been investigated further using postembedding immunogold methods for electron microscopy. This paper provides ultrastructural evidence that GABA-immunoreactive profiles are presynaptic to the ganglion cell soma, axon hillock, and axon initial segment in the GCL and its border with the nerve fiber layer (NFL). Some axonal profiles have a dense undercoat and fasciculated microtubules, features that are characteristic of the axon initial segment in many neurons of both central and peripheral nervous systems. These features are confined to small- and medium-diameter (0.2-0.6 microns) axon profiles located near the GCL/NFL border and are not found on axonal profiles lying deep in the NFL, suggesting that the dense-coated region does not extend far along the axon and that the dense-coated region may be narrower than the distal part of the axon. The dense-coated region may correspond to the ganglion cell "narrow segment" recently described in a variety of species using light microscopic methods. The results presented here strengthen our previous hypothesis that GABA-immunoreactive neurons in the GCL provide direct synaptic input to ganglion cells near the site of action potential initiation.