The synaptic input to OFF-center alpha ganglion cells in the cat retina was analyzed by electron microscopic reconstruction to quantify the bipolar and amacrine cell input and to determine the neurotransmitter content of the presynaptic cells. Cone bipolar cells were found to comprise 11% of the total input with their dyad synapses distributed across the dendritic tree. The remaining contacts were conventional synapses indicative of amacrine cells; postembedding immunogold labeling was used to characterize these cells as either GABA- or glycine-immunoreactive. Results showed the amacrine input to be equally divided between GABA and glycinergic contacts at each order of dendritic branching of the alpha cells. Among the GABA-positive neurons were A19 amacrine cells, the processes of which are characterized by a dense array of neurotubules. A major source of glycinergic input was from lobular appendages of AII amacrine cells with lesser contributions from other glycine-positive amacrine cells. The physiological role(s) of these amino acids must be interpreted in view of the multiple subpopulations of amacrine cells, which provide input to OFF-alpha cells, and the diversity in receptors at their synapses.
Copyright 1999 Wiley-Liss, Inc.