Glycinergic contacts in the outer plexiform layer of the Xenopus laevis retina characterized by antibodies to glycine, GABA, and glycine receptors

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Abstract
Electrophysiological experiments have predicted a direct synaptic input from glycinergic interplexiform cells (IPCs) to GABAergic horizontal cells in the Xenopus retina. However, previous ultrastructural studies failed to demonstrate this input. Here, we used three immunocytochemical approaches to investigate this issue. First, double‐label postembedding immunocytochemistry with GABA‐ and glycine‐like immunoreactivity (GABA‐LI and glycine‐LI) was used to study possible interactions of the glycinergic IPC with GABAergic horizontal cells. Processes postsynaptic to glycine‐LI IPC terminals in the outer plexiform layer (OPL) fell into two groups, small microtubule‐filled processes and larger electron‐lucent processes with sparse microtubules and occasional mitochondria. In no case did we find glycine‐LI synapses onto GABA‐LI cells or processes. Second, pre‐embedding immunocytochemistry was used to label GABA‐LI cells and processes in the OPL. GABA‐LI was sparse in horizontal cell axons and more intense in horizontal cell somas and in small processes. In agreement with our first set of experiments, GABA‐LI profiles did not receive input from conventional synapses. Third, we localized glycine‐receptor‐like immunoreactivity (GlyR‐LI) to several types of apparent synapses in the OPL. As expected, it was found at IPC synapses. Unexpectedly, GlyR‐LI was also subsynaptic at photoreceptor synapses onto second order neurons, both at ribbon and basal junction type synapses. At least some of the GlyR‐LI photoreceptor synapses were from cones. Also, GlyR‐LI was apposed to photoreceptors and to unidentified small diameter processes, where no other indication of synaptic input was evident. Because glycine‐LI is not found in photoreceptors, we suggest that glycine receptors at photoreceptor synapses are stimulated by glycine that diffuses from other sites, possibly from IPCs. This interpretation is consistent with available physiological studies of glycinergic effects in this retina.

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