Localization of synaptic and nonsynaptic nicotinic‐acetylcholine receptors in the goldfish retina

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Abstract
The localization of nicotinic‐cholinergic receptors in the inner plexiform layer (IPL) of goldfish retina was studied by electron microscopic analysis of the binding pattern of a conjugate of horseradish peroxidase and αbungarotoxin (HRP‐αBTx). Specific HRP reaction product (blockable by 1mm curare) was found at both synaptic and nonsynaptic sites. Synaptic binding sites for HRP‐αBTx, which accounted for only 16% of the total specific reaction product sites, always involved an amacrine process as the presynaptic element, whereas amacrine, ganglion, and bipolar cells could be postsynaptic elements at labeled synapses. Only 17.5% of the total number of amacrine synapses were labeled by HRP‐αBTx. Labeled synapses showed the same distribution in the IPL as unlabeled synapses: bimodal for amacrine‐to‐bipolar synapses with peak concentrations at the 20% and 80% layers and unimodal for amacrine‐to‐nonbipolar synapses with a peak concentration at the 60% layer. Nonsynaptic binding sites for HRP‐αBTx (84% of total) were seen on the dendrites of ganglion, amacrine, and bipolar cells. The distribution of the nonsynaptic sites in the IPL largely accounts for the trilaminar binding pattern of 125I‐αBTx as observed in light microscopic autoradiographs. If, as appears likely, the distribution of synapses is the relevant variable in determining the sites of neuronal interaction for a given transmitter system, then this study further illustrates the importance of distinguishing synaptic from nonsynaptic binding when using receptorligand probes to localize sites of chemical synaptic transmission.