Localization of synaptic and nonsynaptic nicotinic-acetylcholine receptors in the goldfish retina.
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 or horseradish peroxidase and alpha bungarotoxin (HRP-alpha BTx). Specific HRP reaction product (blockade by 1mM curare) was found at both synaptic and nonsynaptic sites. Synaptic binding sites for HRP-alpha BTx, 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 post-synaptic elements at labeled synapses. Only 17.5% of the total number of amacrine synapses were labeled by HRP-alpha BTx. 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-alpha BTx (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-alpha BTx 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 receptor-ligand probes to localize sites of chemical synaptic transmission.
D000818:Animals; D001345:Autoradiography; D002038:Bungarotoxins / Q000378:metabolism; D003530:Cyprinidae / Q000033:anatomy & histology*; D006054:Goldfish / Q000033:anatomy & histology*; D006735:Horseradish Peroxidase / Q000378:metabolism; D008854:Microscopy, Electron; D011950:Receptors, Cholinergic / Q000302:isolation & purification*; D017981:Receptors, Neurotransmitter / Q000302:isolation & purification*; D011978:Receptors, Nicotinic / Q000302:isolation & purification*; D012160:Retina / Q000378:metabolism / Q000648:ultrastructure*; D013569:Synapses / Q000378:metabolism
D002038:Bungarotoxins; D011950:Receptors, Cholinergic; D017981:Receptors, Neurotransmitter; D011978:Receptors, Nicotinic; D006735:Horseradish Peroxidase