Extrasynaptic localization of ?-bungarotoxin receptors in the rat superior cervical ganglion.

Binding of [(125)I]?-bungarotoxin to nicotinic cholinergic receptors (?-bungarotoxin receptors) was investigated in the rat superior cervical ganglion by light and electron microscope autoradiography. Both techniques indicated that labelling, which was inhibited by d-tubocurarine, occurred around and/or over neuronal perikarya. In particular, ultrastructural autoradiography showed that the synapses were devoid of radioactivity, suggesting that ?-bungarotoxin receptors in the rat superior cervical ganglion are molecules distinct from the nicotinic (postsynaptic) receptors normally involved in ganglionic transmission. By contrast, specific labelling was found in extrasynaptic areas of the neuronal membrane in contact with satellite cells (neuron-satellite cell boundary). Quantitative analysis indicated that at that level silver grains were present on both the neuronal membrane and satellite cells. Furthermore, beside neuronal perikarya, radioactivity was also found around nerve fibres, probably in relation to both the axonal and interstitial sides of the ensheathing Schwann cells. Only a few grains were clearly accumulated inside nerve fibres. Finally, significant amounts of specific radioactivity were detected in the neuronal cytoplasm, especially at the level of rough endoplasmic reticulum and Golgi apparatus. However, parallel diffusion experiments with [(125)I]?-bungarotoxin and [(3)H]inulin (a marker for the extracellular space) provided no evidence that the toxin enters the neuronal cytoplasm. Thus, the intraneuronal (specific) labeling was probably a reflection of ?-bungarotoxin binding to membrane receptors and the subsequent internalization of the toxin-receptor complex in the neurons. We conclude that in the rat superior cervical ganglion extrasynaptic nicotinic acetylcholine receptors (?-bungarotoxin receptors) may be widely located on the neuronal membrane as well as on the plasma membrane of satellite and Schwann cells. The physiological significance of this molecular architecture is discussed.