Surface AChE in the chick ciliary ganglion neurons: Ultrastructural localization and possible relations to ?-bungarotoxin receptors

The localization of acetylcholinesterase activity in the chick ciliary ganglion was investigated by ultrastructural cytochemistry. Both ganglionic cell populations, i.e. the ciliary and the choroid neurons, showed similar distribution patterns of the enzymic activity in the cytoplasm as well as at the neuronal surface. As indicated by specific inhibition tests, the whole enzymic activity was attributable to specific acetylcholinesterase. While the endocellular activity was mainly localized in the rough endoplasmic reticulum, the surface activity occurred at postsynaptic level and at extrasynaptic areas, where the neuronal membrane comes into contact with the plasma membrane of the satellite cell (boundary neuron-satellite cell). Enzymic activity also uniformly occurred at the surface of preganglionic nerve terminals. The surface localization of specific acetylcholinesterase recalls that recently described for ?-bungarotoxin receptors, which suggests that acetylcholinesterase and ?-bungarotoxin receptors can be distributed together, not only at postsynaptic level but also in extrasynaptic neuronal areas and at presynaptic level. The possibility that ?-bungarotoxin receptors and acetylcholinesterase form a .receptive' system not engaged in ganglionic transmission and not exclusively confined to postsynaptic level is discussed in relation to the electrophysiological data existing in literature.