Regulation of acetylcholine receptors on chick ciliary ganglion neurons by components from the synaptic target tissue.

Chick ciliary ganglion neurons have nicotinic acetylcholine receptors (AChRs) that mediate synaptic transmission through the ganglion. A soluble component of about 50 kDa from embryonic eye tissue, the synaptic target of the ganglion, increases the development of ACh sensitivity by the neurons 10-fold over a 1-week period in culture. The increased sensitivity does not arise from a change in agonist affinity or esterase activity. Both the basal ACh response obtained in the absence of the 50-kDa component and the elevated responses obtained with it can be inhibited by neuronal bungarotoxin (nBgt) but not by alpha-bungarotoxin (alpha Bgt). Increases of less than twofold are observed for the binding of anti-AChR monoclonal antibody 35 (mAb 35), nBgt, and alpha Bgt to the neurons under these conditions. Extract fractions containing the 50-kDa component also enable the neurons to enhance their ACh responses through a cAMP-dependent mechanism. Either the 50-kDa fraction induces the appearance of a new type of AChR regulated by cAMP, or it alters the function of existing AChRs. The 50-kDa fraction produces no change in neuronal growth but can increase GABA responses sixfold, indicating that its effects are not confined to AChRs. It is not clear whether a single molecular species is responsible for the diverse regulatory effects or whether several types of active components are present in the fraction. The component which enhances ACh sensitivity is trypsin-sensitive and heat-labile, as expected for a protein. The component may be widely distributed since the 50-kDa fraction from a number of tissues can increase the ACh response. The fraction from eye tissue, however, has a specific activity 5-10 times greater than that of the liver fraction. A wide distribution would suggest multiple targets and roles for the component during development.