Journal of Neuroscience, Vol 11, 2177-2186, Copyright © 1991 by Society for Neuroscience
Regulation of acetylcholine receptors on chick ciliary ganglion neurons by components from the synaptic target tissue
SW Halvorsen, HA Schmid, AE McEachern and DK Berg
Department of Biology, University of California, San Diego, La Jolla 92093.
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.