Our recent studies of chick parasympathetic ciliary ganglion (CG) neurons d
emonstrate a unique postsynaptic receptor microheterogeneity - under one pr
esynaptic terminal, excitatory nicotinic acetylcholine receptor (nAChR) clu
sters and separate inhibitory glycine receptor (GlyR) clusters coexist in d
istinct membrane microregions. Gephyrin, a peripheral membrane protein that
is required for GlyR clustering at synapses in the rodent central nervous
system, is also expressed in chick CG neurons where it codistributes with G
lyRs, but not nAChRs. We now extend these findings by characterizing the re
gulation of gephyrin expression in chick CG neurons in vivo. We show that d
evelopmental increases in gephyrin transcript levels occur during pre- and
postganglionic synapse formation. The increases are induced by both innerva
tion and target tissue interactions, with the target tissues having the gre
ater regulatory influence. The time course of the developmental rise in gep
hyrin mRNA levels most closely resembles that reported for functional GlyR
expression, but not that of functional nAChRs nor GABA(A) receptors. We als
o demonstrate that gephyrin is concentrated in the postsynaptic density of
a subset of synapses on both the ciliary and choroid neurons in the CG and
is stably expressed from embryonic to adult stages. Altogether, our results
suggest that gephyrin is a synapse organizing molecule that functions to l
ocalize GlyRs, but not nAChRs, to discrete postsynaptic membrane microregio
ns in chick CC neurons in vivo. (C) 2000 Elsevier Science Ltd. All rights r
eserved.