Synaptic interactions regulate gephyrin expression in avian cholinergic neurons in vivo

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.