We have identified the only proteins known to function in localizing nAChRs to neuronal synapses.  The two key synapse organizers are one particular nAChR subunit, a3, and adenomatous polyposis coli (APC), a protein known best for its tumor-suppressing activity (Williams et al., 1998, 1999; Temburni et al., 2004).  APC’s neural function was previously undefined.  The importance of APC’s neural role is highlighted by reports that APC gene deletions occur in numerous forms of human mental retardation (Raedle et al., 2001). We are currently defining the specific role of APC and its binding partners in orchestrating the differentiation of pre-synaptic and postsynaptic specializa-tions in parasympathetic ciliary ganglion (CG) neurons.

In addition, our recent studies have identified a novel postsynaptic receptor heterogeneity on the CG neuron surface (Tsen et al., 2000).  We show that excitatory nAChR clusters are separate from, but proximal to, inhibitory glycine receptor clusters in discrete postsynaptic membrane microregions on the neurons- all under one presynaptic terminal. Further, our data suggest that two distinct release mechanisms, vesicular-mediated ACh release and carrier-mediated glycine efflux, both occur at the synapse in vivo and jointly regulate synaptic transmission.  The presence under one presynaptic terminal of separate clusters of receptors that respond to different fast-acting transmitters with opposing actions is likely to represent a new mechanism for modulating synaptic activity.

Our current studies have identified even greater receptor heterogeneity.  We have found that, in addition to expressing nAChRs and glycine receptors, CG neurons also express glutamate receptors during synapse formation.  We are defining the role of glutamate receptors in directing the proper formation and function of ganglionic synaptic connections.