CLUSTERING OF GEPHYRIN AT GABAERGIC BUT NOT GLUTAMATERGIC SYNAPSES INCULTURED RAT HIPPOCAMPAL-NEURONS

The molecular mechanisms underlying the establishment of a postsynapti c receptor mosaic on CNS neurons are poorly understood. One protein th ought to be involved is gephyrin, a peripheral membrane protein that b inds to the inhibitory glycine receptor and functions in clustering th is receptor at synapses in cultured rat spinal cord neurons. We invest igated the possible association of gephyrin with synapses in cultured rat hippocampal neurons, where glutamate and GABA but not glycine are the principal transmitters. Gephyrin immunoreactivity was detected in axons as well as dendrites, changing from a predominantly axonal to a more dendritic distribution with time in culture. Gephyrin staining wa s not distributed uniformly, but always took the form of clusters. Sma ll clusters of gephyrin (0.2 mu m(2)), present throughout development, were distributed widely and not restricted to synaptic sites. Larger clusters of gephyrin (0.4-10.0 mu m(2), sometimes composed of groups o f small clusters), which developed in older cells, were localized to a subset of contacts between axons and dendrites. These large clusters were not present at glutamatergic synapses (marked by immunostaining f or GluR1), but were closely associated with GABAergic synapses (marked by immunostaining for GABA and glutamic acid decarboxylase). These re sults, together with previous findings, suggest that gephyrin may func tion to anchor GABA and glycine receptors, but not glutamate receptors , at postsynaptic sites on central neurons. They also raise the possib ility that gephyrin has additional functions, independent of its role at synapses.