Synaptic control of glycine and GABA(A) receptors and gephyrin expression in cultured motoneurons

We have evaluated the influence of the secretory phenotype of presynaptic b outons on the accumulation of postsynaptic glycine receptors (GlyRs), type A GABA receptors (GABA(A)Rs), and gephyrin clusters. The cellular distribut ion of these components was analyzed on motoneurons cultured either alone o r with glycinergic and/or GABAergic neurons. In motoneurons cultured alone, we observed gephyrin clusters at nonsynaptic sites and in front of choline rgic boutons, whereas glycine and GABA(A) receptors formed nonsynaptic clus ters. These receptors are functionally and pharmacologically similar to tho se found in cultures of all spinal neurons. Motoneurons receiving GABAergic innervation from dorsal root ganglia neurons displayed postsynaptic cluste rs of gephyrin and GABA(A)R beta but not of GlyR alpha/beta subunits. In mo toneurons receiving glycinergic and GABAergic innervation from spinal inter neurons, gephyrin, GlyR alpha/beta, and GABA(A)R beta formed mosaics at syn aptic loci. These results indicate that (1) the transmitter phenotype of th e presynaptic element determines the postsynaptic accumulation of specific receptors but not of gephyrin and (2) the postsynaptic accumulation of geph yrin alone cannot account for the formation of GlyR-rich microdomains.