THE DEVELOPMENT OF EXCITATORY SYNAPSES IN CULTURED SPINAL NEURONS

Immunohistochemical studies of synapses in the CNS have demonstrated t hat glutamate receptors (GluRs) are concentrated at postsynaptic sites in vivo and in vitro (Baude et al., 1995). The mechanisms leading to receptor clustering at excitatory synapses are far less understood tha n those governing acetylcholine receptor accumulation at the neuromusc ular junction (Hall and Sanes, 1993) or glycine receptor aggregation a t central inhibitory synapses (Kirsch et al., 1993). Using cultured ra t spinal cord neurons, we demonstrate that clustering of the AMPA rece ptor subunit GluR1 is among the earliest events in excitatory synapse formation in vitro, coincident with the onset of miniature EPSCs and i n many cases preceding presynaptic vesicle accumulation. Postsynaptic receptor clustering is induced in a highly specific and reiterative pa ttern, independent of receptor activation, by contact with a subset of axons capable of inducing receptor clusters. The subunit composition of AMPA receptor clusters varied significantly between neurons but was invariant within a given neuron. The presence of either GluR2 or GluR 3 was common to all receptor clusters. Neither high-affinity glutamate transporters nor NMDA receptors appeared to be concentrated with AMPA receptor subunits at these excitatory synapses.