Distinct roles for ionotropic and metabotropic glutamate receptors in the maturation of excitatory synapses

We used the single-cell culture preparation to study the role of activity i n the development of glutamatergic synapses in vitro. Rat hippocampal cells grown in isolation on glial islands formed functional autaptic connections and continued to elaborate new synapses throughout the 2 week investigatio n, resulting in increases in both the evoked AMPA receptor (AMPAR) and NMDA receptor (NMDAR) components of the EPSC. Synaptogenesis was not prevented by chronic blockade of sodium channels or all of the known glutamate recept ors. Analysis of miniature EPSCs revealed that AMPAR quantal size doubled o ver time in vitro whereas NMDAR quantal size remained constant. However, th e proportion of synaptic responses mediated only by NMDARs increased over t ime in vitro. The increase in AMPAR quantal size was prevented by TTX and i onotropic glutamate receptor antagonists, whereas the increase in the propo rtion of NMDAR-only synapses was prevented by metabotropic glutamate recept or antagonists. Notably, chronic NMDAR blockade incubation did not block th e formation of the AMPAR EPSC, indicating that NMDAR-dependent plasticity i s not necessary for the onset of AMPAR synaptic transmission in this system . We conclude that action potentials and ionotropic glutamate receptor acti vation are necessary for the developmental increase in AMPAR quantal size a nd that metabotropic glutamate receptor activation is required for the prod uction of NMDAR-only synapses, but none of these is essential for synapse f ormation.