Interaction between metabotropic and NMDA subtypes of glutamate receptors in sprout suppression at young synapses.

Recently, NMDA receptors (NMDARs) have been implicated in a cell contact-dependent suppression of sprouting in cultured Xenopus tectal neurons during an early period when neither AMPA/kainate (KA) receptors nor action potentials play a prominent role in cell-cell communication. We asked how the NMDA receptors function in the absence of the depolarizing effect of AMPA/KA receptor activity. We show that type II metabotropic glutamate receptors (mGluRs) can operate synergistically with NMDA receptors in the absence of AMPA/KA receptor function to suppress an early neurite sprouting response of the tectal neurons. Calcium imaging with fluo-3 AM and morphological analyses after exposure to glutamate receptor antagonists show that a combination of AMPA/KA receptor and type II mGluR blockers mimics the decrease in intracellular free calcium in response to glutamate and the structural effects produced by NMDA receptor antagonists in these cultures. Patch-clamp analyses confirmed a decrease in NMDA receptor-mediated currents with type II mGluR blockade, and 8-bromo cAMP application produced a decrease in NMDA receptor-mediated calcium influx. These data suggest that type II mGluRs potentiate NMDA receptor function by decreasing cAMP levels in tectal neurons. We also show that NMDARs exhibit low magnesium sensitivity in tectal neurons during the first few days in culture. Thus both metabotropic and ionotropic glutamate receptors can play a role in the contact-mediated suppression of ongoing sprouting at early neuron-neuron contacts before action potential activity.