The mechanism underlying the upregulation of NMDA receptor function by group I metabotropic glutamate receptors (mGluRs), including mGluR1 and 5, is not known. Here we show that in cortical neurons, brief selective activation of group I mGluRs with (S)-3,5-dihydroxy-phenylglycine (DHPG) induced a Ca(2+)-calmodulin-dependent activation of Pyk2/CAKbeta and the Src-family kinases Src and Fyn that was independent of protein kinase C (PKC). Activation of Pyk2 and Src/Fyn kinases led to increased tyrosine phosphorylation of NMDA receptor subunits 2A and B (NR2A/B) and was blocked by a selective mGluR1 antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, but not an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine. Functional linkage between mGluR1 activation and NR2A tyrosine phosphorylation through Pyk2 and Src was also demonstrated after expression of these elements in human embryonic kidney 293 cells. Supporting functional consequences, selective activation of mGluR1 by DHPG induced a potentiation of NMDA receptor-mediated currents that was blocked by inhibiting mGluR1 or Src-family kinases. Furthermore, antagonizing calmodulin or mGluR1, but not PKC, reduced the basal tyrosine phosphorylation levels of Pyk2 and Src, suggesting that mGluR1 may control the basal activity of these kinases and thus the tyrosine phosphorylation levels of NMDA receptors.

Neuronal death induced by activating N-methyl-D-aspartate (NMDA) receptors has been linked to Ca2+ and Na+ influx through associated channels. Whole-cell recording from cultured mouse cortical neurons revealed a NMDA-evoked outward current, INMDA-K, carried by K+ efflux at membrane potentials positive to -86 millivolts. Cortical neurons exposed to NMDA in medium containing reduced Na+ and Ca2+ (as found in ischemic brain tissue) lost substantial intracellular K+ and underwent apoptosis. Both K+ loss and apoptosis were attenuated by increasing extracellular K+, even when voltage-gated Ca2+ channels were blocked. Thus NMDA receptor-mediated K+ efflux may contribute to neuronal apoptosis after brain ischemia.

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