MEMBRANE-DELIMITED MODULATION OF NMDA CURRENTS BY METABOTROPIC GLUTAMATE-RECEPTOR SUBTYPES-1 5 IN CULTURED MOUSE CORTICAL-NEURONS/

1. Modulation of NMDA receptors by metabotropic glutamate receptors (m GluRs) in cultured mouse cortical neurons was investigated using whole -cell and single-channel recordings. 2. NMDA whole-cell current was re versibly attenuated by selective mGluR1/5 agonists (S)-3-hydroxyphenyl glycine (3HPG; 10-200 mu M), (S)-3,5-dihydroxyphenylglycine (S-DHPG; 1 00 mu M) and other mGluR agonists: (1S,3R)-1-aminocyclopentane-1,3-dec arboxylic acid (1S,3R-ACPD; 200 mu M), quisoualate (10 mu M) and (2S,1 'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 100 mu M). 3. The atte nuation of NMDA current by 3HPG was totally eliminated by the mGluR an tagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG; 500 mu M) and by the selective mGluR1/5 antagonist (S)-4-carboxyphenylglycine (4CPG ; 300 mu M). 4. mGluR2/3 agonists S,1'R,2'R'3'R)-2-(2,3-dicarboxycyclo propyl)glycine (DCG-IV; 3 mu M), (S)-4-carboxy-3-hydroxyphenylglycine (4C3HPG; 100-200 mu M) and (S)-4-carboxyphenylglycine (4CPG; 300 mu M) did not reduce NMDA current. 5. The NMDA-induced increase in intracel lular free Ca2+ measured by fura-2 Ca2+ imaging was attenuated by 3HPG (300 mu M). 6. The suppression of NMDA current by 3HPG was not affect ed by treatments that altered intracellular Ca2+ or cAMP levels, or by the protein kinase inhibitor, staurosporine (0.1-0.5 mu M). 7. The op en probability (NPo) of the NMDA receptor channel in excised outside-o ut patches was attenuated by 3HPG but not by 4C3HPG. This 3HPG effect was blocked by MCPG. 8. The 3HPG-induced reduction of NMDA whole-cell and single-channel currents was prevented by GDP beta S (200-400 mu M) . Intracellular dialysis of GTP gamma S (100 mu M) also reduced NMDA w hole-cell current, and rendered irreversible further reduction induced by 3HPG. 9. These data suggest that a selective activation of mGluR1/ 5 downmodulates the NMDA receptor channel in a membrane-delimited mann er, mediated by G proteins, but not by diffusible second messengers.