Mechanisms involved in the metabotropic glutamate receptor‐enhancement of NMDA‐mediated motoneurone responses in frog spinal cord

The metabotropic glutamate receptor (mGluR) agonist trans ‐(±)‐1‐amino‐1,3‐cyclopentanedicarboxylic acid ( trans ‐ACPD) (10–100 μ M ) depolarized isolated frog spinal cord motoneurones, a process sensitive to kynurenate (1.0 m M ) and tetrodotoxin (TTX) (0.783 μ M ). In the presence of NMDA open channel blockers [Mg 2+ ; (+)‐5‐methyl‐10,11‐dihydro‐5 H ‐dibenzo[ a,d ]cyclohepten‐5,10‐imine hydrogen maleate (MK801); 3,5‐dimethyl‐1‐adamantanamine hydrochloride (memantine)] and TTX, trans ‐ACPD significantly potentiated NMDA‐induced motoneurone depolarizations, but not α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐proprionate (AMPA)‐ or kainate‐induced depolarizations. NMDA potentiation was blocked by ( RS )‐α‐methyl‐4‐carboxyphenylglycine (MCPG) (240 μ M ), but not by α‐methyl‐(2 S ,3 S ,4 S )‐α‐(carboxycyclopropyl)‐glycine (MCCG) (290 μ M ) or by α‐methyl‐( S )‐2‐amino‐4‐phosphonobutyrate (L‐MAP4) (250 μ M ), and was mimicked by 3,5‐dihydroxyphenylglycine (DHPG) (30 μ M ), but not by L(+)‐2‐amino‐4‐phosphonobutyrate (L‐AP4) (100 μ M ). Therefore, trans ‐ACPD's facilitatory effects appear to involve group I mGluRs. Potentiation was prevented by the G‐protein decoupling agent pertussis toxin (3–6 ng ml −1 , 36 h preincubation). The protein kinase C inhibitors staurosporine (2.0 μ M ) and N ‐(2‐aminoethyl)‐5‐isoquinolinesulphonamide HCl (H9) (77 μ M ) did not significantly reduce enhanced NMDA responses. Protein kinase C activation with phorbol‐12‐myristate 13‐acetate (5.0 μ M ) had no effect. Intracellular Ca 2+ depletion with thapsigargin (0.1 μ M ) (which inhibits Ca 2+ /ATPase), 1,2‐bis( O ‐aminophenoxy)ethane‐ N,N,N′,N′ ‐tetracetic acid acetyl methyl ester (BAPTA‐AM) (50 μ M ) (which buffers elevations of [Ca 2+ ] i ), and bathing spinal cords in nominally Ca 2+ ‐free medium all reduced trans ‐ACPD's effects. The calmodulin antagonists N ‐(6‐aminohexyl)‐5‐chloro‐1‐naphthalenesulphonamide (W7) (100 μ M ) and chlorpromazine (100 μ M ) diminished the potentiation. In summary, group I mGluRs selectively facilitate NMDA‐depolarization of frog motoneurones via a G‐protein, a rise in [Ca 2+ ] i from the presumed generation of phosphoinositides, binding of Ca 2+ to calmodulin, and lessening of the Mg 2+ ‐produced channel block of the NMDA receptor.British Journal of Pharmacology (1999) 126 , 333–341; doi: 10.1038/sj.bjp.0702263