Intracellular Mechanisms Underlying the Suppression of AMPA Responses by trans-ACPD in Cultured Chick Purkinje...

Intracellular mechanisms underlying the functional suppression of ionotropic glutamate receptors by activation of metabotropic glutamate receptors were investigated in cultured chick Purkinje neurons. The intracellularly recorded depolarization induced by L-AMPA (an ionotropic glutamate receptor agonist, (S)-alpha-amino-4-hydroxy-5-methyl-4-isoxazolepropionic acid) and the L-AMPA-induced inward current recorded by whole-cell voltage clamping were used. L-AMPA responses were suppressed by trans-ACPD (a selective agonist of metabotropic glutamate receptor, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid) for various durations, with the longest duration about 60 min. This trans-ACPD effect was antagonized by l-(+)-2-amino-3-phosphonopropionic acid (a metabotropic glutamate receptor antagonist) and N(G) -monomethyl-L-arginine (a nitric oxide synthase inhibitor). Sodium nitroprusside, 3-morpholinosydnonimine (nitric oxide donors), and potassium ferricyanide mimicked trans-ACPD, and effects of trans -ACPD, sodium nitroprusside, and 3-morpholinosydnonimine were blocked by hemoglobin (a nitric oxide scavenger) but not by methemoglobin, while the effect of potassium ferricyanide was not affected by either hemoglobin or methemoglobin. 8-Bromo-cGMP also suppressed L-AMPA responses. KT5823 (a protein kinase G inhibitor) antagonized effects of trans-ACPD, 8-bromo-cGMP, and sodium nitroprusside. Phorbol 12,13-diacetate (a protein kinase C activator) also suppressed L-AMPA responses, and phorbol 12,13-diacetate plus trans-ACPD or phorhol 12,13-diacetate plus sodium nitroprusside showed an additive effect. Calphostin C and polymyxin B (protein kinase C inhibitors) antagonized the effect of trans-ACPD. These results suggest that activation of metabotropic glutamate receptors leads to the functional suppression of L-AMPA-sensitive ionotropic glutamate receptors in chick Purkinje neurons, and trans-ACPD-induced suppression of L-AMPA responses can be mimicked by activation of protein kinase G and/or protein kinase C. The involvement of nitric oxide in the trans-ACPD effect is discussed.