Co-stimulation of cyclic-AMP-linked metabotropic glutamate receptors in rat striatum attenuates excitotoxin-induced nuclear factor-kappaB activation and apoptosis.

Interactions between glutamatergic mechanisms mediated by receptors of the ionotropic and metabotropic classes in the central nervous system are complex and incompletely understood. To explore the consequences of these interactions on excitotoxicity, we examined the influence of group II and group III selective metabotropic glutamate receptor agonists on the N-methyl-D-aspartate-induced apoptotic destruction of GABAergic neurons in rat striatum. The intrastriatal administration of a group III metabotropic glutamate receptor agonist (amino-4-phosphonobutyric acid, 900-1800 nmol), but not of a group II agonist [(2S,1'S,2'S)-(carboxycyclopropyl)glycine, 100-1800 nmol] produced internucleosomal DNA fragmentation. Similarly, amino-4-phosphonobutyric acid (600 nmol) but not (2S,1'S,2'S)-(carboxycyclopropyl)glycine (100-1800 nmol) destroyed some striatal neurons as indicated by a loss of D1 dopamine receptors and 67,000 mol. wt glutamate decarboxylase (glutamate decarboxylase-67) messenger RNA. On the other hand, the intensity of internucleosomal DNA fragmentation induced by N-methyl-D aspartate (150 nmol) was substantially decreased by the intrastriatal co-administration of either (2S,1'S,2'S)-(carboxycyclopropyl)glycine or amino-4-phosphonobutyric acid (100-600 nmol). Both (2S, 1'S,2'S)-(carboxycyclopropyl)glycine and amino-4-phosphonobutyric acid also reduced the N-methyl-D-aspartate-induced loss of striatal D1 dopamine receptors by 67% and 68% (both P < 0.001), and glutamate decarboxylase-67 messenger RNA by 68% and 61%, respectively. Furthermore, both (2S,1'S,2'S)-(carboxycyclopropyl)glycine and amino-4-phosphonobutyric acid also attenuated the N-methyl-D-aspartate-induced decline in striatal IKB-alpha protein levels by 62% and 37%, as well as the increase in nuclear transcription factor nuclear factor-kappaB binding activity by 135% and 94% (both P < 0.001), and the subsequent rise in p53 and c-Myc protein levels. These results suggest that stimulation of cyclic-AMP-linked metabotropic glutamate receptors inhibits ionotropic glutamate receptor-mediated activation of apoptotic cascades involving IkappaB-alpha degradation and nuclear factor-kappaB nuclear translocation, as well as p53 and c-Myc induction. Certain selective metabotropic glutamate receptor agonists might thus find utility as adjuncts to N-methyl-D-aspartate antagonists in the protection against the neurotoxicity initiated by excessive ionotropic glutamate receptor stimulation.