Ca2+-permeable AMPA receptors and intracellular Ca2+ determine motoneuron vulnerability in rat spinal cord in vivo

Excitotoxicity mediated by overactivation of glutamate receptors, particularly the α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) type, has been implicated in motoneuron degeneration. AMPA receptors lacking the GluR2 subunit are permeable to Ca2+ and the entrance of this cation might be responsible for the selective vulnerability of spinal motoneurons in amyotrophic lateral sclerosis (ALS). To evaluate this hypothesis in vivo, we have used a model of motoneuron death in which AMPA, perfused by microdialysis in the rat lumbar spinal cord, produces ipsilateral paralysis and a remarkable loss of spinal motoneurons. Perfusion of 1-naphthyl acetyl spermine, a selective blocker of the Ca2+-permeable AMPA receptors, and of the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM), prevented the AMPA-induced paralysis and reduced by about 50% the loss of motoneurons. In addition, perfusion of pyruvate, an energy metabolic substrate, similarly prevented the paralysis and the motoneuron death. These results suggest that functional AMPA receptors lacking the GluR2 subunit are present in the rat spinal cord, and that motoneuron death is triggered by an increase of intracellular Ca2+ via such Ca2+-permeable AMPA receptors. Our finding that pyruvate also protected against the excitotoxic effects of AMPA suggests that the increased intracellular Ca2+ probably interferes with the mitochondrial energetic metabolism.