NMDA and non-NMDA receptor-mediated differential Ca super(2+) load and greater vulnerability of motor neurons in spinal cord cultures

Glutamate receptor activated neuronal cell death has been implicated in the pathogenesis of motor neuron disease but the molecular mechanism responsible for neuronal dysfunction needs to be elucidated. In the present study, we examined the contribution of NMDA and non-NMDA sub-types of glutamate receptors in selective vulnerability of motor neurons. Glutamate receptor activated Ca super(2+) signaling, mitochondrial functions and neurotoxicity in motor neurons and other spinal neurons were studied in mixed spinal cord primary cultures. Exposure of cells to glutamate receptor agonists glutamate, NMDA and AMPA elevated the intracellular Ca super(2+), mitochondrial Ca super(2+) and caused mitochondrial depolarization and cytotoxicity in both motor neurons and other spinal neurons but a striking difference was observed in the magnitude and temporal patterns of the [Ca super(2+)] sub(i) responses between the two neuronal cell types. The motor neurons elicited higher Ca super(2+) load than the other spinal neurons and the [Ca super(2+)] sub(i) levels were elevated for a longer duration in motor neurons. AMPA receptor stimulation was more effective than NMDA. Both the NMDA and non-NMDA receptor antagonists APV and NBQX inhibited the Ca super(2+) entry and decreased the cell death significantly; however, NBQX was more potent than APV. Our results demonstrate that both NMDA and non-NMDA sub-types of glutamate receptors contribute to glutamate-mediated motor neuron damage but AMPA receptors play the major role. AMPA receptor-mediated excessive Ca super(2+) load and differential handling/regulation of Ca super(2+) buffering by mitochondria in motor neurons could be central in their selective vulnerability to excitotoxicity.