Loss of nitric oxide-mediated down-regulation of NMDA receptors in neurofilament aggregate-bearing motor neurons in vitro: implications for motor neuron disease

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder in which excitotoxicity has been implicated as a cause for cell death. To examine neurofilament (NF) aggregate-mediated sensitization of motor neurons to NMDA excitotoxicity, we examined NMDA receptor expression and the impact of NO donors (NOC12 or NOC5) or sodium cyanide (NaCN) on calcium influx and viability in dissociated motor neurons derived from wt and hNFL+/+ (NF aggregate-forming) mice. Alterations in intracellular calcium were assayed using Oregon Green calcium dye and the extent of apoptosis using active caspase-3 immunoreactivity. Although NF aggregate-bearing neurons demonstrated increased intracellular calcium levels and enhanced cell death in response to NMDA receptor activation, this was not associated with increased NMDA receptor expression. The down-regulation of the NMDA receptor using NO donors decreased calcium influx and caspase-3 activation in aggregate-bearing neurons, but had no effect on wt cultures. The converse was observed with NaCN in which intracellular calcium levels increased significantly in wt cultures in association with increased cell death. No effect was observed in aggregate-bearing neurons. These findings suggest that the presence of NF aggregates renders motor neurons more susceptible to NMDA-mediated excitotoxicity, and that this can be reversed by NO.