Mechanisms of excitotoxicity: AMPA-mediated calcium dynamics in spinal motoneurons

Introduction: ALS is a neurodegenerative disease, which is characterized by rapid loss of motoneurons. Chronic AMPA- receptor mediated excitotoxicity seems to play a major role in the pathogenetic process, but the mechanisms of the selective vulnerability of motoneurons are still unknown. Apoptosis like degeneration of motoneurons after excitotoxic insults can be imitated in vitro, proposing extensive mitochondrial calcium uptake, increased production of reactive oxygen species and delayed collaps of calcium homeostasis as mechanisms involved.

Methods: Ventral spinal chord was prepared from 14- day- old rat embryos. Motoneurons were enriched and cultivated on a schwann- cell feeder layer. FURA-2 fluorescent imaging was used to monitor the cytosolic calcium concentration [Ca2+]cyt under application of drugs and short defined 2 second stimuli of the AMPA- receptor- agonist kainate with a fast solution exchange system.

Results: Short kainate application produced large calcium transients in motoneurons, which were eliminated under either CNQX, EGTA or dantrolene exposure. The reduction of the extracellular calcium concentration from 3.2 mM (normal) to 0.3 mM did not diminish the peaks in [Ca2+]cyt after kainate stimuli.

Conclusion: Calcium transients in motoneurons after AMPA- receptor stimulation are caused by ryanodin- receptor mediated calcium release, most likely from the endoplasmatic reticulum. Low extracellular calcium concentrations are sufficient to trigger this mechanism in an „all or nothing“ manner. In this context the ryanodin- receptor antagonist dantrolene could turn out to be a neuroprotective drug in ALS.