‘Calcium bombs' as harbingers of synaptic pathology and their mitigation by magnesium at murine neuromuscular junctions

Excitotoxicity is thought to be an important factor in the onset and
progression of amyotrophic lateral sclerosis (ALS). Evidence from human
and animal studies also indicates that early signs of ALS include
degeneration of motor nerve terminals at neuromuscular junctions
(NMJs), before degeneration of motor neuron cell bodies. Here we
used a model of excitotoxicity at NMJs in isolated mouse muscle,
utilizing the organophosphorus (OP) compound omethoate, which inhibits
acetylcholinesterase activity. Acute exposure to omethoate (100 μM) induced
prolonged motor endplate contractures in response to brief tetanic nerve
stimulation at 20–50 Hz. In some muscle fibers, Fluo-4 fluorescence showed
association of these contractures with explosive increases in Ca2+ (“calcium
bombs”) localized to motor endplates. Calcium bombs were strongly and
selectively mitigated by increasing Mg2+ concentration in the bathing medium
from 1 to 5 mM. Overnight culture of nerve-muscle preparations from WldS
mice in omethoate or other OP insecticide components and their metabolites
(dimethoate, cyclohexanone, and cyclohexanol) induced degeneration of
NMJs. This degeneration was also strongly mitigated by increasing [Mg2+]
from 1 to 5 mM. Thus, equivalent increases in extracellular [Mg2+] mitigated
both post-synaptic calcium bombs and degeneration of NMJs. The data
support a link between Ca2+ and excitotoxicity at NMJs and suggest that
elevating extracellular [Mg2+] could be an effective intervention in treatment
of synaptic pathology induced by excitotoxic triggers.