Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the progressive loss of spinal cord and brainstem motoneurons. Although the pathogenesis is incompletely understood, oxidative stress and excitotoxicity are suggested to play a pivotal role. In 25 % of cases, ALS has a predominantly bulbar onset primarily affecting the nucleus hypoglossus, responsible for tongue muscle innervation. Since it is unclear how hypoglossal motoneurons (HMs) respond to oxidative stress in functional terms, the present study sought to investigate it by using, as a model, a slice preparation of the neonatal rat brainstem.

HMs were recorded with whole-cell patch clamping before and during application of H₂O₂ as a donor of free oxygen radicals and also examined (with confocal microscopy) for their oxidative stress condition and damage.

In voltage clamp configuration (holding potential = -70 mV), bath application of 1 mM H₂O₂ (30 min) produced a slowly developing inward current in addition to a significant rise in motoneuron input resistance and large depression of all synaptic events. The effects elicited by H₂O₂ were insensitive to blockers of excitatory or inhibitory synaptic transmission. Miniature synaptic events frequency did not change, while the voltage-activated persistent Ca²⁺ current (I_CaP) evoked by depolarizing ramps was reduced. Under current clamp, H₂O₂ induced a slow depolarization accompanied by increased spike threshold and firing frequency in response to current injection from similar baseline membrane potential. These effects were also resistant to blockers of fast synaptic transmission and were poorly reversible on washout. Oxidative stress condition induced by H₂O₂ application was assessed on the basis of the rhodamine 123 marker. Propidium iodide and hoechst 33342 stainings were instead used as indices of injured and viable cells, respectively.

These data suggest that even a short application of H₂O₂ to HMs could evoke a persistent electrophysiological deficit associated with a demonstrable oxidative stress, without altering short-term cell survival. Oxidative stress paradoxically made motoneurons more isolated from their network because of impaired synaptic transmission, yet more intrinsically excitable. One can surmise that this sequence of events might be occurring at an early stage of ALS and lead to severe cell dysfunction.