Abstract
In peripheral myelinated axons of mammalian spinal motor neurons, Ca(2+) influx was thought to occur only in pathological conditions such as ischaemia. Using Ca(2+) imaging in mouse large motor axons, we find that physiological stimulation with trains of action potentials transiently elevates axoplasmic [C(2+)] around nodes of Ranvier. These stimulation-induced [Ca(2+)] elevations require Ca(2+) influx, and are partially reduced by blocking T-type Ca(2+) channels (e.g. mibefradil) and by blocking the Na(+)/Ca(2+) exchanger (NCX), suggesting an important contribution of Ca(2+) influx via reverse-mode NCX activity. Acute disruption of paranodal myelin dramatically increases stimulation-induced [Ca(2+)] elevations around nodes by allowing activation of sub-myelin L-type (nimodipine-sensitive) Ca(2+) channels. The Ca(2+) that enters myelinated motor axons during normal activity is likely to contribute to several signalling pathways; the larger Ca(2+) influx that occurs following demyelination may contribute to the axonal degeneration that occurs in peripheral demyelinating diseases. Activity-dependent Ca(2+) signalling is well established for somata and terminals of mammalian spinal motor neurons, but not for their axons. Imaging of an intra-axonally injected fluorescent [Ca(2+)] indicator revealed that during repetitive action potential stimulation, [Ca(2+)] elevations localized to nodal regions occurred in mouse motor axons from ventral roots, phrenic nerve and intramuscular branches. These [Ca(2+)] elevations (∼ 0.1 μm with stimulation at 50 Hz, 10 s) were blocked by removal of Ca(2+) from the extracellular solution. Effects of pharmacological blockers indicated contributions from both T-type Ca(2+) channels and reverse mode Na(+)/Ca(2+) exchange (NCX). Acute disruption of paranodal myelin (by stretch or lysophosphatidylcholine) increased the stimulation-induced [Ca(2+)] elevations, which now included a prominent contribution from L-type Ca(2+) channels. These results suggest that the peri-nodal axolemma of motor axons includes multiple pathways for stimulation-induced Ca(2+) influx, some active in normally-myelinated axons (T-type channels, NCX), others active only when exposed by myelin disruption (L-type channels). The modest axoplasmic peri-nodal [Ca(2+)] elevations measured in intact motor axons might mediate local responses to axonal activation. The larger [Ca(2+) ] elevations measured after myelin disruption might, over time, contribute to the axonal degeneration observed in peripheral demyelinating neuropathies.