AXOTOMY-INDUCED AXONAL DEGENERATION IS MEDIATED BY CALCIUM INFLUX THROUGH ION-SPECIFIC CHANNELS

We examined the role of extracellular calcium entry, the possible invo lvement of axonal calcium channels, and the potential protective effec t of calcium channel and calpain antagonists in axotomy-induced axonal degeneration using murine dorsal root ganglia in cell culture. We fou nd that calcium entry is both necessary and sufficient to induce axona l degeneration after axotomy, and may be inhibited by cobalt, manganes e, dihydropyridines, and bepridil. Tetrodotoxin and omega-conotoxin ar e ineffective in preventing axonal degeneration. The activation of cal pains also appears to be necessary and sufficient for axonal degenerat ion to proceed, and can be blocked with membrane-permeant leupeptin an alogs and the oxirane aloxistatin. Although other calcium-activated ev ents may occur, it appears that inhibition of calpain is sufficient to preserve the axon at the light microscope lever, and to prevent axona l cytoskeleton degradation as detected by immunofluorescent staining. Our results suggest that axonal degeneration after axotomy involves th e following sequence of events: (1) a lag-period after axotomy prior t o the onset of axonal degeneration, (2) entry of calcium into the axon through an intact axolemma via a calcium-specific ion transport mecha nism, (3) activation of calcium-dependent effector molecules such as c alpains, (4) degradation of the axonal cytoskeleton. The details of th e second step require further elucidation, and are of particular inter est because this step is a potential target for therapies directed tow ards peripheral neuropathies.