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The presence of L-type Ca2+ channels at the frog neuromuscular junction (nmj) was studied by monitoring changes in intracellular Ca2+ evoked in presynaptic terminals and perisynaptic Schwann cells (PSCs) and by studying the distribution of Ca2+ channels using a monoclonal antibody directed against the alpha 2/delta subunit of L channels. L-type Ca2+ channel agonist and antagonist had no effect on resting level of fluorescence and nerve-evoked Ca2+ responses in presynaptic terminals. However, depolarization of PSCs induced by KCl (25 mM) produced entry of Ca2+, which was prevented by L-type Ca2+ channel blockers, in (+)R Bay K 8644 of nimodipine. Labeling of Ca2+ channels revealed an intracellular epitope with an irregular and spotty distribution along the endplate. Similar results were obtained with a fluorescent phenylalkylamine [(-)DM-BODIPY-PAA], a blocker of L-type Ca2+ channels. Ca2+ channel labeling remained in absence of nerve terminals but was absent after mechanical removal of nerve terminals and PSCs. Most Ca2+ channel spots were distributed in between bands of cholinergic receptors labeled with alpha-bungarotoxin-TRITC. Cross sections of motor endplates revealed that labeling of Ca2+ channels was found only at the level of the synaptic cleft and not all around the PSCs. We conclude that L-type Ca2+ channels are located in perisynaptic glial cells in an appropriate location to sense depolarization induced by neurotransmitters and thus may support possible roles of glial cells on synaptic function.

Localization of L-type Ca2+ channels at perisynaptic glial cells of the frog neuromuscular junction