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

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.