LOCALIZATION OF L-TYPE CA2-CELLS OF THE FROG NEUROMUSCULAR-JUNCTION( CHANNELS AT PERISYNAPTIC GLIAL)

The presence of L-type Ca2+ channels at the frog neuromuscular junctio n (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 antibod y directed against the alpha 2/delta subunit of L channels. L-type Ca2 + channel agonist and antagonist had no effect on resting level of flu orescence and nerve-evoked Ca2+ responses in presynaptic terminals. Ho wever, depolarization of PSCs induced by KCl (25 mM) produced entry of Ca2+, which was prevented by L-type Ca2+ channel blockers, (+)R Bay K 8644 or nimodipine. Labeling of Ca2+ channels revealed an intracellul ar epitope with an irregular and spotty distribution along the endplat e. Similar results were obtained with a fluorescent phenylalkylamine [ (-)DM-BODIPY-PAA], a blocker of L-type Ca2+ channels. Ca2+ channel lab eling remained in absence of nerve terminals but was absent after mech anical removal of nerve terminals and PSCs. Most Ca2+ channel spots we re distributed in between bands of cholinergic receptors labeled with alpha-bungarotoxin-TRITC. Cross sections of motor endplates revealed t hat labeling of Ca2+ channels was found only at the level of the synap tic cleft and not all around the PSCs. We conclude that L-type Ca2+ ch annels are located in perisynaptic glial cells in an appropriate locat ion to sense depolarization induced by neurotransmitters and thus may support possible roles of glial cells on synaptic function.