NOVEL MODULATORY EFFECT OF L-TYPE CALCIUM CHANNELS AT NEWLY FORMED NEUROMUSCULAR-JUNCTIONS

This study aimed to examine changes of presynaptic voltage-sensitive c alcium channel (VSCC) subtypes during synapse formation and regenerati on in relation to transmitter release at the neuromuscular junction (N MJ). Synaptic potentials were recorded from developing rat NMJs and fr om regenerating mouse and frog NMJs. As in normal adult NMJs, evoked t ransmitter release was reduced by an N-type VSCC blocker in the frog a nd by a P/Q-type VSCC blocker in the mammal at immature NMJs; however, various L-type VSCC blockers, both dihydropyridine and nondihydropyri dine antagonists, increased evoked but not spontaneous release in a do se-dependent manner at newly formed NMJs. This presynaptic potentiatio n disappeared as NMJs matured. A rapid intracellular Ca2+ buffer, bis( O-aminophenoxy)ethane-N,N,N',N'-tetra-acid-AM, prevented the potentiat ion effect of nifedipine, but a slow Ca2+ buffer, EGTA-AM, did not. Th us, the potentiation effect of L-type blockers requires Ca2+ transient s. Pretreatment with Ca2+-activated K+ channel blockers, iberiotoxin o r charybdotoxin, did not prevent potentiation by nifedipine at regener ating frog NMJs. Thus, Ca2+-activated K+ channels were not likely invo lved in this potentiation. In contrast, no additional potentiation by nifedipine was seen in muscles pretreated with pertussis toxin (PTX), a G-protein blocker, which by itself enhances evoked transmitter relea se at regenerating frog NMJs. These results suggest the existence of m ultiple subtypes of VSCCs at newly formed motor nerve terminals. In ad dition to the normal N- or P/Q-type VSCCs that mediate transmitter rel ease, L-type VSCCs may play a novel modulatory role in evoked transmit ter release by activating a mechanism linked to PTX-sensitive G-protei ns during synapse maturation.