Calcium channel isoforms underlying synaptic transmission at embryonic Xenopus neuromuscular junctions.

Studies on the amphibian neuromuscular junction have indicated that N-type calcium channels are the sole mediators of stimulus-evoked neurotransmitter release. We show, via both presynaptic and postsynaptic voltage-clamp measurements, that dihydropyridine (DHP)-sensitive calcium channels also contribute to stimulus-evoked release at developing Xenopus neuromuscular junctions. Whereas inhibition of postsynaptic responses by omega-conotoxin (omega-Ctx) GVIA has been taken previously as evidence that only N-type channels mediate transmitter release, we find that both N-type and DHP-sensitive calcium currents are sensitive to this toxin. The unusual sensitivity of DHP-sensitive calcium channels to omega-Ctx GVIA in presynaptic terminals raises the possibility that this channel type may have escaped detection in previous physiological studies on adult frog neuromuscular junctions. Alternatively, the additional channel isoforms may be present only during early development, when they may serve to strengthen collectively presynaptic release during critical periods of synaptogenesis.