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 meas urements, that dihydropyridine (DHP)-sensitive calcium channels also contri bute to stimulus-evoked release at developing Xenopus neuromuscular junctio ns. 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 ca lcium currents are sensitive to this toxin. The unusual sensitivity of DHP- sensitive calcium channels to omega -Ctx GVIA in presynaptic terminals rais es the possibility that this channel type may have escaped detection in pre vious physiological studies on adult frog neuromuscular junctions. Alternat ively, the additional channel isoforms may be present only during early dev elopment, when they may serve to strengthen collectively presynaptic releas e during critical periods of synaptogenesis.