We previously reported that omega Conus geographus toxin (ωCgTX), blocks evoked-release of transmitter at synapses in frog and attenuates the Ca2+ component of the action potential of chick dorsal root ganglion neurons. We report here voltage-clamp experiments on cultured chick dorsal root ganglion neurons which demonstrate that ωCgTX produces a persistent block of voltage-gated Ca2+ currents. Thus, we conclude that ωCgTX inhibits synaptic transmission by blocking Ca2+ channels in the presynaptic nerve terminal. The toxin had no effect on K+ currents; however, in some but not all neurons, ωCgTX reduced Na+ currents by 10–25%. These findings suggest that ωCgTX should be useful as a probe to examine synaptic Ca2+ channels.
