Characterization of a calcium current in a vertebrate cholinergic presynaptic nerve terminal

Calcium currents were recorded from a cholinergic presynaptic nerve terminal in the chick ciliary ganglion using the whole-cell voltage-clamp technique. The presynaptic element of this synapse is in the form of a calyx that envelops the postsynaptic ciliary neuron. A method was developed to isolate the ciliary neuron, expose the calyx, and apply patch-clamp electrodes under visual control. The presynaptic Ca current activated at +30 mV with a fast time constant of about 1.5 msec and deactivated at -80 mV with a time constant of about 0.5 msec, values that are consistent with a role in action-potential-dependent transmitter release. The calyx Ca current was blocked by 0.1 mM Cd or 2 microM omega-conotoxin and was resistant to voltage-dependent inactivation. The presynaptic Ca channel exhibits similarities to the N-type group but differs from these by the minimal voltage-dependent inactivation. This type of channel, designated CaN-PT (N-like, presynaptic terminal), may play a key role in transmitter release at many vertebrate fast-transmitting synapses.