Calcium channels in the high resistivity axonal membrane of photoreceptors of the giant barnacle.

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Abstract
The distribution of Ca channels in the cell membrane of the photoreceptor neuron of the giant barnacle B. nubilus was studied by recording intracellularly in or near the soma, in the axon and near the presynaptic terminals. The membrane properties of these different regions of the cell could be studied by separately superfusing each region with test salines or by cutting the axon between 2 regions. In the presence of tetraethylammonium (TEA) or 3-aminopyridine (3-AP), but not in their absence, Ca-dependent action potentials could be evoked with depolarizing current pulses in the somatic, axonal and terminal regions. Voltage-sensitive Ca channels and TEA-sensitive channels are present in all 3 regions of the cell. The action potentials recorded from the 3 regions were similar in their slow times-to-peak (30-300 ms), long durations (0.2-2 s in 100 mM-TEA) and long-lasting (0.2-10 s) undershoots. The action potentials were inhibited by extracellular Co. Clear differences were consistently observed between terminal action potentials and axonal or somatic action potentials in TEA. Terminal action potentials displayed a lower voltage threshold, faster rate of rise and were less sensitive to inhibition by extracellular Co, suggesting that the Ca current is greater in the terminal region. Bathing the receptor axon in low Ca or Co solutions led to a greater attenuation of large depolarizing components of the visual signal as they spread to the presynaptic terminals.