Voltage-gated ionic currents in an identified modulatory cell type controlling molluscan feeding

An important modulatory cell type, found in all molluscan feeding networks, was investigated using two-electrode voltage- and current-clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at -58 ± 2 mV. It was sensitive to tetrodotoxin only in high concentrations (¿ 50% block at 100 µm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low-threshold persistent Na+ current (activation at <¿-90 mV) was also identified. Two purely voltage-sensitive outward K+ currents were also found: (i) a transient A-current type which was activated at -59 ± 4 mV and blocked by 4-aminopyridine; (ii) a sustained tetraethylammonium-sensitive delayed rectifier current which was activated at -47 ± 2 mV. There was also evidence that a third, Ca2+-activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low-voltage (-65 ± 2 mV) activated T-type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at ¿¿-50 mV; (ii) A sustained high voltage (-40 ± 1 mV) activated current, which was blocked in CdCl2 (100 µm) but not in ¿-conotoxin GVIA (10 µm), ¿-agatoxin IVA (500 nm) or nifedipine (10 µm). This current was enhanced in Ba2+ saline. Current-clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide-acting modulatory influence of this neuron on the rest of the feeding network.