Rapidly activating hydrogen ion currents in perfused neurones of the snail, Lymnaea stagnalis.

Cells from the circumoesophageal nerve ring of the pond snail Lymnaea stagnalis were internally perfused with solutions containing Cs aspartate, EGTA and pH buffers. Time-dependent, voltage-dependent 'residual' outward currents were observed at positive potentials. They were found to be carried largely by H+. The outward H+ currents were reduced by high internal pH, low external pH, external Cd2+ and 4-aminopyridine. External tetraethylammonium ions reduced the H+ currents but had a more effective blocking action on the K+ currents in these cells. All five agents reduced the maximum H+ conductance. In addition Cd2+, low external pH and high internal pH were found to shift the voltage dependence of the H+ current to more positive potentials. There was no significant difference between H+ currents recorded with the internal pCa2+ about 7 and those recorded with the internal pCa2+ near 5. It is likely that the H+ channel described here provides the basis for the increase in H+ permeability described by Thomas & Meech (1982) in depolarized Helix neurones. As judged by their sensitivity to different antagonists, H+ channels are unlike any other previously described channel. They are highly selective for protons and we suggest that their role in molluscan neurones is to compensate for the rapid intracellular acidification which is generated by trains of action potentials (Ahmed & Connor, 1980).