Mechanism of Excitation of Aplysia Neurons by Carbon Dioxide

The abdominal ganglion of Aplysia californica was perfused with artificial seawater equilibrated at different P (CO(CO2))'s and pH's for 5 min or less. 5% CO(2) dropped perfusate pH from 8.0 to 6.5 and produced depolarization and increased discharge rate in visceromotor neurons. Half the giant cells studied had a similar response, whereas the other half were hyperpolarized. Pacemaker neurons showed little, if any, response to such changes in pH or CO(2). Membrane conductance of responsive cells was always increased. The effect of CO(2) occurred even when synaptic transmission was blocked by low calcium and high magnesium, and therefore must have been a direct result of CO(2) or the concomitant fall in pH. When extracellular pH was lowered to 6.5 using HCl or H(2)SO(4) and no CO(2), the same effects were observed. Also, local application of HCl or H(2)SO(4) to the external surface of the cell soma elicited depolarization and spike discharge. When extracellular pH was held constant by continual titration, 5–50% CO(2) had no effect. Intracellular pH was probably decreased at least one pH unit under these circumstances. Thus CO(2) per se, decreased intracellular pH, and increased bicarbonate ion were without effect. It is concluded that CO(2) acts solely through a decrease in extracellular pH.