Mechanism of Excitation of Aplysia Neurons by Carbon Dioxide

The abdominal ganglion of Aplysia californica was perfused with artificial seawater equilibrated at different P_COCO2's and pH's for 5 min or less. 5% CO₂ 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 CO₂. Membrane conductance of responsive cells was always increased. The effect of CO₂ occurred even when synaptic transmission was blocked by low calcium and high magnesium, and therefore must have been a direct result of CO₂ or the concomitant fall in pH. When extracellular pH was lowered to 6.5 using HCl or H₂SO₄ and no CO₂, the same effects were observed. Also, local application of HCl or H₂SO₄ to the external surface of the cell soma elicited depolarization and spike discharge. When extracellular pH was held constant by continual titration, 5–50% CO₂ had no effect. Intracellular pH was probably decreased at least one pH unit under these circumstances. Thus CO₂ per se, decreased intracellular pH, and increased bicarbonate ion were without effect. It is concluded that CO₂ acts solely through a decrease in extracellular pH.