Central chemoreceptor stimulus in the terrestrial, pulmonate snail, Helix aspersa

We studied the effect of hypercapnic and fixed acid central chemoreceptor stimulation on the pneumostone in the pulmonate snail, Helix aspersa. We found that focal stimulation of the central chemoreceptor area of the pulmonate snail brain with hypercapnic solutions more effectively increased the pneumostonal area than did fixed acid stimulation at the same extracellular pH. Disrupting intracellular pH regulation by inhibiting Cl − transport, either pharmacologically (DIDS) or by ion substitution (Cl −-free perfusate), enhanced pneumostomal responses to CO 2. While maintaining a constant perfusate pH, addition of NH 4Cl to the perfusate resulted in pneumostomal closure; whereas removal of NH 4Cl from the bath resulted in pneumostomal opening. In conclusion, the ventilatory response to CO 2 in H. aspersa does not require Cl − transport or conductance. Furthermore, changing pH i alone is an adequate stimulus for the central chemoreceptors in the snail.