CENTRAL CHEMORECEPTOR STIMULUS IN THE TERRESTRIAL, PULMONATE SNAIL, HELIX-ASPERSA

We studied the effect of hypercapnic and fixed acid central chemoreceptor stimulation on the pneumostome 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 pneumostomal 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 CO2. While maintaining a constant perfusate pH, addition of NH4Cl to the perfusate resulted in pneumostomal closure; whereas removal of NH4Cl from the bath resulted in pneumostomal opening. In conclusion, the ventilatory response to CO2 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.