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

We studied the effect of hypercapnic and fixed acid central chemorecep tor stimulation on the pneumostome in the pulmonate snail, Helix asper sa. We found that focal stimulation of the central chemoreceptor area of the pulmonate snail brain with hypercapnic solutions more effective ly increased the pneumostomal area than did fixed acid stimulation at the same extracellular pH. Disrupting intracellular pH regulation by i nhibiting Cl- transport, either pharmacologically (DIDS) or by ion sub stitution (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 f rom the bath resulted in pneumostomal opening. In conclusion, the vent ilatory response to CO2 in H. aspersa does not require Cl- transport o r conductance. Furthermore, changing pH(i) alone is an adequate stimul us for the central chemoreceptors in the snail.