Chemical nature of functional cholinoreceptor groups of Lymnaea stagnalis neurons

1. The effect of pH and the influence of some alkylating agents on the properties of the cholinoreceptive membrane of the mollusc Lymnaea stagnalis neurons have been studied using the microelectrode voltage clamp technique. 2. Lowering below 7.5 of the pH of the bathing solution had to decrease the neuronal responses to ACh. A twofold decrease in cholinoreceptive membrane conductivity was found at the pH 6.7 +/- 0.1 (n=10). Raising the pH to 10.6 did not influence the response to ACh. 3. The pH effect is not associated with the influence on the properties of ionic channels but appears to be due to reduction of a functional group at the ChR active site by proton. 4. No highly reactive SH-groups were found at the ChR active site, but some functionally important carboxyl groups have been discovered. 5. The effect of pH is probably connected with reduction of --COO-- or imidazol group with a pKa of about 6.7.