ACETYLCHOLINE-INDUCED HYPERPOLARIZATION IN IDENTIFIED NEURONS OF THE AFRICAN GIANT SNAIL (ACHATINA-FULICA FERUSSAC)

Three types of hyperpolarization in response to applied acetylcholine (ACh) were observed in 4 identified neurones (RP1, RP2, RP3 and RP4) in the right parietal ganglion of the African giant snail, Achatina fulica Ferussac. In order to clarify the ionic mechanisms of these responses, the neurones were voltage-clamped using two microelectrodes and the reversal potential of the ACh-induced currents was measured under several external ionic environments. Experiments revealed the existence of two different ACh-induced currents (Cl- and K+). It was concluded that the 3 types of hyperpolarizing responses were due to the relative sizes of the two currents in a neurone. Effects of some cholinergic antagonists on these currents were studied. Benzoquinonium blocked both currents at nearly the same concentration. D-Tubocurarine, decamethonium and atropine blocked Cl- current but non K+ current. These drugs did not show any clear voltage-dependency on their blocking actions. Tetraethylammonium blocked the ACh-induced K+ current at a lower concentration than that needed to block the other K+ channels in molluscan neurones, but it had little effect on Cl- current. These results suggest that two currents may be mediated by different ACh receptors.