The effect of anion injection and changes in the external potassium and chloride concentration on the reversal potentials of the IPSP and acetylcholine

Abstract

1.1. Certain neurones in the abdominal ganglion of the snail Helix aspersa show spontaneous inhibitory post-synaptic potentials (IPSP).2.2. Addition of acetylcholine normally causes a hyperpolarization of such neurones.3.3. Fifteen different anions, of increasing hydrated volume, were injected into the neurone. Those with a hydrated diameter greater than 1·24 times the hydrated potassium ion had no effect on the reversal potential of the IPSP and ACh. These anions were fluoride, bisulphite, bicarbonate, acetate, chromate, sulphate, phosphate and propionate.4.4. Injection of small anions, such as bromide, chloride, nitrate, thiocyante, chlorate, formate, altered the reversal potentials of the IPSP and ACh.5.5. Alteration of the external chloride concentration or the external potassium concentration affected the reversal potentials of the IPSP and ACh.6.6. Experiments in which the external concentrations of potassium and chloride were altered so that they either opposed or augmented each other indicate that the effect of the chloride ion was more marked and more rapid than that of the potassium ion on the IPSP and ACh reversal potentials.7.7. It is suggested that during the IPSP or when ACh is applied to the cell, pores open in the membranes so that approximately 90 per cent of the current is carried by the chloride ion and 10 per cent by the potassium ion.8.8. It is probable that acethylcholine is the transmitter at the snail inhibitory synapse.