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1. Four giant neurones, designated G-H cells, in the right pleural ganglion of the marine pulmonate mollusc, Onchidium verruculatum, showed characteristic membrane hyperpolarization during applications of either acetylcholine (ACh) or L-glutamate. In the presence of ACh the membrane was hyperpolarized only transiently, while in the presence of glutamate the response was maintained. Significant increases in membrane conductance accompanied the changes in membrane potential.

2. In excess potassium sea water, a slight hyperpolarization occurred when the normal concentration was increased between one- and twofold. However, depolarization usually occurred when the concentration was increased tenfold except on a few occasions when a slight but definite hyperpolarization occurred. These changes were all accompanied by a substantial increase in the membrane conductance. This hyperpolarization was in all probability the result of an increase in chloride ion permeability caused by the release of an ACh-like transmitter from depolarized presynaptic nerve terminals.

3. The reversal levels for glutamate- and ACh-induced hyperpolarization respectively were approximately — 20 and — 17 mV with respect to the resting membrane potential.

4. By changing the external ion composition, glutamate- and ACh-induced hyperpolarization were shown to be the result of an increased permeability of the subsynaptic membrane to potassium and chloride ions respectively. It appears therefore that inhibition in the same G—H cells can be activated by two different transmitter substances and that each of them activates a change in the membrane permeability to a different ion.

5. The relationship between the concentration of glutamate and the membrane conductance change was suggestive of two glutamate molecules reacting with a single receptor site.