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P. Ascher, Diana Kunze^* and T. O. Neild

Laboratoire de Neurobiologie, Ecole Normale Supérieure, 46, rue d'Ulm, Paris 75005, France

1. The intracellular Cl^- concentration (Cl_i) and the membrane potential (E_m) were measured in the medial pleural neurones of Aplysia under various experimental conditions designed to determine the Cl^- conductance of the neurones and investigate the possibility of an active Cl^- transport.

2. The magnitude of the Cl^- conductance of the cell depends on the experimental conditions.

3. In normal sea water, large changes of E_m produced by passing current across the cell membrane caused no change of Cl_i, suggesting that the Cl^- conductance was low. Similarly, moderate changes of E_Cl produced by decreasing Cl_o or increasing Cl_i had little or no effect on E_m.

4. A high Cl^- conductance was observed in high K_o or very low Cl_o. It was greatly reduced if the external Ca²⁺ was replaced by Co²⁺, or in the presence of tubocurarine, or if the experiment was performed on an isolated cell soma. The high Cl^- conductance is therefore attributed to the release of ACh and perhaps other transmitters from synaptic terminals.

5. High concentrations of tetraethylammonium ions or procaine induced a depolarization of the cell, but a decrease of Cl_i. The rate of fall of Cl_i was increased by lowering external K⁺ or raising external Ca²⁺, and was decreased by replacing external Ca²⁺ by Co²⁺.

6. NH₄⁺ ions applied externally had effects similar to those of K⁺ ions. In situations in which intracellular NH₄⁺ might be increased a fall in Cl_i was observed.

7. The changes of Cl_i caused by TEA, procaine, or internal NH₄⁺ occur against the driving force for passive Cl^- movements. They are still observed in isolated cell bodies, and cannot be attributed to the activation of synaptic channels.

8. Some interpretations of these anomalous Cl^- movements are discussed which could also account for the difference between E_Cl and E_m observed under normal conditions.

Present address: Department of Physiology, Monash University, Clayton, Victoria 3168, Australia.

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