The different actions of chloride and potassium on postsynaptic inhibition of an isolated neurone.

Isolated stretch receptor neurones from freshwater crayfish were examined in solutions containing different concentrations of chloride and potassium. In normal solution the inhibitory reversal potential (Eipsp) of this preparation was strictly negative with respect to the resting potential and even to the reversal potential of spike after-hyperpolarization. The time courses of resting potential and Eipsp following rapid solution change suggest that the current generating the IPSP is mainly carried by chloride ions and that the participation of potassium is very small. This has also been confirmed by the calculated conductances of the activated inhibitory membrane in the different solutions. The results add further evidence for an outwardly directed pumping of chloride ions which keeps the intracellular concentration of this anion at the low level necessary for hyperpolarizing inhibition.