The role of intracellular chloride in hyperpolarizing post‐synaptic inhibition of crayfish stretch receptor neurones

Abstract

The intracellular Cl- activity .**GRAPHIC**. of isolated crayfish stretch receptor neurons was mesaured using liquid ion exchanger Cl--selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl- activity .**GRAPHIC**. and .**GRAPHIC**. (VCl) was compared with the simultaneously measured reversal potential of the inhibitory post-synaptic potential (Ei.p.s.p.) to further clarify the ionic basis of the i.p.s.p. In normal Ringer solution, VCl (63.3 .+-. 2.3 mV) was close to the resting membrane potential (Em, 62.6 .+-. 3.9 mV) while Ei.p.s.p. (74.5 .+-. 1.9 mV) was more negative than either. The VCl value corresponds to an .**GRAPHIC**. of 12.7 .+-. 1.3 mM, which is .apprx.4 mM more than required for a Cl- governed Ei.p.s.p. of 74.5 mV. Reducing .**GRAPHIC**. caused smaller changes in VCl than predicted for passive Cl- re-distributions. On complete removal of extracellular Cl- .**GRAPHIC**. VCl increased to 84.6 .+-. 2.7 mV, equivalent to an apparent .**GRAPHIC**. of .apprx. 5 mM-Cl-. This value can be used as an estimate of the level of intracellular interference on the Cl--selective micro-electrode. Increasing extracellular K+ .**GRAPHIC**. decreased both VCl and Ei.p.s.p. Decreasing .**GRAPHIC**. had the converse effect. The time course of the changes in VCl and Ei.p.s.p. was much the same. The difference between VCl and Ei.p.s.p. decreased to .apprx. 3 mV in high .**GRAPHIC**. and increased to .apprx. 30 mV in low .**GRAPHIC**. This variation in the difference between Ei.p.s.p. and VCl is consistent with the assumption that anions other than Cl- contribute to the recorded VCl rather than that another ion contributes to the inhibitory current. Application of 5 mM-NH4+ or of frusemide (6 .times. 10-4 M) decreased VCl and Ei.p.s.p.. The difference between VCl and Ei.p.s.p. was also decreased. The .**GRAPHIC**. is lower than predicted from a passive distribution and the chloride equilibrium potential (ECl) is more negative than Em. If a constant intracellular interference equivalent to .apprx. 4 mM-Cl- is assumed to contribute to the recorded VCl, ECl was approximately equal to Ei.p.s.p. in all the experimental conditions. The i.p.s.p. is apparently solely generated by Cl- ions.