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Department of Neurophysiology, Max-Planck Institute for Psychiatry, D-8000 Munich 40, F.R.Germany
1. The intracellular Cl- activity (aCli) of isolated crayfish stretch receptor neurones was measured using liquid ion exchanger Cl--selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl- activity (aClo) and aCli (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.
2. In normal Ringer solution, VCl (63·3 ± 2·3 mV) was found to be 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 apparent aCli of 12·7 ± 1·3 mM, which is about 4 mM more than required for a Cl- governed Ei.p.s.p. of 74·5 mV.
3. Reducing aClo caused smaller changes in VCl than predicted for passive Cl- re-distributions. On complete removal of extracellular Cl- (Clo-), VCl increased to 84·6 ± 2·7 mV, equivalent to an apparent aCli of about 5 mM-Cl-. This value can be used as an estimate of the level of intracellular interference on the Cl--selective micro-electrode.
4. Increasing extracellular K+ (K0+) decreased both VCl and Ei.p.s.p.. Decreasing Ko+ 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 about 3 mV in high Ko+, and increased to about 30 mV in low Ko+. 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 another ion contributes to the inhibitory current.
5. Application of 5 mM-NH4+ or of frusemide (6 x 10-4 M) decreased VCl and Ei.p.s.p.. The difference between VCl and Ei.p.s.p. was also decreased.
6. We conclude that aCli is lower than predicted from a passive distribution and thus the chloride equilibrium potential (ECl) is more negative than Em. If a constant intracellular interference equivalent to about 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. Therefore we suggest that the i.p.s.p. is solely generated by Cl- ions.
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