Aim: The aim of the present study was to investigate the electrophysiological actions of cyclosporin A (CsA) and tacrolimus (FK506) on neurons in the brain, and to elucidate the relevant mechanisms.
Methods: Whole-cell current-clamp recording was made in CA1 pyramidal neurons in rat hippocampal slices; whole-cell voltage-clamp recording was made in dissociated hippocampal CA1 pyramidal neurons of rats.
Results: CsA (100 micromol/L) and FK506 (50 micromol/L) did not significantly alter the passive electrical properties of hippocampal CA1 pyramidal neurons, but slowed down the repolarizing phase of the action potential. CsA (10-100 micromol/L) selectively inhibited the delayed rectifier K+ current (IK) in a concentration-dependent manner. CsA did not affect the kinetic properties of IK. Intracellular dialysis of CsA (100 micromol/L) had no effect on IK. The inhibition of IK by CsA (100 micromol/L) persisted under the low Ca2+ conditions that blocked the basal activity of calcineurin.
Conclusion: CsA exerted calcineurin-independent inhibition on the IK in rat hippocampal pyramidal neurons. Taken together with our previous finding with FK506, it is conceivable that the spike broadening caused by the immunosuppressant drugs is due to direct inhibition on the IK.