Ca2+-dependent inactivation of high-threshold Ca2+ currents in hippocampalgranule cells of patients with chronic temporal lobe epilepsy

Intracellular Ca2+ represents an important trigger for various second-messe nger mediated effects. Therefore a stringent control of the intracellular C a2+ concentration is necessary to avoid excessive activation of Ca2+- depen dent processes. Ca2+-dependent inactivation of voltage-dependent calcium cu rrents (VCCs) represents an important negative feedback mechanism to limit the influx of Ca2+ that has been shown to be altered in the kindling model of epilepsy. We therefore investigated the Ca2+-dependent inactivation of h igh-threshold VCCs in dentate granule cells (DGCs) isolated from the hippoc ampus of patients with drug-refractory temporal lobe epilepsy (TLE) using t he patch-clamp method. Ca2+ currents showed pronounced time-dependent inact ivation when no extrinsic Ca2+ buffer was present in the patch pipette. In addition, in double-pulse experiments, Ca2+ entry during conditioning prepu lses caused a reduction of VCC amplitudes elicited during a subsequent test pulse. Recovery from Ca2+-dependent inactivation was slow and only complet e after 1 s. Ca2+-dependent inactivation could be blocked either by using B a2+ as a charge carrier or by including bis-(o-aminophenoxy)-N,N,N',N'-tetr aacetic acid (BAPTA) or EGTA in the intracellular solution. The influence o f the cytoskeleton on Ca2+-dependent inactivation was investigated with age nts that stabilize and destabilize microfilaments or microtubules, respecti vely. From these experiments, we conclude that Ca2+-dependent inactivation in human DGCs involves Ca2+-dependent destabilization of both microfilament s and microtubules. In addition, the microtubule-dependent pathway is modul ated by the intracellular concentration of GTP, with lower concentrations o f guanosine triphosphate (GTP) causing increased Ca2+-dependent inactivatio n. Under low-GTP conditions, the amount of Ca2+-dependent inactivation was similar to that observed in the kindling model. In summary, Ca2+ dependent inactivation was present in patients with TLE and Ammon's horn sclerosis (A HS) and is mediated by the cytoskeleton similar to rat pyramidal neurons. T he similarity to the kindling model of epilepsy may suggest the possibility of altered Ca2+-dependent inactivation in patients with AHS.