Intracellular Ca2+ represents an important trigger for various second-messenger mediated effects. Therefore a stringent control of the intracellular Ca2+ concentration is necessary to avoid excessive activation of Ca2+-dependent processes. Ca2+-dependent inactivation of voltage-dependent calcium currents (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 high-threshold VCCs in dentate granule cells (DGCs) isolated from the hippocampus of patients with drug-refractory temporal lobe epilepsy (TLE) using the patch-clamp method. Ca2+ currents showed pronounced time-dependent inactivation when no extrinsic Ca2+ buffer was present in the patch pipette. In addition, in double-pulse experiments, Ca2+ entry during conditioning prepulses caused a reduction of VCC amplitudes elicited during a subsequent test pulse. Recovery from Ca2+-dependent inactivation was slow and only complete after 1 s. Ca2+-dependent inactivation could be blocked either by using Ba2+ as a charge carrier or by including bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid (BAPTA) or EGTA in the intracellular solution. The influence of the cytoskeleton on Ca2+-dependent inactivation was investigated with agents that stabilize and destabilize microfilaments or microtubules, respectively. From these experiments, we conclude that Ca2+-dependent inactivation in human DGCs involves Ca2+-dependent destabilization of both microfilaments and microtubules. In addition, the microtubule-dependent pathway is modulated by the intracellular concentration of GTP, with lower concentrations of guanosine triphosphate (GTP) causing increased Ca2+-dependent inactivation. 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 (AHS) and is mediated by the cytoskeleton similar to rat pyramidal neurons. The similarity to the kindling model of epilepsy may suggest the possibility of altered Ca2+-dependent inactivation in patients with AHS.
Antiepileptic Effects of Tiagabine, a Selective GABA Uptake Inhibitor, in the Rat Kindling Model of Temporal Lobe Epilepsy
