Ca²⁺-Dependent Inactivation of High-Threshold Ca²⁺ Currents in Hippocampal Granule Cells of Patients With Chronic Temporal Lobe Epilepsy

Abstract

Intracellular Ca(2+) represents an important trigger for various second-messenger mediated effects. Therefore a stringent control of the intracellular Ca(2+) concentration is necessary to avoid excessive activation of Ca(2+)-dependent processes. Ca(2+)-dependent inactivation of voltage-dependent calcium currents (VCCs) represents an important negative feedback mechanism to limit the influx of Ca(2+) that has been shown to be altered in the kindling model of epilepsy. We therefore investigated the Ca(2+)-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. Ca(2+) currents showed pronounced time-dependent inactivation when no extrinsic Ca(2+) buffer was present in the patch pipette. In addition, in double-pulse experiments, Ca(2+) entry during conditioning prepulses caused a reduction of VCC amplitudes elicited during a subsequent test pulse. Recovery from Ca(2+)-dependent inactivation was slow and only complete after 1 s. Ca(2+)-dependent inactivation could be blocked either by using Ba(2+) 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 Ca(2+)-dependent inactivation was investigated with agents that stabilize and destabilize microfilaments or microtubules, respectively. From these experiments, we conclude that Ca(2+)-dependent inactivation in human DGCs involves Ca(2+)-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 Ca(2+)-dependent inactivation. Under low-GTP conditions, the amount of Ca(2+)-dependent inactivation was similar to that observed in the kindling model. In summary, Ca(2+)-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 Ca(2+)-dependent inactivation in patients with AHS.

Effect of calcium channel blocker as anticonvulsant and its potentiating effect when used along with sodium valproate in pentylenetetrazole induced seizures in Albino rats

“…6 Satyanarayana et al, also reported decrease in duration of THLE in MES model with amlodipine pretreatment in graded doses (1mg/kg, 2mg/kg, 4mg/kg) and also observed delay in the onset of clonic convulsions in PTZ model with similar pretreatments. 4 Results of present study are comparable to this study. 9 Jagathi devi et al, showed the combination of amlodipine and indomethacin showed a superior anticonvulsant effect than the use of Amlodipine alone, in both electrically and chemically induced seizures with picrotoxin in mice.…”

“…3 A fall in the extracellular calcium concentrations occurs prior to onset of seizure activity followed by an increase in the intracellular calcium concentrations. 4 Amlodipine belongs to the 1,4-dihydropyridine (DHP) group of CCBs. Amlodipine exhibits unique features that afford a smooth gradual onset of action and sustained effect that provides for continuous and consistent activity throughout a 24-hour period.…”

Influence of Ca2+-binding proteins and the cytoskeleton on Ca2+-dependent inactivation of high-voltage activated Ca2+ currents in thalamocortical relay neurons

“…For instance, involvement of the cytoskeleton has been found in cardiac myocytes [34], hippocampal cells [4,30] and snail neurons [23,29] and calmodulin has been identified as the Ca 2+ sensor for CDI of L-type and P/Q-type Ca 2+ channels in expression systems [35,53,55,65], whereby these channels are regulated by calmodulin in a lobespecific manner [39]. To fulfil this function Ca 2+ -free calmodulin and L-type Ca 2+ channels are preassociated as has been shown by fluorescence resonance energy transfer (FRET) two-hybrid mapping [20].…”

Activity-Induced Expression of Common Reference Genes in Individual CNS Neurons

“…One hippocampus was immediately frozen in liquid nitrogen for in situ hybridization, whereas the contralateral hippocampus was used for preparation of 400-m transverse vibratome slices. From these slices, single CA1 neurons were harvested with a glass pipette in less than 10 l of extracellular solution (Beck et al, 1999). mRNA was isolated using the Dynabead mRNA Direct Micro Kit (Dynal, Hamburg, Germany) according to the manufacturer's protocol.…”