Effects of Phenytoin, Carbamazepine, and Gabapentin on Calcium Channels in Hippocampal Granule Cells from Patients with Temporal Lobe Epilepsy

Summary: Purpose: The anticonvulsants phenytoin (PHT), carbamazepine (CBZ), and gabapentin (GBP) are commonly used in the treatment of temporal lobe epilepsy. Ca" current modulation has been proposed to contribute to the antiepileptic activity of these drugs. The purpose of this study was to determine the effects of these anticonvulsants on voltage-dependent calcium channels in pathologically altered neurons from patients with chronic temporal lobe epilepsy. Methods: Acutely isolated human hippocampal granule cells were examined by using the whole-cell configuration of the patch-clamp technique. Results: PHT and CBZ produced a reversible, concentrationdependent inhibition of high-voltage-activated (HVA) Ca2' currents without affecting voltage-dependent activation. The concentration-response curves of PHT and CBZ indicated maximal inhibition of 35 and 65%, respectively, with halfmaximal inhibition being obtained at 89 and 244 ph4, respectively. At therapeutic cerebrospinal fluid (CSF) concentrations, HVA currents were not significantly altered by PHT and CBZ. However, PHT but not CBZ showed a reduction of HVA currents of 16% at a therapeutic whole-brain concentration of 80 FM. In contrast to CBZ, PHT produced a small hyperpolarizing shift in the voltage dependence of steady-state inactivation. PHT, 80 ph4, shifted the potential of half-maximal inactivation by -3.1 f 0.5 mV (p < 0.05). GBP, which was recently found to bind to the a$ subunit of a neuronal Ca" channel, showed no modulation of Ca2+ conductances. Conclusions: These results suggest that, in contrast to GBP and CBZ, modulation of postsynaptic Ca2+ channels can contribute to the anticonvulsant action of PHT in human hippo