CALCIUM CURRENTS IN ACUTELY ISOLATED HUMAN NEOCORTICAL NEURONS

1. Ca2+ currents were investigated in neurons acutely isolated from adult human temporal neocortex. The aim was to compare the basic characteristics of the currents with those previously described in animals and to examine the effects of dihydropyridine Ca2+ antagonists and antiepileptic drugs. The tissue, obtained from patients undergoing temporal lobe surgery for medically intractable epilepsy, was sliced, incubated in papain, and triturated. 2. Most of the isolated neurons (34 of 36) were judged to be pyramidal cells by their morphology. Whole-cell voltage-clamp recordings revealed two components of Ca2+ current: 1) a low-threshold (T-type) current that was transient, small in amplitude, and required hyperpolarization more negative than -70 mV for removal of inactivation and 2) a high-threshold current that was slowly inactivating and was available for activation from more positive potentials. The characteristics of the Ca2+ currents were very similar to those in the neocortical neurons of young rats, although the low-threshold current was less prominent in the human cells. 3. Subcomponents of the high-threshold current were identified by pharmacology. About 20% of the peak current was blocked by omega-conotoxin GVIA (presumed N current) and 40-50% of the peak current was blocked by micromolar concentrations of the dihydropyridine Ca2+ antagonists nifedipine and nimodipine (presumed L current). In two neurons tested with a range of nimodipine concentrations, the threshold for suppression of the high-threshold current was approximately 10 nM. 4. The antiepileptic agents ethosuximide, carbamazepine, and valproate did not affect the Ca2+ currents at therapeutically relevant concentrations. Phenytoin marginally reduced the low- and high-threshold Ca2+ currents at 8 muM (a concentration corresponding to the upper therapeutic range). The results do not support the hypothesis that inhibition of Ca2+ currents in neocortical pyramidal neurons is a major action of these drugs.