Ketamine effects on human neuronal Na+ channels.

BACKGROUND AND OBJECTIVE: Similar doses of ketamine are employed in regional and general anaesthesia. In contrast to commonly used local anaesthetic agents, accidental systemic application of local anaesthetic doses of ketamine will not result in seizure, dysrhythmia or cardiovascular depression. However, there is some doubt about the quality of regional analgesia induced by ketamine. As human sodium channels constitute an important molecular target of local anaesthetics, the study was designed to establish concentration-dependent effects of ketamine on conductance, steady-state activation and steady-state inactivation of human neuronal sodium channels. This information--that has, so far, not been published--will help to characterize further local anaesthetic properties of ketamine. METHODS: Whole-cell patch-clamp recordings were made of sodium channels natively expressed in human neuroblastoma SH-SY5Y cells. RESULTS: The sodium channels activated at a threshold of -60 mV and exhibited a maximal peak current at -10 mV. The voltage of half-maximal activation was -20 mV. The Na+ currents depended on the prepulse potential. The voltage of half-maximal inactivation was -80 mV. Ketamine inhibited the sodium conductance in a concentration-dependent manner (IC50 = 1140 micromol). A concentration-dependent hyperpolarizing shift of both steady-state activation and steady-state inactivation accounted for at most 5 mV. CONCLUSIONS: The effects of ketamine on these human ion channels occur at clinical concentrations. They are consistent with the local anaesthetic action of ketamine. Whether ketamine helps to decrease the incidence of severe side-effects during regional anaesthesia needs to be addressed in further clinical studies.

Carbamazepine effects on Na+ currents in human dentate granule cells from epileptogenic tissue.

PURPOSE: Carbamazepine (CBZ) is a well-established drug in the therapy of temporal lobe epilepsy (TLE). The anticonvulsant action of CBZ has been explained mainly by use-dependent effects on voltage-dependent Na+ channels in various nonhuman cell type. However, it is unclear whether Na+ currents in neurons within the focal epileptogenic area of patients with medically intractable TLE show similar characteristics. METHODS: Therefore we used the whole-cell patch-clamp technique to investigate the effects of CBZ on voltage-dependent Na+ currents in 23 acutely isolated dentate granule cells (DGCs) from the resected hippocampus of eight patients with medically intractable TLE. RESULTS: As in findings in animal preparations, CBZ significantly reduced the amplitude of the Na+ current and significantly shifted the current-voltage dependence of the steady-state inactivation in the hyperpolarizing direction. In contrast, the rapid component of the recovery from inactivation of the Na+ currents was not affected by CBZ. In addition, the reduction of the Na+ current amplitude observed during repetitive stimulation with depolarizing pulses was not significantly altered by CBZ. CONCLUSIONS: In summary, CBZ strongly affects the voltage-dependent steady-state inactivation, with no effects on the removal of inactivation in Na+ currents of human DGCs. In spite of the lack of suitable control material, the CBZ insensitivity of the removal of inactivation may be an interesting concept to explain the medically intractable TLE in these patients.

Electrophysiological characterization of Na+ currents in acutely isolated human hippocampal dentate granule cells.

1. Properties of voltage-dependent Na+ currents were investigated in forty-two dentate granule cells (DGCs) acutely isolated from the resected hippocampus of twenty patients with therapy-refractory temporal lobe epilepsy (TLE) using the whole-cell patch-clamp technique. 2. Depolarizing voltage commands elicited large, rapidly activating and inactivating Na+ currents (140 pS microm-2; 163 mM extracellular Na+) that were reduced in amplitude by lowering the Na+ gradient (43 mM extracellular Na+). At low temperatures (8-12 C), the time course of Na+ currents slowed and could be well described by the model of Hodgkin & Huxley. 3. Na+ currents were reversibly blocked by tetrodotoxin (TTX) and saxitoxin (STX) with a half-maximal block of 4.7 and 2.6 nM, respectively. In order to reduce series resistance errors, the Na+ current was partially blocked by low toxin concentrations (10-15 nM) in the experiments described below. Under these conditions, Na+ currents showed a threshold of activation of about -50 mV, and the voltages of half-maximal activation and inactivation were -29 and -55 mV, respectively. 4. The time course of recovery from inactivation could be described with a double-exponential function (time constants, 3-20 and 60-200 ms). The rapid and slow time constants showed a distinct voltage dependence with maximal values around -55 and -80 mV, respectively. These properties contributed to a reduction of the Na+ currents during repetitive stimulation that was more pronounced with higher stimulation frequencies and also showed a dependence on the holding potential. 5. In summary, the most striking features of DGC Na+ currents were the large current density and the presence of a current component showing a slow recovery from inactivation. Our data provide a basis for comparison with properties of Na+ currents in animal models of epilepsy, and for the study of drug actions in therapy-refractory epilepsy.