Block of sodium channel current by anticonvulsant U-54494A in mouse neuroblastoma cells

(+/- ) -cis-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzamide (U-54494A), structurally related to a kappa opioid agonist U-50448H, is a potent anticonvulsant without analgesic or sedative effects of the opioid agonist in intact animal studies (VonVoigtlander et al., 1987). To explore the mechanism of its anticonvulsant action, we investigated the interaction of U-54494A with the voltage-gated sodium channel using the whole cell patch clamp technique in mouse neuroblastoma cells (NIE-115). The drug reversibly and dose-dependently reduced the tetrodotoxin-sensitive inward Na current without affecting its activation or inactivation kinetics or the reversal potential. Nearly half of this resting block by 50 microM U-54494A at a holding potential of -80 mV was reversed upon further hyperpolarization to -120 mV. We also observed a hyperpolarization shift (9.3 mV) of the steady-state slow inactivation curve in the presence of 50 microM drug with no shift in the steady-state activation or the fast inactivation curves. These results indicate that the drug interacts with the resting and the slowly inactivated channels. The drug appears not to interact with the open state, judging from the absence of a time-dependent block in chloramine-T-treated cells. The recovery rate of the inactivated channel was markedly delayed by the drug, and apparently is responsible for its use-dependent block upon repetitive depolarizations. Our results suggest that voltage- and use-dependent block of the Na channel by U-54494A may be an important pharmacological basis for its anticonvulsant action.