ACTIONS OF THE NOVEL NEUROPROTECTIVE AGENT, LIFARIZINE (RS-87476), ON VOLTAGE-DEPENDENT SODIUM CURRENTS IN THE NEUROBLASTOMA CELL-LINE, N1E-115

1 The actions of the neuroprotective agent, lifarizine (RS-87476-190), on voltage-dependent Na+ currents have been examined in the neuroblastoma cell line, N1E-115, using the whole-cell variant of the patch clamp technique. 2 At a holding potential of -80 mV, lifarizine reduced the peak Na+ current evoked by a 10 ms depolarizing step with an IC50 of 1.3 mu M. At holding potentials of -100 and -60 mV the IC50 concentrations of lifarizine were 7.3 mu M and 0.3 mu M, respectively. 3 At a holding potential of -100 mV, most channels were in the resting state and the IC50 value for inhibition of Na+ current should correspond to the dissociation constant of lifarizine for resting channels (K-R). K-R was therefore estimated to be 7.3 mu M. 4 In the absence of lifarizine, recovery from inactivation following a 20 s depolarization from -100 mV to 0 mV was complete within 2 s. However in the presence of 3 mu M lifarizine recovery took place in a biexponential fashion with time constants of 7 s and 79 s. 5 Lifarizine (1 mu M) had no effect on steady-state inactivation curves when conditioning pre-pulses of 1 s duration were used. However, when pre-pulse durations of 1 min were used the curves were shifted to the left by lifarizine by about 10 mV. Analysis of the shifts induced by a range of lifarizine concentrations revealed that the apparent affinity of lifarizine for the inactivated state of the channel (K-1) was 0.19 mu M. 6 Lifarizine (1 mu M) had no effect on chloramine-T-modified Na+ currents, suggesting no significant open channel interaction. 7 Taken together, these data show that lifarizine is a potent voltage-dependent inhibitor of Na+ currents in N1E-115 cells and that the voltage-dependence arises from an interaction of the compound with the inactivated state of the channel. The possible contribution of Na+ current inhibition to the neuroprotective actions of lifarizine is discussed.