Voltage- and use-dependent inhibition of Na+ channels in rat sensory neurones by 4030W92, a new antihyperalgesic agent

1 Whole cell patch clamp techniques were used to study the effects of 4030W92 (2,4-diamino-5-(2,3-dichlorophenyl)-6-fluoromethylpyrimidine), a new antihyperalgesic agent, on rat dorsal root ganglion (DRG) neurones. 2 In small diameter, presumably nociceptive DRG neurones under voltage-clamp, 4030W92 (1-100 mu M) produced a concentration-related inhibition of slow tetrodotoxin-resistant Na+ currents (TTXR). From a holding potential (V-h) Of -90 mV, currents evoked by test pulses to 0 mV were inhibited by 4030W92 with a mean IC50 value of approximately 103 mu M. 3 The inhibitory effect of 4030W92 on TTXR was both voltage- and use-dependent. Currents evoked from a V-h Of -60 mV were inhibited by 4030W92 with a mean IC50 value of 22 mu M, which was 5 fold less than the value obtained at -90 mV. Repeated activation of TTXR by a train of depolarizing pulses (5 Hz, 20 ms duration) enhanced the inhibitory effects of 4030W92. These data could be explained by a preferential interaction of the drug with inactivation states of the channel. In support of this hypothesis 4030W92 (30 mu M) produced a significant hyperpolarizing shift of 10 mV in the slow inactivation curve for TTXR and markedly slowed the recovery from channel inactivation. 4 Fast TTX-sensitive Na+ currents (TTXs) were also inhibited by 4030W92 in a voltage-dependent manner. The IC50 values obtained from V(h)s Of -90 mV and -70 mV were 37 mu M and 5 mu M, respectively. 4030W92 (30 mu M) produced a 13 mV hyperpolarizing shift in the steady-state inactivation curve of TTXs. 5 High threshold voltage-gated Ca2(+) currents were only weakly inhibited by 4030W92. The reduction in peak Ca2+ current amplitude produced by 100 mu M 4030W92 was 20+/-6% (n=6). Low threshold T-type Ca2+ currents were inhibited by 17 +/- 8% and 43 +/- 3% by concentrations of 4030W92 of 30 mu M and 100 mu M, respectively (n=6). 6 Under current clamp, some cells exhibited broad TTX-resistant action potentials whilst others showed fast TTX-sensitive action potentials in response to a depolarizing current injection. In most cells a long duration (800 ms) supramaximal current injection evoked a train of action potentials. 4030W92 (10-30 mu M) had little effect on the first spike in the train but produced a concentration-related inhibition of the later spikes. The number of spikes per train was significantly reduced from 9.7+/-1.5 to 4.2+/-1.0 and 2.6 +/- 1.1 in the presence of 10 mu M and 30 mu M 4030W92, respectively (n = 5). 7 Thus, 4030W92 is a potent voltage- and use-dependent inhibitor of Na+ channels in sensory neurones. This profile can be explained by a preferential action of the drug on a slow inactivation state of the channel that results in a delayed recovery to the resting state. This state-dependent modulation by 4030W92 of Na+ channels that are important in sensory neurone function may underlie or contribute to the antihyperalgesic profile of this compound observed in vivo.