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 microM) produced a concentration-related inhibition of slow tetrodotoxin-resistant Na+ currents (TTXR). From a holding potential (Vh) of -90 mV, currents evoked by test pulses to 0 mV were inhibited by 4030W92 with a mean IC50 value of approximately 103 microM. 3. The inhibitory effect of 4030W92 on TTX(R) was both voltage- and use-dependent. Currents evoked from a Vh of -60 mV were inhibited by 4030W92 with a mean IC50 value of 22 microM, which was 5 fold less than the value obtained at -90 mV. Repeated activation of TTX(R) 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 microM) produced a significant hyperpolarizing shift of 10 mV in the slow inactivation curve for TTX(R) 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 Vhs of -90 mV and -70 mV were 37 microM and 5 microM, respectively. 4030W92 (30 microM) 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 microM 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 microM and 100 microM, 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 microM) 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 microM and 30 microM 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.
Trezise
D J
DJ
Neurosciences Unit, Glaxo Wellcome Research and Development, Stevenage, Hertfordshire.
John
V H
VH
Xie
X M
XM
D000200:Action Potentials / Q000187:drug effects; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects; D005260:Female; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D008297:Male; D009475:Neurons, Afferent / Q000187:drug effects* / Q000502:physiology; D018408:Patch-Clamp Techniques; D011743:Pyrimidines / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin / Q000494:pharmacology
C114513:2,4-diamino-5-(2,3-dichlorophenyl)-6-fluoromethylpyrimidine; D018712:Analgesics, Non-Narcotic; D015220:Calcium Channels; D011743:Pyrimidines; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin