PHENYTOIN AND CARBAMAZEPINE - DIFFERENTIAL INHIBITION OF SODIUM CURRENTS IN SMALL-CELLS FROM ADULT-RAT DORSAL-ROOT GANGLIA

We determined the effects of carbamazepine and phenytoin, anticonvulsa nt drugs used to treat neuropathic pain, on the heterogeneous populati on of Na+ channels in patch-clamped small cells from adult rat dorsal root ganglia. Both fast tetrodotoxin-sensitive (ITX-S) and slow TTX-re sistant (TTX-R) currents were inhibited by 10-100 mu M drug. TTX-R cur rents were divided into two classes. Control type I currents had a ver y depolarized voltage for 50% availability (V-h) of ca. -29 mV and 17% reduction in current by the 20th pulse at 1 Hz. Control type II curre nts had a V-h closer to -46 mV and 49% reduction in current at 1 Hz. A t 0.1 Hz, which gave relatively little loss of control current, 100 mu M drug caused 53 +/- 4% (n = 5) block of type I current and 88 +/- 2% inhibition of type II current (n = 4). Strong 1 s hyperpolarizing pre pulses relieved most of the fast channel block but had much less effec t on blocked TTX-R channels. (C) 1997 Elsevier Science Ireland Ltd.