Abstract

The inhibitory action of a number of clinically effective anticonvulsants on neurotoxin-activated sodium channels in cultured neuroblastoma cells and rat brain synaptosomes has been examined. Diphenylhydantoin (KI = 35 microM) and carbamazepine (KI = 41 microM) inhibited batrachotoxin-activated 22Na+ influx in N18 cells. Similarly, batrachotoxin-activated 22Na+ influx in rat brain synaptosomes was also inhibited by diphenylhydantoin (KI = 38 microM) and carbamazepine (KI = 22 microM). Comparison of KI values with mean brain levels of these drugs achieved during prevention of electroshock seizures indicates that diphenylhydantoin and carbamazepine occupy 35% and 50%, respectively, of their receptor sites associated with sodium channels at mean therapeutic concentrations. Diazepam (KI = 51 to 63 microM) and phenobarbital (KI = 1.2 to 1.3 mM) inhibited batrachotoxin-activated 22Na+ flux in N18 cells and synaptosomes at concentrations in excess of mean therapeutic central nervous system levels. Carbamazepine, like diphenylhydantoin, acts as a competitive inhibitor of sodium channel activation by the full agonist batrachotoxin, but produces mixed inhibition of veratridine-activated channels. This finding is consistent with the conclusion that both carbamazepine and diphenylhydantoin act as allosteric inhibitors of neurotoxin-activated sodium channels. The dose-response relationships for carbamazepine and diphenylhydantoin inhibition of 22Na+ flux in N18 cells are shifted 1.5-fold to higher concentrations when 22Na+ flux measurements are made in the presence of physiological concentrations of sodium and calcium ions. These results suggest that anticonvulsant inhibition of neurotoxin-activated 22Na+ flux in our standard ion flux media, containing low concentrations of Na+ and no Ca2+, is likely to reflect an effect of these agents expected in vivo. The results of this study provide further evidence to support the hypothesis that diphenylhydantoin and carbamazepine, both of which possess similar therapeutic profiles in the treatment of grand mal and partial seizures, may exert their pharmacological effects by occupancy of receptor sites associated with the activation of voltage-sensitive sodium channels in the central nervous system.