The cannabinoid receptor agonist CP-55,940 and ethyl arachidonate interfere with [3H]batrachotoxinin A 20 α-benzoate binding to sodium channels and inhibit sodium channel function

Recent investigations in our laboratory showed that voltage-gated sodium channels (VGSCs) in brain are sensitive to inhibition by various synthetic cannabinoids and endocannabinoids. The present experiments examined the effects of the cannabinoid-1 (CB1) receptor agonist CP-55,940 and ethyl arachidonate on [3H]batrachotoxinin A 20 α-benzoate ([3H]BTX-B]) binding and VGSC-dependent depolarization of the nerve membrane in synaptoneurosomes isolated from mouse whole brain. CP-55,940 acted as a full inhibitor of [3H]BTX-B binding and its IC50 was established at 22.3 µM. At its maximum effect concentration, ethyl arachidonate achieved partial (approximately 70%) inhibition and was less effective than CP-55,940 as an inhibitor of binding (IC50 = 262.7 µM). The potent CB1 receptor antagonist AM251 (2 µM) had no significant effect on the displacement of [3H]BTX-B by either compound (P > 0.05). Scatchard analyses showed that CP-55,940 and ethyl arachidonate reduce the binding of [3H]BTX-B by lowering its Bmax but ethyl arachidonate also increased the Kd of radioligand binding. In kinetic experiments, CP-55,940 and ethyl arachidonate were found to boost the dissociation of [3H]BTX-B from VGSCs to rates that exceed the maximum velocity achievable by veratridine, indicating they operate as allosteric inhibitors of [3H]BTX-B binding. Neither compound was effective at changing the initial rate of association of [3H]BTX-B with sodium channels. CP-55,940 and ethyl arachidonate inhibited veratridine-dependent (TTX-suppressible) depolarization of the plasma membrane of synaptoneurosomes with IC50s of 3.2 and 50.1 µM respectively. These inhibitory effects were again not influenced by 2 µM AM251. Our data demonstrate that the potent cannabinoid receptor agonist CP-55,940 and the ethyl ester of arachidonic acid have the ability to associate with VGSCs and inhibit their function independently of effects on CB1 receptors. Binding data comparisons using mouse brain preparations indicate CP-55,940 is approximately 10,000 times more potent as a CB1 receptor ligand than a sodium channel ligand while ethyl arachidonate shows a much smaller differential. Ethyl arachidonate has been shown previously to be the principal metabolite of ethanol in the brains of intoxicated individuals and effects of this ester on VGSCs and CB1 receptors may contribute to the depressant effects of alcohol.