Inhibition of Voltage‐Sensitive Sodium Channels by the Cannabinoid 1 Receptor Antagonist AM 251 in Mammalian Brain

Abstract

The cannabinoid 1 receptor antagonist AM 251 is known to block the inhibitory effects of endocannabinoids and synthetic cannabinoid agonists on transmitter release through an action at presynaptic cannabinoid 1 receptors in brain. We examined the ability of AM 251 to inhibit sodium channel-dependent functions and the binding of [3H]batrachotoxinin A 20-alpha-benzoate to sodium channels in mouse brain synaptic preparations. Depolarization of synaptoneurosomes by the sodium channel site 2-specific neurotoxin veratridine, which is abolished by tetrodotoxin, was found to be inhibited in a concentration-dependent fashion by AM 251 (IC50=8.9 microM). Veratridine-dependent (tetrodotoxin suppressible) release, of L-glutamic acid and GABA from synaptosomes was also reduced by AM 251 [IC50s=8.5 microM (L-glutamic acid), 9.2 microM (GABA)]. The binding of the radioligand [3H]batrachotoxinin A 20-alpha-benzoate to site 2 on sodium channels was displaced by AM 251 (IC50=11.2 microM). Scatchard analysis of binding showed that at its IC50, AM 251 increased (by 2.3 times) the KD of radioligand without altering Bmax, suggesting a competitive mechanism of inhibition by AM 251. Kinetic experiments indicated that AM 251 inhibits equilibrium binding by allosterically accelerating the dissociation of the [3H]-batrachotoxinin A 20-alpha-benzoate:sodium channel complex. Our data suggest that micromolar concentrations of AM 251 are capable of reducing neuronal excitability and inhibiting release of excitatory and inhibitory transmitters through blockade of voltage-sensitive sodium channels in brain.