Anandamide suppression of Na⁺ currents in rat dorsal root ganglion neurons

Anandamide, the ethanolamide of arachidonic acid, is an endogenous cannabinoid. It is an agonist at CB₁ and CB₂ cannabinoid receptors as well as the vanilloid receptor, VR1. It is analgesic in inflammatory and neuropathic pain. Both central and peripheral mechanisms are considered to participate in its analgesia. Primary sensory neurons express Na⁺ currents that are involved in the pathogenesis of pain. We examined the effect of anandamide on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na⁺ currents in rat dorsal root ganglion neurons. Anandamide inhibited both Na⁺ currents in a concentration-dependent manner. At a membrane potential of -80 mV, the current inhibition was greater in TTX-S than TTX-R currents (K_d; 5.4 μM vs. 38.4 μM). The activation and inactivation became faster in TTX-R current but not in TTX-S current. Anandamide did not alter the activation voltage in either type of current. It, however, produced a hyperpolarizing shift of the steady-state inactivation voltage in both types of currents. The maximum availability at a large negative potential was not reduced by anandamide. Thus, anandamide seems to affect inactivated Na⁺ channels rather than resting channels. The inhibition of Na⁺ currents was not reversed by AM 251 (a CB₁ antagonist), AM 630 (a CB₂ antagonist) or capsazepine (a VR1 antagonist), suggestive of a direct action of anandamide on Na⁺ channels. The inhibition of Na⁺ currents in sensory neurons may contribute to the anandamide analgesia.