Although the
cannabinoid agonists R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)-(1-naphthalenyl) methanone
mesylate [
WIN 55,212-2 (WIN)] and (R,S)-3-(2-iodo-5-nitrobenzoyl)-1-(1-methyl-2-piperidinylmethyl)-1H-indole (
AM1241) exert peripheral antihyperalgesia in inflammatory
pain models, the mechanism for
cannabinoid-induced inhibition of nociceptive sensory neurons has not been fully studied. Because TRPV1 and TRPA1 channels play important roles in controlling
hyperalgesia in inflammatory
pain models, we investigated their modulation by WIN and
AM1241. The applications of WIN (>5 microM) and
AM1241 (>30 microM) inhibit responses of sensory neurons to
capsaicin and mustard oil. To determine potential mechanisms for the inhibition, we evaluated
cannabinoid effects on nociceptors. WIN and
AM1241 excite sensory neurons in a concentration-dependent manner via a nonselective Ca2+-permeable channel. The expression of TRP channels in CHO cells demonstrates that both WIN and
AM1241 activate TRPA1 and, by doing so, attenuate
capsaicin and mustard oil responses. Using TRPA1-specific
small interfering RNA or TRPA1-deficient mice, we show that the
TRPA1 channel is a sole target through which WIN and mustard oil activate sensory neurons. In contrast,
AM1241 activation of sensory neurons is mediated by TRPA1 and an unknown channel. The knockdown of TRPA1 activity in neurons completely eliminates the desensitizing effects of WIN and
AM1241 on
capsaicin-activated currents. Furthermore, the WIN- or AM1241-induced inhibition of
capsaicin-evoked nocifensive behavior via peripheral actions is reversed in TRPA1 null-mutant mice. Together, this study demonstrates that certain
cannabinoids exert their peripheral antinocifensive actions via activation of the
TRPA1 channel on sensory neurons.