Cannabinoids as Potential Analgesics

Several reports attribute the role of CB1 for the peripheral effects of cannabinoids. Additionally, we have shown that TRPA1 party mediates the peripheral effects of a synthetic cannabinoid, WIN in that WIN at peripherally restricted doses can inhibit a TRPV1 agonist, capsaicin (CAP). (Akopian et al., 2008). These studies suggest heterologous desensitization of TRPV1 by a TRPA1 agonist in vivo. Moreover, using CHO cells, as well as sensory neurons, application of a TRPV1 and CB1 cannabinoid agonist, arachidonoly-2 chloroethanolamine (ACEA) significantly inhibited MO-evoked inward currents (Akopian et al., 2008). Moreover, ACEA, at peripherally restricted doses, attenuated MO-induced nocifensive behavior in mice. However, this effect was abolished in TRPV1-/- mice.

Objectives: and Methods: evaluate 1) potential sites required for functional ACEA activation of TRPV1 using five capsaicin-insensitive mutations(S511Y, Y512A, Arg-114, Glu-761 and T550I) and two anandamide-insensitive (S511Y, Y512A) TRPV1 mutations; and 2) the effects of selected TRPV1 mutations on ACEA-induced inhibition of MO responses in vitro using calcium imaging. Data were analyzed using ANOVA.

Results: Our results demonstrate that 1) in CHO cells transfected with TRPV1 mutations alone, all mutations lost >90% of ACEA activation of TRPV1; 2) in CHO cells transfected with WT TRPV1 and TRPA1, 100uM ACEA fully abolished MO-induced calcium influx. Interestingly, mutations such as S511Y, Arg-114 and Glu-761, when doubly transfected with TRPA1, led to restoration of ACEA function including the ability to inhibit MO-induced calcium influx.

Conclusion: Collectively, we show that ACEA inhibits MO response in vitro and in vivo. Moreover, ACEA gating of TRPV1 is nearly abolished in TRPV1 mutants and that co-expression of TRPV1 mutations with TRPA1 restored ACEA function. These studies suggest a possible conformational change in TRPV1 upon physical interaction with TRPA1. Overall, these studies provide insight into the potential mechanisms by which certain cannabinoids mediate peripheral anti-nociceptive effects via TRP channels.