Cannabinoid CB2 Agonist GW405833 Suppresses Inflammatory and Neuropathic Pain through a CB1 Mechanism that is Independent of CB2 Receptors in Mice.

GW405833, widely accepted as a cannabinoid receptor 2 (CB2) agonist, suppresses pathologic pain in preclinical models without the unwanted central side effects of cannabinoid receptor 1 (CB1) agonists; however, recent in vitro studies have suggested that GW405833 may also behave as a noncompetitive CB1 antagonist, suggesting that its pharmacology is more complex than initially appreciated. Here, we further investigated the pharmacologic specificity of in vivo antinociceptive actions of GW405833 in models of neuropathic (i.e., partial sciatic nerve ligation model) and inflammatory (i.e., complete Freund's adjuvant model) pain using CB2 and CB1 knockout (KO) mice, their respective wild-type (WT) mice, and both CB2 and CB1 antagonists. GW405833 (3, 10, and 30 mg/kg i.p.) dose dependently reversed established mechanical allodynia in both pain models in WT mice; however, the antiallodynic effects of GW405833 were fully preserved in CB2KO mice and absent in CB1KO mice. Furthermore, the antiallodynic efficacy of GW405833 (30 mg/kg i.p.) was completely blocked by the CB1 antagonist rimonabant (10 mg/kg i.p.) but not by the CB2 antagonist SR144528 (10 mg/kg i.p.). Thus, the antinociceptive properties of GW405833 are dependent on CB1 receptors. GW405833 (30 mg/kg i.p.) was also inactive in a tetrad of tests measuring cardinal signs of CB1 activation. Additionally, unlike rimonabant (10 mg/kg i.p.), GW405833 (10 mg/kg, i.p.) did not act as a CB1 antagonist in vivo to precipitate withdrawal in mice treated chronically with Δ9-tetrahydrocannabinol. The present results suggest that the antiallodynic efficacy of GW405833 is CB1-dependent but does not seem to involve engagement of the CB1 receptor's orthosteric site.