LY-2828360

CB₂ cannabinoid receptor agonists suppress pathological pain in animal models and lack unwanted side effects commonly associated with direct activation of CB₁ receptors. However, the types of pain most responsive to CB₂ agonists are incompletely understood and cell types which underlie CB₂-mediated therapeutic efficacy remain largely unknown. We previously reported that the CB₂ receptor agonist LY2828360 reduced neuropathic nociception induced by toxic challenge with chemotherapeutic and anti-retroviral agents in mice. Whether these findings generalize to models of inflammatory pain is not known. Here we show that LY2828360 (10 mg/kg i.p.) reversed the maintenance of carrageenan-induced mechanical allodynia in female mice. Anti-allodynic efficacy was fully preserved in global CB₁ knock out (KO) mice but absent in CB₂ KO mice. The anti-allodynic efficacy of LY2828360 was absent in conditional KO (cKO) mice lacking CB₂ receptors in peripheral sensory neurons (Advillin^CRE/+; CB₂^f/f) and preserved in cKO mice lacking CB₂ receptors in microglia/macrophages expressing C-X3-C Motif Chemokine Receptor 1 (CX3CR1^CRE/+; CB₂^f/f). Intraplantar administration of LY2828360 (30 μg i.pl.) reversed carrageenan-induced mechanical allodynia in CB₂^f/f but not Advillin^CRE/+; CB₂^f/f mice of both sexes. Thus, CB₂ receptors in peripheral sensory neurons likely underlie the therapeutic effects of LY2828360 injection in the paw. Lastly, qRT-PCR analyses revealed that LY2828360 reduced carrageenan-induced increases in IL-1β and IL-10 mRNA in paw skin. Our results suggest that LY2828360 suppresses inflammatory nociception in mice through a neuronal CB₂-dependent mechanism that requires peripheral sensory neuron CB₂ receptors and suggest that the clinical applications of LY2828360 as an anti-hyperalgesic agent should be re-evaluated.

Painful peripheral neuropathy is a common neurological complication associated with human immunodeficiency virus (HIV) infection and anti-retroviral therapy. We characterized the impact of two CB2 cannabinoid agonists (AM1710 and LY2828360 - ligands differing in signaling bias and CNS penetration) on neuropathic nociception induced by the antiretroviral agent Zalcitabine (2',3'-dideoxycytidine; ddC). We also used a conditional knockout approach to identify cell types mediating CB2 agonist-induced antinociceptive efficacy and sparing of morphine tolerance. AM1710 and LY2828360 alleviated ddC-induced neuropathic nociception in mice of both sexes. These benefits were absent in global CB2 knockout mice, which exhibited robust morphine antinociception. Like morphine, AM1710 blunted ddC-induced increases in proinflammatory cytokine (IL-1β, TNF-α) and chemokine (CCL2) mRNA expression levels. We generated advillin^Cre/+;CB2^f/f conditional knockout mice to ascertain the role of CB2 localized to primary sensory neurons in CB2-mediated therapeutic effects. Antinociceptive efficacy of both AM1710 and LY2828360, but not reference analgesics, were absent in advillin^Cre/+;CB2^f/f mice, which exhibited robust ddC-induced neuropathy. In ddC-treated CB2^f/f mice, LY2828360 suppressed development of morphine tolerance and reversed established morphine tolerance, albeit with greater efficacy in male compared to female mice. LY2828360 failed to block or reverse morphine tolerance in advillin^Cre/+;CB2^f/f mice. The present studies indicate that CB2 activation may alleviate HIV-associated antiretroviral neuropathy and identify a previously unreported mechanism through which CB2 activation produces antinociceptive efficacy. Our results also provide the first evidence that a CB2 agonist can reverse established morphine tolerance and demonstrate that CB2 localized to peripheral sensory neurons mediates the opioid tolerance sparing efficacy of CB2 agonists.