Protective role of neuronal and lymphoid cannabinoid CB2 receptors in neuropathic pain

Cannabinoid CB2 receptor (CB2r) agonists are potential painkillers void of psychotropic effects. Peripheral immune cells, neurons and glia express CB2r, however the involvement of CB2r from these cells in neuropathic pain remains unresolved. We explored spontaneous neuropathic pain through on-demand self-administration of the selective CB2r agonist JWH133 in wild-type and knockout mice lacking CB2r in neurons, monocytes or constitutively. Operant self-administration reflected drug-taking to alleviate spontaneous pain, nociceptive and affective manifestations. While constitutive deletion of CB2r disrupted JWH133-taking behavior, this behavior was not modified in monocyte-specific CB2r knockouts and was increased in mice defective in neuronal CB2r knockouts suggestive of increased spontaneous pain. Interestingly, CB2r-positive lymphocytes infiltrated the injured nerve and possible CB2r transfer from immune cells to neurons was found. Lymphocyte CB2r depletion also exacerbated JWH133 self-administration and inhibited antinociception. This work identifies a simultaneous activity of neuronal and lymphoid CB2r that protects against spontaneous and evoked neuropathic pain.