The cannabinoid CB2 receptor as a target for the treatment of neuropathic pain

Cannabinoids are cannabis-like drugs that are moderately efficacious in the treatment of some types of chronic pain, including pain of a neuropathic origin. Limiting their therapeutic application, however, are adverse psychoactive effects as a result of cannabinoid receptor activation in the central nervous system (CNS). Cannabinoids act via two receptors: cannabinoid receptor I (CB1) and cannabinoid receptor II (CB2), which are respectively known as the central and peripheral cannabinoid receptors. While CB1 receptors are thought to mediate the antinociceptive effects of cannabinoids, they are also responsible for the adverse effects. Selective targeting of CB2 receptors has shown promise as a treatment in neuropathic pain models in animals, without the associated psychoactivity seen following CB1 receptor activation, however the cellular mechanisms involved have not been elucidated. While initially considered a peripheral receptor, recent evidence has suggested that CB2 receptors may be upregulated in the CNS following neuropathic pain. Using a well established rodent model of neuropathic pain, this study aimed to assess the efficacy of CB2 selective agonists in the treatment of allodynia (pain in response to a normally innocuous stimulus), and investigate the presence and function of CB2 receptors in the spinal cord, a key structure in nociceptive transmission and processing. In the chronic constriction injury (CCI) model of sciatic neuropathy, CB2 selective agonists were efficacious when delivered systemically at high doses, but not when delivered by intrathecal cannulation to the spinal cord. Using immunohistochemistry and Western blot, labeling was detected for CB2 receptors in the superficial dorsal horn of the spinal cord, which was not modulated by CCI surgery or drug administration. Attempted validation of the antibody used in these approaches, however, indicated that this antibody was not specific for CB2 protein, and detects at least one unspecified cytosolic protein in addition to CB2. These findings cast doubt on the validity of this primary antibody which has been widely used, both in these immunohistochemistry and Western blot studies, as well as in previous reports of CB2 receptor protein expression. To circumvent this issue, a functional receptor assay, the [35S]GTPγS assay, was employed to assess the presence of functional CB2 receptors in the spinal cord. Employing this assay on membrane preparations and tissue slices in situ, no evidence for functional CB2 receptors was found in sham or neuropathic spinal cords. This study found that while efficacious in the treatment of neuropathic pain in this model, CB2 selective agonists are not acting via spinal CB2 receptors. Furthermore, no evidence was found for functional CB2 receptors above the threshold for detection in the healthy or neuropathic spinal cord, suggesting that spinal CB2 receptors are not a rational target for the treatment of neuropathic pain following peripheral nerve injury