CB2 cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB2 receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB2 receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB2 receptormediated inhibition of pain responses. Here, we test the hypothesis that CB2 receptor activation stimulates release from keratinocytes of the endogenous opioid ␤-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB2 receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to ␤-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that ␤-endorphin is necessary for CB2 receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in -opioid receptordeficient mice. Hindpaw injection of ␤-endorphin was sufficient to produce antinociception. AM1241 stimulated ␤-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB2 cannabinoid receptorselective antagonist and was not observed in skin from CB2 cannabinoid receptor-deficient mice. These data suggest that CB2 receptor activation stimulates release from keratinocytes of ␤-endorphin, which acts at local neuronal -opioid receptors to inhibit nociception. Supporting this possibility, CB2 immunolabeling was detected on ␤-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of ␤-endorphin, where CB2 receptors are present, leading to anatomical specificity of opioid effects.␤-endorphin ͉ nociception ͉ pain ͉ keratinocyte ͉ skin C B 2 cannabinoid receptor-selective agonists are very promising candidates for the treatment of pain. Activation of CB 2 cannabinoid receptors inhibits nociception to thermal and mechanical stimuli (1, 2), thermal and tactile hypersensitivity produced by peripheral inflammation (2-4), and tactile and thermal hypersensitivity produced in a neuropathic pain model (5). Experimental findings suggesting that activation of peripheral (noncentral nervous system) CB 2 receptors is necessary and sufficient to inhibit pain responses come from site-specific injections of CB 2 receptor-selective agonists and antagonists (1, 3, 4). Importantly, CB 2 cannabinoid receptor-selective agonists do not cause central nervous system (CNS) effects (1, 6), consistent with the inability to demonstrate the expression of CB 2 receptors in the normal CNS (7-10). The lack of CNS effects is an important feature of this class of drug candidates because the efficacy of current pain therapies is frequently limited by CNS side effects. However, enthusiasm for this therapeutic approach has been tempered by the lack of information regarding the mechanism underlying the inhibition of n...

Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence implicates the CB(1) receptor in the production of antinociception. However, the capacity of CB(2) receptors, which are located outside the central nervous system (CNS), to produce antinociception is not known. Using AM1241, a CB(2) receptor-selective agonist, we demonstrate that CB(2) receptors produce antinociception to thermal stimuli. Injection of AM1241 in the hindpaw produced antinociception to a stimulus applied to the same paw. Injection of an equivalent dose of AM1241 into the paw contralateral to the side of testing did not. The antinociceptive actions of AM1241 were blocked by the CB(2) receptor-selective antagonist AM630, but not by the CB(1) receptor-selective antagonist AM251. AM1241 also produced antinociception when injected systemically (intraperitoneally). The antinociceptive actions of systemic AM1241 were blocked by injection of AM630 into the paw where the thermal stimulus was applied, but not the contralateral paw. These findings demonstrate the local, peripheral nature of CB(2) cannabinoid antinociception. AM1241 did not produce the CNS cannabinoid effects of hypothermia, catalepsy, inhibition of activity or impaired ambulation, while this tetrad of effects was produced by the mixed CB(1)/CB(2) receptor agonist WIN55,212-2. Peripheral antinociception without CNS effects is consistent with the peripheral distribution of CB(2) receptors. CB(2) receptor agonists may have promise clinically for the treatment of pain without CNS cannabinoid side effects.

We designed AM1241, a selective CB2 cannabinoid receptor agonist, and used it to test the hypothesis that CB2 receptor activation would reverse the sensory hypersensitivity observed in neuropathic pain states. AM1241 exhibits high affinity and selectivity for CB 2 receptors. It also exhibits high potency in vivo. AM1241 dose-dependently reversed tactile and thermal hypersensitivity produced by ligation of the L5 and L6 spinal nerves in rats. These effects were selectively antagonized by a CB 2 but not by a CB1 receptor antagonist, suggesting that they were produced by actions of AM1241 at CB 2 receptors. AM1241 was also active in blocking spinal nerve ligation-induced tactile and thermal hypersensitivity in mice lacking CB 1 receptors (CB1 ؊/؊ mice), confirming that AM1241 reverses sensory hypersensitivity independent of actions at CB 1 receptors. These findings demonstrate a mechanism leading to the inhibition of pain, one that targets receptors localized exclusively outside the CNS. Further, they suggest the potential use of CB 2 receptor-selective agonists for treatment of human neuropathic pain, a condition currently without consistently effective therapies. CB 2 receptor-selective agonist medications are predicted to be without the CNS side effects that limit the effectiveness of currently available medications. N europathic pain is defined as pain initiated or caused by a primary lesion or dysfunction in the nervous system (1). It affects Ϸ1% of the population and results from a variety of etiologies including trauma, infection, diabetes, immune deficiencies, ischemic disorders, and toxic neuropathies (1, 2). It can be excruciating, and some patients are unable to work or to perform normal daily activities. Neuropathic pain often responds poorly to medical therapy (3,4). This may be due, in part, to adverse side effects of available medications that limit drug dosage (5). Medications currently used for the treatment of neuropathic pain act on neurotransmitter systems or ion channels and typically produce significant CNS side effects. For example, gabapentin, a drug commonly used to treat neuropathic pain because of its modest side effect profile compared with other therapeutic options, produces somnolence in 19% of patients and dizziness in 17% (Neurontin prescribing information, Parke-Davis). A therapy directed at targets not found in the CNS would avoid these problems. CB 2 cannabinoid receptors are one such potential target.CB 2 receptor mRNA is not detected in brain (6, 7). In addition, the CB 2 receptor-selective antagonist SR144528 did not displace the nonselective cannabinoid ligand [ 3 H]CP55,940 from binding to rat brain (7). Finally, binding of [ 3 H]CP55,940 to mouse brain was eliminated by disruption of the CB 1 receptor gene (8) but was not affected by disruption of the CB 2 receptor gene (9). These studies suggest that CB 2 receptors are not found in the normal CNS, although they do not fully exclude the possibility that CB 2 receptors are expressed in the CNS in small, but functionally signi...