The CB2 Cannabinoid Receptor-mediated Antinociception And Its Expression In Rats With Neuropathic Pain

The CB2 cannabinoid receptor-mediated antinociception and its expression in rats with neuropathic painBackground: Neuropathic pain is defined as pain initiated or caused by a primary lesion or dysfunction in the nervous system. The mechanism underlying neuropathic pain is not clear and the pain often responds poorly to medical therapy. This may be due, in part, to adverse side effects of available medications that limit drug dosage. So it is needed to investigate the mechanism and explore drugs acting upon spinal cord or periphery without CNS side effects. CB2 cannabinoid receptors are expressed mainly on peripheral immune cells. It is suggested that selective CB2 agonists inhibit acute, inflammatory pain without producing CNS side effects. But study concerning their effects on neuropathic pain is scarce. In addition, reports on whether CB2 receptors are expressed on spinal cord and dorsal root ganglion are conflicting, making further investigation necessary.Objectives: To investigate the effects of selective CB2 receptor agonist AM1241 on neuropathic pain models; To investigate changes of CB2 receptor expression in the spinal cord and DRG during the development of neuropathic pain; To explore the molecular mechanisms by which selective CB2 receptor agonists are effective in neuropathic pain.Methods: Part one: Thirty-six male Sprague-Dawley rats were randomly divided into six groups(n=6):①CCI+ placebo, chronic constriction injury of the common sciatic nerve was performed on the rats, ten days later(the same as follows), placebo was administered peritoneally;②CCI+AM1241 (100μg/kg), the selective CB2 receptor agonist AM1241 was administered with a dose of 100μg/kg;③CCI+AM1241 (3001μg/kg);④CCI+AM1241 (1000μg/kg);⑤CCI+AM1241+AM630, AM1241 (1000μg/kg) and selective CB2 receptor antagonist AM630 (300μg/kg) were administered;⑥CCI+AM1241+AM251, AM1241 (1000μg/kg) and selective CB1 receptor antagonist AM251 (300μg/kg) were administered. CCI was performed by loosely tying the common sciatic nerve. Prior to and post-CCI and drug therapy, tactile withdrawal threshold was measured in the ipsilateral hind paw by using von Frey filaments.Part two: Forty-two male Sprague-Dawley rats were divided into seven groups(n=6):①C group, control group;②S3 group, 3 days after sham operation;③C3 group, 3 days after CCI procedure;④S7 group, 7 days after sham operation;⑤C7 group, 7 days after CCI procedure;⑥S14 group, 14 days after sham operation;⑦C14 group, 14 days after CCI procedure. CCI was performed by loosely tying the common sciatic nerve. The sham groups received the same procedure except for nerve ligation. After the tactile withdrawal threshold was measured with von Frey filaments, rats were killed at scheduled time and the ipsilateral spinal dorsal hom and DRG were harvested. Western blot analysis was used to investigate the changes of CB2 receptor expression in the spinal cord and DRG during development of neuropathic pain.Results: CCI induced a decrease in the tactile withdrawal threshold in the ipsilateral hind paw. AM1241 produced a dose-dependent inhibition of CCI-induced tactile allodynia in rats. The effect of AM1241 was completely blocked by the CB2 receptor-selective antagonist AM630 but was not affected by the CB1 receptor-selective antagonist AM251. CB2 receptor was expressed in the spinal cord and DRG of all groups. Compared with that of the control group, CB2 receptor expression was up-regulated in the ipsilateral spinal dorsal horn and DRG for the C7, C14 and C3, C7, C14 group, respectively. There were no significant changes in the CB2 receptor expression in the spinal dorsal hom and DRG for all sham groups.Conclusion: The selective CB2 receptor agonist AM1241 could alleviate CCI-induced tactile allodynia in a CB2-mediated manner. Neuropathic pain states could induce an up-regulation of the CB2 receptor expression in the spinal dorsal hom and DRG, suggesting that spinal cord and DRG might be the targets for CB2 receptor agonist.