Test-retest reliability of thermal temporal summation using a novel individualized protocol

Abstract

Chronic constriction injury (CCI) of the rodent sciatic nerve causes light touch mechanical sensitivity (allodynia) and leads to activation of spinal astrocytes and microglia. The endocannabinoid system includes the cannabinoid receptor 1 (CB1R) present on neurons and the cannabinoid receptor 2 (CB2R) present mostly on immune cells. CB2R agonists, like AM1710, believed to exert analgesic effects through CB2R actions, are effective in controlling pathological pain states in animal models. However, activation of CB1R could be partly responsible for the analgesic effects of AM1710 through non-specific binding. We sought to determine whether AM1710 could lead to anti-allodynia and produce changes in spinal pro-and anti-inflammatory protein IR in mice lacking functional CB1R (CB1R-/-). We first characterized the allodynia profile of CB1R+/+, CB1R+/-and CB1R-/-mice. Following CCI, CB1R-/-mice displayed allodynia similar to wild-type and heterozygous littermates, with reliable 27-day chronic allodynia. Following baseline threshold assessment, mice underwent sham or CCI surgery, and thresholds were reassessed 5 and 12 days later. Mice were given an intraperitoneal (i.p.) injection of either AM1710 (25 mg/kg/ml) or equivolume vehicle. Following behavioral reassessment at 25 min after injection, spinal tissues were harvested. AM1710, given i.p. reversed CCI-induced allodynia in CB1R-KO mice to control levels. Next, we sought to determine an effective timecourse for AM1710 (5 mg) delivered intrathecally (i.t.) to reverse allodynia in wild type mice. Allodynia reversal occurred 2 hours after administration. Finally, we sought to determine if spinal CB1R were responsible for intrathecal AM170 effects. We found AM1710 robustly reversed chronic allodynia in CB1R-/-mice. Ongoing studies using spectral analysis of IR will examine glial activation, and pro-and anti-inflammatory cytokine changes in these tissues.