Cannabinoid CB2 receptors represent a therapeutic target that circumvents unwanted central side effects (e.g., psychoactivity and/or addiction) associated with activation of CB1 receptors. One of the primary investigative tools used to study functions of the CB2 receptor is the aminoalkylindole (R,S)-AM1241. However, (R,S)-AM1241 has been described as an atypical CB2 agonist which produces antinociception mediated indirectly by opioid receptors. (R,S)-AM1241 and its enantiomers, (R)-AM1241 and (S)-AM1241, were evaluated for antinociception in response to thermal (Hargreaves) and mechanical (von Frey) stimulation. Pharmacological specificity was established using antagonists for CB1 (rimonabant [SR141716]) and CB2 (SR144528). The opioid antagonist naloxone was administered locally in the paw or systemically to evaluate the contribution of opioid receptors to CB2-mediated antinociception produced by (R,S)-AM1241, (R)-AM1241, and (S)-AM1241. Comparisons were made with the opioid analgesic morphine. (R,S)-AM1241, (R)-AM1241, and (S)-AM1241 (0.033–10 mg/kg i.p.) produced antinociception to thermal, but not mechanical, stimulation of the hindpaw in naive rats. Antinociception produced by (R,S)-AM1241 and (S)-AM1241 exhibited an inverted U-shaped dose response curve. (R)-AM1241 produced greater antinociception than either (S)-AM1241 or (R,S)-AM1241 at the lowest (0.033 and 0.1 mg/kg i.p.) and highest (10 mg/kg i.p.) doses. Similar levels of antinociception were observed at intermediate doses. (R,S)-AM1241, (R)-AM1241, and (S)-AM1241 each produced CB2-mediated antinociception that was blocked by SR144528 but not by rimonabant. Local and systemic naloxone blocked morphine-induced antinociception but did not block antinociceptive effects of (R,S)-AM1241, (R)-AM1241, or (S)-AM1241. The antinociceptive effects of the CB2-selective cannabinoid (R,S)-AM1241 and its enantiomers, (R)-AM1241 and (S)-AM1241, are not dependent upon opioid receptors.
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