Abstract

Pain affects > 30% of the U.S. population and currently available analgesics are not adequate to meet the clinical needs. Opioids are effective for treating moderate to severe pain; however, their use is limited due to the unwanted effects. We recently found that imidazoline I2 receptor ligands produce antinociceptive effects in rat models of chronic pain and also enhance the antinociceptive effects of morphine. However, little is known of their interaction with other opioids. This study examined the anti‐hyperalgesic effects of selective I2 receptor ligands (2‐BFI and phenyzoline) alone and in combination with oxycodone using the von Frey filament test in rats with complete Freund's adjuvant (CFA)‐induced inflammatory pain. In CFA‐treated rats, 2‐BFI (1‐10 mg/kg, i.p.) and phenyzoline (17.8‐56 mg/kg, i.p.) dose‐dependently produced significant antinociceptive effects. When studied as combinations, 2‐BFI and oxycodone produced additive interactions, while phenyzoline and oxycodone produced synergistic interactions for their effects against mechanical hyperalgesia in CFA‐treated rats. Taken together, these studies suggest that imidazoline I2 receptor ligands alone have anti‐hyperalgesic effects and also enhance the effects of opioids in a rat model of chronic pain. Therefore, drugs acting on the imidazoline I2 receptor may represent a novel class of pharmacotherapy for the management of chronic pain.

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