Accumulating evidence indicates that imidazoline I(2) receptor agonists enhance the antinociceptive effects of opioids and therefore may be suitable for combination therapy with opioids for pain treatment. However, little is known of the effects of I(2) receptor agonists on other behavioral effects of opioids. This study used schedule-controlled responding and dose-addition analyses to examine interactions between the μ opioid receptor agonist morphine and two imidazoline I(2) receptor agonists, 2-BFI and BU224. In 8 rats responding under a fixed ratio 10 schedule of food presentation, morphine (3.2-17.8 mg/kg), 2-BFI (3.2-17.8 mg/kg), and BU224 (5.6-17.8 mg/kg) each dose-dependently decreased responding. The addition of fixed proportions of 2-BFI or BU224 shifted the morphine dose-effect curves leftward. The interactions between morphine and 2-BFI or BU224 were infra-additive when the same proportions of morphine and I(2) receptor agonists were mixed; however, the interaction between morphine and I(2) receptor agonists was additive when the drugs were mixed at other proportions. These results provide quantitative evidence that I(2) receptor agonists do not enhance the response rate-decreasing effect of morphine and suggest that the enhancement of morphine antinociception is selective. Together, these results further support the therapeutic potential of combining I(2) receptor agonists and opioids for pain control.
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