Effects of imidazoline I2 receptor agonists and morphine on schedule-controlled responding in rats

► Imidazoline I2 receptor agonists decreased food-maintained responding. ► I2 receptor agonists and morphine produced additive or infra-additive interactions in decreasing food-maintained responding. ► The nature of the interaction depended on the proportions of morphine and I2 receptor agonists in the mixture. ► That I2 receptor agonists enhance the antinociceptive but not response-decreasing effects of morphine supports combining I2 agonists and morphine for pain treatment.