The effect of adenosine receptor agonists on analgesic effects of morphine.

The effect of adenosine, S-phenylisopropyladenosine (S-PIA) and dipyridamole (an adenosine reuptake inhibitor) on the analgesic action of morphine in mice and rats was investigated in the hot-plate (56 degrees C) and tail immersion (52 degrees C) tests. Adenosine, 50 and 100 mg/kg, induced analgesia in mice and rats in the hot-plate test and potentiated the action of morphine (particularly in mice). The analgesic effects of adenosine were completely abolished by caffeine (10 mg/kg in mice and rats), and partially inhibited by naloxone (1 mg/kg, only in mice). S-PIA given alone (0.6 mg/kg) produced in mice some analgesic effect in the hot-plate test: the effect was abolished by caffeine and partially by naloxone. The effect of S-PIA on the action of morphine depends on the dose and the animal species. Dipyridamole alone did not affect the reactivity of animals in tests for analgesia, but potentiated the action of morphine.