Amphetamine-induced analgesia does not involve brain opioids.

The intrinsic analgesic properties of amphetamine were studied in rats. Subcutaneous injection of amphetamine exerted an additive effect on morphine-induced analgesia in the hot-plate test. Amphetamine itself showed intrinsic analgesic activity in a dose-dependent manner. Administration of naloxone failed to affect the analgesia induced by amphetamine. However, injection of haloperidol totally suppressed the amphetamine-induced change in pain response latency. Both naloxone and haloperidol failed to affect the pain threshold when injected alone, but inhibited morphine-induced analgesia. It is concluded that amphetamine possesses intrinsic analgesic properties which involve catecholamine but not opioid transmission in the brain.