Endogenous opiates and nociception: a possible functional role in both pain inhibition and detection as revealed by intrathecal naloxone.

Naloxone, the opiate antagonist, was injected into the intrathecal space of rats in doses of 15, 30 and 60 micrograms to gauge its effect on the nociceptive threshold as measured by the vocalization test. Whereas 60 micrograms of naloxone produced hyperalgesia, injections of 15 micrograms lead to hypoalgesia. These opposite effects of naloxone depending on the dose used do not support the idea that endogenous opiates have unequivocal effects on pain transmission, and an alternative hypothesis of their role is discussed.