Mixed opioid/nonopioid effects of dynorphin and dynorphin related peptides after their intrathecal injection in rats.

Dynorphin dose-dependently increased the tail flick latency of rats to radiant heat following its intrathecal injection. This effect was accompanied by an alteration in motor function that was characterized by a flaccid extension of the hindlimbs and flaccidity of the tail. Naloxone (10 but not 1 mg/kg) blocked the antinociceptive effect and motor disturbance produced by dynorphin. The non-opioid analogue des-Tyr1-dynorphin(1-13) also increased tail flick latency and produced paralysis. Dynorphin(1-8) significantly elevated tail flick latency without affecting motor function. Furthermore, the effect of dynorphin(1-8) was blocked by 1 mg/kg naloxone. These data suggest a possible physiological role of dynorphin in influencing motor function in the spinal cord and a role of dynorphin(1-8) in modulating pain transmission. Another finding of the present study was that dynorphin was approximately ten times more potent in producing its effects when injected one day after surgery compared to when it when it was injected one week or more after surgery.