Dynorphin: analgesic effect via kappa receptors in spinal cord of rats

Intrathecal injection of 5 nmol of dynorphin into the spinal subarachnoid space of rats produced a strong and long-lasting analgesic effect, which was partially reversed only by a large dose (10 but not 1 mg/kg,sc) of naloxone. Rats made tolerant to dynorphin by repeated intrathecal injections showed a diminished analgesic response to the kappa agonist ethylketocyclazocine, with an unaffected analgesic response to the mu agonist morphine. Moreover, the analgesic effect of dynorphin was almost fully retained in rats made tolerant to morphine. These results indicate that the analgesic effect of dynorphin in the spinal cord of rats is mediated by kappa opioid receptors.