Possible regulatory role of dynorphin-(1-13) on narcotic-induced changes in naloxone efficacy.

Pretreatment of mice with a single injection of morphine, beta-endorphin, Leu-enkephalin, FK33824 or [D-Ala2-D-Leu5]enkephalin, for 3 h increased the ability of naloxone to antagonize the analgesic effects of morphine. However, dynorphin-(1-13) can only antagonize morphine, beta-endorphin or Leu-enkephalin induced increased naloxone efficacy but not FK33824 or [D-Ala2-D-Leu2]enkephalin. Dynorphin itself can also increase naloxone efficacy when treated alone. However, this effect can be prevented when animals are pretreated with dynorphin and naloxone together.