Pain inhibition by endomorphins.

Spinal analgesic effects of endomorphin-1 and endomorphin-2 were studied during acute, inflammatory, and neuropathic pain in rats chronically implanted with intrathecal cannulas. Endomorphin-1 and endomorphin-2 (2.5-10 micrograms i.t.), as well as their analogues, increased the tail-flick and the paw pressure latencies. In a model of inflammatory pain, the formalin-induced behavior was attenuated by endomorphins; however, the effect studied was not dose-dependent and was less pronounced in comparison with that evoked by morphine. On the other hand, in rats with a sciatic nerve injury (crush), endomorphins antagonized allodynia in a dose-dependent manner, whereas morphine was found to be ineffective in a similar dose range. Endomorphins also exhibited an antinociceptive potency in rats tolerant to morphine. In conclusion, our results show a powerful analgesic action of endomorphins at the spinal level. The most interesting finding is a strong effect of endomorphins in neuropathic pain, which opens up a possibility of using these compounds in pain therapy.