Increase in antinociceptive effect of [leu5]enkephalin after intrathecal administration of mixture of three peptidase inhibitors.

)enkephalin during incubation with cerebral membrane preparations is almost completely prevented by a mixture of three peptidase inhibitors such as amastatin, captopril, and phosphoramidon. The present in vivo study was performed to examine the antinoci- ceptive effect of (Leu 5 )enkephalin administered intrathecally under pretreatment with these three peptidase inhibitors. Methods: A tail-flick test was used to determine the nociceptive threshold after administration of (Leu5)enkepha- lin. The time-course profiles of the latency to flick the tail and the ar ea under the time response curve were evalu- ated for the antinociceptive action of the drug. Results: The antinociceptive effect of (Leu 5 )enkephalin administered intrathecally on the tail-flick test was increased more than 100-fold under i.t. pretreatment with three peptidase inhibitors. The antinociceptive effect produced by (Leu 5 )enkephalin in rats pretreated with any single peptidase inhibitor increased antinociception compared to that with saline. The antinociceptive potency of (Leu 5 )enkephalin under pretreatment with any combination of two peptidase inhibitors was smaller than that in rats pretreated with three peptidase inhibitors together. These results indicate that any residual single peptidase inactivates signifi cant amounts of (Leu 5 )en- kephalin. Conclusion: The present data, together with those of earlier studies, clearly demonstrate that amastatin-, cap- topril-, and phosphoramidon-sensitive enzymes play an important role in the inactivation of (Leu 5 )enkephalin