Antinociceptive effect and enzymatic degradation of endomorphin-1 in newborn rat spinal cord

Recently discovered endomorphin-1 and -2 are the first endogenous agonists selective for the mu-opioid receptor. We examined the antinociceptive effect and enzymatic degradation of endomorphin-1 in the newborn rat spinal cord. Endomorphin-1 inhibited the binding of [H-3][D-Ala(2), N-Me-Phe(4), Gly-ols] enkephalin (DAMGO) to the membrane fraction of the newborn rat spinal cord as potently as DAMGO and morphine. Endomorphin-1 at 1-1,000 nM reduced the slow ventral root potential, which reflects noxious transmission in the isolated newborn rat spinal cord, concentration-dependently via the Cl-opioid receptor. A similar effect was observed with endomorphin-2. The newborn rat spinal cord homogenate degraded endomorphin-1 in a 120-min incubation procedure, while it degraded [Leu(5)]enkephalin even in a 30-min incubation procedure. The degradation of endomorphin-1 was inhibited by actinonin but not by thiorphan. These results showed that in the newborn rat spinal cord, endomorphins had high affinity for the mu-opioid receptor and exerted mu-opioid-receptor-mediated inhibitory effects on noxious responses. Endomorphin-1 was degraded by peptidases, but slowly compared with [Leu(5)]enkephalin degradation, and the degrading enzymes were actinonin-sensitive peptidases.