Enhancement of a kappa-opioid receptor agonist-induced analgesia by L-tyrosine and L-tryptophan.

The effects of the methyl esters of L-tyrosine (L-Tyr-OMe) and L-tryptophan (L-Trp-OMe) on the analgesic action of trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidin)cyclohexyl]-benzene acetamide methane sulfonate (U-50,488H), a kappa-opioid receptor agonist, were determined in male Swiss-Webster mice using the tail-flick test. Intraperitoneal injections of U-50,488H produced a dose-dependent analgesic response. The analgesic response to all doses of U-50,488H was potentiated by L-Tyr-OMe at 200 mg/kg injected intraperitoneally 30 min prior to the injection of U-50,488H. The effect of various doses of L-Tyr-OMe (50, 100 and 200 mg/kg) on the analgesia produced by 20 mg/kg of U-50,488H was also determined. The lowest dose (50 mg/kg) of L-Tyr-OMe did not modify U-50,488H-induced analgesia but the two higher doses enhanced it significantly. L-Tyr-OMe by itself at all the doses tested had no effect on the tail-flick latency. L-Trp-OMe (200 mg/kg) enhanced the analgesic action of 10 and 20 mg/kg doses of U-50,488H but not that induced by a 5 mg/kg dose. The analgesia induced by 20 mg/kg of U-50,488H was potentiated by L-Trp-OMe at 100 and 200 mg/kg but not by a 50 mg/kg dose. L-Trp-OMe by itself also did not alter the tail-flick latency. Previously, the studies in this laboratory have shown that L-Try-OMe potentiates morphine, a mu-opioid receptor agonist-induced analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)