Abstract

Involvement of μ1-opioid receptor on the antinociception induced by dermorphin tetrapeptide analogues Try-D-Arg-Phe-β-Ala (TAPA) and Tyr-D-Arg-Phe-β-Ala-NH_2 (TAPA-NH_2) were determined in mice, using a tail-pressure test and formalin test. TAPA and TAPA-NH_2 injected i. c. v. and i. t. produced dose-dependent antinociception in both assays. In the tail-pressure test, the antinociception induced by i. c. v. and i. t. injected TAPA, but not TAPA-NH_2, was significantly attenuated by the pretreatment with naloxonazine, selective antagonist for μ1-opioid receptor. Moreover, naloxonazine also significantly attenuated the antinociception induced by i. c. v. injected TAPA, but not TAPA-NH_2 in formalin test. In contrast, the antinociception induced by both TAPA and TAPA-NH_2 given i. t. was significantly attenuated by the pretreatment with naloxonazine in formalin test. The present results suggest that TAPA and TAPA-NH_2 should be considered to be selective agonist for μ1-and μ2-opioid receptors, respectively. The C-terminal amidation may be the critical portion for TAPA-NH_2 to distinguish μ1-and μ2-opioid receptors

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