Abstract

Novel bioactive opioid mimetic agonists containing 2′,6′-dimethyl-l-tyrosine (Dmt) and a pyrazinone ring interact with μ-and δ-opioid receptors. Compound 1 [3-(4′ -Dmt-aminobutyl)-6-(3′-Dmt-aminopropyl)-5-methyl-2(1H)pyrazinone] exhibited high μ-opioid receptor affinity and selectivity (K_iμ = 0.021 nM and K_iδ/K_iμ = 1,519, respectively), and agonist activity on guinea pig ileum (IC₅₀ = 1.7 nM) with weaker δ-bioactivity on mouse vas deferens (IC₅₀ = 25.8 nM). Other compounds (2-4) had μ-opioid receptor affinities and selectivities 2- to 5-fold and 4- to 7-fold less than 1, respectively. Intracerebroventricular administration of 1 in mice exhibited potent naloxone reversible antinociception (65 to 71 times greater than morphine) in both tail-flick (TF) and hot-plate (HP) tests. Distinct opioid antagonists had differential effects on antinociception: naltrindole (δ-antagonist) partially blocked antinociception in the TF, but it was ineffective in the HP test, whereas β-funaltrexamine (irreversible antagonist, μ₁/μ₂-subtypes) but not naloxonazine (μ₁-subtype) inhibited TF test antinociception, yet both blocked antinociception in the HP test. Our data indicated that 1 acted through μ- and δ-opioid receptors to produce spinal antinociception, although primarily through the μ₂-receptor subtype; however, the μ₁-receptor subtype dominates supraspinally. Subcutaneous and oral administration indicated that 1 crossed gastrointestinal and blood-brain barriers to produce central nervous system-mediated antinociception. Furthermore, daily s.c. dosing of mice with 1 for 1 week developed tolerance in a similar manner to that of morphine in TF and HP tests, implicating that 1 also acts through a similar mechanism analogous to morphine at μ-opioid receptors.