Abstract

Recent studies suggest that δ-opioid receptors play a role in the development of opioid tolerance and led us to hypothesize that highly selective μ-opioid agonists may produce less tolerance. H-2′,6′-dimethyltyrosine-d-Arg-Phe-Lys-NH₂([Dmt¹]DALDA) has extraordinary selectivity for μ-receptors (K_i^δ/K_i^μ> 14,000). Daily administration of [Dmt¹]DALDA (5 times ED₅₀; s.c.) for 7 days increased ED₅₀ 3.6-fold from 0.16 to 0.58 μmol/kg. A higher dose of [Dmt¹]DALDA (10 times ED₅₀, every 12 h) for 2.5 days resulted in a 11.7 times increase in the ED₅₀ (1.9 μmol/kg). Complete cross-tolerance to morphine was observed, with a 3.4- and 15.1-fold shift in the morphine ED₅₀, respectively. We also compared the extent of spinal versus supraspinal tolerance after repeated s.c. [Dmt¹]DALDA administration. Five doses of [Dmt¹]DALDA (10 times ED₅₀, every 12 h) resulted in a 3.4 times shift in the i.c.v. ED₅₀ (15.4 versus 4.6 pmol/mouse) but a 44 times shift in the i.t. ED₅₀ (52.9 versus 1.2 pmol/mouse). Tolerance to [Dmt¹]DALDA was associated with 30 to 35% reduction in [³H][Dmt¹]DALDA binding in brain and spinal cord. Coadministration of [Dmt¹]DALDA with δ-antagonist naltriben (NTB) reduced spinal tolerance by 50%. Even after spinal tolerance had been established, addition of a δ-antagonist (NTB or H-Tyr-TicΨ[CH₂NH]Phe-Phe-OH) significantly enhanced the potency of i.t. [Dmt¹]DALDA 2- to 4-fold. These results suggest that agonist activation of δ-receptors is not necessary for the development of opioid tolerance; however, δ-receptors play a modulatory role in the maintenance of the tolerant state.