Vol. 302, Issue 1, 188-196, July 2002

Profound Spinal Tolerance after Repeated Exposure to a Highly Selective µ-Opioid Peptide Agonist: Role of -Opioid Receptors

Guo-Min Zhao, Dunli Wu, Yi Soong, Megumi Shimoyama, Irena Berezowska, Peter W. Schiller and Hazel H. Szeto

Department of Pharmacology, Joan and Sanford I. Weill Medical College of Cornell University, New York, New York (G.-M.Z., D.W., Y.S., H.H.S.); Department of Physiology, Chiba University School of Medicine, Chuo-ku, Chiba-shi, Chiba-ken, Japan (M.S.); and Laboratory of Chemical Biology and Peptide Research, Clinical Research Institute of Montreal, Montreal, Quebec, Canada (I.B., P.W.S.)

Recent studies suggest that -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/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 -antagonist naltriben (NTB) reduced spinal tolerance by 50%. Even after spinal tolerance had been established, addition of a -antagonist (NTB or H-Tyr-Tic[CH₂NH]Phe-Phe-OH) significantly enhanced the potency of i.t. [Dmt¹]DALDA 2- to 4-fold. These results suggest that agonist activation of -receptors is not necessary for the development of opioid tolerance; however, -receptors play a modulatory role in the maintenance of the tolerant state.