Vol. 305, Issue 2, 696-702, May 2003

Endogenous Opioid Peptides Contribute to Antinociceptive Potency of Intrathecal [Dmt¹]DALDA

Hazel H. Szeto, Yi Soong, Dunli Wu, XuanXuan Qian and Guo-Min Zhao

Department of Pharmacology, Joan and Sanford I. Weill Medical College of Cornell University, New York, New York

[Dmt¹]DALDA (H-Dmt-D-Arg-Phe-Lys-NH₂; Dmt = 2',6'-dimethyltyrosine) is a dermorphin analog that shows high affinity and selectivity for the µ opioid receptor. The intrathecal potency of [Dmt¹]DALDA far exceeded its affinity at µ receptors and suggests that other mechanisms must be involved in its action in the spinal cord. The affinity and selectivity of [Dmt¹]DALDA was determined using cell membranes expressing cloned human µ, , and  opioid receptors. Competitive displacement binding with [³H][Dmt¹]DALDA, [³H]DPDPE (H-Tyr-D-Pen-Gly-Phe-D-Pen), and [³H]U69,593 [(5,7,8)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec-8-yl)-benzeneacetamide] revealed K_i of 156 ± 26 pM for µ opioid receptor (MOR), 1.67 ± 0.04 µM for  opioid receptor (DOR), and K_i of 4.4 ± 1.7 nM for  opioid receptor (KOR), respectively. [Dmt¹]DALDA increased guanosine 5'-O-(3-[³⁵S]thiotriphosphate) binding in MOR, DOR, and KOR membranes, with EC₅₀ being 17 (8.8-33) nM, 2 (1.2-3.2) µM, and 124 (15-1000) nM, respectively. Intrathecal [Dmt¹]DALDA inhibited the tail-flick response in mice with ED₅₀ = 1.22 (0.59-2.34) pmol. Intrathecal administration of an antiserum against dynorphin A(1-17) or [Met⁵]enkephalin significantly attenuated the response to i.t. [Dmt¹]DALDA, resulting in ED₅₀ of 6.2 (3.6-12.6) pmol and 6.6 (3.5-19.6) pmol, respectively. Neither antisera had any effect on the response to i.t. morphine. Intracerebroventricular (i.c.v.) [Dmt¹]DALDA was not affected by previous i.c.v. administration of anti-Dyn or anti-ME. Pretreatment with norbinaltorphimine or naltriben also attenuated the antinociceptive response to i.t., but not i.c.v., [Dmt¹]DALDA. These data suggest that i.t. [Dmt¹]DALDA causes the release of dynorphin and [Met⁵]enkephalin-like substances that act at  and  receptors, respectively, to contribute to the extraordinary potency of [Dmt¹]DALDA.