TY - JOUR T1 - Endogenous Opioid Peptides Contribute to Antinociceptive Potency of Intrathecal [Dmt<sup>1</sup>]DALDA JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 696 LP - 702 DO - 10.1124/jpet.102.048561 VL - 305 IS - 2 AU - Hazel H. Szeto AU - Yi Soong AU - Dunli Wu AU - XuanXuan Qian AU - Guo-Min Zhao Y1 - 2003/05/01 UR - http://jpet.aspetjournals.org/content/305/2/696.abstract N2 - [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, δ, and κ opioid receptors. Competitive displacement binding with [3H][Dmt1]DALDA, [3H]DPDPE (H-Tyr-d-Pen-Gly-Phe-d-Pen), and [3H]U69,593 [(5α,7α,8β)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec-8-yl)-benzeneacetamide] revealed Ki of 156 ± 26 pM for μ opioid receptor (MOR), 1.67 ± 0.04 μM for δ opioid receptor (DOR), and Ki of 4.4 ± 1.7 nM for κ 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 κ and δ receptors, respectively, to contribute to the extraordinary potency of [Dmt1]DALDA. The American Society for Pharmacology and Experimental Therapeutics ER -