Endogenous opioid peptides contribute to antinociceptive potency of intrathecal [Dmt1]DALDA

Dmt(1)] DALDA (H-Dmt-D-Arg-Phe-Lys-NH2; Dmt = 2', 6'-dimethyltyrosine) is a dermorphin analog that shows high affinity and selectivity for the mu opioid receptor. The intrathecal potency of [Dmt(1)] DALDA far exceeded its affinity at mu 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 mu, delta, and kappa opioid receptors. Competitive displacement binding with [H-3][Dmt(1)] DALDA, [H-3] DPDPE (H-Tyr-D-Pen- Gly-Phe-D-Pen), and [H-3] U69,593 [(5alpha,7alpha,8beta)- (+)-N-methyl-N-( 7-[1-pyrrolidinyl]-1-oxaspiro[4.5] dec-8-yl)-benzeneacetamide] revealed K-i of 156 +/- 26 pM for mu opioid receptor (MOR), 1.67 +/- 0.04 muM for delta opioid receptor (DOR), and K-i of 4.4 +/- 1.7 nM for kappa opioid receptor (KOR), respectively. [Dmt(1)] DALDA increased guanosine 5'-O-(3-[S-35] thiotriphosphate) binding in MOR, DOR, and KOR membranes, with EC50 being 17 (8.8-33) nM, 2 (1.2-3.2) muM, and 124 (15-1000) nM, respectively. Intrathecal [Dmt(1)] 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 [Met(5)] enkephalin significantly attenuated the response to i.t. [Dmt(1)] 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.) [Dmt(1)] 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., [Dmt(1)] DALDA. These data suggest that i.t. [Dmt(1)] DALDA causes the release of dynorphin and [Met(5)] enkephalin-like substances that act at kappa and delta receptors, respectively, to contribute to the extraordinary potency of [Dmt(1)] DALDA.