Abstract
Guinea pig ileum longitudinal muscle/myenteric plexus preparations contain functional μ- and κ-opioid receptors. A preparation with blocked κ receptors was obtained by pretreating guinea pigs with 0.1–10.0 mg kg intraperitoneal doses of a new κ-selective affinity label, DIPPA (2-(3,4-dichlorophenyl)- N-methyl- N-[1S-1-(-3-isothiocyanatophenyl)-2-(-1-pyrrolidinyl) ethyl] acetamide). Determination of IC 50 values for the μ-selective agonist DAMGO ([D-Ala 2,MePhe 4,Gly(ol) 5]-enkephalin) and the κ-selective agonist U 69,593 ([5α,7α,8β]-(+)- N-methyl- N-[7-(1-pyrrolidinyl)-1-oxaspiro-(4,5)-dec-8-yl]) showed that 0.5 mg kg DIPPA at 48 h produced reliable, near-complete blockade of κ-opioid activity but a minimal shift for μ-opioid agonism. This new assay should be useful for studying mixed agonists/antagonists that produce strong κ-opioid receptor agonism, which prevents determination of μ-opioid receptor antagonism.
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