Pharmacological properties of endogenous opioid peptides on dre-µ. Competition binding curves of endogenous opioid peptides on dre-µ membrane homogenates are shown. Data were fit to the one-site competition model, and each point represents the mean ± standard error of the mean (SEM) (capped bars) of three independent experiments performed in triplicate. Morphine, a µ agonist, and naloxone, a non-specific antagonist, were added for comparison. □, Met-ENK; Δ, Leu-ENK; ∇, β- END; ○, Met-ENK-GlyTyr (MEGY); +, (d-Ala2)-MEGY; ×, (d-Ala2, Val6)-MEGY; *, morphine; ■ naloxone.