Fig. 2.

δ receptor ligands enhance μ receptor activity in cell lines expressing μ-δ receptors. Agonist-mediated [³⁵S]GTPγS binding in CHO cells coexpressing μ and δ receptors or in SK-N-SH cells endogenously expressing μ and δ receptors. Cells were permeabilized with 0.5% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in 50 mM Tris·HCl, pH 7.5, and were treated with indicated doses of morphine or damgo with or without 10 nM Delt II, TIPPψ, naltriben, or ICI 174,864, and the [³⁵S]GTPγS binding was measured as described (13). Basal values, determined in the absence of the μ agonist, but in the presence of δ ligands, were taken as 100%. Results are the mean ± SEM of three experiments in quadruplicate. ^**, P < 0.005; ^*, P < 0.01, Student's t test.