Fig. 4.

The δ receptor antagonist enhances μ receptor activity in vitro and potentiates morphine analgesia in vivo. (A) Binding of 6 nM ³H-morphine to SK-N-SH cells endogenously expressing μ and δ receptors in the absence or presence of 10 nM TIPPψ. Specific binding (fmol/mg protein) obtained in the absence of TIPPψ is taken as 100%. Values represent the mean ± SEM of three experiments in triplicate. ^***, P < 0.001, t test. (B) Morphine (10^-7 M)-mediated [³⁵S]GTPγS binding to SK-N-SH cells endogenously expressing μ and δ receptors in the absence or presence of 10 nM TIPPψ. Basal values obtained in the absence of agonist treatment (but presence of antagonist) are taken as 100%. Results are the mean ± SEM of three experiments in quadruplicate. ^***, P < 0.001, t test. (C) Morphine (10^-9 M)-mediated inhibition of the levels of intracellular cAMP in SK-N-SH cells endogenously expressing μ and δ receptors in the absence or presence of 10 nM TIPPψ. Basal values obtained in the absence of agonist treatment (but presence of antagonist) are taken as 100%. Results are the mean ± SEM of three experiments in quadruplicate. ^***, P < 0.001, t test. (D) Morphine-mediated intrathecal analgesia in mice. Intrathecal analgesia was measured by the tail-flick assay 30 min after the injection of morphine (0.3 nmol, a dose that gives 20% maximal possible effect) in the absence or presence of 2 nmol of TIPPψ. Results are the mean ± SEM of 20 animals per group. ^**, P < 0.01, t test.