Stereoselective interaction of ketamine with recombinant μ, κ, and δ opioid receptors expressed in Chinese hamster ovary cells

Background The authors examined the interaction of ketamine with recombinant mu, kappa, and delta opioid receptors and recombinant orphan opioid receptors expressed in Chinese hamster ovary cells (CHO-mu, CHO-kappa, CHO-delta, and CHOORL1, respectively). Methods: CHO-mu, CHO-kappa, and CHO-delta membranes were incubated with the opioid receptor radioligand [H-3]diprenorphine at room temperature. Ketamine (racemic, R(-) and S(+)) was included at concentrations covering the clinical range. CHOORL1 membranes were incubated with [I-125]Tyr(14) nociceptin and racemic ketamine at room temperature. The effects of racemic ketamine and selective opioid receptor agonists (mu: [D-Ala(2), MePhe(4), Gly(ol)(5)] enkephalin (DAMGO); kappa: spiradoline or delta: [D-pen(2), D-pen(5)] enkephalin (DPDPE)) on forskolin-stimulated cyclic adenosine monophosphate formation also were examined. Data are mean + SEM. Results: Racemic ketamine increased the radioligand equilibrium dissociation constant for [H-3]diprenorphine from 85 +/- 5 to 273 +/- 11, 91 +/- 6 to 154 +/- 16, and 372 +/- 15 to 855 +/- 42 pM in CHO-mu, CHO-kappa, and CHO-delta, respectively. The concentration of radioligand bound at saturation was unaffected. In CHO-mu and CHO-kappa cells, racemic ketamine did not slow the rate of naloxone-induced [H-3]diprenorphine dissociation. Ketamine and its isomers also displaced [H-3]diprenorphine binding to mu, kappa, and delta receptors in a dose-dependent manner, with pK(i) values for racemic ketamine of 4.38 +/- 0.02, 4.55 +/- 0.04, and 3.57 +/- 0.02, respectively. SCC) -ketamine was two to three times more potent than R(-)-ketamine at mu and kappa receptors, Racemic ketamine displaced [I-125]Tyr(14)nociceptin with an estimated affinity constant of 0.5 mM. Racemic ketamine inhibited the formation of cyclic adenosine monophosphate (naloxone insensitive) in a dose-dependent manner (concentration producing 50% inhibition similar to 2 mM) in all cell Lines, including untransfected CHO cells. Ketamine (100 mu M) reversed DAMGO (mu) and spiradoline (kappa) inhibition of formation of cyclic adenosine monophosphate. Conclusions: Ketamine interacts stereoselectively with recombinant mu and kappa opioid receptors.