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The authors examined the interaction of ketamine with recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] opioid receptors and recombinant orphan opioid receptors expressed in Chinese hamster ovary cells (CHO-[micro sign], CHO-[Greek small letter kappa, CHO-[small delta, Greek], and CHOORL1, respectively).CHO-[micro sign], CHO-[small kappa, Greek], and CHO-[small delta, Greek] membranes were incubated with the opioid receptor radioligand [(3) H] diprenorphine at room temperature. Ketamine (racemic, R(-) and S(+)) was included at concentrations covering the clinical range. CHOORL1 membranes were incubated with [(125) I]Tyr14 nociceptin and racemic ketamine at room temperature. The effects of racemic ketamine and selective opioid receptor agonists ([micro sign] [D-Ala2, MePhe4, Gly(ol)5] enkephalin (DAMGO); [Greek small letter kappa spiradoline or [small delta, Greek]: [D-pen2, D-pen5] enkephalin (DPDPE)) on forskolin-stimulated cyclic adenosine monophosphate formation also were examined. Data are mean +/− SEM.Racemic ketamine increased the radioligand equilibrium dissociation constant for [(3) H]diprenorphine from 85 +/− 5 to 273 +/− 11, 91 +/− 6 to 154 +/− 16, and 372 +/− 15 to 855 +/− 42 pM in CHO-[micro sign], CHO-[small kappa, Greek], AND CHO-[small delta, Greek], respectively, The concentration of radioligand bound at saturation was unaffected. In CHO-[micro sign] and CHO-[small kappa, Greek], cells, racemic, ketamine did not slow the rate of naloxone-induced [(3) H]diprenorphine dissociation. Ketamine and its isomers also displaced [(3) H]diprenorphine binding to [micro sign], [small kappa, Greek], and [small delta, Greek] receptors in a dose-dependent manner, with pKi values for racemic ketamine of 4.38 +/− 0.02, 4.55 +/− 0.04, and 3.57 +/− 0.02, respectively. S(+)-ketamine was two to three times more potent than R(-)-ketamine at [micro sign] and [small kappa, Greek] receptors. Racemic ketamine displaced [(125) I]Tyr14 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 [tilde operator] 2 mm) in all cell lines, including untransfected CHO cells. Ketamine (100 [micro sign]M) reversed DAMGO ([micro sign]) and spiradoline ([small kappa, Greek]) inhibition of formation of cyclic adenosine monophosphate.Ketamine interacts stereoselectively with recombinant [micro sign] and [small kappa, Greek] opioid receptors.