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 μ, κ, and δ opioid receptors and recombinant orphan opioid receptors expressed in Chinese hamster ovary cells (CHO-μ, CHO-κ, CHO-δ, and CHO_ORL1, respectively). Methods: CHO-μ, CHO-κ, and CHO-δ membranes were incubated with the opioid receptor radioligand [³H]diprenorphine at room temperature. Ketamine (racemic, R(-) and S(+)) was included at concentrations covering the clinical range. CHO_ORL1 membranes were incubated with [¹²⁵I]Tyr¹⁴nociceptin and racemic ketamine at room temperature. The effects of racemic ketamine and selective opioid receptor agonists (μ: [D-Ala², MePhe⁴, Gly(ol)⁵] enkephalin (DAMGO); κ: spiradoline or δ: [D-pen², D-pen⁵] 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 [³H]diprenorphine from 85 ± 5 to 273 ± 11, 91 ± 6 to 154 ± 16, and 372 ± 15 to 855 ± 42 pM in CHO-μ, CHO-κ, and CHO-δ, respectively. The concentration of radioligand bound at saturation was unaffected. In CHO-μ and CHO-κ cells, racemic ketamine did not slow the rate of naloxone-induced [³H]diprenorphine dissociation. Ketamine and its isomers also displaced [³H]diprenorphine binding to μ, κ, and δ 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. S(+)-ketamine was two to three times more potent than R(-)-ketamine at μ and κ receptors. Racemic ketamine displaced [¹²⁵I]Tyr¹⁴nociceptin with an estimated affinity constant of 0.5 mM. Racemic ketamine inhibited the formation of cyclic adenosine monophosphatc (naloxone insensitive) in a dose-dependent manner (concentration producing 50% inhibition ∼ 2 mM) in all cell lines, including untransfected CHO cells. Ketamine (100 μM) reversed DAMGO (μ) and spiradoline (κ) inhibition of formation of cyclic adenosine monophosphate. Conclusions: Ketamine interacts stereoselectively with recombinant μ and κ opioid receptors.