Background The neuronal mechanisms responsible for dissociative anesthesia remain controversial. N-methyl-D-aspartate (NMDA) receptors are inhibited by ketamine and related drugs at concentrations lower than those required for anesthetic effects. Thus, the authors studied whether ligand-gated ion channels other than NMDA receptors might display a sensitivity to ketamine and dizocilpine that is consistent with concentrations required for anesthesia.
Methods: Heteromeric human neuronal nicotinic acetylcholine receptors (hnAChR channels alpha2beta2, alpha2beta4, alpha3beta2, alpha3beta4, alpha4beta2 and alpha4beta4), 5-hydroxytryptamine3 (5-HT3), alpha1beta2gamma2S gamma-aminobutyric acid type A (GABAA) and alpha1 glycine receptors were expressed in Xenopus oocytes, and effects of ketamine and dizocilpine were studied using the two-electrode voltage-clamp technique.
Results: Both ketamine and dizocilpine inhibited hnAChRs in a noncompetitive and voltage-dependent manner. Receptors containing beta1 subunits were more sensitive to ketamine and dizocilpine than those containing beta2 subunits. The inhibitor concentration for half-maximal response (IC50) values for ketamine of hnAChRs composed of beta4 subunits were 9.5-29 microM, whereas those of beta2 subunits were 50-92 microM. Conversely, 5-HT3 receptors were inhibited only by concentrations of ketamine and dizocilpine higher than the anesthetic concentrations. This inhibition was mixed (competitive/noncompetitive). GABAA and glycine receptors were very resistant to dissociative anesthetics.
Conclusions: Human nAChRs are inhibited by ketamine and dizocilpine at concentrations possibly achieved in vivo during anesthesia in a subunit-dependent manner, with beta subunits being more critical than alpha subunits. Conversely, 5-HT3, GABAA, and glycine receptors were relatively insensitive to dissociative anesthetics.