Ketamine Psychedelic and Antinociceptive Effects Are Connected.
Ketamine produces potent analgesia combined with psychedelic effects. It has been suggested that these two effects are associated and possibly that analgesia is generated by ketamine-induced dissociation. The authors performed a post hoc analysis of previously published data to quantify the pharmacodynamic properties of ketamine-induced antinociception and psychedelic symptoms. The hypothesis was that ketamine pharmacodynamics (i.e., concentration-effect relationship as well as effect onset and offset times) are not different for these two endpoints. Seventeen healthy male volunteers received escalating doses of S- and racemic ketamine on separate occasions. Before, during, and after ketamine infusion, changes in external perception were measured together with pain pressure threshold. A population pharmacokinetic-pharmacodynamic analysis was performed that took S- and R-ketamine and S- and R-norketamine plasma concentrations into account. The pharmacodynamics of S-ketamine did not differ for antinociception and external perception with potency parameter (median 95% CI) C50, 0.51 (0.38 to 0.66) nmolml blood-effect site equilibration half-life, 8.3 5.1 to 13.0 min), irrespective of administration form (racemic ketamine or S-ketamine). R-ketamine did not contribute to either endpoint. For both endpoints, S-norketamine had a small antagonistic effect. The authors conclude that their data support an association or connectivity between ketamine analgesia and dissociation. Given the intricacies of the study related to the pain model, measurement of dissociation, and complex modeling of the combination of ketamine and norketamine, it is the opinion of the authors that further studies are needed to detect functional connectivity between brain areas that produce the different ketamine effects.