(2 R,6 R)-Hydroxynorketamine promotes dendrite outgrowth in human inducible pluripotent stem cell-derived neurons through AMPA receptor with timing and exposure compatible with ketamine infusion pharmacokinetics in humans

The mechanisms underlying the prolonged antidepressant effects after a single infusion of ketamine are only partially understood. Ketamine half-life of about 2 hours cannot explain antidepressant effects that last for one week, suggesting the triggering of long lasting neuroplasticity. Recent human pharmacokinetics (PK) data indicate that a ketamine metabolite, (2R,6R)-hydroxynorketamine (HNK), persists in the high submicromolar range for additional 6-12 hours. Since in rodents HNK can induce dendrite outgrowth via AMPA receptor-mediated mechanisms, in this work we aimed to show that HNK produces similar effects in human neurons at concentrations and exposure-time compatible with human PK after ketamine infusion. Human dopaminergic neurons were differentiated in vitro from iPSCs obtained from healthy donors. Exposure to submicromolar HNK for 6 hours produced dendritic outgrowth when measured 3 days after exposure. These neuroplasticity effects were similar to those obtained with exposure to micromolar concentrations of ketamine for 1 or 6 hours and were blocked by pretreatment with the AMPA receptor antagonists NBQX and GYKI 52466, as well as by the mTOR pathway blocker rapamycin. It is reasonable to conclude that the mechanistic similarity of the effects produced by ketamine and HNK and their diachronic brain exposure due to their different plasma PK observed after single therapeutic infusion can contribute to the final sustained antidepressant action.