Corticosterone as a potential confounding factor in delineating mechanisms underlying ketamine's rapid antidepressant actions
Although ketamine represents a new line of antidepressants with unique clinical advantages, its use as a long-term treatment has limitations, particularly its dissociative/psychotomimetic effects and abuse potential. In rats, we observed that a subanesthetic dose of ketamine (10mg/kg) induced a 3-fold increase in corticosterone (CORT) levels in both serum and brain tissue, within an hour of administration. This increase took place in both male and female rats, in both naive and stressed animals. However, no CORT increase was detected in rats injected with (2R, 6R)-hydroxynorketamine (HNK), an active metabolite of ketamine, that is believed to contribute to ketamines antidepressant effect. In response to the release in CORT, ketamine injected animals displayed a significant increase in the expression of sgk1, a downstream effector of glucocorticoid receptor signaling, in the hippocampus indicating the initiation of a transcriptional program. We hypothesized this surge in CORT release was a manifestation of stress experienced by the rat in response to ketamines psychotropic effects. When sensory perception was blocked under isoflurane anesthesia, administration of ketamine did not increase circulating CORT levels as compared to animals injected with saline. Taken together, ketamine administration triggers a behavioral stress response that has downstream molecular consequences. The resulting CORT release, virtually concurrent with the timing of ketamines rapid-acting antidepressant actions, necessitates the consideration of this pathways potential involvement when trying to dissect out the relevant molecular mechanisms underlying ketamines action.