Immediate and Long-Term Electrophysiological Biomarkers of Antidepressant Behavioral Effects After Subanesthetic Ketamine and Medial Prefrontal Cortex Deep Brain Stimulation Treatment

1 Ketamine and medial prefrontal cortex (mPFC) deep brain stimulation (DBS) are emerging depression treatments, however their mechanisms and efficacy biomarkers have not been established. This study investigates immediate and long-term mPFC electrophysiological correlates of their antidepressant effects, and explores the effects of combinatorial treatment. Local field potential electrodes were implanted into the mPFC of male Sprague Dawley rats, then corticosterone was administered for 21 days, except in the control group (n = 8). In the last week of depression induction, the remaining groups received mPFC DBS (n = 8), ketamine (n = 9), both (n = 9), or neither (n = 8). All groups then performed a behavioral assay. We found that DBS treatment immediately suppressed relative low gamma power and improved cognition in the long-term, which correlated with rescued apathy-like behavior in the Groom Test (GT). In contrast, ketamine treatment suppressed sample entropy in the long-term, which correlated with its immediate decrease in sample entropy as well as rescued GT behavior. In the combinatorial group these changes were inhibited, which correlated with a failure to fully rescue GT behavior. Despair-like behavior in the Forced Swim Test was rescued by all three treatments, which correlated with increased relative high frequency oscillation (HFO) power immediately after ketamine and combinatorial treatment. Our findings support the utility of low gamma and HFO power, sample entropy, and cognitive tests in the search for mechanisms and biomarkers of depression and novel treatments. 2 Significance Statement There is growing interest in psychedelic pharmacological agents, such as ketamine, and direct neurostimulation techniques, like deep brain stimulation, as treatments for depression. However, their mechanisms are poorly understood, particularly those underlying their sustained benefits. Our study addresses this by correlating behavioral rescue with immediate and long-term changes in the function of the medial prefrontal cortex. These antidepressant correlates may represent therapeutically relevant functional changes, and therefore also have potential as biomarkers. Clinically, such biomarkers enable predicting and monitoring of treatment efficacy, and thereby contribute to the personalization of care. In research, utilizing these translatable biomarkers enhances the etiological validity of disease models, and enables the development of more targeted, next-generation treatment strategies.