Title: Elucidating the mechanisms of (R,S)-ketamine as a prophylactic against stress-induced psychiatric disorders
Description: Mental illness has been a perplexing mystery for centuries, inciting both fear and stigmatization. Yet, the knowledge that the brain gives rise to the mind transformed the field of psychiatry; biological studies of aberrant human behavior has revealed that mental disorders are rooted in physical abnormalities that may be targeted to alleviate symptoms. Even with recent progress, there remains many open questions, one of which is: how exactly are some individuals more susceptible to developing these disorders than others? Excess, or traumatic, stress can lead to the onset of maladaptive disorders such as major depressive disorder (MDD) and post-traumatic stress disorder (PTSD). But what if it were possible to prevent these diseases from occurring in the first place? What if there was a prophylactic or vaccine-like approach to increase resilience against environmental stressors? Would we be able to target susceptible populations and administer this prophylactic? In this thesis, I present our work demonstrating the potential for prophylactic pharmaceuticals to enhance stress resilience and protect against stress-induced psychopathology. (R,S)-ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has been demonstrated to be a viable candidate drug to administer as a prophylactic against stress-induced psychopathology.

It was serendipitously discovered to rapidly (in as little as a half hour) and persistently (up to 2 weeks) alleviate depressive symptoms in patients with MDD. Since its discovery as an effective antidepressant, research has been focused on its mechanism of action with the goal of ultimately developing more efficacious, rapid-acting, long-lasting antidepressant drugs. However, in our lab, we made a truly unexpected discovery in 2016, described in Chapter 2: (R,S)-ketamine prevents the development of psychiatric symptoms when administered before a stressor. We found that prophylactic (R,S)-ketamine is effective against behavioral despair and buffers against learned fear in a time- and dose-specific manner, described in Chapter 3. This was the first indication that a drug can be administered before stress to prevent stress-induced psychopathology, opening a novel field of preventative psychopharmaceuticals. Follow-up studies in our and other labs have consistently replicated this effect using different stressors and mouse strains, in rats, and in both males and females.

These data demonstrate that (R,S)-ketamine can effectively enhance resilience pre-clinically. To address how (R,S)-ketamine is inducing long-lasting protection, in Chapter 4, I describe a study that used a metabolomics platform to uncover the long-term effects of (R,S)-ketamine in buffering against learned fear. We found that (R,S)-ketamine alters purine and pyrimidine metabolism in brain and, most notably, the periphery. These data suggest the potential to conduct a simple blood test to screen for biomarkers of prophylactic efficacy in the clinic. However, while these data revealed the end-products of therapeutic efficacy, it was unknown what brain mechanisms may mediate such long-lasting protection against a psychological stressor. In a separate study, the ventral CA3 (vCA3) region of the hippocampus was uncovered to be necessary for (R,S)-ketamine’s prophylactic fear buffering effects, and that targeting this region both mimics and occludes its effects. It was then discovered that 1 week after a single administration of (R,S)-ketamine or FENM, AMPA bursts were attenuated in vCA3.

These data reveal vCA3 a central node for prophylactic (R,S)-ketamine efficacy. The biggest limitation of these preliminary studies is that they each only assessed changes at single timepoints rather than mapped out what occurs throughout treatment, during stress, and during recall of a stressor. It remained unknown whether (R,S)-ketamine alters the experience or recollection of a stressor to induce long-lasting protection. The next goal was to use in vivo technologies such as 1-photon Ca2+ imaging in freely-moving mice to develop a more thorough understanding of how exactly (R,S)-ketamine is acting on vCA3 to confer its prophylactic fear buffering effects, which is outlined in Chapter 5. Mice were imaged in the ventral hippocampus throughout a prophylactic (R,S)-ketamine administration paradigm. We found that prophylactic (R,S)-ketamine administration buffered against the experience of the stressor specifically in vCA3 and reduced ventral hippocampal correlated network activity to ultimately buffer against learned fear.

These data indicate that (R,S)-ketamine actively buffers against learned fear in the ventral hippocampal at the time of stress. The promise of (R,S)-ketamine is that it is also beneficial as a prophylactic in other settings beyond MDD and PTSD, such as in patients with traumatic brain injury (TBI). In Chapter 6, I describe a study that sought to determine whether (R,S)-ketamine can be useful as a prophylactic for TBI-induced neuropsychiatric effects. Here, TBI mice developed fear generalization, or the inability to distinguish between fear-inducing and neutral stimuli. To understand how TBI alters fear memory traces to promote fear generalization, we used the ArcCreERT2 x enhanced yellow fluorescent protein (eYFP) activity-dependent memory tagging strategy developed by Dr. Christine Ann Denny and found that TBI-induced fear generalization is partially mediated by dentate gyrus (DG) memory trace dysregulation. To reverse this fear generalization phenotype, a single administration of (R,S)-ketamine 1 hour after a TBI prevented the fear generalization phenotype.

These data reveal the possibility of administering (R,S)-ketamine or other prophylactics in the clinic as part of post-operative care for TBI patients to prevent long-term fear generalization deficits. Altogether, this thesis demonstrates the potential for pharmacotherapies for stress resilience enhancement and reveals potential targets for prophylactic drug development. We have uncovered the long-term metabolomic changes that occur after a single dose of (R,S)-ketamine, revealed a central node for prophylactic efficacy, mapped the dynamic changes that occur throughout treatment, and applied the prophylactic paradigm to a model of TBI to demonstrate the broad range of applications of this approach. This work paves the way for the novel field of preventative psychiatry and opens new avenues to explore ways to reduce the devastating impact of mental illness on individuals and society.