There is a strong corollary between stress and the development of depression and suicidal behaviors. The neurobiological mediators of stress are primarily controlled by the noradrenergic and corticotropin-releasing factor (CRF) median eminence systems. Furthermore, stress directly and indirectly, through the Hypothalamic Pituitary Axis, activates the Locus Coeruleus (LC), which is the primary producer of NE in the central nervous system (CNS). Directly, glutaminergic neurons send excitatory signals to the LC via interaction with N-Methyl-D-Aspartate (NMDA) receptors NMDA antagonists, such as ketamine, can dampen the glutaminergic system, which has been implicated during states of depression and low moods.

The neurobiological commonality between multiple psychiatric disorders and depression, suicide, and attempted suicide is a decrease in serotonergic activity. It has been shown that patients who died of suicide, have decreased serotonin transporter in the ventromedial prefrontal cortex and anterior cingulate, which are areas that control decision making or willful action. The prefrontal cortex is important for inhibitory behavioral control. A potential treatment modality of ketamine, is that it produces activation of this region.35 The anterior cingulate cortex has been shown to be associated with impulsive aggression compared to control. Clinical studies have shown that low CSF 5-HIAA, metabolite in serotonin system, has been implicated and positively correlated to aggression scores and impulsivity. This is interesting because, suicides in the military are thought to have an impulsivity component, triggered by one or more major life stressors.

Another region associated with suicide is the infralimbic cortex. A recent study, based on neuroimaging techniques, demonstrated that glucose metabolism in this region was associated with SI at baseline, and decreases in SI was observed after ketamine infusion. This is the same region target by deep brain stimulation, for depression treatment. Additionally, the infralimbic cortex has been implicated in behavioral flexibility. Implicating that ketamine's anti-suicidal properties may stem from its ability to promoting cognitive flexibility. Most likely due to its ability to increase brain-derived neurotrophic factor (BDNF), which is a major contributor to neuronal plasticity. BDNF also plays key roles in synaptic and long-term potentiation, which may counteract the decreased levels in Mood Disorder (MDD) patients. Furthermore, ketamine infusion has been shown to change sleep slow wave activity. This biomarker is functionally related to increased synaptic strengthening and cortical synchronization. These factors, combined, may be implicated in not only ketamine's antidepressant effects and counteraction of decreased synaptic plasticity seen mood disorders, but also its ability to have week long lasting effects.

This information leads us to hypothesize that treatment of acutely suicidal patients with ketamine would: 1) decrease suicidal ideationto a clinically significant degree, and 2) the effect of ketamine will be seen for as long as one-week post administration.

To the best of our knowledge, this study does not duplicate any ongoing work. Instead, it would strengthen power to past studies and current work. There are four clinical trials investigating ketamine's effect on SI. One has an unknown status. There are two that are investigating ketamine in relationship to psychiatric standard of care, whereas this study is investigating its effects against a saline placebo. Finally, the last clinical trial is investigating the Neurobiology of Suicide. Their phase 2 component, which utilizes a similar protocol, uses ketamine as a tool to identify potential biomarkers for suicide. Furthermore, this study differs from Janssen Research \& Development's clinical trial in administration route and study design. Their study focuses on using ketamine through intranasal administration. Their primary outcomes are the long-term safety and efficacy, and the design of their study is an open label multicenter trial.

This research does not duplicate any prior work. To date, there is only one study, from Iran, that evaluates the effectiveness of ketamine in high risk patients, or those that present as acutely suicidal to the ED. This was a single blinded trial that utilized 0.2mg/kg infused over one minute. The study indicated significant decrease in their measurement outcomes. However, they concluded that ketamine is not a good choice for treatment because it did not meet their cut off values. Their results might have been influenced by the rapid infusion over one minute, which differs from our study as well as the vast majority of the literature. We believe the slower 40-minute infusion is necessary for optimal results, as the larger dosage has produced more clinically meaningful results in prior studies, and the slower infusion produced less negative side effects. They chose the minimal dose shown to diminish SI41 200 ng/ml (0.2 mg/kg), which provokes lateral nystagmus.35 This protocol utilizes a higher dose, 0.5 mg/kg, which is the ED50 for narcosis. Our study is medically relevant because dosage effects on SI have not been studied. Comparison of our studies may address questions regarding the optimal dose and infusion rate.

The BSSI will measure the severity of SI. It is based on the interviewer rated version of the original Scale for Suicidal Ideation (SSI), which is one of the few document suicide assessment tools with predictive validity for suicide completion. The internal reliability, test and retest validity, as well as invariance over time has been demonstrated for the BSS. Furthermore, the first five items of the BSSI are a common clinical screening for the presence of suicidal thoughts. For these reasons, the BSSI was chosen as our primary outcomes measure. Two studies have indicated that the cut off between high and low risk is a BSS ≥ 2. A recent investigation has determined BSSI ≥ 6 is predictive of future suicide attempts. These two values will serve in our analysis.

The Montgomery- Åsberg Depression Rating Scale (MADRS) is a widely known 10 item clinician administered measure of depression severity. Since it's development in the late 1970s, it has become more popular than the gold standard, Hamilton rating scale for Depression (HAM-D). It is considered to be more sensitive to change, just as effective, and simpler to use clinically. However, the reliability depends on good interrater agreement. Difficulties in clinical trial to show signal detection for known effective drugs have implicated clinician administered measurement as a possible source of error. To avoid poor interrater reliability, rater bias, and variable interview quality, this trial will utilize the self-administered version of the MADRS-S. This has 9 items and a total score ranging from 0 to 27.50 The scoring of MADRS-S has shown to be similar to that of physician scoring.

The emotional pain of the suicidal patient requires empathetic care that may not always be possible with the time pressures, volume, and pragmatic nature of the ED environment. A pharmacologic intervention with rapid effects to decrease SI would play a vital role in improving the standard of care for this vulnerable population.