Kasane V. Tonegawa
It is estimated that around 300 million people worldwide have been diagnosed with depression (World Health Organization, 2018). Depression is a mental illness that involves feelings of worthlessness, a lack of interest in previously enjoyable activities, thoughts of guilt, and changes in sleeping and eating patterns (National Alliance on Mental Illness, 2017). Depression can sometimes result in suicidal ideations (i.e., thoughts or plans to kill oneself; NAMI, 2017), which often act as a precursor to suicide, a leading cause of death in the United States (Murphy, Xu, Kochanek, Curtin, & Arias, 2017). Around one-third of those with depression have treatment-resistant depression (TRD); in other words, there is no reduction in depressive symptoms or the symptoms relapse, despite the use of common forms of treatment, such as antidepressant medication or psychotherapy (Rajkumar, Fam, Yeo, & Dawe, 2015; Warden, Rush, Trivedi, Fava, & Wisniewski, 2006). It is imperative that alternative forms of treatment are explored for the 100 million individuals around the world with TRD (WHO, 2018). Recently, commonly used recreational hallucinogens, such as Lysergic acid diethylamide (LSD) and psilocybin mushrooms, have been examined for their benefits for depression, obsessivecompulsive disorder, and post-traumatic stress disorder (Roseman, Nutt & Carhart-Harris, 2018). In 2019, the Food and Drug Administration approved of ketamine, an anesthetic typically used for pain relief and sedation, as a possible form of treatment for TRD (Berman et al., 2000; Niciu et al., 2018; Price, Nock, Charney, & Mathews, 2009). As such, this review aims to describe current and alternative treatment options for TRD, including a promising new avenue of research involving the use of low doses of ketamine.
Prevalent Treatment Options for TRD
Depression is typically treated with pharmacological drugs, psychotherapy, or a combination of the two (McConnell, Carter, & Patterson, 2019). Most antidepressant drugs increase the levels of neurotransmitters negatively correlated with depression, such as serotonin, dopamine, and norepinephrine (Epstein, Szpindel, & Katzman, 2014). The symptoms typically improve after two weeks of treatment, with a reduction in almost all symptoms in two months (McConnell et al., 2019). However, the symptoms might return or might not be alleviated at all, warranting a need for different treatment approaches that involve an increase in dosage, change of the drug type, or a combination of drugs (Epstein et al., 2014; McConnell et al., 2019; Shelton, Osuntokun, Heinloth, & Corya, 2010). Furthermore, physical side effects, such as weight gain or fatigue, and cognitive side effects, such as apathy and anhedonia, are also possible with pharmacological antidepressant treatments (Epstein et al., 2014; McConnell et al., 2019).
In order to increase the likelihood of efficacy, pharmacological treatment for depression is typically combined with a form of psychotherapy, in which the individual collaborates with a therapist to achieve relief of symptoms (Shelton et al., 2010). The most common form of psychotherapy used to treat depression is cognitive-behavioral therapy (CBT). CBT is an approach that is based on the assumption that the negative thought patterns and beliefs are what contribute to depressive symptoms (Dorasamy, 2019). In this form of treatment, a therapist and the client practice role playing and restructure thought patterns to stop or change habitual negative beliefs (Dorasamy, 2019). Although the combined treatment of CBT and antidepressants is typically more effective, relapses or failure to alleviate symptoms might occur (Dorsamy, 2019). When changes in dosage, drug types, and combinations of psychotherapy do not work, alternative forms of treatment— either pharmacological or non-pharmacological—can be used to treat TRD (Shelton et al., 2010).
Common non-pharmacological treatments include electroconvulsive therapy (ECT) and transcranial magnetic stimulation (TMS; Kayser et al., 2011; Shelton et al., 2010). These forms of therapy are reserved as a last-resort or for urgent cases, such as risk of suicidality due to the negative cognitive side effects, including memory loss and confusion (Kayser et al., 2011; Kellner et al., 2012). ECT consists of inducing a brief seizure in the brain to increase levels of neurotransmitters dopamine and norepinephrine (Shelton et al., 2010). While ECT works faster and has a higher response rate than pharmacological treatments, it has a high rate of relapses, meaning the depressive symptoms may return (Kayser et al., 2011; Shelton et al., 2010). Alternatively, TMS works by regulating the neural activity in brain regions associated with depression (O’Reardon et al., 2007). Although TMS has fewer side effects compared to ECT, results from TMS typically appear only after two weeks and might even require several weeks for significant alleviation of symptoms (O’Reardon et al., 2007; Shelton et al., 2010). Therefore, although ECT and TMS are both effective forms of treatment, there are still cognitive side effects, high relapse rates, and long periods of time until alleviation is seen, thus meriting the need to examine unorthodox forms of treatment that are faster and more effective.
Alternative Treatments for TRD
Hallucinogens, or drugs that cause hallucinations when consumed, have recently become considered as a possible form 16 | Literature Reviews of treatment for TRD (Roseman et al., 2018). Some forms of therapy involving naturally-occurring hallucinogen—such as psilocybin mushrooms—are known as psychedelic therapy, in which small dosages of the drugs are administered in the presence and under the guidance of a therapist (Roseman et al., 2018). As with other forms of depression treatments, the exact mechanisms underlying the efficacy of the treatment are unclear. However, improvement in symptoms is related to the conscious “mystical” experience of the drug’s effects, rather than the targeting of specific neurotransmitters (Cahart-Harris et al., 2012; Roseman et al., 2018). In other words, the effects of the drugs are related to reported feelings of unity and awe that are experienced while on the drug (Cahart-Harris et al., 2012). This type of therapy is being examined for its unique approach to treating depression, as well as the quick alleviation of symptoms, which can be experienced in as little as an hour with two dosing sessions (Cahart-Harris et al., 2012). While the mechanisms underlying the efficacy of most antidepressant drugs occur on a neurochemical basis, the benefits of psychedelic therapy derive from the philosophical experiences that are induced by the hallucinogenic properties of the drugs (Cahart-Harris et al., 2012).
Another hallucinogenic drug is ketamine, an anesthetic that has historically been used to treat agitation and pain via sedation and numbing from low doses (Nichols & Paciullo, 2019). At a dissociative or high dose, the drug is known to have psychedelic effects, such as experiences of hallucinations and dissociation (Wolfson, 2014). Ketamine has the distinct benefit of alleviating suicidality in those with TRD and is, therefore, being examined more rigorously as a treatment option (Price et al., 2009, 2014). Nevertheless, because the mechanisms underlying the benefits are still unclear when treating TRD, ketamine and other hallucinogens are reserved as a last resort and require further research in order to better understand its effects (Ionescu & Papakostas, 2016).
How Ketamine Functions as an Antidepressant
Typical antidepressant drugs target neurotransmitters serotonin and norepinephrine, but this mechanism does not always permanently alleviate depressive symptoms in individuals with TRD (Rajkumar et al., 2015). As a result, alternative treatments, such as ketamine, might be a solution given that it targets a different neurotransmitter called glutamate (Gould, Zarate, & Thompson, 2019; Trullas & Skolnick, 1990). Glutamate is a neurotransmitter that has been linked to depression and suicidality when at abnormally low levels (Deutschenbauer et al., 2016) The receptors that glutamate latches onto are so active in those with depression that it decreases the amount of glutamate in the synapses, or the gaps between neurons (Deutschenbaur et al., 2016; Krystal, Sanacora, & Duman, 2013; Reinstatler & Youssef, 2015). Ketamine is an N-Methyl-D-Aspartate (NMDA) receptor antagonist, meaning it inhibits the activity of the glutamate receptors, which results in higher levels of glutamate between neurons (Deutschenbaur et al., 2016; Luckenbaugh et al., 2014). Because ketamine works to increase glutamate levels, it is hypothesized that its inhibitory effects on the receptors is one of the key factors in the drug’s antidepressant effects since it increases the neurotransmitter associated with elevated mood (Gould et al., 2019). Because ketamine targets a different neurotransmitter than typical antidepressant drugs, it is being explored as a promising alternative treatment to TRD.
Ketamine’s potential use as a treatment option is especially unique given its rapid-acting effects that specifically reduce suicidal ideations (Ballard et al., 2014; DiazGranados et al., 2010; Price et al., 2009, 2014; Murrough et al., 2015). In fact, a reduction in suicidal ideations was observed in as little as 40 minutes, suggesting that ketamine might be especially suited for immediate reductions in depressive symptoms, possibly due to the drug’s hallucinogenic and dissociative properties (Ballard et al., 2014; DiazGranados et al., 2010; Price et al., 2009, 2014). However, the reduction of suicidal ideations via ketamine treatment has not been fully examined, because research on the drug’s antidepressant use is still in its beginning stages (Rajkumar et al., 2015). Even so, there might be a relation between reduction in brain regions affected by ketamine’s dissociative properties and the drug’s role as an NMDA receptor antagonist, perhaps contributing to its antidepressant effects in individuals with TRD (Erhardt et al., 2013; Luckenbaugh et al., 2014; Sowa-Kucma et al., 2013).
Examining the brains of individuals who have attempted suicide has elucidated that the level of quinolonic acid (QUIN) is uniquely related to suicidal ideations (Erhardt et al., 2013; Rajkumar et al., 2015; Sowa-Kucma et al., 2013). QUIN is an NMDA receptor agonist that has opposite effects of ketamine, such that it activates, rather than inhibits, the glutamate receptors, resulting in lower levels of glutamate (Erhardt et al., 2013; Gould et al., 2019). Increased levels of QUIN, which decreases glutamate, was found in suicide attempters, indicating that glutamate is strongly related to suicidal ideations (Erhardt et al., 2013; Sowa-Kucma et al., 2013). Accordingly, if reduced levels of glutamate resulting from QUIN are connected to suicidal ideations, then the consequential increased levels of glutamate resulting from ketamine might be key to its antidepressant effects.
While ketamine is not the only NMDA receptor antagonist, other NMDA-antagonist drugs, such as phencyclidine and methadone, have not had the same rapid-acting reductions in suicidal ideations that ketamine has (Niciu et al., 2018). In particular, unlike other NMDA receptor antagonists that do not result in such a robust decrease in suicidal ideations, ketamine can lead to dissociation, or a sense of a detachment from oneself and one’s surroundings (Luckenbaugh et al., 2014; Niciu et al., 2018; Sos et al., 2013). In fact, the dissociative side effects have been positively correlated with the antidepressant response and elevated mood (Niciu et al., 2018; Sos et al., 2013). Additionally, brain regions related to depression and suicidal ideations—the hippocampus and prefrontal cortex—are particularly affected by the occurrence of dissociation induced by ketamine (Erhardt Literature Reviews | 17 Alternative Treatments for TRD et al., 2013; Rajkumar et al., 2015; Sowa-Kucma et al., 2013). In particular, the brains of suicide attempters have reduced hippocampal volume and show signs of dysfunction in the prefrontal cortex (Erhardt et al., 2013; Sowa-Kucma et al., 2013). The regions of the brain where abnormalities are related to suicidality are also related to dissociation, which is prompted by ketamine’s increase in glutamate levels. It is possible, then, that the drug’s effects on suicidality are linked, at least in part, to its dissociative effects in these brain areas (Rajkumar et al., 2015). Given that it is rapid, effective, and specifically targets suicidality, ketamine may be a promising treatment for those with TRD.
Conclusion
For the one-third of individuals with TRD, alternative treatments that provide faster alleviation and higher efficacy are being explored. These treatments include ECT, TMS, and psychedelic therapy. While ECT and TMS are faster than pharmacological treatment, there are still cognitive side effects to consider (Kayser et al., 2011; Shelton et al., 2010). With psychedelic therapies, such as psilocybin mushrooms, the unique experience-based characteristic may be key in its efficacy for TRD treatment (Cahart-Harris et al., 2012). One of f the most promising psychedelic treatments is ketamine, which works to reduce suicidality in TRD by specifically targeting the NMDA glutamate receptors in the hippocampus and prefrontal cortex to increase levels of glutamate, the neurotransmitter responsible for suicidal ideations (Erhardt et al., 2013; Luckenbaugh et al., 2014; Rajkumar et al., 2015). Much of the research on ketamine’s effects on TRD began with a focus on its general antidepressant use, but the drug may have unique benefits in terms of reducing suicidality given its dissociative properties (Rajkumar et al., 2015). Given the relatively recent discovery of ketamine’s possible medical use along with other psychedelic therapies, many studies have not yet been placebo-controlled, randomized, double-blinded, or used a large sample, thus making conclusion of causality difficult (Ballard et al., 2015; DiazGranados et al., 2010; Murrough et al., 2015, Rajkumar et al., 2015). The current literature review was explored from a theoretical perspective given the dearth of clinical trials with the drugs. Therefore, it is necessary to not only conduct studies with larger samples but to also measure suicidality with scales specifically targeting changes in suicidal ideation over short periods of time. Another consideration for future studies is examining the long-term effects of repeated administration of ketamine and other psychedelic drugs, such as potential dependency and tolerance. However, given the urgency required in treating suicidal TRD patients, it is likely that the potential benefits are greater than the costs (Rajkumar et al., 2015).
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