1. Neuroscience
Download icon

Antidepressants: Where ketamine and dopamine collide

  1. David J Marcus
  2. Michael R Bruchas  Is a corresponding author
  1. Department of Anesthesiology and Pain Medicine and Center for Neurobiology of Addiction, Pain, and Emotion, University of Washington, United States
  2. Department of Anesthesiology and Pain Medicine, Center for Neurobiology of Addiction, Pain, and Emotion, and Department of Pharmacology, University of Washington, United States
Insight
  • Cited 0
  • Views 1,542
  • Annotations
Cite this article as: eLife 2021;10:e70148 doi: 10.7554/eLife.70148

Abstract

Ketamine strengthens connections between two brain regions that are involved in the production and regulation of dopamine, which may explain how the drug can alleviate depression.

Main text

1987 was a watershed year in the history of antidepressants, with Prozac being the first selective serotonin reuptake inhibitor drug to be approved for use in the US. Prozac and other drugs that limit the reuptake of neurotransmitters such as serotonin, norepinephrine and dopamine would dominate the market for the next three decades. These treatments were marked improvements over their predecessors, but it has become clear that they are not the silver bullet they were once touted to be: moderate efficacy, insensitive populations, and untenable side effects have all limited their clinical utility (Warden et al., 2007).

Enter Ketamine. First synthesized over 75 years ago, this small, unassuming compound had so far been relegated to veterinary clinics as a pet anesthetic, while also doubling as a club drug that could induce dissociation and euphoria. In the early 2000s, however, reports started to emerge suggesting that a single dose of ketamine could have profound and lasting antidepressant effects (Berman et al., 2000). After over a decade of research, the US Food and Drug Administration (FDA) finally approved ketamine, in the form of ‘esketamine’, for the treatment of depression (Carboni et al., 2021). However, the mechanisms that drive the antidepressant effects of ketamine are poorly understood: this is unusual for an FDA-approved drug, although not unheard of for molecules used to treat affective disorders such as depression. How can a compound used to anesthetize cats or induce a psychedelic-like high have clinical utility?

Dopamine, the so-called ‘pleasure or reward neuromodulator’, is indispensable for regulating responses to rewards such as delicious foods, sex or addictive substances; however, it has also been implicated in certain mood disorders (Berridge, 2018). In fact, the emergence of depression has been linked to disruption in the activity of the dopamine-producing neurons present in the ventral tegmental area (VTA) of the brain, but few studies have examined whether ketamine elicits its antidepressant effects by altering the activity of these cells (Hamon and Blier, 2013). Now, in eLife, Yevgenia Kozorovitskiy and colleagues at Northwestern University – including Mingzheng Wu as first author – report how ketamine can strengthen brain circuits that include the VTA (Wu et al., 2021).

The researchers used a ‘learned helplessness’ experimental mouse model which mimics the blunted behavioral or emotional reactions that are one of the hallmarks of depression (Bylsma et al., 2008). The rodents were repeatedly exposed to mild electric shocks to the foot that were impossible to escape: over time, they learn that it was pointless to try to avoid this stressor, and they froze rather than try to escape. Antidepressants – including ketamine, as Wu et al. now show – alleviate this helplessness and restore escape behaviors. The researchers had also genetically manipulated the rodents to introduce a bioengineered molecule that emits light when neurons become activated, with the change in fluorescence being used as a proxy for neuronal activity (Inoue, 2020). The experiments revealed that in mice with learned helplessness, the activity pattern of VTA neurons was abnormal: however, ketamine treatment could reverse this disruption (Figure 1).

A new mechanism for the antidepressant effects of ketamine.

Over time, mice that are exposed to repeated mild electric shocks from which it is not possible to escape stop trying to avoid this stressor; instead, they develop learned helplessness and ‘freeze’ rather than attempt to escape, a behavior reminiscent of the reduced behavioral responses observed in depression (top). This exposure to stress disrupts the activity of a circuit formed by dopamine-producing projections (blue lines) and glutamate-producing projections (red lines) that connect the ventral tegmental area (VTA) and the medial prefrontal cortex (mPFC). Receiving ketamine (bottom), however, has an antidepressant effect. The drug alleviates learned helplessness and allows the mice to display active coping behaviors such as trying to escape the stimuli, while also reversing the disruption in the circuits between VTA and mPFC.

These data clearly demonstrated that VTA dopamine neurons are involved in responding to stressors, but whether ketamine acts by changing the activity of the neurons themselves remained unresolved. To test for a causal link, Wu et al. selectively inhibited VTA neurons by using another bioengineered molecule, a receptor called DREADD that can only be selectively activated by a synthetic molecule (Dobrzanski and Kossut, 2017). Ketamine had no effect when the VTA neurons were inhibited, demonstrating that VTA dopamine neuron activity is necessary for the drug to have an antidepressant effect.

Further physiology experiments were then conducted in the VTA to determine how ketamine could impact neuronal activity, with, somewhat unexpectedly, few noticeable effects emerging. This suggested that rather than acting locally, ketamine most likely worked by modulating the activity of an upstream brain region that influences VTA activity. The medial prefrontal cortex (mPFC for short) represented an attractive target because it sends dense projections to the VTA, and because it mediates, in part, the antidepressant effects of katamine (Vertes, 2004; Moda-Sava et al., 2019).

Indeed, Wu et al. discovered that injecting ketamine specifically into the mPFC (but not other VTA-projecting brain regions) replicated the improvements observed when the mice were administered the drug systemically. In addition, antidepressant-like effects that mirrored those induced by ketamine emerged when mPFC neurons that express the receptor for dopamine were artificially activated using excitatory DREADDs.

The research presented by Wu et al. provides a unique understanding of the antidepressant effects of ketamine on canonical reward circuits. Specifically, the results suggest that the drug acts by strengthening a recurrent neural circuit between the VTA and the mPFC, which allows ketamine’s antidepressant properties to persist long after the compound has been cleared from the body. The data pave the way for future studies that directly examine how the effects of ketamine are mediated by mPFC projections to the VTA. Ultimately, this knowledge will help to find and design more selective compounds which can target these circuits to treat depression and related disorders.

References

Article and author information

Author details

  1. David J Marcus

    David J Marcus is in the Department of Anesthesiology and Pain Medicine and the Center for Neurobiology of Addiction, Pain, and Emotion, University of Washington, Seattle, United States

    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-0767-4249
  2. Michael R Bruchas

    Michael R Bruchas is in the Department of Anesthesiology and Pain Medicine, the Center for Neurobiology of Addiction, Pain, and Emotion, and the Department of Pharmacology, University of Washington, Seattle, United States

    For correspondence
    mbruchas@uw.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-4713-7816

Publication history

  1. Version of Record published: June 17, 2021 (version 1)

Copyright

© 2021, Marcus and Bruchas

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 1,542
    Page views
  • 111
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Download citations (links to download the citations from this article in formats compatible with various reference manager tools)

Open citations (links to open the citations from this article in various online reference manager services)

Further reading

    1. Neuroscience
    Aurelio Cortese et al.
    Research Article Updated

    The human brain excels at constructing and using abstractions, such as rules, or concepts. Here, in two fMRI experiments, we demonstrate a mechanism of abstraction built upon the valuation of sensory features. Human volunteers learned novel association rules based on simple visual features. Reinforcement-learning algorithms revealed that, with learning, high-value abstract representations increasingly guided participant behaviour, resulting in better choices and higher subjective confidence. We also found that the brain area computing value signals – the ventromedial prefrontal cortex – prioritised and selected latent task elements during abstraction, both locally and through its connection to the visual cortex. Such a coding scheme predicts a causal role for valuation. Hence, in a second experiment, we used multivoxel neural reinforcement to test for the causality of feature valuation in the sensory cortex, as a mechanism of abstraction. Tagging the neural representation of a task feature with rewards evoked abstraction-based decisions. Together, these findings provide a novel interpretation of value as a goal-dependent, key factor in forging abstract representations.

    1. Cell Biology
    2. Neuroscience
    Francois Singh et al.
    Research Article

    Parkinson’s disease (PD) is a major and progressive neurodegenerative disorder, yet the biological mechanisms involved in its aetiology are poorly understood. Evidence links this disorder with mitochondrial dysfunction and/or impaired lysosomal degradation – key features of the autophagy of mitochondria, known as mitophagy. Here, we investigated the role of LRRK2, a protein kinase frequently mutated in PD, in this process in vivo. Using mitophagy and autophagy reporter mice, bearing either knockout of LRRK2 or expressing the pathogenic kinase-activating G2019S LRRK2 mutation, we found that basal mitophagy was specifically altered in clinically relevant cells and tissues. Our data show that basal mitophagy inversely correlates with LRRK2 kinase activity in vivo. In support of this, use of distinct LRRK2 kinase inhibitors in cells increased basal mitophagy, and a CNS penetrant LRRK2 kinase inhibitor, GSK3357679A, rescued the mitophagy defects observed in LRRK2 G2019S mice. This study provides the first in vivo evidence that pathogenic LRRK2 directly impairs basal mitophagy, a process with strong links to idiopathic Parkinson’s disease, and demonstrates that pharmacological inhibition of LRRK2 is a rational mitophagy-rescue approach and potential PD therapy.