For decades, psychiatry has focused on neurotransmitters. Depression was a serotonin problem. Schizophrenia was a dopamine problem. Bipolar disorder was a mood stabilizer problem. This concept, while clinically useful, has always been incomplete. Now research suggests we may have been looking at the wrong part of the cell. The mitochondrion, which is often described as “the powerhouse of the cell,” turns out to be a complex regulator of mood, cognition, stress resilience, and neuroplasticity. And it turns out that mitochondria don’t always stay inside cells.
The Mitochondrion as Energy Producers
The human brain accounts for approximately 2% of the body’s weight but uses about 20% of the body’s energy. Neurons are among the most energy-consuming cells in our body, and their survival depends on mitochondria. When mitochondria’s energy-producing efficiency decreases, the consequences are much more than just fatigue. Research has demonstrated that mitochondrial dysfunction occurs in major depressive disorder, bipolar disorder, and schizophrenia.
But, mitochondria control much more than energy production. They regulate calcium, initiate cell death (apoptosis), and produce reactive oxygen species (ROS). When ROS accumulate, they produce oxidative stress. This occurs during chronic psychological stress, and it destabilizes mitochondrial membranes. This impairs the neurons that regulate emotion, executive function, and social cognition. So, mental illness can be understood, at least in part, as a disease of the mitochondria.
Stress, the HPA Axis, and Mitochondrial Remodeling
The relationship between psychological stress and mitochondria works in both directions. In a paper in Nature Reviews Neuroscience, Martin Picard and Bruce McEwen proposed that mitochondria act as stress-response hubs. They further suggest these organelles receive signals from other cells and change their shape, either fusing with other mitochondria or breaking apart, and these changes directly affect the resilience of our cells. Stress hormones, such as cortisol and catecholamines, bind to receptors on mitochondrial membranes and influence their energy output.
Chronic activation of the hypothalamic-pituitary axis (HPA), which occurs during sustained psychological trauma, depression, and PTSD, causes mitochondria to fragment. This fragmentation impairs neuroplasticity and increases inflammation throughout the brain. The result is similar to what we see in individuals suffering with treatment-resistant depression. Their cognition slows. Their emotions are blunted. Their ability to adapt is decreased.
Neuroinflammation
One of the most important functions of mitochondria is their role in regulating immunity. Damaged mitochondria release mitochondrial damage-associated molecular patterns (DAMPs). This provides a direct link between metabolic stress and the neuroinflammation that is increasingly recognized to contribute to depression, schizophrenia, and post-COVID psychiatric syndromes.
Microglia, which are the brain’s immune cells, are very sensitive to signals from mitochondria. When the structure of mitochondria is compromised, microglia become activated and release inflammatory molecules known as “cytokines.” These cytokines then inhibit the growth of new neurons in the brain and reduce the connections between neurons (synapses). They also produce less serotonin. This combination of reduced energy production and inflammation may explain why conventional antidepressants are so frequently ineffective.
The Startling Discovery of Extracellular Mitochondria
One of the most surprising recent findings in neuroscience is the discovery that mitochondria can exist and send signals outside of cells. For most of the twentieth century, the idea of extracellular mitochondria was viewed as heretical. Now we know better.
In 2020, researchers found mitochondria circulating in human blood, encapsulated within extracellular vesicles or as “free” organelles, that could be taken up by cells in order to restore their respiratory function.
Also, support cells in the nervous system known as astrocytes transfer mitochondria to damaged neurons following injury. This is a type of intercellular rescue, almost like a rocket bringing supplies to a damaged space station. Platelets, which help blood clot, also release mitochondria in response to injury and inflammation.
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The implications for psychiatry are only beginning to be explored. Cell-free mitochondrial DNA is elevated in the cerebrospinal fluid and blood of individuals with depression compared to healthy controls, suggesting that stressed neural tissue may be expelling mitochondrial contents as a distress signal, although the reason for this release remains unknown.
Even more intriguing is the possibility that extracellular mitochondria might serve as messengers, traveling to distant cells and delivering messages to them. If this is confirmed to occur in the human central nervous system, this would demonstrate an entirely new level of brain-body communication that operates in addition to classical neurotransmission and hormonal signaling.
Therapeutic Possibilities
If mitochondrial dysfunction underlies much of psychiatric illness, the therapeutic implications are huge. Exercise remains the most validated mitochondrial intervention. Aerobic activity stimulates mitochondrial production, enhances antioxidant defenses, and promotes the growth of new neurons. These effects may partially explain the antidepressant properties of physical activity. Ketogenic and caloric restriction diets provide a shift from carbohydrates to fats as a source of fuel, which reduces oxidative stress. Several agents, including ketamine, stimulate the production of new mitochondria, which may contribute to their antidepressant effects by restoring mitochondrial functioning.
The field of mitochondrial transfer therapy is growing. If mitochondria can be delivered into damaged neurons, the prospect of reversing the energy deficits associated with treatment-resistant mental illness becomes theoretically possible. We are not there yet. But the evidence suggests we might benefit from moving in that direction.
Rethinking the Cause of Mental Illness
Psychiatry has long tried to blame mental illness on problems at the level of the synapse. The mitochondrial hypothesis doesn’t replace that hypothesis. Dopamine and serotonin still matter. But the mitochondrial hypothesis suggests dysfunctional synapses may be only one part of the problem. Neurons that lack sufficient energy cannot release neurotransmitters reliably, or support the neuroplastic changes that underlie learning and memory.
And now, with the discovery that mitochondria travel between cells, carrying energy and possibly signals across the brain, our understanding of mental health must expand. The mind is not merely a chemical event. It is a mitochondrial one too.