Low testosterone promotes anxiety through astrocytic mitochondrial remodelling at the nucleus accumbens blood-brain barrier
This study demonstrates that low testosterone promotes anxiety in male rats by disrupting androgen receptor signaling in nucleus accumbens astrocytes, which impairs mitochondrial organization and blood-brain barrier integrity, while testosterone restoration or astrocyte-specific mitochondrial rescue reverses these effects.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
For decades, scientists have understood that the brain does not exist in a vacuum. It is a protected organ, shielded by a sophisticated border control system known as the blood-brain barrier. This barrier acts like a highly selective gatekeeper, allowing essential nutrients to pass from the blood into the brain while keeping out toxins and harmful substances. When this barrier functions correctly, the brain's environment remains stable, allowing neurons to communicate clearly. However, when this barrier becomes leaky or dysfunctional, the brain's delicate chemistry can be disrupted, a condition increasingly linked to mood disorders like anxiety. While researchers have long studied the neural circuits and genetic factors that drive anxiety, a newer line of inquiry asks how the body's broader physical state, particularly its hormonal balance, might shape the brain's vulnerability to stress. One hormone in particular, testosterone, has long been associated with mood and behavior in men, but the precise biological mechanism connecting low levels of this hormone to persistent anxiety has remained a mystery.
A team of researchers at the École Polytechnique Fédérale de Lausanne and collaborating institutions has now uncovered a specific chain of events that links low testosterone to anxiety, revealing that the problem begins not in the brain's neurons, but in the support cells that line its blood vessels. By studying male rats that naturally displayed either high or low levels of anxiety, the scientists discovered that the anxious animals had significantly lower levels of circulating testosterone. More importantly, they found that this hormonal deficit was not just a side effect of their mood; it was a cause. When the researchers lowered testosterone levels in normally calm rats, those animals became anxious. Conversely, when they restored testosterone levels in anxious rats, the animals became calmer. This established a direct, causal relationship: low testosterone drives high anxiety.
The next step was to find where this hormonal signal was being translated into brain changes. The researchers focused on the nucleus accumbens, a deep brain region critical for processing emotions and stress. They examined the blood vessels in this area and found a striking difference between the anxious and calm rats. In the anxious animals, the blood-brain barrier in the nucleus accumbens was more permeable, meaning it was leaking. This leakiness was accompanied by a physical breakdown in the structure of the barrier. Specifically, the "feet" of star-shaped support cells called astrocytes, which normally wrap tightly around blood vessels to seal the barrier, were pulling away. This retraction left gaps in the vessel walls, allowing substances to seep into the brain tissue where they did not belong.
Digging deeper into the cellular machinery of these astrocytes, the team found that the root of the problem lay in the mitochondria, the tiny power plants inside cells that generate energy. In the anxious rats, the mitochondria within the astrocyte feet were disorganized and smaller than usual. They were also failing to make proper contact with another cellular structure called the endoplasmic reticulum, a network involved in protein and lipid production. This disconnection was linked to a shortage of a specific protein called mitofusin 2, or Mfn2, which acts as a molecular glue, helping mitochondria fuse together and maintain their shape. The anxious rats had significantly less of this protein in their astrocytes.
The researchers then traced the source of this cellular disarray back to the hormone itself. They showed that testosterone normally binds to receptors on these astrocytes to instruct them to maintain healthy mitochondria and a tight seal around blood vessels. When they blocked these receptors in the nucleus accumbens, the protective effects of testosterone disappeared, and the rats remained anxious despite having normal hormone levels in their blood. To prove that fixing the mitochondria was enough to solve the problem, the team used a viral vector to artificially increase the levels of Mfn2 specifically in the astrocytes of anxious rats. This single intervention, without changing the hormone levels at all, was sufficient to restore the blood-brain barrier and significantly reduce the animals' anxiety-like behaviors.
The study also confirmed that these findings were not limited to young animals. In older male rats, whose natural testosterone levels had declined with age, the same pattern emerged: low testosterone, leaky blood vessels in the nucleus accumbens, and disorganized mitochondria in the supporting astrocytes. Restoring testosterone in these older animals repaired the cellular damage and reduced their anxiety. The researchers also tested whether this mechanism applied to female rats. While they could separate female rats into high and low anxiety groups, they did not find the same link between anxiety and blood-brain barrier leakage in the nucleus accumbens, suggesting that this specific pathway is unique to males and driven by the presence of testosterone.
These findings offer a new perspective on how systemic physiology shapes mental health. Rather than viewing anxiety solely as a malfunction of brain circuits, this research highlights how the body's hormonal state can physically remodel the brain's protective barriers. The study demonstrates that low testosterone triggers a cascade of events starting in the astrocytes, where a lack of a specific protein leads to mitochondrial dysfunction, which in turn causes the blood-brain barrier to fail. This failure creates a vulnerable environment in the nucleus accumbens that sustains anxiety. By identifying this specific molecular pathway, the research suggests that the health of the brain's border control system is intimately tied to the body's hormonal balance, providing a concrete biological explanation for why low testosterone is a risk factor for anxiety in men.
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