Bipolar disorder doesn’t come from a single broken mechanism in the brain. It arises from a combination of genetic vulnerability, chemical signaling problems, structural brain differences, and disrupted stress responses that together destabilize mood regulation. Twin studies estimate heritability at around 93%, making it one of the most genetically influenced psychiatric conditions. But genes alone don’t tell the whole story. What’s happening inside the brain involves several interconnected systems going wrong at once.
Neurotransmitter Imbalances During Mood Episodes
The brain communicates through chemical messengers, and in bipolar disorder, three of them are consistently out of balance: dopamine, GABA, and glutamate. Each plays a different role depending on whether someone is in a manic or depressive episode.
Dopamine is the most strongly implicated in the shift between depression and mania. During depressive episodes, levels of dopamine byproducts in spinal fluid drop. During mania, they rise. This pattern suggests that manic episodes involve a flood of dopamine activity, while depression reflects a deficit. Some researchers believe the problem isn’t just how much dopamine is present, but how sensitive the brain’s dopamine receptors become. Hypersensitive receptors could amplify normal dopamine signals into the racing thoughts, euphoria, and impulsivity that define mania.
GABA, the brain’s primary calming signal, also runs low during depressive episodes. Patients with depression show reduced activity of the enzyme responsible for making GABA, which means the brain has less ability to put the brakes on excitatory signals. In manic episodes, rising GABA levels appear to correlate with treatment response, suggesting that restoring this chemical balance is part of what brings mania under control.
Brain Structure Changes With Mood State
The physical architecture of the brain shifts in bipolar disorder, and these changes aren’t static. They fluctuate with mood episodes. Two regions stand out: the prefrontal cortex and the amygdala.
The prefrontal cortex sits behind your forehead and handles planning, impulse control, and decision-making. In people with bipolar disorder who are currently depressed, this region shows lower volume and reduced gray matter density compared to when those same individuals are in a stable mood. This matters because a smaller, less active prefrontal cortex means less capacity to regulate emotional responses, which helps explain why depressive episodes feel so consuming and difficult to manage.
The amygdala, the brain’s emotional alarm center, shows a similar pattern. People scanned during bipolar depression have significantly smaller amygdala volumes than those in a stable phase or healthy controls. Because the amygdala processes fear, reward, and social signals, these volume changes likely contribute to the emotional extremes that characterize the disorder. The fact that brain structure changes with mood state, rather than remaining fixed, suggests that bipolar disorder is an active, cycling process in the brain rather than a permanent structural deficit.
Genetic Vulnerability and Neuronal Excitability
Bipolar disorder is polygenic, meaning dozens or hundreds of genes each contribute a small amount of risk. Two of the most consistently identified are CACNA1C and ANK3, both of which control how neurons fire electrical signals.
CACNA1C encodes a component of calcium channels, the gates that open when a neuron becomes electrically active. These channels control a cascade of functions inside the cell: neurotransmitter release, gene expression, and communication between neurons. Variants in this gene alter how readily those channels open, potentially making neurons more reactive to stimulation.
ANK3 produces a protein called Ankyrin-G, which anchors sodium channels in place along nerve fibers. Sodium channels are what generate the electrical impulses neurons use to communicate. Rare variants of ANK3 can change how excitable neurons are, effectively lowering the threshold for firing. Together, these two genes point to a core problem in bipolar disorder: neurons that are too easily tipped into high-activity states, which could set the stage for manic episodes, and that crash into low-activity states afterward.
The Stress Response System
The body’s stress system, known as the HPA axis, is consistently overactive in bipolar disorder. This system controls cortisol, the hormone released during stress. A large meta-analysis found that people with bipolar disorder have significantly elevated cortisol levels, both at baseline and in response to stress challenges. The elevation is most pronounced during manic episodes.
This matters because chronically high cortisol damages the brain over time. The overproduction of cortisol seen in conditions like Cushing’s disease directly causes depressive symptoms, manic symptoms, and cognitive problems, essentially mimicking bipolar disorder through hormonal excess alone. In bipolar disorder, progressive HPA axis dysfunction appears to drive a worsening cycle: each episode further disrupts cortisol regulation, which increases the risk of future relapses and cognitive decline. This is one reason why the disorder tends to become more severe over time if untreated. Stressful life events don’t cause bipolar disorder on their own, but in a genetically vulnerable brain, they can trigger episodes by pushing an already dysregulated stress system past its tipping point.
Cellular Energy Problems
A less well-known but increasingly important piece of the puzzle involves mitochondria, the structures inside every cell that produce energy. Brain cells are extraordinarily energy-hungry, and in bipolar disorder, their power supply is compromised.
Post-mortem brain studies show that many mitochondria-related genes are less active in people with bipolar disorder compared to healthy controls. Specific components of the energy production chain, particularly in the cerebellum, show reduced expression and activity. During depressive episodes, blood platelet studies reveal decreased activity in several parts of this energy chain. When neurons can’t produce enough energy, they become more vulnerable to damage and less able to maintain stable signaling. At the same time, impaired mitochondria produce higher levels of oxidative stress, which is essentially chemical damage from unstable molecules. This creates a feedback loop: energy deficits make neurons fragile, oxidative stress damages them further, and the brain’s capacity for stable mood regulation erodes.
Reduced Brain Growth Signals
The brain relies on a protein called BDNF (brain-derived neurotrophic factor) to maintain healthy neurons, grow new connections, and repair damage. People with bipolar disorder have markedly lower levels of this protein in their blood, and the deficit is worst during active mood episodes.
During depressive episodes, BDNF levels drop below roughly 33,000 pg/ml, which distinguishes depressed patients from healthy individuals with 84% accuracy. During mania, levels fall even further, below about 29,500 pg/ml, with 96% accuracy in separating manic patients from controls. Even during stable periods between episodes, some evidence suggests BDNF remains lower than normal regardless of medication. Low BDNF means the brain has a reduced ability to adapt, repair, and maintain its neural networks. Over time, this contributes to the gray matter loss and cognitive difficulties that accumulate with repeated episodes.
How Treatment Targets These Mechanisms
Understanding what goes wrong in the brain also explains why certain treatments work. Lithium, the oldest and most effective mood stabilizer, acts on multiple dysfunctional systems simultaneously. One of its key actions is inhibiting an enzyme called GSK-3, which is involved in cell signaling, gene expression, and the structural maintenance of neurons. By blocking this enzyme, lithium appears to protect neurons from damage, reduce the sensitivity of overactive signaling pathways, and stabilize the cellular machinery that goes haywire during mood episodes.
This multi-target action is part of why no single theory fully explains bipolar disorder. The condition involves neurotransmitter imbalances, structural brain changes, genetic excitability problems, stress hormone dysregulation, energy production failures, and reduced growth signals, all interacting with each other. The most effective treatments are the ones that address several of these layers at once, which is also why finding the right medication often takes time. Each person’s version of the disorder involves a slightly different combination of these disruptions.