02/16/2021
The goal in treating the biology of depression is to improve the brain's ability to regulate mood. We now know that neurotransmitters are not the only important part of the machinery. But let's not diminish their importance either. They are deeply involved in how nerve cells communicate with one another. And they are a component of brain function that we can often influence to good ends.
Neurotransmitters are chemicals that relay messages from neuron to neuron. An antidepressant medication tends to increase the concentration of these substances in the spaces between neurons (the synapses). In many cases, this shift appears to give the system enough of a nudge so that the brain can do its job better.
How the system works. If you trained a high-powered microscope on a slice of brain tissue, you might be able to see a loosely braided network of neurons that send and receive messages. While every cell in the body has the capacity to send and receive signals, neurons are specially designed for this function. Each neuron has a cell body containing the structures that any cell needs to thrive. Stretching out from the cell body are short, branchlike fibers called dendrites and one longer, more prominent fiber called the axon.
A combination of electrical and chemical signals allows communication within and between neurons. When a neuron becomes activated, it passes an electrical signal from the cell body down the axon to its end (known as the axon terminal), where chemical messengers called neurotransmitters are stored. The signal releases certain neurotransmitters into the space between that neuron and the dendrite of a neighboring neuron. That space is called a synapse. As the concentration of a neurotransmitter rises in the synapse, neurotransmitter molecules begin to bind with receptors embedded in the membranes of the two neurons.
The release of a neurotransmitter from one neuron can activate or inhibit a second neuron. If the signal is activating, or excitatory, the message continues to pass farther along that neural pathway. If it is inhibitory, the signal will be suppressed. The neurotransmitter also affects the neuron that released it. Once the first neuron has released a certain amount of the chemical, a feedback mechanism (controlled by that neuron's receptors) instructs the neuron to stop pumping out the neurotransmitter and start bringing it back into the cell. This process is called reabsorption or reuptake. Enzymes break down the remaining neurotransmitter molecules into smaller particles.
When the system falters. Brain cells usually produce levels of neurotransmitters that keep senses, learning, movements, and moods perking along. But in some people who are severely depressed or manic, the complex systems that accomplish this go awry. For example, receptors may be oversensitive or insensitive to a specific neurotransmitter, causing their response to its release to be excessive or inadequate. Or a message might be weakened if the originating cell pumps out too little of a neurotransmitter or if an overly efficient reuptake mops up too much before the molecules have the chance to bind to the receptors on other neurons. Any of these system faults could significantly affect mood.
Kinds of neurotransmitters. Scientists have identified many different neurotransmitters. Here is a description of a few believed to play a role in depression:
Acetylcholine enhances memory and is involved in learning and recall.
Serotonin helps regulate sleep, appetite, and mood and inhibits pain. Research supports the idea that some depressed people have reduced serotonin transmission. Low levels of a serotonin byproduct have been linked to a higher risk for su***de.
Norepinephrine constricts blood vessels, raising blood pressure. It may trigger anxiety and be involved in some types of depression. It also seems to help determine motivation and reward.
Dopamine is essential to movement. It also influences motivation and plays a role in how a person perceives reality. Problems in dopamine transmission have been associated with psychosis, a severe form of distorted thinking characterized by hallucinations or delusions. It's also involved in the brain's reward system, so it is thought to play a role in substance abuse.
Glutamate is a small molecule believed to act as an excitatory neurotransmitter and to play a role in bipolar disorder and schizophrenia. Lithium carbonate, a well-known mood stabilizer used to treat bipolar disorder, helps prevent damage to neurons in the brains of rats exposed to high levels of glutamate. Other animal research suggests that lithium might stabilize glutamate reuptake, a mechanism that may explain how the drug smooths out the highs of mania and the lows of depression in the long term.
Gamma-aminobutyric acid (GABA) is an amino acid that researchers believe acts as an inhibitory neurotransmitter. It is thought to help quell anxiety.
Genes' effect on mood and depression
Every part of your body, including your brain, is controlled by genes. Genes make proteins that are involved in biological processes. Throughout life, different genes turn on and off, so that — in the best case — they make the right proteins at the right time. But if the genes get it wrong, they can alter your biology in a way that results in your mood becoming unstable. In a person who is genetically vulnerable to depression, any stress (a missed deadline at work or a medical illness, for example) can then push this system off balance.
Mood is affected by dozens of genes, and as our genetic endowments differ, so do our depressions. The hope is that as researchers pinpoint the genes involved in mood disorders and better understand their functions, depression treatment can become more individualized and more successful. Patients would receive the best medication for their type of depression.
Another goal of gene research, of course, is to understand how, exactly, biology makes certain people vulnerable to depression. For example, several genes influence the stress response, leaving us more or less likely to become depressed in response to trouble.
Perhaps the easiest way to grasp the power of genetics is to look at families. It is well known that depression and bipolar disorder run in families. The strongest evidence for this comes from the research on bipolar disorder. Half of those with bipolar disorder have a relative with a similar pattern of mood fluctuations. Studies of identical twins, who share a genetic blueprint, show that if one twin has bipolar disorder, the other has a 60% to 80% chance of developing it, too. These numbers don't apply to fraternal twins, who — like other biological siblings — share only about half of their genes. If one fraternal twin has bipolar disorder, the other has a 20% chance of developing it.
The evidence for other types of depression is more subtle, but it is real. A person who has a first-degree relative who suffered major depression has an increase in risk for the condition of 1.5% to 3% over normal.
One important goal of genetics research — and this is true throughout medicine — is to learn the specific function of each gene. This kind of information will help us figure out how the interaction of biology and environment leads to depression in some people but not others.
Stressful life events
At some point, nearly everyone encounters stressful life events: the death of a loved one, the loss of a job, an illness, or a relationship spiraling downward. Some must cope with the early loss of a parent, violence, or sexual abuse. While not everyone who faces these stresses develops a mood disorder — in fact, most do not — stress plays an important role in depression.
As the previous section explained, your genetic makeup influences how sensitive you are to stressful life events. When genetics, biology, and stressful life situations come together, depression can result.
Stress has its own physiological consequences. It triggers a chain of chemical reactions and responses in the body. If the stress is short-lived, the body usually returns to normal. But when stress is chronic or the system gets stuck in overdrive, changes in the body and brain can be long-lasting.
How stress affects the body
Stress can be defined as an automatic physical response to any stimulus that requires you to adjust to change. Every real or perceived threat to your body triggers a cascade of stress hormones that produces physiological changes. We all know the sensations: your heart pounds, muscles tense, breathing quickens, and beads of sweat appear. This is known as the stress response.
The stress response starts with a signal from the part of your brain known as the hypothalamus. The hypothalamus joins the pituitary gland and the adrenal glands to form a trio known as the hypothalamic-pituitary-adrenal (HPA) axis, which governs a multitude of hormonal activities in the body and may play a role in depression as well.
When a physical or emotional threat looms, the hypothalamus secretes corticotropin-releasing hormone (CRH), which has the job of rousing your body. Hormones are complex chemicals that carry messages to organs or groups of cells throughout the body and trigger certain responses. CRH follows a pathway to your pituitary gland, where it stimulates the secretion of adrenocorticotropic hormone (ACTH), which pulses into your bloodstream. When ACTH reaches your adrenal glands, it prompts the release of cortisol.
The boost in cortisol readies your body to fight or flee. Your heart beats faster — up to five times as quickly as normal — and your blood pressure rises. Your breath quickens as your body takes in extra oxygen. Sharpened senses, such as sight and hearing, make you more alert.
CRH also affects the cerebral cortex, part of the amygdala, and the brainstem. It is thought to play a major role in coordinating your thoughts and behaviors, emotional reactions, and involuntary responses. Working along a variety of neural pathways, it influences the concentration of neurotransmitters throughout the brain. Disturbances in hormonal systems, therefore, may well affect neurotransmitters, and vice versa.
Normally, a feedback loop allows the body to turn off "fight-or-flight" defenses when the threat passes. In some cases, though, the floodgates never close properly, and cortisol levels rise too often or simply stay high. This can contribute to problems such as high blood pressure, immune suppression, asthma, and possibly depression.
Studies have shown that people who are depressed or have dysthymia typically have increased levels of CRH. Antidepressants and electroconvulsive therapy are both known to reduce these high CRH levels. As CRH levels return to normal, depressive symptoms recede. Research also suggests that trauma during childhood can negatively affect the functioning of CRH and the HPA axis throughout life.
Early losses and trauma
Certain events can have lasting physical, as well as emotional, consequences. Researchers have found that early losses and emotional trauma may leave individuals more vulnerable to depression later in life.
Profound early losses, such as the death of a parent or the withdrawal of a loved one's affection, may resonate throughout life, eventually expressing themselves as depression. When an individual is unaware of the wellspring of his or her illness, he or she can't easily move past the depression. Moreover, unless the person gains a conscious understanding of the source of the condition, later losses or disappointments may trigger its return.
Traumas may also be indelibly etched on the psyche. A small but intriguing study in the Journal of the American Medical Association showed that women who were abused physically or sexually as children had more extreme stress responses than women who had not been abused. The women had higher levels of the stress hormones ACTH and cortisol, and their hearts beat faster when they performed stressful tasks, such as working out mathematical equations or speaking in front of an audience.
Many researchers believe that early trauma causes subtle changes in brain function that account for symptoms of depression and anxiety. The key brain regions involved in the stress response may be altered at the chemical or cellular level. Changes might include fluctuations in the concentration of neurotransmitters or damage to nerve cells. However, further investigation is needed to clarify the relationship between the brain, psychological trauma, and depression.
Medical problems
Certain medical problems are linked to lasting, significant mood disturbances. In fact, medical illnesses or medications may be at the root of up to 10% to 15% of all depressions.
Among the best-known culprits are two thyroid hormone imbalances. An excess of thyroid hormone (hyperthyroidism) can trigger manic symptoms. On the other hand, hypothyroidism, a condition in which your body produces too little thyroid hormone, often leads to exhaustion and depression.
Heart disease has also been linked to depression, with up to half of heart attack survivors reporting feeling blue and many having significant depression. Depression can spell trouble for heart patients: it's been linked with slower recovery, future cardiovascular trouble, and a higher risk of dying within about six months. Although doctors have hesitated to give heart patients older depression medications called tricyclic antidepressants because of their impact on heart rhythms, selective serotonin reuptake inhibitors seem safe for people with heart conditions.
The following medical conditions have also been associated with depression and other mood disorders:
degenerative neurological conditions, such as multiple sclerosis, Parkinson's disease, Alzheimer's disease, and Huntington's disease
stroke
some nutritional deficiencies, such as a lack of vitamin B12
other endocrine disorders, such as problems with the parathyroid or adrenal glands that cause them to produce too little or too much of hormones.
certain immune system diseases, such as lupus
some viruses and other infections, such as mononucleosis, hepatitis, and HIV
cancer
erectile dysfunction in men.
When considering the connection between health problems and depression, an important question to address is which came first, the medical condition or the mood changes. There is no doubt that the stress of having certain illnesses can trigger depression. In other cases, depression precedes the medical illness and may even contribute to it. To find out whether the mood changes occurred on their own or as a result of the medical illness, a doctor carefully considers a person's medical history and the results of a physical exam.
If depression or mania springs from an underlying medical problem, the mood changes should disappear after the medical condition is treated. If you have hypothyroidism, for example, lethargy and depression often lift once treatment regulates the level of thyroid hormone in your blood. In many cases, however, the depression is an independent problem, which means that in order to be successful, treatment must address depression directly.
Depression medications
Sometimes, symptoms of depression or mania are a side effect of certain drugs, such as steroids or blood pressure medication. Be sure to tell your doctor or therapist what medications you take and when your symptoms began. A professional can help sort out whether a new medication, a change in dosage, or interactions with other drugs or substances might be affecting your mood.
Keep in mind the following regarding drugs that may affect depression and mood:
Researchers disagree about whether a few of these drugs — such as birth control pills or propranolol — affect mood enough to be a significant factor.
Most people who take the medications listed will not experience mood changes, although having a family or personal history of depression may make you more vulnerable to such a change.
Some of the drugs cause symptoms like malaise (a general feeling of being ill or uncomfortable) or appetite loss that may be mistaken for depression.
Even if you are taking one of these drugs, your depression may spring from other sources.
