Biology of depression

Scientific studies have found that numerous brain areas show altered activity in depressed patients. It has not been possible to pin down the cause of depression to a single specific flaw in a single part of the brain. Instead it seems likely that depression usually involves a disturbed pattern of interaction between multiple parts of the brain. Here are the areas that are most strongly affected:

Raphe nuclei
The raphe nuclei are a group of small nuclei in the upper brain stem, located directly at the midline of the brain. They are the sole source of serotonin in the brain. Despite their small size, they project very widely, and are involved in a very diverse set of functions. Most antidepressants are agonists for serotonin. Serotonin system dysfunction cannot be the sole cause of depression, though: antidepressants usually bring serotonin levels up to normal very quickly, but it often takes at least two to four weeks before mood improves significantly.

The functions of serotonin are difficult to describe in a simple way. In some circumstances serotonin seems to act as a signal of "repletion" or "satisfaction". Thus, satiation after eating, and orgasm following sex, both produce release of serotonin. In animals that have hierarchical social structures, dominant individuals show higher levels of serotonin metabolites than lower-status individuals. In the brain, serotonin exerts a suppressive effect on both the reward and punishment systems, and therefore is likely to reduce the intensity of motivation whether aversive or appetitive. (One of the most common but least-discussed side effects of antidepressants is to reduce sex drive.)

Suprachiasmatic nucleus (SCN)
The Suprachiasmatic nucleus (SCN) is the control center for the body's "biological clock". It contains neurons whose activity waxes and wanes throughout the day. The output from the SCN controls the sleep/wake cycle as well as a number of other biological rhythms, such as fluctuations in body temperature. Disturbances of these cycles are a consistent symptom of depression, especially of the melancholic type. The "classic" pattern is for depressed people to have great difficulty falling asleep at night, and then to wake bolt upright at around 3 AM. The waking is usually preceded by a rise in body temperature, which in non-depressed people does not usually occur until several hours later. It is a common observation that antidepressants produce a return to normal sleep patterns before they produce an improvement in mood: if good sleep does not return, it is a strong sign that the treatment is not going to be effective. Conversely, disruptions to sleep are often the first indication of impending relapse.

There is a powerful interaction between the Raphe nuclei and the SCN. On one hand, the Raphe nuclei send a strong serotonergic projection to the SCN. In animal studies, this input has been shown to modulate the ability of light to reset the timing of the biological clock: the more serotonin, the stronger the effects of light. On the other hand, the biological clock exerts a strong influence on the Raphe nuclei: serotonin levels drop during sleep, and fall almost to nothing during REM (dreaming) sleep. It is worth noting that one of the characteristics of sleep in depressed people is that REM tends to appear very soon after sleep onset, whereas in non-depressed people it does not usually dominate sleep until the last hours, in the early morning. Antidepressants are powerful suppressors of REM.

Hypothalamic-Pituitary-Adrenal (HPA) axis
The Hypothalamic-pituitary-adrenal axis is a chain of structures that are activated during the body's response to stressors of various sorts. It often shows increased activation in depressed people, and drugs that reduce its activate are sometimes effective in reducing symptoms. The HPA influences many parts of the brain, including the Raphe nuclei.

Ventral tegmental area (VTA)
The Ventral tegmentum or Ventral tegmental area is a small area in the basal forebrain which is a critical part of the brain's reward system. It sends projections to the nucleus accumbens that use the neurotransmitter dopamine. Addictive drugs universally increase the effects of dopamine in this system, whereas drugs that oppose dopamine produce anhedonia of the sort seen in depressed people. Dopamine-enhancers such as cocaine often relieve the lack-of-pleasure in dopamine, but the effects only last as long as a drug is present in the body: that is, they temporarily alleviate one of the main symptoms, but do not help to cure the disease.

Anterior cingulate cortex (ACC)
The anterior cingulate cortex is activated by negative experiences of many types, and consistently shows higher levels of activity in depressed people than in non-depressed people. The functions of the ACC are controversial, but one proposal is that it mediates the conscious experience of suffering. Several decades ago, trials were made of ablating parts of the ACC in an attempt to relieve intolerable pain in patients who were terminally ill. These patients reported that after the surgery, they could still perceive the physical sensations of pain, but they no longer found them distressing. (The effects of heroin and morphine are sometimes described in the same way.) Very recently, clinical experiments were made in using deep brain stimulation to temporarily inactivate the ACC in severely depressed patients. This was not effective in all cases, but in some patients very striking results were achieved, with a perceptible lifting of mood immediately apparent to the patient as soon as the stimulus was applied.

Monoamine theories
Most antidepressants increase synaptic levels of the monoamine neurotransmitter serotonin. They may also enhance the levels of two other neurotransmitters, norepinephrine and dopamine. This observation gave rise to the monoamine theory of depression. In its contemporary formulation, the monoamine theory postulates that the deficit of certain neurotransmitters is responsible for the corresponding features of depression: "Norepinephrine may be related to alertness and energy as well as anxiety, attention, and interest in life; [lack of] serotonin to anxiety, obsessions, and compulsions; and dopamine to attention, motivation, pleasure, and reward, as well as interest in life." The proponents of this theory recommend choosing the antidepressant with the mechanism of action impacting the most prominent symptoms. The anxious and irritable patients should be treated with SSRIs or norepinephrine reuptake inhibitors, and the ones with the loss of energy and enjoyment of life—with norepinephrine and dopamine enhancing drugs.

Consistent with the monoamine theory, a longitudinal study uncovered a moderating effect of the serotonin transporter (5-HTT) gene on stressful life events in predicting depression. Specifically, depression seems especially likely to follow stressful life events, but even more so for people with one or two short alleles of the 5-HTT gene. Serotonin may help to regulate other neurotransmitter systems, and decreased serotonin activity may "permit" these systems to act in unusual and erratic ways. Facets of depression may be emergent properties of this dysregulation.

In the past two decades, research has uncovered multiple limitations of the monoamine theory, and its inadequacy has been criticized within the psychiatric community. Intensive investigation has failed to find convincing evidence of a primary dysfunction of a specific monoamine system in patients with major depressive disorders. The antidepressants that do not act through the monoamine system, such as tianeptine and opipramol, have been known for a long time. Experiments with pharmacological agents that cause depletion of monoamines have shown that this depletion does not cause depression in healthy people nor does it worsen the symptoms in depressed patients. Already limited, the monoamine theory has been further oversimplified when presented to the general public.

An offshoot of the monoamine theory suggests that monoamine oxidase A (MAO-A), an enzyme which metabolizes monoamines, may be overly active in depressed people. This would, in turn, cause the lowered levels of monoamines. This hypothesis received support from a PET study, which found significantly elevated activity of MAO-A in the brain of some depressed people. In genetic studies, the alterations of MAO-A-related genes have not been consistently associated with depression. Contrary to the assumptions of the monoamine theory, lowered but not heightened activity of MAO-A was associated with the depressive symptoms in youth. This association was observed only in maltreated youth, indicating that both biological (MAO genes) and psychological (maltreatment) factors are important in the development of depressive disorders. In addition, some evidence indicates that problems in information processing within neural networks, rather than changes in chemical balance, might underlie depression.