Biology of depression

Scientific studies have found that numerous brain areas show altered activity in patients suffering from depression, and this has encouraged advocates of various theories that seek to identify a biochemical origin of the disease, as opposed to theories that emphasize psychological or situational causes. Several theories concerning the biologically based cause of depression have been suggested over the years, of which the most prominent and widely researched is the monoamine hypothesis.

Genetic factors

In contrast to other psychiatric disorders such as autism and schizophrenia, genetic factors involved in depression have been more difficult to identify. In 2003 Science published an influential[1] study of Avshalom Caspi et al. who found that a gene-environment interaction (GxE) may explain why life stress is a predictor for depressive episodes in some individuals, but not in others, depending on an allelic variation of the serotonin-transporter-linked promoter region (5-HTTLPR).[2] Soon after, the results were replicated by Kenneth Kendler's group, raising hopes in the psychiatric genetics community.[3][4] By 2007 there were 11 replications, 3 partial replication and 3 non-replications of this proposed GxE. However, two of the largest studies[5][6] were negative.[7] Two 2009 meta-analyses were also negative; one included 14 studies,[8] the other just five, owing to different study selection criteria.[9] A 2010 review of studies in this area found 17 replications, 8 partial replications (interaction only in females or only with one of several types of adversity), and 9 non-replications (no interaction or an interaction in the opposite direction). It also found a systematic relationship between the method used to assess environmental adversity and the results of the studies; all studies using objective indicators or structured interviews to assess stress replicated the gene–environment interaction fully or partially, whereas all non-replications relied on brief self-report measures of adversity. This review also found that both 2009 meta-analyses were significantly biased toward negative studies.[10]

Other hypothesized genomic influences are BDNF polymorphisms, but the replications studies have been mixed and insufficient as of 2005 for a meta-analysis.[11] Studies also indicate an association of BDNF to suicidal behavior.[12] However, findings from the gene-environment interactions studies suggest that the current BDNF models of depression are too simplistic.[13] A 2008 study found interactions (biological epistasis) in the signaling pathways of the BDNF and the serotonin transporter; the BDNF Val66Met allele, which was predicted to have reduced responsitivity to serotonin, was found to exercise protective effects in individuals with the short 5-HTTLPR allele that is otherwise believed to predispose individuals to depressive episodes after stressful events.[14] Thus, the BDNF-mediated signalling involved in neuroplastic responses to stress and antidepressants is influenced by other genetic and environmental modifiers.[13]

The largest genome-wide study to date failed to identify variants with genome-wide significance in over 9000 cases.[15]

Recently, the first genetics study has been published with positively identified two variants with genome-wide association with major depressive disorder.[16] This study, conducted in Chinese Han woman, identified two variants in intronic regions near SIRT1 and LHPP.

Circadian rhythm

Depression may be related to the same brain mechanisms that control the cycles of sleep and wakefulness.

Depression may be related to abnormalities in the circadian rhythm,[17] or biological clock. For example, rapid eye movement (REM) sleep—the stage in which dreaming occurs—may be quick to arrive and intense in depressed people. REM sleep depends on decreased serotonin levels in the brain stem,[18] and is impaired by compounds, such as antidepressants, that increase serotonergic tone in brain stem structures.[18] Overall, the serotonergic system is least active during sleep and most active during wakefulness. Prolonged wakefulness due to sleep deprivation[19] activates serotonergic neurons, leading to processes similar to the therapeutic effect of antidepressants, such as the selective serotonin reuptake inhibitors (SSRIs). Depressed individuals can exhibit a significant lift in mood after a night of sleep deprivation. SSRIs may directly depend on the increase of central serotonergic neurotransmission for their therapeutic effect, the same system that impacts cycles of sleep and wakefulness.[18]

Research on the effects of light therapy on seasonal affective disorder suggests that light deprivation is related to decreased activity in the serotonergic system and to abnormalities in the sleep cycle, particularly insomnia. Exposure to light also targets the serotonergic system, providing more support for the important role this system may play in depression.[20] Sleep deprivation and light therapy both target the same brain neurotransmitter system and brain areas as antidepressant drugs, and are now used clinically to treat depression.[21] Light therapy, sleep deprivation and sleep time displacement (sleep phase advance therapy) are being used in combination quickly to interrupt a deep depression in hospitalized patients.[20]

Monoamines

Illustration of the major elements in a prototypical synapse. Synapses are gaps between nerve cells. These cells convert their electrical impulses into bursts of chemical relayers, called neurotransmitters, which travel across the synapses to receptors on adjacent cells, triggering electrical impulses to travel down the latter cells.

Monoamines are neurotransmitters and neuromodulators that include serotonin, dopamine, norepinephrine, and epinephrine.[22] Many antidepressant drugs increase synaptic levels of the monoamine neurotransmitter, serotonin, but they may also enhance the levels of two other neurotransmitters, norepinephrine and dopamine. The observation of this efficacy led to the monoamine hypothesis of depression, which 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 hypothesis recommend choosing the antidepressant with the mechanism of action impacting the most prominent symptoms. Anxious or 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.[23]

Monoamine receptors affect phospholipase C and adenylyl cyclase inside of the cell. Green arrows means stimulation and red arrows inhibition. Serotonin receptors are blue, norepinephrine orange, and dopamine yellow. Phospholipase C and adenylyl cyclase start a signaling cascade which turn on or off genes in the cell. Many variations of the monoamine hypothesis involve the neurotransmitter, serotonin, regulated by the serotonin transporter, which assists the modulation of feelings and behavior such as anxiety, anger, appetite, sexuality, sleep, mood, etc. People with depression may have differences in serotonin transporter gene length.[24] People with both alleles that are long are less likely to become depressed, while people with one short and one long or two short alleles are more likely to develop depression.[25] The 5HT-3 receptor is associated with gastrointestinal adverse effects and has no relationship to the other monoamine receptors.

Consistent with the monoamine hypothesis, 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.[26] 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.[27]

An offshoot of the monoamine hypothesis 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.[28] In genetic studies, the alterations of MAO-A-related genes have not been consistently associated with depression.[29][30] Contrary to the assumptions of the monoamine hypothesis, 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.[31] In addition, some evidence indicates that problems in information processing within neural networks, rather than changes in chemical balance, might underlie depression.[32]

Since the 1990s, research has uncovered multiple limitations of the monoamine hypothesis, and its inadequacy has been criticized within the psychiatric community.[33] For one thing, serotonin system dysfunction cannot be the sole cause of depression; antidepressants usually bring serotonin levels up to normal very quickly, but it often takes at least two to four weeks before mood improves significantly. 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.[34][35] Already limited, the monoamine hypothesis has been further oversimplified when presented to the general public.[36]

Abnormalities by brain region

Research on the brains of depressed patients usually shows disturbed patterns of interaction between multiple parts of the brain. Several areas of the brain are implicated in studies seeking to more fully understand the biology of depression:

Raphe nuclei

The sole source of serotonin in the brain is the raphe nuclei, a group of small nerve cell nuclei in the upper brain stem, located directly at the mid-line of the brain. There is some evidence for neuropathological abnormalities in the rostral raphe nuclei in depression.[37][38][39] Despite their small size, they reach very widely through their projections, and are involved in a very diverse set of functions. Most antidepressants are serotonergic.

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.

Ventral tegmental area (VTA)

The ventral tegmentum (or ventral tegmental area) is a small area in the basal midbrain 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 depression, 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.

Nucleus accumbens (NAc)

Long-term exposure to various unavoidable stress factors decreases dopamine release in the NAc shell,[40][41] as it was shown in the forced swimming test, an animal model of depression.[42]

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.

Subgenual cingulate

Recent studies have shown that Brodmann area 25, also known as Subgenual cingulate is metabolically overactive in treatment-resistant depression.[43] This region is extremely rich in serotonin transporters and is considered as a governor for a vast network involving areas like hypothalamus and brain stem, which influences changes in appetite and sleep; the amygdala and insula, which affect the mood and anxiety; the hippocampus, which plays an important role in memory formation; and some parts of the frontal cortex responsible for self-esteem.[44][45] Thus disturbances in this area or a smaller than normal size of this area contributes to depression. Deep Brain Stimulations of this area have been successful in reducing its elevated activity and thus curing depression in patients that could not be cured by anti-depressants.[46]

Hypothalamic-pituitary-adrenal (HPA) axis

The hypothalamic-pituitary-adrenal axis is a chain of endocrine structures that are activated during the body's response to stressors of various sorts. It often shows increased activation in depressed people, but the mechanism behind this is not yet known.[47]

Ventricles

Multiple studies have found evidence of ventricular enlargement in people who have depression, particularly enlargement of the third ventricle.[48][49][50] These observations are interpreted as indicating loss of neural tissue in brain regions adjacent to the enlarged ventricle, leading to suggestions that cytokines and related mediators of neurodegeneration may play a role in giving rise to the disease.[51][52][53]

Altered neuroplasticity

Recent studies have called attention to the role of altered neuroplasticity in depression. A review found convergence of three phenomena:

  1. Chronic stress reduces synaptic and dendritic plasticity
  2. Depressed subjects show evidence of impaired neuroplasticity (e.g. shortening and reduced complexity of dendritic trees)
  3. Anti-depressant medications enhance neuroplasticity at both a molecular and dendritic level.

The conclusion is that disrupted neuroplasticity is an underlying feature of depression, and is reversed by antidepressants.[54]

Large-scale brain network theory

Instead of studying one brain region, studying large scale brain networks is another approach to understanding psychiatric and neurological disorders,[55] supported by recent research that has shown that multiple brain regions are involved in these disorders. Understanding the disruptions in these networks may provide important insights into interventions for treating these disorders. Recent work suggests that at least three large-scale brain networks are important in psychopathology:[55]

Central executive network

The executive network is made up of fronto-parietal regions, including dorsolateral prefrontal cortex and lateral posterior parietal cortex.[56][57] This network is crucially involved in high level cognitive functions such as maintaining and using information in working memory, problem solving, and decision making.[55][58][59][60][61] Deficiencies in this network are common in most major psychiatric and neurological disorders, including depression.[62][63] Because this network is crucial for everyday life activities, those who are depressed can show impairment in basic activities like test taking and being decisive.[64]

Default mode network

The default mode network includes hubs in the prefrontal cortex and posterior cingulate, with other prominent regions of the network in the medial temporal lobe and angular gyrus.[55] The default mode network is usually active during mind-wandering and thinking about social situations. In contrast, during specific tasks probed in cognitive science (for example, simple attention tasks), the default network is often deactivated.[65][66] Research has shown that regions in the default mode network (including medial prefrontal cortex and posterior cingulate) show greater activity when depressed participants ruminate (that is, when they engage in repetitive self-focused thinking) than when typical, healthy participants ruminate.[67] Individuals suffering from major depression also show increased connectivity between the default mode network and the subgenual cingulate and the adjoining ventromedial prefrontal cortex in comparison to healthy individuals, individuals with dementia or with autism. Numerous studies suggest that the subgenual cingulate plays an important role in the dysfunction that characterizes major depression.[68] The increased activation in the default mode network during rumination and the atypical connectivity between core default mode regions and the subgenual cingulate may underlie the tendency for depressed individual to get “stuck” in the negative, self-focused thoughts that often characterize depression.[69] However, further research is needed to gain a precise understanding of how these network interactions map to specific symptoms of depression.

Salience network

The salience network is a cingulate-frontal operculum network that includes core nodes in the anterior cingulate and anterior insula.[56] A salience network is a large-scale brain network involved in detecting and orienting the most pertinent of the external stimuli and internal events being presented.[55] Individuals who have a tendency to experience negative emotional states (scoring high on measures of neuroticism) show an increase in the right anterior insula during decision-making, even if the decision has already been made.[70] This atypically high activity in the right anterior insula is thought to contribute to the experience of negative and worrisome feelings.[71] In major depressive disorder, anxiety is often a part of the emotional state that characterizes depression.[72]

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