Biological Causal Factors in Unipolar Depression - Exploring the Biology of Depression

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Hormones, Neurotransmitters and Mood

The causes of unipolar depression are complex and are often the result of a mixture of biological, environmental and social factors. Dramatic events, stress at work and unhealthy lifestyle can all contribute to depression, but genetics and biological rhythms which affect hormones and neurotransmitters can make some people more prone to depression than others.

Hormones are chemicals released into the blood which affect other cells in other parts of the body. An example of a hormone linked to depression is melatonin, which is a light-sensitive hormone that affects our ability to sleep. Hormones can also affect the production of neurotransmitters, which are similar to hormones but which act specifically on the nervous system and affect a variety of bodily functions. Serotonin in particular is a neurotransmitter which has been linked to mood and many antidepressant drugs are believed to work by increasing concentrations of serotonin in the brain.

The biological causal factors in unipolar mood disorders are those which affect the levels of hormones such as cortisol and melatonin, or neurotransmitters such as serotonin, endorphins, gamma-aminobutyric acid and norepinephrine, all of which can alter mood and may be potential causes of depression.

Genetic Causes of Depression

One such biological factor is the existence of a specific gene known as 5-HTT, which is known to be related to serotonin production. About 20% of people possess the ‘short’ version of the gene and this makes them more predisposed to experience imbalances in serotonin levels and thus experience depression after a stressful life event.

Such experiences should, however, be distinguished from bipolar depression and other forms of genetic mood disorder. Bipolar depression is experienced by perhaps 2-5% of the population but has distinctive biological causes; anybody can experience unipolar depression, the short 5-HTT gene simply means some people are more likely to do so.

Circadian Rhythms and the Menstrual Cycle

Our body chemistries are constantly changing, on a daily or even hourly basis. Hormone levels are responsible for managing several internal cycles, such as the circadian rhythm which controls our sleep and wakefulness. Levels of melatonin (related to sleep) and cortisol (often associated with stress) change throughout the day and a disruption to rhythms, perhaps one that causes a loss of sleep, can lead not only to depression, but also several other disorders such as anxiety or panic attacks.

The menstrual cycle is another example where hormone levels can affect mood. Levels of oestrogen and progesterone affect levels of various neurotransmitters associated with depression and are linked to the experience of premenstrual syndrome (PMS). Pregnancy, miscarriage and menopause are all examples of events that can cause disruptions in the menstrual cycle and levels of hormones which can, in turn, lead to extended periods of unipolar depression.

The relationship between such cycles and unipolar depression is, however, more complex than simple cause and effect – for example, in the case of Circadian Rhythm Disorder (CRD) and associated depression, it is often not clear whether it is depression causing the rhythmic disruption or the CRD which precipitated the depression. Often, however, a change in lifestyle to address the rhythmic disruption can help to lift the patient out of their depression.

Lifestyle Factors and Other Biological Causes

While there are a number of potential biological causes of depression, it is often actually lifestyle or environmental factors that serve to trigger them. Stress at work can create an excess of cortisol in the blood which can in turn lead to a disruption in the sleep-wake cycle. Exercise triggers neurotransmitters such as endorphins and a lack of exercise can also result in a slide into depression.

Poor diet and sleep routines are two more examples of lifestyle factors which can result in deficiencies in certain mood-affective chemicals in the body.

So in summary, hormones and neurotransmitters regulate a number of bodily functions and many of them have a direct or indirect effect on mood. Serotonin, endorphins, cortisol and melatonin are all examples of substances which have been shown to relate to the experience of depression.

Genetic factors which inhibit the production of neurotransmitters like serotonin, or disruptions to natural bodily cycles that affect hormone levels are both examples of potential biological causes of depression. It is rare, however, that biological causes alone are to blame and there is often a social or environmental trigger. And often routine or cognitive changes can help to lift somebody out of a malaise.

References

Boyce, P & Barriball, E “Circadian rhythms and depression”, https://www.racgp.org.au/afp/201005/201005boyce.pdf

Kvetnansky et al (2009), “Stress, Neurotransmitters and Hormones”, Annals of the New York Academy of Sciences

NHS, “Depression - causes”, https://www.nhs.uk/Conditions/Depression/Pages/Causes.aspx