Everyone suffers from states of sadness and fatigue. However, chronic and persistent unhappiness may lead to a clinical diagnosis of Major Depressive Disorder. According to mental health guidelines provided by the American Psychiatric Association, clinical depression includes disturbances in appetite and sleep, sexual dysfunction and suicide contemplation. While some causes of depression remain unknown, several underlying biological mechanisms are now understood.
Genetics
Many mental health disorders tend to "run-in-families" and major depression is no exception. In Biological Psychology, James Kalat reports a moderate degree of heritability between close relatives. Measures of heritability for the disorder remain relatively constant even when studying adopted children. Genes that may code for depression however do not appear to be specific to that disorder. In the study, "Shared genetic risk of major depression, alcohol dependence, and marijuana dependence", Qiang Fu and colleagues found that close relatives of people diagnosed with depression also proved likely to suffer from anxiety disorders, attention-deficit hyperactivity disorder, eating disorders and substance abuse. Thus, genetic markers for depression may be non-specific to a particular mental health disorder but rather a "sick gene" with a variety of potential negative outcomes.
Brain Chemicals
The brain consists of hundreds of thousands of nerve cells called neurons. Neurons communicate with each other by means of chemicals called neurotransmitters. There is a normal range for the levels of neurotransmitters present in the brain for optimal health. However, according to Stephen Stahl's Essential Psychopharmacology, the field of neuroscience has identified neurotransmitters with abnormal levels associated with depression. As a result, deficits or excesses in dopamine, serotonin and norepinephrine have become common targets for many pharmaceutical interventions. Research on antidepressant drug treatments remains challenging as some neurotransmitter levels need to be increased or decreased depending on the location of the nerve cells in the brain.
Hormones
The American Psychological Association reports higher rates of depression in women, and approximately 1 in 5 mothers experience post-partum depression. The neuroscientist James Kalat reports that drug-induced deficits in progesterone and estradiol hormones may trigger symptoms of depression in women with a history of post-partum depression. In addition, Kalat suggests that excessive levels of the hormone melatonin have also been implicated in the disorder. Persons suffering from a type of depression known as "Seasonal Affective Disorder" experience cyclical states of depression that coincide with the changing of the seasons. In general, people residing in colder climates with shorter duration of sunlight present a higher risk for this type of winter-time depression. Shorter days appear to trigger the early release of melatonin (normally released in higher quantities during night-time) which may account for some of the fatigue factors associated with depression.
References
- Diagnostic and Statistical Manual of Mental Disorders, American Psychological Association, 2000
- Stahl's Essential Psychopharmacology, 3rd Edition, Stephen M. Stahl, MD, PhD.; 2008
- Biological Psychology, 9th Edition, James Kalat; 2007
- "Shared genetic risk of major depression, alcohol dependence, and marijua dependence",Qiang Fu, et al.; 2002
