Many factors have been recognized which contribute to the etiology of depression. The predisposing factors are genetics, personality and environmental factors. Research has been ongoing in all these areas and the focus of research these days is on various genes and neurotransmitters which lead to depression.
Studies have shown that there is 2-3 times higher risk of depression if there is family history of depression in first degree relatives. Twin studies have found 37% concordance rates for depression in monozygotic and dizygotic twins (Sullivan, 2000). Studies on families have shown that it is not a single gene or factor which increases the risk of depression. Instead, it is a multifactorial disease.
Defects in the serotonin transporter have been found in depressed patients (KP, 1996). There are also anatomical differences in the emotional areas of brain in individuals who are depressed (Pezawas, 2005). Recent studies have also found that the enzyme tryptophan hydroxylase is increased by as much as 30% in depressed individuals (Meyer, 2006). It has also been found that reduction in the levels of serotonin and norepinephrine can lead to recurrence of symptoms in individuals treated for depression (Ruhe, 2007). This highlights the importance of these two neurotransmitters in the etiology of depression. The levels of neurotransmitters can also be affected by impairment of function of second messenger system.
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