Twin studies Research using twins indicates a genetic basis for depression. The basis of twin studies is that identical (or monozygotic, MZ) twins are naturally occuring clones of each other, having all of their genes in common. On the other hand, non-idnetical (or dizygotic, DZ) twins share on average half of their genes. If we assume that the environment shared by twins is roughly the same for both types of twin, then any greater similarities in MZ than DZ pairs shows the action of genes. McGuffin et al (1996) studied 177 probands with depression and their same-sex co-twins. The concordance rate was 46% for MZ twins and 20% for DZ twins, suggesting that depression has a substantial heritable component.
Genes as diatheses Genetic factors are thought to ac vas diatheses in a diathesis-stress relationship. Such a view would see a enetic predisposition for depression recation. We might expect, therefore, such environmental stressors to affect those with the genetic predisposition differently to those without it. Kendler et al (1995) found that women who were the co-twin of depressed sibling were more likely to have become depressed than those without this presumed genetic vulnerability. Most significantly, the highest levels of depression werwe found in the group that was exposed to significant negative life events and was most genetically at risk for depression.
Which genes? If depression has a genetic basis then researchers should be able to locate the genes responsible. One candidate is the serotonin transporter gene, which is responsible for producing serotonin in the brain. This gene comes in three forms (alleles), varying in the length of its two strands: long-long, long-short and short-short. It is belived that the short-short form leads to inefficient serotonin production.
It is widely belived that a group of neurotransmitters called monoamies, which include noradrenaline and serotonin, exist in lower loevels in the brains of depressed individuals.
Noradrenaline In the 1960's, it was proposed that depression stems from a deficiency of the neurotransmitter noradrenaline in certain brain circuits. Among the findings linking low levels of noradrenaline to depression was the disocvery hat indirect makes of noradrenaline levels (e.g. by-products found in urine) were often low in depressed individuals (Bunney and Davis 1965).
Serotonin Among the findings supporting a link beween low synaptic serotonin levls and depression is that serebrospinal fluid in depressed, and especially in suicidal, patients conains reduced amounts of major serotonin by-product, signifying reduced levels of serotonin in the brain itslef (McNeal and Cimbolic, 1986). The introduction of Prozac and other antidepressant drugs that selectively block serotonin reuptake confirmed the association between serotonin and depression. Furhermore, Delgado et al (1990) gave depressed patients who were receiving antidepressant medication a special diet tha lowered their levels of one of the precursors of serotonin - typtophan. The majority of patients experienced a return of their depressive symptoms, which disappeaqred again when their diet was returned to normal.
Freud (1917) believed that…