What is Serotonin?

Figure 5. Structure of Serotonin

Serotonin or 5-hydroxytrptamine (5-HT) is a monoamine neurotransmitter, which is biochemically derived from tryptophan (Schloss & Williams 1998). It is mainly found in the gastrointestinal tract and the central nervous system. In the CNS, serotonin has various functions, which includes the regulation of mood, appetite, temperature regulation, and sleep (Schloss & Williams 1998). It also has a minor function in some cognitive functions such as memory and learning. It has also been noted that it causes an inhibitory effect on the nervous system that generates feelings of general contentment and satiation (Ho 1999). Also, it does not only have a role in controlling physiological aspects of the body, it also has associations with behaviours such as aggression and sadness (Ho 1999). A lot of studies even showed that a deficiency with it is one of the factors that can trigger the development of anxiety and depressive disorders (Wurtman 2005).

Video 2. Serotonin Structure and Function

Role on Depression

Various studies have determined the relationship of serotonin related genes and the development of depression. A twin study by Meltzer et al. (1981) showed that the velocity of the serotonin uptake on patients with major depressive disorder (MDD) is lower compared to normal individuals; this study also stated that this characteristic is heritable because it was highly correlated among twins, especially among monozygotic twins. Subsequent studies also showed that the brain serotonin reuptake of individuals with MDD is relatively slower as compared to normal individuals (Wurtman 2005). This reduction or complete ablation of 5-HTT reuptake is an underlying neurobiological predisposition of depression because it causes an increase in serotonin levels in the synaptic cleft, which can lead to the downregulation of serotonin activity or the desensitisation of the binding of serotonin to its receptors (Rachalski et. al 2009).

Figure 6. Changes in the SL6CA4 causes changes in the serotonin re-uptake velocity.

The most common genetic marker that causes low serotonin uptake in MDD patients is the serotonin transporter linked polymorphic region (5-HTTLPR), a region in the SL6CA4, which is the gene that codes for serotonin transporters (Ho 1999). It was found that defects in these gene locus could greatly affect the production as well as the speed of the reuptake of serotonin in the pre-synpaptic cell (Wurtman 2005). This changes in reuptake velocity is considered to be greatly associated with the development of depression (Wurtman 2005). For example, a study by Lyons-Ruth et al. (2007) showed that young adults with short 5-HTTLPR regions have greater tendencies to develop antisocial and depressive behaviours.

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