Serotonin… a word that brings to mind a commercial that might show our happy brain neurons bouncing serotonin between them.
There is a lot more to this molecule than most of us realize! This article covers the way that serotonin is made in the body, the transport of serotonin, and the receptors needed to complete the pathway.
About 90% of serotonin is made in the gut and helps to regulate motility.[ref] Serotonin also regulates other functions such as; bone mass, cardiovascular health, the endocrine system, and appetite.
In the brain, serotonin acts in several ways:
Some researchers believe that imbalances in serotonin may play a role in depression or anxiety. Common antidepressants include SSRIs (selective serotonin reuptake inhibitors), which are thought to increase serotonin levels in the brain. Although, the method through which they work is still not completely understood.[ref]
Ultimately, balance is the key to serotonin. Too much serotonin is known as serotonin syndrome. Symptoms include restlessness, confusion, shivering, diarrhea, and, potentially, death.[ref] Serotonin syndrome is usually caused by drugs such as MAOIs or SSRIs that affect the rate of serotonin break down. [ref]
Serotonin, also known as 5-hydroxytryptamine or 5-HT, is synthesized from the amino acid tryptophan using tryptophan hydroxylase (TPH1 and TPH2 gene).
It is transported by SLC6A4, which is also known as SERT or sodium-dependent serotonin transporter.
The serotonin signal is received by serotonin receptors on the cell membrane: HTR1A, HTR1B, and HTR2A.
All of these work in concert: from the creation of serotonin from amino acids to the transport of serotonin to the receptors that are necessary to receive this chemical messenger.
Below is a compilation of studies on genetic variants that affect serotonin. It is not an exhaustive list; instead, treat this as a starting point for you to find out more about your genes.
Tryptophan hydroxylase is an enzyme that catalyzes the reaction that produces serotonin from the amino acid tryptophan. Iron is a co-factor, and BH4 is also used in the reaction.
There are two genes that code for tryptophan hydroxylase:
A good overview of TPH2 can be found in the introduction of this study which also explains the link with GABA levels as well.
Several TPH2 genetic variants have been linked to psychiatric issues such as obsessive-compulsive disorder, depression, and bipolar disorder. These variants affect the rate at which serotonin is being produced in the brain.
Check your genetic data for rs4570625 (23andMe v4, v5; AncestryDNA):
Interactions between different genetic variants can be important as well. There are a couple of interesting studies that look at the combination of carrying the rs4570625 genotypes with the BNDF Val66Met (rs6265) genotypes. Those with rs4570625 G/G and rs6265 T/T were more likely to have “impaired inhibition of negative emotional content”. [ref] (The next time you start yelling at someone be sure to think about your “impaired inhibition of negative emotional content“!)
While there is still debate among researchers on this topic, one recent paper explains that the function of the brain serotonin receptors is to moderate stress and anxiety through patience and coping.
The HTR1 receptors are thought to mediate the ability to tolerate a source of stress, ‘passive coping’.
The HTR2 receptors mediate the ability to actively cope and improve one’s ability to change due to adversity.
The HTR1A gene codes for a serotonin receptor.
Check your genetic data for rs6295 C1019G (23andMe v4, v5):
The HTR1B gene codes for another serotonin receptor.
Check your genetic data for rs6296 (23andMe v4, v5; AncestryDNA):
HTR2A gene: The serotonin 2A receptor (HTR2A) also has several well-studied variants including rs6314, also known as C1354T.
Check your genetic data for rs6314 C1354T(23andMe v4, v5; AncestryDNA):
Other studies on rs6314 show:
Check your genetic data for rs6311 (23andMe v4, v5; AncestryDNA):
SLC6A4 is the gene that encodes the serotonin transporter. It is also called SERT or 5-HTT in some of the research. The 5-HTTLPR variant is a variable number tandem repeat. This just means there is a short version of the gene and a long version – due to repeated parts of the gene.
A lot of researchers have studied the 5-HTTLPR short/short and long/long forms. Initial studies in the ’90s and early 2000s found that those with the short version of the gene were more prone to depression due to stressful events. Here is a nice article summarizing the research that has been done on this gene: Controversial Gene-Depression Link Confirmed in New Study.
Variable number tandem repeats are not included in 23andMe or AncestryDNA data, but there are SNPs that usually go along with the serotonin transporter repeats. So you can get a pretty good idea of whether you carry the long or short version. A couple of studies have found that two SNPs predict the long or short version of 5-HTTLPR fairly well – around 95+% of the time. [ref][ref][ref] These are found in both v.4 and v.5 for 23andMe data.
(23andMe v4, v5; AncestryDNA)
(23andMe v4, v5; AncestryDNA)
The above table can be kind of confusing – here is an example: If your rs2129785 genotype is T/T and your rs11867581 genotype is A/G, you would carry one copy of the 5-HTTLPR short and one copy of the long version.
Studies on personality traits and genes tend to show all kinds of different results — with the results of one study often contradicting other studies. So take the following with a grain of salt:
A recent study may explain some of the contradictory results from previous studies. [ref] The researchers found that people who carried two copies of the long allele were more affected by environmental influences on mood (e.g. good or bad life events). Thus people who carried the long/long genotype were more likely to have a positive mood in a positive environment, but they were also more negative when bad things happened.
Studies show the long or short form may influence how well depression medications work for you:
Serotonin plays a major role in the gut, and IBS patients with a short allele had worse symptoms than those with a long/long version of this gene.[ref]
The short/short genotype is also tied to an increased risk of gastrointestinal intolerance of metformin.[ref]
Below are research-supported ways of affecting your serotonin levels. If you are needing psychiatric help, please be sure to contact a health care provider!
While your genes have an effect on serotonin levels in the body, there are also several environmental factors that also influence serotonin production. There is a correlation between higher serotonin levels and exposure to bright light or sunlight.[ref]
Physical activity may also increase serotonin levels. Eating foods high in tryptophan may also increase serotonin levels, but it is hard to know how much of an effect it has on serotonin in the brain.[ref]
Serotonin also plays a big role in the gut. Here is a great research article on serotonin and the gut microbiome: Serotonin, tryptophan metabolism and the brain-gut-microbiome axis
There are quite a few studies (mostly animal studies) showing the effect of Lactobacillus species on serotonin production in the gut. This not only seems to have an effect on gut motility and gut issues, but it also has an effect, perhaps through the gut-brain axis, on anxiety and depression.[ref][ref][ref][ref]
A good overview of the gut-brain axis and the recent research on the effect of gut microbes on mood and anxiety can be found in the March 2018 journal article “Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders“.
Quite a few clinical trials show specific probiotics are effective for depression and anxiety (whether due to serotonin problems or other reasons).
A supplemental form of the precursor to serotonin, 5-HTP is readily available in most health food stores. Many studies show positive effects of supplementing with 5-HTP, but always use caution when taking supplements that can affect your neurotransmitters. With this in mind, if you are under a doctor’s care or on other medications, be sure to check with your doctor. A case report shows that mixing MAOI prescription with over the counter 5-HTP supplement caused an adverse reaction (manic episode).[ref] [ref][ref]
Bipolar Disorder, Depression, Circadian Clock Genes
New research shows that depression and bipolar disorder are linked to changes or disruption in circadian genes. Some people carry genetic variants in the circadian genes that make them more susceptible to circadian disruption.
Tryptophan is an amino acid that the body uses to make serotonin and melatonin. Genetic variants can impact the amount of tryptophan that is used for serotonin. This can influence mood, sleep, neurotransmitters, and immune response.