From an evolutionary perspective, serotonin (also known sa 5-hydroxytryptamine or 5-HT) is one of the oldest central neurotransmitters .
It has many highly important roles in the body that are not solely restricted to the central nervous system. For example, serotonin carries signals between nerve cells in the brain, but is also implicated in appetite, mood, digestion, sleep, bone health, blood clotting, and sexual function [2-5].
Although it has been widely believed that serotonin levels play a role in depression, there are no diagnostic criteria (for instance, in the setting of serotonin deficiency, whereby there are no threshold values and no methods to directly measure its concentration in the brain of a living person). However, the ability to measure serotonin levels would offer substantial therapeutic value and insights for the management of a wide range of conditions.
This article will take a look at the methods for testing serotonin levels and the challenges associated with them.
How Is Serotonin Made?
Briefly, serotonin is synthesized by a series of reactions using enzymes, beginning with the essential amino acid tryptophan as a substrate. Essential amino acids cannot be produced by the body, hence the name, and must therefore be consumed through the food we eat. As a serotonin precursor, the dietary intake of tryptophan is closely related to the suppression of depressed and anxious moods .
The initial rate-limiting step in the metabolic pathway that produces serotonin is tryptophan hydroxylase (TPH) . TPH converts tryptophan into 5-hydroxytryptophan (5-HTP) .
There are two isoforms of TPH in mammals. TPH1 is mainly expressed by specialized gut endocrine cells called enterochromaffin cells , whereas TPH2 is found in the neurons of raphe nuclei in the brainstem .
After the conversion of tryptophan to 5-HTP, aromatic L-amino acid decarboxylase (AAAD) facilitates the decarboxylation of 5-HTP to form serotonin [11,12].
Perhaps counterintuitively, majority (over 90%) of the serotonin produced by the body is made in the gut as opposed to the brain .
For a more in-depth look into how and where serotonin is produced, read here.
How Can We Test for Serotonin?
Testing for serotonin levels in the brain is very difficult. Although blood tests are available for serotonin, doctors tend to diagnose low serotonin based on symptoms, not blood concentrations. In the case of serotonin syndrome (high levels of serotonin caused by medication use), doctors use symptomatology and a thorough assessment of medications to arrive at a diagnosis [14,15].
The serotonin present in the blood is synthesized in the gut and cannot cross the blood-brain barrier. Therefore, the serotonin acting in the periphery has little relevance to that involved in emotional status and brain-related outcomes .
Very recent research has estimated acute serotonin release in the living human brain using positron emission tomography, in a non-invasive manner, however this was first accomplished in 2020 in 17 participants .
What About Testing for Serotonin Precursors?
The essential amino acid tryptophan is an indispensable building block in the synthesis of serotonin. It cannot be produced by the body and must be consumed through dietary means (food or supplementation). Tryptophan is a serotonin precursor, and therefore its consumption or lack of in the diet is associated with different mood states [6,18]. The levels of tryptophan in blood plasma are determined by the balance between dietary intake and its removal from plasma for protein biosynthesis, a core biological process for life .
Brain cells synthesize their own serotonin from dietary tryptophan, in a manner distinct from the gut synthesis of peripheral serotonin.
Research from the early 1990s discovered that acutely depleting tryptophan in the diet brought about a depressive episode in 14 out of 21 patients using antidepressants with previously remitted symptoms [20,21]. On returning to regular food intake, the depressive episode resolved, suggesting that serotonin availability likely plays a part in the therapeutic mechanism of antidepressant medication .
Another notable observation from these studies is that brain serotonin content is dependent on the levels of tryptophan in the blood plasma , and there is a negative association between plasma tryptophan and depression score (as free plasma tryptophan increases, depression score decreases) during acute tryptophan depletion .
Therefore, testing for plasma tryptophan might be a worthwhile and useful surrogate biomarker.
The Role of the Gut-Brain Axis
The gut-brain axis is a two-way, or bi-directional, system that links the brain and gastrointestinal tract. The influence of gut microbiota (and dysregulation of this environment) on behavior is emerging as a potential avenue to address conditions such as depression and anxiety [24,25].
Recent evidence has suggested that the gut microbiome can produce neurotransmitters, including serotonin, GABA, norepinephrine, and dopamine. Specifically, Candida, Escherichia coli, Streptococcus, and Enterococcus may secrete serotonin [26,27]. This production of neurotransmitters by the gut microbiota may dysregulate neurotransmitter levels and cause the development of pathogenic bacteria. Alterations in the gut microbiota in response to inflammatory bowel syndrome (IBS) could be linked with dysregulated neurotransmitters and emotional distress .
Different protein diets have been found to alter specific gut microbes, and ultimately affect the levels of serotonin found in the blood serum .
In this article, we have explained how serotonin, a monoamine neurotransmitter with diverse physiological functions and roles, is produced through the conversion of tryptophan in both the central nervous system and gastrointestinal tract.
We have also covered how directly testing for serotonin in the brain is extremely difficult, even with cutting-edge methods, whereas the measurement of serotonin precursors such as tryptophan could offer some value as a biomarker.
Lastly, we presented information on the gut-brain axis and its role in modulating different mood states, depression, and anxiety. We also explored the possibility that the gut-brain axis is implicated in the manifestation of IBS, as well as the influence of dietary intake on the gut microbiota and blood serum serotonin.
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