7.1 General discussion
The aim of the study was to investigate if an orally given SCFA, butyrate, stimulates the formation
and release of serotonin and dopamine from different parts of the gastrointestinal tract in an in vivo
rat model. These questions are of interest, due to the scarcity of in vivo investigations on the effects
of butyrate in the entire gastrointestinal tract. The study showed that the effect of butyrate on tissue
concentration of serotonin varies in different segments of the gastrointestinal tract. Butyrate tends to
increase the tissue level of serotonin in the duodenum but decrease the level of serotonin in the
ileum and colon. The differences were, however, not statistically significant. An important finding was
that the basal production of serotonin appears to be high in the stomach, duodenum and jejunum,
compared to the lower concentrations in the ileum and colon. This study showed that a 40 mM
butyrate solution is not able to affect the urinary excretion of 5-HIAA, the main stable metabolite of
serotonin. However, there was a clear tendency of butyrate to increase the urinary excretion of HVA,
the main stable metabolite of dopamine. This is an important finding since the scarce in vivo
investigations on the effect of butyrate on dopamine. Furthermore, the study showed that a 40 mM
butyrate solution has no negative effect on the rats’ general well-being. Butyrate did not affect the
animals’ food intake, total energy intake nor the body weight gain during 10 days.
There are conflicting results regarding the effect of butyrate on the release of serotonin. In vitro
investigations have shown that sodium butyrate (0.5 mM and 1 mM) significantly increases the
expression of TPH1 mRNA in BON cells, a human enterochromaffin cell model (Reigstad et al. 2015).
However, higher levels of sodium butyrate (8 mM and 16 mM) tend to supress the transcription of
TPH1. Moreover, Martin et al. (2017) showed that butyrate (1-30 mM) is not able to acutely stimulate
the release of serotonin from isolated mouse duodenal and colonic enterochromaffin cells. Ex vivo
investigations have shown that luminal perfusion of a mixture of SCFA (100-200 mM) causes a
significant increase in serotonin concentration in the rat colon (Fukumoto et al. 2003). In a similar
way, Grider and Piland (2007) showed that a mixture of SCFA with concentrations of 0.5-100 mM
increases the release of serotonin from a three-compartment flat-sheet preparation of the rat colon.
In vivo investigations, however, show that healthy adults have a decreased plasma serotonin level
after intake of whole grain rye bread compared with intake of refined wheat bread (Keski-Rahkonen
et al. 2019). Keski-Rahkonen et al. (2019) showed, moreover, that intake of wheat aleurone and rye
bran decreases the serotonin level in the jejunum and colon of mice. Kundi et al. (2021) showed in
line with the previous study, that mice fed a western diet have significantly increased ileal TPH1
mRNA-expression, while intake of rye and oat fibres reduce the expression. Thus, an increased
expression of TPH1 was inversely associated with the SCFA concentration.
This study supports the theory of butyrate decreasing the level of serotonin in the colon. In line with
previous in vivo investigations (Keski-Rahkonen et al. 2019, Kundi et al. 2021), we showed that 40
mM butyrate administered in the drinking water for 10 d decreases the level of serotonin in the rat
colon. Our results are also in line with the in vitro investigations by Martin et al. (2017) and Reigstad
et al. (2015). The butyrate concentration used in the in vitro investigations was lower than 40 mM,
but instead directly applied on the isolated cells. Our results are, however, in contrast with the ex
vivo investigations by Fukumoto et al. (2003) and Grider and Piland (2007). Fukumoto et al. (2003)
showed an increase in the serotonin concentration in the rat colon after luminal perfusion of a
mixture of SCFA (100-200 mM). The mixture consisted of acetic acid, propionic acid and butyric acid
in the molar ratio 65:20:15, which means a butyrate concentration of 15 mM-30 mM. Likewise, Grider
and Piland (2007) showed that a mixture of SCFA with concentrations of 0.5-100 mM increases the
release of serotonin from the rat colon. This contrast might be due to differences in the methods
used as well as the quality of SCFA. We used an in vivo rat model and administered only butyrate. In
the in vivo model butyrate was probably partly absorbed by gastrointestinal cells already in the
duodenum and used as an energy source especially by colonocytes. This would lead to a lower
butyrate concentration than 40 mM reaching the colon. In our in vivo model was also the vasculature
present and able to transport away some of the serotonin released. This is important to take into
account as we analysed the tissue level of serotonin in the colon. Serotonin is primarily released by
exocytosis from the basal membrane into small blood vessels and the lamina propria (Szeitz and
Bandiera 2018). Only a small amount of serotonin is releases from the apical membrane of the
enterochromaffin cells.
This study showed that butyrate tends to increase the level of serotonin in the duodenum. Duodenum
is abundant with receptors FFA2 and FFA3 for butyrate, on the enterochromaffin cells. Butyrate
activates the receptors upon binding, and stimulates the production and release of serotonin. This
might have led to the increased serotonin level in the duodenum, compared to the other parts of the
gastrointestinal tract. The result is, however, in contrast to the in vitro study by Martin et al. (2017)
that showed that butyrate is not able to acutely stimulate the release of serotonin from isolated
mouse duodenal enterochromaffin cells. This contrast might be due to differences in the methods
used as well as differences in the concentration of butyrate used.
We showed a clear tendency of butyrate to increase the urinary excretion of HVA, the main stable
metabolite of dopamine. The result is in line with previous studies: Nankova et al. (2014) showed that
butyrate (1 mM) activates transcription of the TH gene in PC12 cells. Laurent et al. (2013) showed
that sodium butyrate (10 mM) increases the dopamine level in the brain of an alpha-synuclein
Drosophila transgenic model of familial Parkinson’s disease. Furthermore, Hou et al. (2021) showed
that orally given butyrate reaches the brain in a mice model of Parkinson’s disease and increases the
level of TH. The effect of butyrate on dopamine is an interesting field, since the scarce number of
investigations on the topic.