Table 1.
Common neurotransmitters and short chain fatty acids, their production, and functionality.
| Neurotransmitter | Endogenous Production | Exogenous Production | Function | Remarks | References |
|---|---|---|---|---|---|
| Nitric oxide (NO) | Enteric inhibitory neurons | Enterobacteria, some lactobacilli and bifidobacteria, and some oral anaerobes | Gut motility, brain development, memory, and anti-anxiety | Enteric-produced NO does not play a role in anxiety | [38,39,40] |
| γ-aminobutyric acid (GABA) | GABA-ergic neurons | Some lactic acid bacteria and bifidobacteria | Neuroprotection, anti-diabetic, antioxidant, anti-inflammatory, anti-allergic, hepatoprotection, renoprotection, anti-depression, and anti-insomnia | Does not cross blood–brain barrier | [41,42,43] |
| Norepinephrine | Enteric nerve cells | E. coli, Bacillus, Saccharomyces spp., S. marcescens and P. vulgaris | Anti-inflammatory, anti-stress, and anti-anxiety | Does not cross blood–brain barrier | [44,45,46,47,48,49] |
| Dopamine | Central nervous system, various other tissues | Bacillus spp. | Locomotion, learning, working memory, cognition, and emotion | Does not cross blood–brain barrier | [46,49,50] |
| Acetylcholine | Cholinergic neurons | L. plantarum | Cognitive function and intestinal motility | Does not cross blood–brain barrier | [51,52,53] |
| Serotonin (5-hydroxytryptamine; 5-HT) | Serotonergic neurons mainly in the gut | Candida, E. coli, Lc. lactis, L. plantarum, S. thermophilus, M. morganii, K. pneumoniae, H. alvei and Enterococcus spp. | Regulation of mood, appetite, sleep, and cognitive function | Does not cross blood–brain barrier | [49,54,55,56,57] |
| Melatonin | Enterochromaffin cells in the gut | - | Regulation of circadian rhythm | Intestinal microbiota may be involved in breakdown | [57,58,59] |
| Indole | - | Actinobacteria, Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, Clostridium, Burkholderia, Streptomyces, Pseudomonas and Bacillus | May influence emotional behavior | Crosses blood–brain barrier | [57,60] |
| Kynurenine and/kynurenic acid | Central nervous system, various other tissues | B. infantis | Associated with depression and schizophrenia | Increased kynurenic acid: kynurenine is neuroprotective. Both can cross blood–brain barrier | [57,61,62] |
| Quinolinic acid | Epithelial cells and intestinal immune cells | - | Associated with depression | Neurotoxic. Does not cross blood–brain barrier. May be blocked by L. helveticus and B. longum | [57,61,63,64] |
| Histamine | Mast cells and other immune cells | Certain lactic acid bacteria fermented foods | Mediates arousal, attention, and reactivity | Does not cross blood–brain barrier | [56,65,66,67] |
| Short chain fatty acids (SCFA) * | Muscle tissue | Most anaerobes in the gut | Regulate inflammation, appetite, depression, and gut motility | Crosses blood–brain barrier | [68,69,70,71,72,73,74,75,76,77,78,79,80,81] |
* Short chain fatty acids are not neurotransmitters. However, as they may modulate the levels of neurotransmitters, they are included here.