The Gut-Brain Connection

Extract of Jenkins, TA. et al. (2016). Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis. Nutrients. 8(56), pp. 1–15
 
I have found this recently published article an amazing read, and learnt much from it for it is the latest in research, so very pertinent for my newsletter and subject of stress, and so very instructive. Really worth the read.
 
1. Introduction 
Tryptophan is an essential amino acid found in many protein-based foods and dietary proteins including meats, dairy, fruits, and seeds. High-glycaemic index and -glycaemic load meals also increase the availability of tryptophan. Levels of plasma tryptophan are determined by a balance between dietary intake, and its removal from the plasma as a part of its essential role in protein biosynthesis. Aside from its role in protein formation, tryptophan is a precursor for a number of metabolites, most notably kynurenine and the neurotransmitter, serotonin, which is the focus of this review.
 
9. Tryptophan, Sleep, Mood and Cognition 
Tryptophan has been shown to have direct effects on sleep, producing an increase in rated subjective sleepiness, and decrease in total wakefulness. This improved quality of sleep is associated with an improvement in hedonic and cognitive measures, improved morning alertness and brain measures of attention.
Acute tryptophan depletion studies in humans demonstrate inhibition of rapid eye movement (REM) latency and prolonged REM sleep, with further work from animal studies demonstrating the importance of serotonin in this association. Serotonin is also a precursor to melatonin in the pineal gland.
Patients with depression suffer from poor sleep quality, with associated antidepressant treatment often exacerbating sleep inefficiency with insomnia and decreased total sleep time being common side-effects. The effect of tryptophan depletion on sleep in depression has largely focused on remitted patients-acute tryptophan depletion in these patients, who were still taking antidepressants, resulted in reduced sleep and REM latencies but increased density, demonstrating that depleting tryptophan did not alter the antidepressant side-effects. Interestingly, in a population of patients with obsessive compulsive disorder, tryptophan depletion induced a worsening of sleep continuity, but no changes of REM or slow wave sleep.
 
10. Tryptophan, Serotonin and the Brain-Gut Axis 
The brain-gut axis is a bi-directional system of communication between the brain and the gastrointestinal tract, linking emotional and cognitive centres of the brain with peripheral control and function of the gut (Figure 1). Serotonin is a key element of this axis, acting as a neurotransmitter in the CNS and in the enteric nervous system that is present in the wall of the gut. In addition, serotonin is produced by endocrine cells and acts as a paracrine hormone in the gut and as an endocrine hormone, carried through the blood bound to platelets. Its role as a hormone acts to link the two ends of the brain-gut axis as well as having systemic effects such as bone density and metabolism. Central serotonin production represents just 5% of total serotonin synthesis, with the vast majority of serotonin made in the periphery. Peripheral synthesis occurs in tissues such as bone, mammary glands, the pancreas, but the gastrointestinal epithelium is by far the largest source. The enterochromaffin cells in the gastrointestinal epithelium account for ~90% of all serotonin synthesis. The peripheral endocrine synthesis pathway only differs from the central and enteric neuronal pathways by the utilisation of tryptophan hydroxylase type 1 instead of type 2. Degradation of serotonin is via monoamine oxidase and aldehyde dehydrogenase to 5HIAA as in the CNS, but in the periphery glucuronidation also plays an important role. ​

Figure 1. The brain-gut axis and the bi-directional system of communication. The brain-gut axis is a bi-directional system of communication between the brain and the gastrointestinal tract. This links emotional and cognitive centres of the brain with peripheral control and function of the gut and its resident microbiota. Serotonin is a key element of this axis, acting as a neurotransmitter in the CNS and in the enteric nervous system that is present in the wall of the gut. A. Neural communication between the gut and brain is via the vagus (stomach and rectum) and dorsal root ganglia (DRG-small and large intestine), via projections from the enteric nervous system to sympathetic ganglia and parasympathetic innervation of the gut. B. Humeral communication is via release of bacterial factors, production of cytokines and circulating hormones. An important advance for future studies will be testable models of a potential mechanism of action (e.g., cutting the vagus can block some effects of changing the gut microbiota in rodent models).

10.1. Tryptophan and the Gut Microbiota 
Another piece of the serotonin puzzle involves the resident community of microorganisms that have colonised the digestive tract. The gut microbiota is primarily found in the large intestine, but smaller numbers can be found throughout the gastrointestinal tract. Cross-talk between the gastrointestinal epithelium and enteric flora contributes to functions such as immune responses and regulation of hormones, and is proving to be critical to the maintenance of both homeostasis and health (Figure 1). How the bacterial community establishes early in life, or changes across the lifespan, can have consequences on the metabolism of tryptophan, and thus the serotonergic system. A balance is needed between bacterial utilization of tryptophan and the tryptophan necessary for serotonin synthesis in both enteric and central nervous systems.
There is both direct and indirect regulation of tryptophan and serotonin in the gut by the resident microbiota. Indirect regulation of tryptophan availability and serotonin formation by the gut microbiota is primarily via the kynurenine pathway. As noted, the synthesis of kynurenine accounts for approximately 90% of tryptophan metabolism. Recent evidence for direct regulation comes from germ-free animals that are laboratory-raised and are gut microbiota-deficient. These animals show increased levels of circulating tryptophan and decreased serotonin. When these animals have tryptophan metabolising bacteria introduced to their gut, circulating levels of tryptophan fall, with this alteration accompanying a sex-specific effect on hippocampal serotonin concentrations in male germ-free animals. Within the brain, an increase in hippocampal serotonin levels and turnover was observed, along with a decrease in anxiety-like behaviour, demonstrating the influence of gut microbiota on both behavioural correlates and brain neurochemistry. Interestingly, these animals also displayed a reduction in brain-derived neurotrophic factor messenger RNA levels and reduced expression of the synaptic signalling genes PSD-95 and synaptophysin in regions of the brain responsible for motor control and anxiety such as the striatum.
In irritable bowel syndrome, changes in the balance of microbiota are associated with symptomatology as well as alterations to both gut and brain serotonin levels. Moreover, the expression of toll-like receptors, which act to alert the body to pathogens, are altered in both plasma and colonic samples from irritable bowel syndrome patients. Recent data also shows that bacterial products such as short chain fatty acids can upregulate serotonin production by the enterochromaffin cells.
 
10.2. Behaviour and the Gut Microbiome 
As discussed, central serotonin plays a major role in mood and cognition. An influence of gut microbiota on behaviour is becoming increasingly evident, via a variety of proposed mechanisms including changes to tryptophan uptake and serotonin synthesis.
Germ-free mice display less anxiety-like behaviours than their traditionally colonised counterparts. Meanwhile, chronic treatment with lactic acid bacteria Lactobacillus rhamnosus to mice induced alterations in GABA receptors in cortical hippocampus, and amygdala in comparison with control-fed mice, while also reducing stress-induced corticosterone levels and anxiety- and depression-related behaviour. Interestingly, these effects were not found in vagotomized mice, identifying the vagus as a major modulatory communication pathway between the gut bacteria and the brain.
In animal models of depression, both environmental and surgical, animals display depressive-like behaviour and an altered intestinal microbial profile. These findings have now been replicated within a clinical population. In a recent study in major depression patients, several predominant genera were found in significantly different levels between the depressive and control groups showing either a predominance of some potentially harmful bacterial groups or a reduction in beneficial bacterial genera.
The influence of the gut microbiota on behaviour also extends to cognitive function in preclinical models, though all animal behavioural testing has an anxiety component, this suggests that cognitive deficits are not observed without a degree of stress. Mice infected with an enteric pathogen exhibited working memory dysfunction, and socially-associated behavioural impairment but only after acute water stress. Clinically, there is discussion of the engagement of gut microbial flora in the pathogenesis of Alzheimer’s disease, but at this stage this is speculative.
 
10.3. Tryptophan Depletion and the Gut-Brain Axis 
The central control of pain is an important component of irritable bowel syndrome and serotonin has been shown to play a role. In healthy women, a painful balloon distension to the rectum resulted in increased brain activity as shown by functional magnetic resonance imaging. When these stimuli were repeated during acute tryptophan depletion, there was an enhanced response in the amygdala, emotional arousal areas, and homeostatic afferent networks. There was also a decrease in negative feedback inhibition of the amygdala. When these tests were repeated in women with constipation-predominant irritable bowel syndrome, a similar pattern of brain activity was observed. This suggests that there are enhanced change in brain activity, namely the homeostatic afferent network and the emotional arousal network, after aversive visceral stimulation.
In addition, cognitive performance is altered in irritable bowel syndrome. Female patients with irritable bowel syndrome and healthy controls underwent a battery of neuropsychological tests after a placebo or acute tryptophan depletion. The results showed that acute tryptophan depletion produces decreased hippocampal-mediated cognitive performance. A similar test in female patients with diarrhea-predominant irritable bowel syndrome and healthy controls showed acute tryptophan depletion was significantly associated with impaired immediate and delayed recall performance in an affective memory test, though there was no difference in scores between patient and control groups. These patients also showed an enhanced visceral perception to an aversive visceral stimulus during acute tryptophan depletion similar to the study by Labus et al.
Interestingly, acute tryptophan depletion has not been shown to have an effect on mucosal concentrations of serotonin or the metabolite 5-hydroxyindoleacetic acid. However, acute tryptophan depletion studies investigating effects on regulation of gastrointestinal motility and sensation have shown lowered plasma tryptophan decreased the sensation of nausea during balloon distension without affecting gastric sensitivity and compliance. Acute tryptophan depletion also enhanced the postprandial intragastric volume increase, but this was not reflected by an increased nutrient intake. In contrast, motor function of the rectum during acute tryptophan depletion was tested in female patients with diarrhea-predominant irritable bowel syndrome. While the patient group had significantly altered rectal motor function, acute tryptophan depletion did not alter this.
Significant associations of tryptophan hydroxylase 1 gene polymorphisms, which may modify levels of circulating serotonin, are observed with irritable bowel syndrome-related cognitions in female patients. Employing the Cognitive Scale for Functional Bowel Disorders, tryptophan hydroxylase 1 gene polymorphisms were associated with negative cognitions regarding pain and anxiety around bowel movement. These polymorphisms were also associated with reductions in quality of life scores, in particular mental health and energy subscales, suggesting that subsets of the tryptophan hydroxylase 1 gene may impact the onset and course of irritable bowel syndrome, along with symptom severity and the emotional consequences of living with this disorder.

11. Concluding Remarks 
As we have outlined in this review, experimental manipulation of tryptophan levels has allowed us to understand the role of central serotonin in mood and cognition. Low serotonin contributes to a lowered mood state, however this should be in concert with a biological or genetic manipulation, producing a predisposition that interacts with lowered serotonin to decrease mood. In addition, depleted serotonin causes cognitive impairments, with reports including deficits in verbal reasoning, episodic, and working memory, while conversely tryptophan supplementation has positive effects on attention and memory. Interestingly, emotional processing, the modification of memory that underlies emotion, is inhibited in subjects with depression, or has a high-risk to develop, after tryptophan depletion.
 
An influence of gut microbiota on behaviour is becoming increasingly evident, as is the extension to effects on tryptophan and serotonin metabolism. There is regulation of tryptophan and serotonin in the gut by the resident microbiota and recent studies show that low-to-no gut microbiota increases levels of tryptophan and serotonin and modifies central higher order behaviour. 
 
Treatments for cognitive and mood disorders are an ongoing focus for neuroscience researchers and pharmaceutical organisations. The suggestion that the gut-microbiota has central influence opens up many new possibilities, especially with the suggestion from Mayer and colleagues that the composition and metabolic activity of the gut microbiota may play a role in such brain disorders as autism, anxiety, and depression. Ongoing studies will, in time, evaluate these assertions and hopefully determine the mechanisms by which the gut microbiota affect mood and cognition. ​