3.3.2. Tryptophan and Serotonin

Upon intestinal absorption into the bloodstream, the essential amino acid tryptophan can cross the blood-brain barrier (BBB). Thereby tryptophan can act as the precursor of the neurotransmitter 5-HT, which plays an important part in the microbiome-gut-brain axis [77]. Although it is still unclear to what extent the microbiome influences the synthesis of 5-HT, it has been established that certain strains of bacteria, such as *Streptococcus* spp., *Enterococcus* spp., and *Escheria* spp. are capable of producing this NT [78]. Most of the 5-HT is produced and stored in gastrointestinal cells and affects peristalsis, nausea, satiety and abdominal pain [79]. Meanwhile, in the brain, it influences other NTs, such as DA, Cholin (CH) and GABA, which influence memory and mood [80].

Banerjee et al. showed that 5-HT may have an influence on hyperactive and impulsive symptoms in ADHD [81]. Another study implied lower levels of 5-HT in the CNS of ADHD patients due to a decreased transport capacity of its precursor, tryptophan, into the brain [82]. Finally, one study showed that inflammation in the intestine affects 5-HT signaling pathways due to a decreased function and expression of the serotonin selective reuptake transporter (SERT) resulting in an increased level of 5-HT in the body [79]. However, it is important to remember that serotonin is not able to cross the BBB, and thus, the 5-HT pools in the CNS and the periphery do not directly interact with each other.

To demonstrate the importance of microbes on the 5-HT system, one study concluded that GF male mice have a 1.3 fold increased level of 5-HT in their hippocampus. This is an important finding as certain therapeutic medications of ADHD, such as escitalopram and lithium increase serotonin levels in a similar amount [83]. Thus, the composition and the modulation of the gut microbiota might become an interesting, future therapeutic intervention strategy.

Although the studies do not allow us to make a precise conclusion in what way bacterial-produced 5-HT influences ADHD, they do make it clear that it is one of the several catecholamines that play an important role in the pathophysiology of ADHD.

## 3.3.3. Kynurenine Pathway

Although tryptophan is the key amino acid for the production of 5-HT, 90% of tryptophan is catabolized by the kynurenine pathway [84]. This process produces nicotinamide adenine dinucleotide (NAD) through the stimulation of inflammatory and glucocorticoid metabolites. The kynurenine pathway has received attention in regards to psychiatric diseases, such as depression and schizophrenia [80,85,86] as it uses most of the tryptophan, and thus, leaves a limiting amount of substrate for the synthesis of serotonin.

Intermediate products, such as kynurenine, kynurenic acid (KA), xanthurenic acid (XA) and quilonoic acid (QA) can influence the immune system and neurotransmission [87]. The three former metabolites have anti-inflammatory properties as KA inhibits the NMDA-gated ion channels [88], and XA interferes with the glutamatergic neurotransmission [89]. Also, these products decrease the amount of pro-inflammatory IFN gamma in comparison to the anti-inflammatory IL-10 [87]. In contrast, QA stimulates microglial cells and increases the ratio of IFN gamma/IL-10 [87], resulting in pro-inflammatory effects [90]. Although KA shows neuroprotective properties, human and animal studies show that high levels of KA are associated with cognitive abnormalities, such as attention and memory issues typically associated with psychiatric disease [91,92].

Studies regarding levels of tryptophan and metabolites of the kynurenine pathway show inconclusive results. A Norwegian study, using 133 adult ADHD patients and 133, did not find that the ADHD group had lower levels of tryptophan and neuroprotective KA and XA [86]. These data were confirmed by another study testing ADHD children, which exhibited lower KA and XA levels [93]. These researchers, however, recorded higher levels of tryptophan in ADHD subjects [93]. These data do suggest an association between low levels of KA and XA in ADHD, but as there are still too few studies on this topic, it is difficult to deduce a definitive connection between tryptophan, its metabolites, and ADHD.

The various steps of the kynurenine pathway are dependent on coenzymes, such as the activated form of vitamin B6, pyridoxal 5 -phosphate (PLP). One study found an inverse correlation between serum levels of vitamin B6 and ADHD including its symptom severity [94]. Similarly, Aarsland et al. also observed a decrease in vitamin B6 in their patient group. Other data suggested that vitamin B6 metabolism plays a key part in the pathophysiology of ADHD, as vitamin B6 dependent enzymes show severe abnormalities in the ADHD test group [95]. Thus, lower levels of intermediate metabolites could be related to a deficiency of enzyme substrate. This data supports the importance of optimal coverage of ADHD patients with vitamin B6. The microbiome could play a potentially important role, as bacteria in the large intestine produce this vitamin [96]. As the correlation between levels of vitamin B6 and ADHD are relatively new, future studies are warranted to asses to what extent the microbiome can influence vitamin B6 levels on a therapeutic level.
