Besides dopamine, the gut bacteria can modulate and produce several neurotrans- mitters, including GABA and serotonin. In this section, we will briefly summarize their function and how gut microbiota composition may affect them. 5.1. Serotonin Serotonin has a key role both in the brain and peripherally, including modulat- ing satiety, anxiety, and mood and stimulating peristalsis, secretion, and vasodilation, respectively [154-156]. There is also evidence suggesting the role of serotonin in the reg- ulation of glucose and lipid metabolism [157,158]. Serotonin cannot pass the BBB; thus production from tryptophan takes place separately within the periphery and the CNS. Tryptophan is an essential amino acid obtained from dietary proteins and is a precursor to several metabolites, including serotonin [159]. It is well established that the majority of peripheral serotonin is produced in the enterochromaffin cells in the (small) intestinal EECs [156,160). Within the hypothalamus, serotonin mediates its food intake suppressant effects through inhibition of NPY/AgRP and activation of POMC neurons. However, the complete interaction between serotonin and satiety signals is not yet fully understood [155]. The serotonin 2C receptor agonist, lorcaserin, was found to significantly improve weight loss in individuals with obesity [161], but was recently withdrawn from the market after reports of increased cancer risk [162]. In obesity, evidence of decreased central serotonin signaling has been reported [147]. In the periphery, increased circulating plasma serotonin concentrations have been found in individuals with obesity [163]. Crane et al. found that inhibition of peripheral serotonin synthesis enhanced thermogenesis in adipose tissue, causing increased energy expenditure and protection against high-fat-diet-induced obesity in mice [164]. Gut mi- crobes have a regulatory effect on the peripheral concentrations of serotonin [165]. The role of serotonin in obesity is complex, and the role the microbiota have on central serotonin is still unclear. However, our recent pilot study using SPECT imaging in humans suggested that the gut microbiota can directly or indirectly affect brain serotonin (and dopamine) transporter binding potential in humans with obesity via sympathetic tone [80]. 5.2. GABA GABA is the main inhibitory neurotransmitter. GABA is produced from its counter glutamate, which functions as an excitatory neurotransmitter. GABA has been implicated in the hypothalamic control of food intake [166,167]. Peripheral GABA production can be stim- ulated by several bacteria of the microbiota, such as Bifidobacterium and Lactobacillus [168]. Animal studies have indicated that treatment modulating the microbiota might improve metabolic health and behavior. For example, treatment with Lactobacillus rhamnosus in- duced alterations in GABA mRNA in the brain and reduced anxiety- and depression-related behavior via the regulation of vagal nerve tone [169], and obese mice that received the GABA-producing Lactobacillus brevis improved metabolically and had less depressive-like behavior [170]. Finally, we recently showed that in humans with obesity treated with fecal microbiota transplantation (FMT) from lean donors, plasma GABA concentrations increased [171.اريد تلخيص شامل بالانجليزي