## 2 Microbiota-Gut-Brain Axis
### Recent Trends on Studies between VN and Microbiota in Different Disease  
VN is the longest and the most important nerve in the body. It innervates all the organs in the body: heart, lungs, liver, pancreas, spleen, stomach, kidneys and obvious it has projections into the brain.VN is involved in almost every chronic disease. If the VN does not function correctly the brain, the brainstem and all the organs could be affected.  
Figure 1: Bi-directional communications between gut and brain.  
If are followed the neurology of every disease, the cause is VN. The VN can also sense the microbiota and can transfer this sensed information to the central nervous system. Then, central system can generate a response; the latter could perpetuate a pathological condition of the digestive tract or promote neurodegenerative disorders [22]. A dysbiosis is observed in obesity and diabetes. Therefore, VN can be seen as the interface of the brain-gut communication axis [23; 33]. When the vagal tone is low, it is very easy to contract disease and it is described in obesity and diabetes too [24]. Stimulation can help improve the VN damage and stimulate the body to repair and also could restore homeostasis in such diseases. Furthermore, VN stimulation used for the treatment of depression and epilepsy [25] and its anti-inflammatory properties such as in obesity and diabetes [26], should be of interest.  
If the VN malfunctions, significant effects can be seen on appetite and eating acting as a regulator of body weight [34]. Microbes control eating behavior by influencing signals that are delivered to the brain and by the VN and two of the most important hormones dopamine and serotonin [35]. Dopamine is associated with motivation, reward [34] and serotonin plays a role in cognition, mood, memory, and executive functions [36]. Approximately 50% of the dopamine in the body is produced in the gut and also L-dopamine that is used to treat Parkinson's disease [37], while all the serotonin in the body is made also in the gut [36]. Serotonin deficiency is linked to depression and neurological disfunction [38]. Therefore microbes by control just these two hormones can influence our mood, behavior, anxiety, relaxation, sleep, and eating.  
Neurotransmitters produced in the gut which regulate appetite, peptide YY, pancreatic polypeptide, glucagon-like peptide-1 and oxyntomodulin suppress appetite, whilst ghrelin increases appetite [39]. There is an equilibrium of these two types that determine the appetite, adjusting eating behavior. The gut microbes can alter the level of these hormones since they are mainly produced in the gut. The probiotic bacteria can raise the level of this amino acid tryptophan which is important because it is involved in producing these hormones (for example leptin, ghrelin, PYY that influence appetite) [6]. In conclusion, microbiota can influence both directly and indirectly the eating behavior and appetite control [40]. Microbiota products that can alter the host behavior and health are: fatty acids that influence energy, metabolism and immunity; biotin that influence cell growth and metabolism; vitamin K that produce blood coagulation; tryptophan, serotonin, dopamine, GABA, noradrenaline, acetylcholine which are neurotransmitters and influence brain functions [41].