**6. Probiotics and enteric nervous system**

Enteric nervous system (ENS), which is located in the wall of the digestive tract, is a neural network called as second brain that is consisted of sensory neurons, motor neurons, inter neurons and glial cells. It regulates complicated reflexes, motility and secretory functions of digestive tract. Although it is connected to central nervous system, ENS can regulate the

The Impact of Probiotics on the Gastrointestinal Physiology 63

Probiotics have been used in the treatments of neuromotoric and sensory functional disorders of digestive tract since their effects on ENS have been revealed. For example, it has been reported that functional disorders such as delayed gastric emptying, increased visceral perception and abnormal feeding pattern which occurs in mice due to *Helikobacter* infection, were treated by supplementation of *Lactobacillus rhamnosus* R0011 and *Lactobacillus helveticus* 

Probiotics in digestive tract affect central nervous system by both ENS and parasympathetic fibers that innervates digestive tract. However this effect is probably species specific. For example, *Lactobacillus reuteri* changes mRNA expressions of GABAA and GABAB receptors in central nervous system. The changes in these receptors have found to be related with anxious and depressive-like behaviors (Cryan & O'Mahony, 2011). Similarly it has been reported that *Bifidobacterium longum* has anxiolytic effect and decreases the excitability of

Intestinal barrier is a morphologic and physiologic structure placed between tissues and intestinal lumen which is known as external environment and it ensures continuing of events such as absorption and secretion between them. Intestinal lumen consists of microclimate on epithelial cells and lamina propria under epithelium. It regulates nutrients absorption, water and ion fluxes, and represents the first defensive barrier against toxins and enteric pathogens. Intestinal barrier consists of internal and external layers; the internal layer includes intestinal epithelial cells and tight junctions (TJ), the external layer includes

The intestinal epithelium is formed by a monolayer epithelial cells, the spaces between epithelial cells is sealed by tight junctions. Tight junctions are specific structures comprised of transmembrane proteins. Microclimate consists of unstirred water layer, glycocalyx, and mucus layer. Lamina propria is a layer existed under epithelial cells. In this layer there are cells of innate and acquired immunity secreting immunoglobulins and cytokines which are

Proper intestinal barrier function is essential for maintaining optimal health and balance throughout the body. The epithelium of the intestinal mucosa prevents the passage of commensal and pathogenic microorganisms. Therefore, it is the first line of defense against luminal antigens and toxins. An impairment of this intestinal barrier is critical for pathogenesis of several diseases such as inflammatory bowel disease, celiac disease

Development of physical and functional intestinal barrier begins during embryonic period. In human, enterocytes appear in intestinal mucosa at 8th weeks, and TJ appear at 10th weeks of pregnancy. Functional immune barrier becomes functional after the formation of panet cells at 12th weeks. In this period, panet cells produce antimicrobial defensins and lysozymes. Mucins, which start to be expressed at 6.5th weeks of pregnancy and increase in time, constitute functional barrier. Although the development of intestinal barrier begins at prenatal period, it continues through postnatal period (Patel & Lin, 2010). Because, intestinal barrier is not yet fully developed in preterm infants, aberrant inflammatory and apoptotic responses to bacteria may occur. When premature infants

**7. The effect of probiotics on the intestinal barrier functions** 

(Chichlowski et al., 2008) and atopic dermatitis (Rosenfeldt et al., 2004).

bacteria and a mucus layer (Catalioto et al., 2011).

substantial for intestinal barrier.

R0052 (Verdu et al., 2008).

ENS (Bercik et al., 2011).

function of its target organ without input from the central nervous system (CNS) (Gershon, 2005).

There is a mutual communication between CNS and microorganisms in the digestive tract. The central nervous system affects microorganisms by chancing motility, secretion, and permeability of digestive tract or via various mediators that are secreted by neuro-endocrine cells (Barbara et al., 2005). Microorganisms in the digestive tract affect functions of ENS and CNS via direct or indirect mechanisms. Microorganisms both affect development of sensory and motor neurons and induce plasticity.

Microorganisms in intestines communicate with nervous system via epithelial cells, various receptors or cells in lamina propria. Enterochromaffin cells play a key role in this communication. They function such as a transducer and provide a link between intestinal lumen and ENS (Indrio & Neu, 2011).

Effects of microorganisms in intestinal tract on nervous system occur via more than one mechanism. They affect development of sensory and motor neurons in gut by secreted substances or fermented products. For example SCFA, which is a fermented product of microorganisms in digestive tract, may affect motor activity in digestive tract (Soret et al., 2010). Furthermore, certain mediators secreted by immune cells which are activated by microorganisms in intestinal tract are effective on the regulation of ENS. Because, enteric neurons have receptors which are responsive to immune cells secreted mediators. For example, secretion of substances such as histamine, interleukin-6, leukotrienes, 5 hydroxytryptamine, platelet activating factor, mast cell proteases, adenosine, interleukin-1β, prostaglandins as a result of stimulation of mast cells affect functions of ENS by connecting to the receptors on the neurons of ENS (Wood, 2007).

Bacteria including probiotics can be considered as a chemical factory producing biologically active substance such as neurotransmitters and neuromodulators (Wang et al., 2010). It has been determined that *Lactobacillus* and *Bifidobacterium* produce GABA, *Escherichia, Bacillus*  and *Saccharomyces* produce norepinephrine, *Candida, Streptococcus, Escherichia* and *Enterococcus* produce serotonin, *Bacillus* produce dopamine, *Lactobacillus* produce acetylcholine (Lyt, 2011).

It has been determined that *Lactobacillus reuteri* increases the excitability of myenteric AH cells in rats by inhibiting calcium dependent potassium channels (Kunze et al., 2009). The same researches have also reported that activity of ENS was inhibited as a result of the effect of *Lactobacillus reuteri* on AH cells (Whang et al., 2010). Because ENS depresses intestines motility, inhibition of ENS causes an increase in motility.

Probiotics reveal their effects by changing neuro-chemical characteristics of enteric neurons. Kamm et al. (2004) have reported that the numbers of neurons containing calbidin, which is a multiple calcium binding protein, decreased in jejunum of pigs supplemented with *Sacharomyces boulardii.* Similarly Giancamillo et al. (2010) have reported that the density of galaninergic and calcitonin gene-related peptide (CGRP) positive neurons increased in submucosal plexus of ileum of *Pediococcus acidilactici* treated pigs. Galanin is effective on peristaltic activity, secretion, blood flow and eating behaviors, and CGRP is effective on the modulation of sensory functions and the regulation of activity of smooth muscle. Furthermore, the same researchers have determined that density of glial cells in ileums' inner and outer sub mucosal plexus was increased in *Pediococcus acidilactici* treated pigs.

function of its target organ without input from the central nervous system (CNS) (Gershon,

There is a mutual communication between CNS and microorganisms in the digestive tract. The central nervous system affects microorganisms by chancing motility, secretion, and permeability of digestive tract or via various mediators that are secreted by neuro-endocrine cells (Barbara et al., 2005). Microorganisms in the digestive tract affect functions of ENS and CNS via direct or indirect mechanisms. Microorganisms both affect development of sensory

Microorganisms in intestines communicate with nervous system via epithelial cells, various receptors or cells in lamina propria. Enterochromaffin cells play a key role in this communication. They function such as a transducer and provide a link between intestinal

Effects of microorganisms in intestinal tract on nervous system occur via more than one mechanism. They affect development of sensory and motor neurons in gut by secreted substances or fermented products. For example SCFA, which is a fermented product of microorganisms in digestive tract, may affect motor activity in digestive tract (Soret et al., 2010). Furthermore, certain mediators secreted by immune cells which are activated by microorganisms in intestinal tract are effective on the regulation of ENS. Because, enteric neurons have receptors which are responsive to immune cells secreted mediators. For example, secretion of substances such as histamine, interleukin-6, leukotrienes, 5 hydroxytryptamine, platelet activating factor, mast cell proteases, adenosine, interleukin-1β, prostaglandins as a result of stimulation of mast cells affect functions of ENS by connecting

Bacteria including probiotics can be considered as a chemical factory producing biologically active substance such as neurotransmitters and neuromodulators (Wang et al., 2010). It has been determined that *Lactobacillus* and *Bifidobacterium* produce GABA, *Escherichia, Bacillus*  and *Saccharomyces* produce norepinephrine, *Candida, Streptococcus, Escherichia* and *Enterococcus* produce serotonin, *Bacillus* produce dopamine, *Lactobacillus* produce

It has been determined that *Lactobacillus reuteri* increases the excitability of myenteric AH cells in rats by inhibiting calcium dependent potassium channels (Kunze et al., 2009). The same researches have also reported that activity of ENS was inhibited as a result of the effect of *Lactobacillus reuteri* on AH cells (Whang et al., 2010). Because ENS depresses intestines

Probiotics reveal their effects by changing neuro-chemical characteristics of enteric neurons. Kamm et al. (2004) have reported that the numbers of neurons containing calbidin, which is a multiple calcium binding protein, decreased in jejunum of pigs supplemented with *Sacharomyces boulardii.* Similarly Giancamillo et al. (2010) have reported that the density of galaninergic and calcitonin gene-related peptide (CGRP) positive neurons increased in submucosal plexus of ileum of *Pediococcus acidilactici* treated pigs. Galanin is effective on peristaltic activity, secretion, blood flow and eating behaviors, and CGRP is effective on the modulation of sensory functions and the regulation of activity of smooth muscle. Furthermore, the same researchers have determined that density of glial cells in ileums' inner and outer sub mucosal plexus was increased in *Pediococcus acidilactici* treated pigs.

2005).

and motor neurons and induce plasticity.

lumen and ENS (Indrio & Neu, 2011).

acetylcholine (Lyt, 2011).

to the receptors on the neurons of ENS (Wood, 2007).

motility, inhibition of ENS causes an increase in motility.

Probiotics have been used in the treatments of neuromotoric and sensory functional disorders of digestive tract since their effects on ENS have been revealed. For example, it has been reported that functional disorders such as delayed gastric emptying, increased visceral perception and abnormal feeding pattern which occurs in mice due to *Helikobacter* infection, were treated by supplementation of *Lactobacillus rhamnosus* R0011 and *Lactobacillus helveticus*  R0052 (Verdu et al., 2008).

Probiotics in digestive tract affect central nervous system by both ENS and parasympathetic fibers that innervates digestive tract. However this effect is probably species specific. For example, *Lactobacillus reuteri* changes mRNA expressions of GABAA and GABAB receptors in central nervous system. The changes in these receptors have found to be related with anxious and depressive-like behaviors (Cryan & O'Mahony, 2011). Similarly it has been reported that *Bifidobacterium longum* has anxiolytic effect and decreases the excitability of ENS (Bercik et al., 2011).
