Probiotic microbes inhabit the gut mucosa where they communicate with the GALT (gut associated lymphoid tissue), pyre's patches and influence the secretion of cytokines by immune cells. Do the probiotic microorganisms have influence on neuronal signals in the gut?
You could read this as a starting point, it reviews the use of probiotics and to the CNS:
Gut-brain axis: how the microbiome influences anxiety and depression. Foster JA, McVey Neufeld KA. Trends Neurosci. 2013 May;36(5):305-12. doi: 10.1016/j.tins.2013.01.005.
That is one of the million dólar question. There is evidence for microbe/immune/neuronal interaction, but few things are certain at the moment... Here is an exemple of gut microbes producing neurotransmitters, and other useful papers:
http://www.ncbi.nlm.nih.gov/pubmed/?term=michael+fischbach+tryptamine
http://www.ncbi.nlm.nih.gov/pubmed/25823690
http://www.ncbi.nlm.nih.gov/pubmed/25469018
http://www.ncbi.nlm.nih.gov/pubmed/23234727
Lyte M. Cryan J.F. 2014. Microbial endocrinology: the microbiota-gut-brain axis in health and disease. Advances in Experimental Medicine and Biology 817, Springer, New York, 435 p.
http://jhc.sagepub.com/content/57/11/1013.short
It is form my ms in press (available soon). I hope it will be interesting to you.......
Microbiota is producing a wide range of compounds, including a variety of mediators involved in the activation or inhibition of various cells and systems of host, including the nervous, cardiovascular, digestive and endocrine systems. For example, bifido- and lactobacilli, and Escherichia coli can synthesize neurotransmitters and neuromodulators such as GABA, serotonin, acetylcholine, dopamine, etc. [3, 5, 33, 34]. It is possible that the neurotransmitters secreted by bacteria may stimulate intestinal epithelial cells to release mediators which modulate the activity of the enteric nervous system (ENS) and act directly on primary afferent axons [35]. The bacteria may also synthesize other neuroactive metabolites, including short chain fatty acids [34, 36, 37], and long-chain fatty acids [37].
The gut-brain axis uses 4 major information carriers for the communication: 1) a direct link between the intestinal bacteria and the nervous system via the vagus nerve [38]; 2) communication via the cytokine release as a result of activation of immune cells [39]; 3) endocrine communication via the hormones of the gastrointestinal tract [5] and 4) via releasing of various signaling molecules synthesized by microbiota and released into the bloodstream [3, 34, 37]. The fourth way may have complex interactions with the other three ways. Moreover, all four ways of communication of "gut-brain" axes work together closely [1].
It should be noted that enteric microbiota do not directly contact with neurons of ENS, which are separated from the luminal contents by a continuous epithelial lining, covered by a layer of mucus. Bacteria may affect nerve fiber of ENS and peripheral nervous system releasing signaling molecules into the gut and/or stimulating immune and/or epithelial (endocrine) cells to release neuroactive substances. The microbiota may also act directly on neurons in the brain via metabolites, which enter the systemic circulation and cross the blood-brain barrier.
Influence of Lactobacilli and/or their metabolites on the isolated nerve cells of the brain is not studied. We investigated the effect of PP enriched with lactobacillus metabolites on isolated rat brain cells. It was found that PP manifested obvious pharmacological reaction at the cellular level and, in particular, an increase in [Ca2+]i in various excitable cells [20, 23, 31]. In brain neurons PP also increased [Ca2+]i by calcium release from intracellular stores, and by extracellular calcium entry into the cell. Increase in [Ca2+]i stimulates intracellular signaling mechanisms.
http://www.scientificamerican.com/article/the-tantalizing-links-between-gut-microbes-and-the-brain/?WT.mc_id=SA_HLTH_20151027
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