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Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The actions of purine and pyrimidine compounds on isolated segments of the mouse intestine were investigated during postnatal development. The localization of P2Y(1), P2Y(2), P2Y(4), P2X(1,) P2X(2) and P2X(3) receptors were examined immunohistochemically, and levels of expression of P2Y(1), P2X(1) and P2X(2) were studied by Western immunoblot. From day 12 onwards, the order of potency for relaxation of longitudinal muscle of all regions was 2-MeSADP>or=alpha,beta-meATP>or=ATP=UTP=adenosine, suggesting P2Y(1) receptors. This was supported by the sensitivity of responses to 2-MeSADP to the selective antagonist MRS 2179 and P2Y(1) receptor immunoreactivity on longitudinal muscle and a subpopulation of myenteric neurons. A further alpha,beta-meATP-sensitive P2Y receptor subtype was also indicated. ATP and UTP were equipotent suggesting a P2Y(2) and/or P2Y(4) receptor. Adenosine relaxed the longitudinal muscle in all regions via P1 receptors. The efficacy of all agonists to induce relaxation of raised tone preparations increased with age, being comparable to adult by day 20, the weaning age. During postnatal development the contractile response of the ileum and colon was via P2Y(1) receptors, while the relaxant response mediated by P2Y(1) receptors gradually appeared along the mouse gastrointestinal tract, being detectable in the stomach from day 3 and in the duodenum from day 6. In the ileum and colon relaxant responses to 2-MeSADP were not detected until days 8 and 12, respectively. 2-MeSADP induced contractions on basal tone preparations from day 3, but decreased significantly at day 12 and disappeared by day 20. At day 8, contractions of colonic longitudinal muscle to ATP showed no desensitisation suggesting the involvement of P2X(2) receptors. Immunoreactivity to P2X(2) receptors only was observed on the longitudinal muscle of the colon and ileum from day 1 and on a subpopulation of myenteric neurons from day 3. These data suggest that P2Y(1) receptors undergo postnatal developmental changes in the mouse gut, with a shift from contraction to relaxation. Such changes occur 1 week before weaning and may contribute to the changes that take place in the gut when the food composition changes from maternal milk to solid food.

The involvement of NMDA glutamate receptors in the effects of glucose/oxygen deprivation (in vitro ischaemia) on spontaneous endogenous acetylcholine and glutamate overflow from the guinea pig ileum was studied. Neurotransmitter overflow was measured by HPLC. Deprivation of glucose in the medium slightly reduced acetylcholine overflow, and did not significantly influence glutamate overflow. During oxygen deprivation and glucose/oxygen deprivation, acetylcholine overflow augmented with a biphasic modality: an early peak was followed by a long lasting increase, whereas glutamate overflow increased with a rapid and sustained modality. The effects of glucose/oxygen deprivation on both acetylcholine and glutamate overflow were abolished after reperfusion with normal oxygenated medium. Acetylcholine and glutamate overflow induced by glucose/oxygen deprivation were significantly reduced in the absence of external Ca(2+) as well as by the addition of the mitochondrial Na(+)-Ca(2+) exchanger blocker, CGP 37157, and of the endoplasmic reticulum Ca(2+)/ATPase blocker, thapsigargin. +/-AP5, an NMDA receptor antagonist, and 5,7-diCl-kynurenic acid, an antagonist of the glycine site associated to NMDA receptor, markedly depressed glucose/oxygen deprivation-induced acetylcholine and glutamate overflow as well. Our results suggest that in vitro simulated ischaemia evokes acetylcholine and glutamate overflow from the guinea pig ileum, which is partly linked to an increase in intracellular Ca(2+) concentration dependent on both Ca(2+) influx from the extracellular space and Ca(2+) mobilization from the endoplasmic reticulum and mitochondrial stores. During glucose/oxygen deprivation, ionotropic glutamate receptors of the NMDA type exert both a positive feedback modulation of glutamate output and contribute to increased acetylcholine overflow.

The actions of adenosine, adenosine 5'-triphosphate (ATP), 2-methylthio adenosine diphosphate ADP (2-MeSADP), 2-methylthio ATP (2-MeSATP), alpha,beta-methylene ATP (alpha,beta-meATP) and uridine triphosphate (UTP) on isolated segments of mouse stomach (fundus), duodenum, ileum and colon were investigated. The localization of P2Y(1), P2Y(2), P2Y(4), P2X(1) and P2X(2) receptors and neuronal nitric oxide synthase (NOS) were examined immunohistochemically, and P2Y(1) mRNA was examined with in situ hybridization. The order of potency for relaxation of longitudinal muscle of all regions was: 2-MeSADP>/=2-MeSATP>alpha,beta-meATP>ATP=UTP=adenosine. This is suggestive of P2Y(1)-mediated relaxation and perhaps a further P2Y receptor subtype sensitive to alpha,beta-meATP. As ATP and UTP are equipotent, the presence of a P2Y(2) receptor is indicated. ATP responses were inhibited by the P2Y(1)-selective antagonist MRS 2179, and suramin. P2Y(1) receptors were visualized immunohistochemically in the smooth muscle of the ileum and in a subpopulation for myenteric neurones, which also stained for NOS. P2Y(1) mRNA was localized in neurones in both myenteric and submucosal ganglia in the ileum. Taken together, these results suggest that ATP was acting on non-adrenergic, non-cholinergic inhibitory neurons, which release both nitric oxide (NO) and ATP. Reduced relaxations to 2-MeSADP by tetrodotoxin and N(omega)-nitro-L-arginine methyl ester, are consistent with this possibility. Adenosine acts via P1 receptors to relax smooth muscle of the mouse gut. Segments of mouse colon (in contrast to the stomach and small intestine) were contracted by nucleotides with the potency order: 2-MeSATP>alpha,betameATP>ATP; the contractions showed no desensitization and were antagonized by suramin and PPADS, consistent with responses mediated by P2X(2) receptors. Immunoreactivity to P2X(2) receptors was demonstrated on both longitudinal and circular muscle of the colon, but not in the other regions of the gut, except for a small subpopulation of myenteric neurones. In summary, neuronal P2Y(1) receptors appear to mediate relaxation, largely through NO in all regions of the mouse gut, and to a lesser extent by P2Y(1), P2Y(2) and a novel P2Y receptor subtype responsive to alpha,beta-meATP in smooth muscle, while P2X(2) receptors mediate contraction of colonic smooth muscle.

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.