A novel calcium-sensitive potassium conductance is coupled to P2X3 subunit containing receptors in myenteric neurons of guinea pig ileum

Purinergic Signalling in the Gut

The article will begin with the discovery of purinergic inhibitory neuromuscular transmission in the 1960s/1970s, the proposal for purinergic cotransmission in 1976 and the recognition that sympathetic nerves release adenosine 5'-triphosphate (ATP), noradrenaline and neuropeptide Y, while non-adrenergic, non-cholinergic inhibitory nerve cotransmitters are ATP, nitric oxide and vasoactive intestinal polypeptide in variable proportions in different regions of the gut. Later, purinergic synaptic transmission in the myenteric and submucosal plexuses was established and purinergic receptors expressed by both glial and interstitial cells. The focus will then be on purinergic mechanosensory transduction involving release of ATP from mucosal epithelial cells during distension to activate P2X3 receptors on submucosal sensory nerve endings. The responses of low threshold fibres mediate enteric reflex activity via intrinsic sensory nerves, while high threshold fibres initiate pain via extrinsic sensory nerves. Finally, the involvement of purinergic signalling in an animal model of colitis will be presented, showing that during distension there is increased ATP release, increased P2X3 receptor expression on calcitonin gene-related peptide-labelled sensory neurons and increased sensory nerve activity.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Deletion of P2X2 and P2X3 receptor subunits does not alter motility of the mouse colon

Purinergic P2X receptors contribute to neurotransmission in the gut. P2X receptors are ligand-gated cation channels that mediate synaptic excitation in subsets of enteric neurons. The present study evaluated colonic motility in vitro and in vivo in wild type (WT) and P2X2 and P2X3 subunit knockout (KO) mice. The muscarinic receptor agonist, bethanechol (0.3–3 μM), caused similar contractions of the longitudinal muscle in colon segments from WT, P2X2 and P2X3 subunit KO mice. Nicotine (1–300 μM), acting at neuronal nicotinic receptors, caused similar longitudinal muscle relaxations in colonic segments from WT and P2X2 and P2X3 subunit KO mice. Nicotine-induced relaxations were inhibited by nitro-l-arginine (NLA, 100 μM) and apamin (0.1 μM) which block inhibitory neuromuscular transmission. ATP (1–1000 μM) caused contractions only in the presence of NLA and apamin. ATP-induced contractions were similar in colon segments from WT, P2X2 and P2X3 KO mice. The mouse colon generates spontaneous migrating motor complexes (MMCs) in vitro. The MMC frequency was higher in P2X2 KO compared to WT tissues; other parameters of the MMC were similar in colon segments from WT, P2X2 and P2X3 KO mice. 5-Hydroxytryptophan-induced fecal output was similar in WT, P2X2 and P2X3 KO mice. These data indicate that nicotinic receptors are located predominately on inhibitory motor neurons supplying the longitudinal muscle in the mouse colon. P2X2 or P2X3 subunit containing receptors are not localized to motor neurons supplying the longitudinal muscle. Synaptic transmission mediated by P2X2 or P2X3 subunit containing receptors is not required for propulsive motility in the mouse colon.

Purinergic receptors and synaptic transmission in enteric neurons

Purines such as ATP and adenosine participate in synaptic transmission in the enteric nervous system as neurotransmitters or neuromodulators. Purinergic receptors are localized on the cell bodies or nerve terminals of different functional classes of enteric neurons and, with other receptors, form unique receptor complements. Activation of purinergic receptors can regulate neuronal activity by depolarization, by regulating intracellular calcium, or by modulating second messenger pathways. Purinergic signaling between enteric neurons plays an important role in regulating specific enteric reflexes and overall gastrointestinal function. In the present article, we review evidence for purine receptors in the enteric nervous system, including P1 (adenosine) receptors and P2 (ATP) receptors. We will explore the role they play in mediating fast and slow synaptic transmission and in presynaptic inhibition of transmission. Finally, we will examine the molecular properties of the native receptors, their signaling mechanisms, and their role in gastrointestinal pathology.