ATP as a putative sensory mediator: activation of intrinsic sensory neurons of the myenteric plexus via P2X receptors

Pharmacology and function of nicotinic acetylcholine and P2X receptors in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). Ligand-gated ion channels are an important class of receptors expressed by enteric neurones. This review will focus on nicotinic acetylcholine receptors (nAChRs) and P2X receptors for ATP, as these receptors contribute to fast synaptic transmission in identified pathways in the ENS. There are multiple subunit proteins that compose nAChRs and P2X receptors in the nervous system. Functional and pharmacological studies indicate that the predominant class of nAChR mediating fast synaptic transmission in enteric neurones is composed of alpha3 and beta4 subunits. P2X receptors mediating fast synaptic excitation are predominately P2X2 homomeric receptors.

Enteric P2X receptors as potential targets for drug treatment of the irritable bowel syndrome

The irritable bowel syndrome (IBS) is a gastrointestinal motility disorder affecting millions of patients. IBS symptoms include diarrhea, constipation and pain. The etiology of IBS is due partly to changes in the function of nerves supplying the gastrointestinal tract, immune system activation and to psychological factors. P2X receptors are multimeric ATP-gated cation channels expressed by neuronal and non-neuronal cells. Sensory nerve endings in the gastrointestinal tract express P2X receptors. ATP released from gastrointestinal cells activates P2X receptors on sensory nerve endings to stimulate motor reflexes and to transmit nociceptive signals. Antagonists acting at P2X receptors on sensory nerves could attenuate abdominal pain in IBS patients. Primary afferent neurons intrinsic to the gut, and enteric motor- and interneurons express P2X receptors. These neurons participate in motor reflexes. Agonists acting at enteric P2X receptors may enhance gastrointestinal propulsion and secretion, and these drugs could be useful for treating constipation-predominant IBS. Antagonists acting at enteric P2X receptors would decrease propulsion and secretion and they might be useful for treating diarrhea-predominant IBS. Current knowledge of P2X receptor distribution and function in the gut of laboratory animals provides a rational basis for further exploration of the therapeutic potential for drugs acting at P2X receptors in IBS patients. However, more information about P2X receptor distribution and function in the human gastrointestinal tract is needed. Data on the distribution and function of P2X receptors on gastrointestinal immune cells would also provide insights into the therapeutic potential of P2X receptor agents in IBS.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Purinergic mechanisms contribute to mechanosensory transduction in the rat colorectum

BACKGROUND & AIMS

Adenosine 5'-triphosphate plays a role in peripheral sensory mechanisms and, in particular, mechanosensory transduction in the urinary system. P2X(3) receptors are selectively expressed on small-diameter sensory neurons in the dorsal root ganglia; sensory neurons from dorsal root ganglia L1 and S1 supply the colorectum. This study investigated whether purinergic signaling contributes to mechanosensory transduction in the rat colorectum.

METHODS

A novel in vitro rat colorectal preparation was used to elucidate whether adenosine 5'-triphosphate is released from the mucosa in response to distention and to evaluate whether it contributes to sensory nerve discharge during distention.

RESULTS

P2X(3) receptor immunostaining was present on subpopulations of neurons in L1 and S1 dorsal root ganglia, which supply the rat colorectum. Distention of the colorectum led to pressure-dependent increases in adenosine 5'-triphosphate release from colorectal epithelial cells and also evoked pelvic nerve excitation, which was mimicked by application of adenosine 5'-triphosphate and alpha,beta-methylene adenosine 5'-triphosphate. The sensory nerve discharges evoked by distention were potentiated by alpha,beta-methylene adenosine 5'-triphosphate and ARL-67156, an adenosine triphosphatase inhibitor, and were attenuated by the selective P2X(1), P2X(3), and P2X(2/3) antagonist 2',3'-O-trinitrophenyl-adenosine 5'-triphosphate and by the nonselective P2 antagonists pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid and suramin. Adenosine, after ectoenzymatic breakdown of adenosine 5'-triphosphate, seems to be involved in the longer-lasting distention-evoked sensory discharge. Single-fiber analysis showed that high-threshold fibers were particularly affected by alpha,beta-methylene adenosine 5'-triphosphate, suggesting a correlation between purinergic activation and nociceptive stimuli.

CONCLUSIONS

Adenosine 5'-triphosphate contributes to mechanosensory transduction in the rat colorectum, and this is probably associated with pain.

P2X2 subunits contribute to fast synaptic excitation in myenteric neurons of the mouse small intestine

P2X receptors are ATP-gated cation channels composed of one or more of seven different subunits. ATP acts at P2X receptors to contribute to fast excitatory postsynaptic potentials (fEPSPs) in myenteric neurons but the subunit composition of enteric P2X receptors is unknown. These studies used tissues from P2X2 wild-type (P2X2+/+) and P2X2 gene knockout (P2X2-/-) mice to investigate the role of this subunit in enteric neurotransmission. Intracellular electrophysiological methods were used to record synaptic and drug-induced responses from ileal myenteric neurons in vitro. Drug-induced longitudinal muscle contractions and peristaltic contractions of ileal segments were also studied in vitro. Gastrointestinal transit was measured as the progression in 30 min of a liquid radioactive marker administered by gavage to fasted mice. RT-PCR analysis of mRNA from intestinal tissues and data from immunohistochemical studies verified P2X2 gene deletion. The fEPSPs recorded from S neurons in tissues from P2X2+/+ mice were reduced by mecamylamine (nicotinic cholinergic receptor antagonist) and PPADS (P2X receptor antagonist). The fEPSPs recorded from S neurons from P2X2-/- mice were unaffected by PPADS but were blocked by mecamylamine. ATP depolarized S and AH neurons from P2X2+/+ mice. ATP depolarized AH but not S neurons from P2X2-/- mice. alpha,beta-Methylene ATP (alpha,beta-mATP)(an agonist at P2X3 subunit-containing receptors) did not depolarize S neurons but it did depolarize AH neurons in P2X2+/+ and P2X2-/- mice. Peristalsis was inhibited in ileal segments from P2X2-/- mice but longitudinal muscle contractions caused by nicotine and bethanechol were similar in segments from P2X2+/+ and P2X2-/- mice. Gastrointestinal transit was similar in P2X2+/+ and P2X2-/- mice. It is concluded that P2X2 homomeric receptors contribute to fEPSPs in neural pathways underlying peristalsis studied in vitro.

ATP and sensory transduction in the enteric nervous system

ATP is a neurotransmitter in the central and peripheral nervous systems and is also involved in peripheral inflammation and transmission of the sensation of pain. Recently, the regulated release of ATP from non-neuronal sources has been shown to play a role in the activation of sensory nerve terminals. Within the enteric nervous system, which is present in the wall of the gastrointestinal tract, ATP plays three major roles. ATP acts as an inhibitory transmitter from the enteric motor neurons to the smooth muscle via P2Y receptors. ATP is released as an excitatory neurotransmitter between enteric interneurons and from the interneurons to the motor neurons via P2Y and P2X receptors. Finally, ATP may act as a sensory mediator, from epithelial sources to the intrinsic sensory nerve terminals. Thus, ATP participates in the transduction of sensory stimuli from the gut lumen and in the subsequent initiation and propagation of enteric reflexes.

Intracellular cross talk and physical interaction between two classes of neurotransmitter-gated channels

Fast chemical communications in the nervous system are mediated by several classes of receptor channels believed to be independent functionally and physically. We show here that concurrent activation of P2X2 ATP-gated channels and 5-HT3 serotonin-gated channels leads to functional interaction and nonadditive currents (47-73% of the predicted sum) in mammalian myenteric neurons as well as in Xenopus oocytes or transfected human embryonic kidney (HEK) 293 cell heterologous systems. We also show that these two cation channels coimmunoprecipitate constitutively and are associated in clusters. In heterologous systems, the inhibitory cross talk between P2X2 and 5-HT3 receptors is disrupted when the intracellular C-terminal domain of the P2X2 receptor subunit is deleted and when minigenes coding for P2X2 or 5-HT3A receptor subunit cytoplasmic domains are overexpressed. Injection of fusion proteins containing the C-terminal domain of P2X2 receptors in myenteric neurons also disrupts the functional interaction between native P2X2 and 5-HT3 receptors. Therefore, activity-dependent intracellular coupling of distinct receptor channels underlies ionotropic cross talks that may significantly contribute to the regulation of neuronal excitability and synaptic plasticity.

Ligand-gated ion channels in the enteric nervous system

There are many cell surface receptors expressed by neurones in the enteric nervous system (ENS). These receptors respond to synaptically released neurotransmitters, circulating hormones and locally released substances. Cell surface receptors are also targets for many therapeutically used drugs. This review will focus on ligand-gated ion channels, i.e. receptors in which the ligand binding site and the ion channel are parts of a single multimeric receptor. Ligand-gated ion channels expressed by enteric nerves are: nicotinic acetylcholine receptors (nAChRs), P2X receptors, 5-hydroxytryptamine3 (5-HT3) receptors, gamma-aminobutyric acid (GABAA) receptors, N-methyl-d-aspartate (NMDA) receptors,alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and glycine receptors. P2X, 5-HT3 and nAChRs participate in fast synaptic transmission in S-type neurones in the ENS. Fast synaptic transmission occurs in some AH-type neurones, and AH neurones express all the ligand-gated ion channels listed above. Ligand-gated ion channels may be localized at extra-synaptic sites in some AH neurones and these extra-synaptic receptors may be useful targets for drugs that can be used to treat disorders of gastrointestinal function.