P2X₂receptors are essential for [Ca²⁺]_iincreases in response to ATP in cultured rat myenteric neurons

Abstract

]i similar to that induced by P2Y agonists. At Ϫ60 mV, ATP evoked a slowly inactivating inward current that was suppressed by the removal of extracellular Na ϩ concentration. The current-voltage relation for ATP showed an inward rectification with the reversal potential of about 0 mV. The apparent rank order of potency for the purinoceptor agonist-induced increases of [Ca 2ϩ ]i was ATP Ն adenosine 5Ј-O-3-triphosphate Ն CTP Ն 2-methylthio-ATP Ͼ benzoylbenzoyl-ATP. A similar potency order was obtained with current responses to these agonists. P2 antagonists inhibited inward currents induced by ATP. Ca 2ϩ and Mg 2ϩ suppressed the ATP-induced current, and Zn 2ϩ , Cu 2ϩ , and protons potentiated it. RT-PCR and immunocytochemical studies showed the expression of P2X 2 receptors in cultured rat myenteric neurons. These results suggest that ATP mainly activates ionotropic P2X 2 receptors, resulting in a [Ca 2ϩ ]i increase dependent on [Ca 2ϩ ]o in rat myenteric neurons. A small part of the ATP-induced [Ca 2ϩ ]i increase may be also mediated via a P2Y receptor-related mechanism. enteric neuron; fura-2; patch clamp; purinoceptor THE ENTERIC NERVOUS SYSTEM (ENS) plays a key role in controlling various gastrointestinal functions, including motor activity, secretion, absorption, and local circulation (60). The ENS is composed of functionally different neurons that contain a variety of potential neurotransmitters and express receptors for these substances (20). Accumulation of ATP in the extracellular space evokes various responses in neurons, immune/inflammatory cells, smooth muscle, and glandular epithelium by activating different P2 purinoceptor subtypes (10, 49). In the gastrointestinal tract, ATP was first recognized as an inhibitory neurotransmitter released from myenteric motor neurons to relax smooth muscles (16). In enteric neurons of the guinea pig, a part of the fast excitatory postsynaptic potentials (fEPSPs) is suppressed by a nicotinic receptor antagonist, and the remaining fEPSPs are further reduced by P2 purinoceptor antagonists (22), indicating the presence of purinergic excitatory neurotransmission in the ENS (45). An experiment using mice lacking the P2X receptor subtype suggested that ATP was involved in fEPSPs (51). Moreover, it has recently been reported that ATP functions as a putative sensory mediator from epithelial sources to the intrinsic sensory nerve terminals (3). Therefore, purinergic signaling in the ENS is considered to be important in the regulation of gastrointestinal functions (17).P2 purinoceptors are identified as ionotropic and metabotropic receptors based on their pharmacological properties, mechanisms of signal transudation, and deduced amino acid sequences (32). Ionotropic P2 purinoceptors, P2X, are ligandgated nonselective cation channels that are permeable for Na ϩ and Ca 2ϩ (44). Seven P2X receptors, identified as P2X 1 -P2X 7 , have been cloned. In the ENS, P2X 2 (11, 58), P2X 3 (48), P2X 2 and P2X 3 (59, 61), and P2X 7 (30) have been detected immunohistochemically. It has...

Structural insights into the nucleotide base specificity of P2X receptors

“…1A,B), but not GTP- and UTP-evoked currents (Fig. 1C,D), consistent with the previous electrophysiological analyses of P2X receptors1617182223242526.…”

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

“…Studies using P2X2 receptor knockout mice showed that P2X2 receptors contribute to fast synaptic excitation of myenteric neurons in small intestine [572]. P2X2 homomeric receptors appear to be the predominant receptors mediating fast synaptic excitation in the gut [248,250,524]. Intrinsic sensory neurons in the gut, identified as Dogiel type II neurons, express P2X2 receptors [237].…”

Pharmacology of P2X channels

“…Several studies have proposed a role for homomeric P2X 2 , and possibly heteromeric P2X 2 -containing channels, in ATP-mediated facilitation of inhibitory γ-amino butyric acid-mediated (GABAergic) synaptic transmission in the hippocampus and dorsal horn [8,22,133,167]. P2X 2 is also heavily expressed in the peripheral nervous system on both sensory and autonomic ganglion neurons [45,59,63,201,202,270,313,330,[336][337][338][339], signifying roles in afferent and efferent signaling pathways, and in the enteric nervous system where homomeric P2X 2 channels are thought to mediate fast synaptic excitation on S-type myenteric neurons [48,94,233,250,340]. Numerous recent studies have implicated both peripheral and central P2X 2 channels in chemosensory transduction in a variety of physiological systems, including the regulation of respiratory control in response to hypoxia and hypercapnia (via sensory neurons within neuroepithelial bodies and the carotid body and the ventrolateral medulla) [107,121,205,242,258,335], and in the detection of chemical stimuli, such as odorants (via trigeminal neurons in the nasal epithelium) [279], and taste (by gustatory nerves) [86].…”