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

Abstract

This study characterized P2X receptors in guinea pig ileum myenteric S neurons (n = 124) in vitro using electrophysiological methods. ATP or alpha,beta-methylene ATP (alpha,beta-mATP), an agonist at P2X(1) and P2X(3) subunit containing receptors, depolarized 103 neurons (85%). Pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid (10 micromol L(-1)) blocked ATP- and alpha,beta-mATP-induced depolarizations. ATP-induced depolarizations and fast excitatory postsynaptic potentials (fEPSPs) were reduced by trinitrophenyl-ATP (10 micromol L(-1)), an antagonist that can block P2X(3) receptors. Ivermectin (10 micromol L(-1)), a modulator of P2X(4) and P2X(4/6) receptors, had no effect on alpha,beta-mATP-induced depolarizations. In 58% of neurons, the alpha,beta-mATP induced-depolarization was followed by an afterhyperpolarization (AHP) (P2X-AHP). Under voltage clamp, alpha,beta-mATP induced an inward current followed by an outward current which reversed polarity at 0 and -80 mV respectively. The P2X-AHP was reduced in low extracellular Ca(2+) solutions. Blockers of large, intermediate and small conductance Ca(2+)-activated K(+) channels or voltage-gated K(+) channels did not inhibit the P2X-AHP. Half of the neurons exhibiting the P2X-AHP contained nitric oxide synthase (NOS)-immunoreactivity (ir). In summary, NOS-ir S neurons express P2X(3) subunit containing P2X receptors. P2X receptors couple to activation of a Ca(2+)-activated K(+) conductance that mediates an AHP. As P2X receptors contribute to fEPSPs, the P2X-AHP may modulate S neuron excitability during purinergic synaptic transmission.

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

“…It was shown that the P2X2 receptor was the dominant P2X receptor subtype in the myenteric plexus. Most myenteric S neurons in guinea pig small intestine expressed P2X3 receptors with about half of these being inhibitory motoneurons [571]. P2X5 receptors were present on nerve fibres that envelop ganglion cell bodies in the myenteric and submucous plexuses in mouse intestine, probably as heteromultimers with P2X2 receptors on enteric sensory neurons [587].…”

Purinergic receptors and synaptic transmission in enteric neurons

“…Inward currents induced by ATP acting at myenteric P2X receptors desensitize by 80% in 7 s [66]. In guinea-pig myenteric plexus, P2X receptors have also been shown to couple to a calcium-dependent conductance that mediates an afterhyperpolarization in some NOS-positive S neurons [67].…”

“…In a parallel study in the guinea pig ileum, all S neurons that were sensitive to ATP were depolarized by α,β-mATP [67] but only 17% of AH neurons were sensitive to α, β-mATP (J. Ren, personal observation). Previous pharmacological studies have found that α,β-mATP-sensitive P2X-receptor subtypes (P2X 1 or P2X 3 ) contribute to fast EPSPs [82].…”

Peripheral Nervous System

“…P2X 5 receptors have been shown to be present on nerve fibres that envelop ganglion cell bodies in the myenteric and submucous plexuses in mouse intestine, perhaps as heteromultimers with P2X 2 receptors on enteric sensory neurons . Most myenteric Sneurons in guinea pig small intestine express P2X 3 receptors and about half of these are on inhibitory motorneurons (Ren and Galligan 2007).…”