On the excitatory effects of ATP and its role as a neurotransmitter in coeliac neurons of the guinea‐pig.

Abstract

SUMMARY1. The effects of ATP on neurons from guinea-pig coeliac ganglia were studied to evaluate the possibility that this nucleotide acts as an excitatory neurotransmitter substance.2. In experiments with intracellular microelectrodes, ATP ( > 100 nM) depolarized coeliac neurons from the resting potential and produced an increase in the membrane conductance. These excitatory effects of ATP were observed in isolated coeliac ganglia, in acutely dissociated neurons or in cultured neurons. ATP also produced membrane conductance increases in neurons clamped at the resting potential using a single electrode voltage clamp.3. When studied in the whole-cell configuration of the patch clamp (intracellular Cs' to block K+ currents; -50 mV holding potential), ATP evoked inward currents in a manner more potent and efficacious than acetylcholine (ACh).4. Whole-cell currents induced by ATP were inwardly rectifying and reversed at -13 mV in normal Na+ solutions. Changes in extracellular Na+ concentration altered the reversal potential in a manner predicted by the Goldman-Hodgkin-Katz bi-ionic equation with a ratio of Na+ to Cs+ permeability (PNa/PCs) = 0-6. 5. Single channel currents were evoked by ATP in excised (outside-out) patches. Current-voltage relationships for single channel currents exhibited inward rectification. The mean single channel conductance was 22 pS at -50 mV.6. Antagonists of ATP-gated channels (suramin, Reactive Blue 2) reduced the effects of ATP but not ACh.7. Antagonists at nicotinic receptors/ion channels (hexamethonium or tubocurarine) reduced the effects of ACh but not ATP.8. Excitatory synaptic currents were observed in cultures of coeliac neurons. Synaptic currents possessed similar current-voltage relationships to currents produced by ATP, were increased in frequency by K+ depolarization in a Ca'+-dependent manner, and were selectively antagonized by ATP antagonists.

ATP ‐ a fast neurotransmitter

“…By this method of defining the receptors, Pzx receptors have an agonist sensitivity order of cc&methylene ATP > ATP > 2-methylthio-ATP while the order for Pzu receptors is reversed. Pzy receptors will not be further considered here, though it is interesting to note that the ligand gated ion channels in ganglia [22,23,24], and locus coeruleus [12,25] have some aspects of the pharmacology of P,, receptors leading Illes and Niirenberg [26] to suggest that neuronal P,, receptors should be subdivided into Pzvo, (ligandgated ion channels) and Pzys (G-protein coupled receptors). We would suggest a functional definition such that P,, receptors are ligand-gated ion channel receptors while the P,, receptors are members of the family of G-protein coupled receptors.…”

“…These two receptor types are thus likely to mediate quite different forms of synaptic transmission. The ion channels formed by the ATP receptors (P2x, by our definition) which have been studied so far are non-selective cation channels [5,23,25] and so are likely to have characteristics similar to several other receptor-channels mediating fast excitatory synaptic transmission. In contrast, synaptic transmission mediated by Pzu receptors is likely to be neuromodulatory, reflecting the slower and more diverse functions performed by the G-protein coupled receptors.…”

“…In contrast, calcium influx through ATP receptor-channels would not be expected to be voltage dependent and so may occur even in the absence of cell depolarization. Moreover, the channels show strong inward rectification [23,27,28] and both the rectification and the driving force for calcium will result in maximal influx of calcium at more negative membrane potentials. The significance of these properties in terms of synaptic transmission will be discussed below.…”

“…The ligand gated ion channels, activated from the extracellular surface by ATP have been shown to be cation selective and to show marked inward rectification such that very little outward current flows at positive potentials [23,24]. In autonomic ganglion cells [22,23,3] the channels show very little selectivity between cations allowing all small monovalent cations (e.g.…”

“…In autonomic ganglion cells [22,23,3] the channels show very little selectivity between cations allowing all small monovalent cations (e.g. Na', K', Cs') to pass approximately equally.…”