Characterization of cultured dorsal root ganglion neuron P2X receptors

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Abstract
P2X receptors for adenosine 5′‐triphosphate (ATP) comprise a family of ligand‐gated cation channels with distinct characteristics which are dependent on the receptor subunits (P2X1–7) expressed, and the homomeric or heteromeric assembly of protein subunits in individual cells. We describe the properties of P2X receptors expressed by cultured adult rat dorsal root ganglion cells on the basis of the time course of responses to ATP, α,β‐methylene adenosine 5′‐triphosphate (α,β‐meATP) and 2‐methyl‐thioadenosine 5′‐triphosphate (2‐meSATP), and using the antagonists 2′,3′‐O‐(2,4,6‐trinitrophenyl) ATP (TNP‐ATP), a novel and highly selective purinoceptor antagonist, suramin and iso‐pyridocalphosphate‐6‐azophenyl‐2′,5′ disulphonic acid (PPADS). ATP (10 μm) evoked inward currents in ≈ 95% of neurons tested and > 80% responded with a fast transient inward current that rapidly inactivated during the continued presence of ATP. Of the remaining neurons, ≈ 4% showed a sustained response and ≈ 10% showed a combination of transient and sustained components. Rapid application of ATP, α,β‐meATP and 2meSATP demonstrated these to be full agonists of the rapidly inactivating P2X receptor (pA50 values = 5.83, 5.86 and 5.55, respectively), whilst uridine triphosphate (UTP) and 1‐β,γ‐methyleneadenosine 5′‐triphosphate (1‐β,γ‐meATP) were ineffective as agonists. These rapidly inactivating responses could be inhibited by TNP‐ATP, suramin and PPADS (pIC50 = 9.5, 6.5, 6.4, respectively). Using inactivation protocols, we demonstrate the presence of homomeric P2X3‐like receptors and non‐inactivating P2X receptors, which indicates that individual subsets of adult dorsal root ganglion neurons have distinct P2X receptor phenotypes, and that individual DRG neurons may express multiple P2X receptor subtypes.