P2X receptor-mediated changes in cochlear potentials arising from exogenous adenosine 5′-triphosphate in endolymph

Abstract

Our previous studies have determined the presence of adenosine 5′-triphosphate (ATP) in the cochlear fluids and shown that extracellular ATP introduced into the endolymphatic compartment of the guinea pig cochlea has a significant dose-dependent suppressive effect on both endocochlear potential (EP) and cochlear microphonic (CM), which is mediated via P2 receptors. In the present study, the influence of P2 receptor agonists and antagonists on this suppressive effect was investigated to characterise the subtypes of P2 receptor mediating the ATP-induced effect on cochlear function. Using a double-barreled pipette attached to a pressure injector, small volumes (2–10 nl) of ATP (0.01–1 mM) and P2 receptor agonists or P2 receptor antagonists in artificial endolymph were introduced into the scala media of the first (basal) and third turns of the guinea pig cochlea, while the EP and CM were monitored. ATP and P2 receptor agonists (5×10 −14–1×10 −11 moles) reversibly decreased the magnitude of EP and CM in a dose-dependent manner, in the order: 2-methylthioATP>ATP>α,β-methyleneATP⪢adenosine 5′-diphosphate. The ATP-induced decline of EP and CM was significantly inhibited by P2 receptor antagonists in the order of potency: pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid⪢suramin>cibacron blue. Neither adenosine nor uridine 5′-triphosphate (2×10 −13–2×10 −11 moles) nor the P2 receptor antagonists on their own had any effect on EP and CM. The ATP effect on the potentials was greater at the third cochlear turn when compared to the first turn. These results provide evidence that in the endolymphatic compartment of the guinea pig, the extracellular ATP effect on cochlear function is likely mediated through an interaction with P2 receptors which assemble as ATP-gated ion channels.