Alcohol action on membrane ion channels gated by extracellular ATP (P2X receptors)

Extracellular adenosine 5'-triphosphate (ATP) has been reported to produce excitatory actions in the nervous system, such as excitatory postsynaptic potentials or currents in both central and peripheral neurons, via activation of a class of ATP-gated membrane ion channels designated P2X receptors. This article reviews studies of alcohol effects on these receptor-channels. Ethanol has been found to inhibit ATP-gated ion channel function by shifting the agonist concentration-response curve to the right in a parallel manner, increasing the EC50 without affecting Emax of this curve. To distinguish whether this inhibition involves competitive antagonism of agonist action or a decrease in the affinity of the agonist binding site, the kinetics of activation and deactivation of agonist-activated current were studied. Ethanol was found to decrease the time-constant of deactivation of ATP-gated ion channels without affecting the time-constant of activation, indicating that ethanol inhibits the function of these receptors by an allosteric decrease in the affinity of the agonist binding site. The inhibition of ATP-gated ion channel function by a number of alcohols was found to exhibit a distinct cutoff effect that appeared to be related to the molecular volume of the alcohols. For alcohols with a molecular volume of < or = 42.2 ml/mol, potency for inhibiting ATP-activated current was correlated with lipid solubility (order of potency: 1-propanol = trifluoroethanol > monochloroethanol > ethanol > methanol). However, despite increased lipid solubility, alcohols with a molecular volume of > or = 46.1 ml/mol (1-butanol, 1-pentanol, trichloroethanol, and dichloroethanol) were without effect on the ATP-activated current. This cutoff effect has been interpreted as evidence that alcohols inhibit the function of ATP-gated ion channels by interacting with a hydrophobic pocket of circumscribed dimensions on the receptor protein. To evaluate the localization of this presumed alcohol binding site, the effect of the intracellular application of ethanol was studied on the inhibition of ATP-activated current by extracellularly applied ethanol. The intracellular application of 100 mM ethanol did not affect the inhibition of current by 100 mM extracellular ethanol, suggesting that the alcohol inhibition of ATP-gated ion channel function involves the extracellular domain of the receptor. Finally, recent studies suggest that the alcohol sensitivity of ATP-gated channels may be regulated by physiological mechanisms.