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

Abstract

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 E_max 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 ≤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 ≥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.

Introduction

Almost a century ago, independent research studies by an English graduate student and docent working in Zürich, Charles E. Overton, and a professor of pharmacology at the University of Marburg, Hans H. Meyer, revealed that the narcotizing potencies of alcohols and anesthetics are proportional to their lipid solubility (hydrophobicity). Such observations led to the idea that alcohols exert their primary action in a non-specific manner on membrane lipids, and only secondarily affect the function of membrane proteins. With many subsequent studies confirming the hydrophobic nature of alcohol’s effects in the nervous system, the ‘lipid theory’ of alcohol action became widely accepted over the past century (cf. Weight, 1992). In recent years, evidence has been accumulating that alcohols can affect the function of a number of different types of membrane ion channels. These membrane proteins mediate the generation of action potentials or postsynaptic potentials (PSPs) in neurons (cf. Hille, 1992). These proteins form a transmembrane ion channel that is opened by membrane voltage or when a specific type of neurotransmitter (or agonist) binds to a site on the extracellular surface of the protein. The open ion channel permits the flow of selective ions down their concentration gradient resulting in depolarization or hyperpolarization of the membrane potential to generate action potentials or postsynaptic potentials. The flow of charged ions through these channels can be detected using electrophysiological techniques, permitting detailed investigation of the function of these proteins. Consequently, these methods have been used in many recent studies on the effects of alcohols on the function of membrane ion channels. The papers in this issue of Neurochemistry International review alcohol actions on various types of membrane ion channels. This paper reviews studies of alcohol effects on membrane ion channels that are gated by extracellular adenosine 5′-triphosphate (ATP).

Membrane ion channels gated by extracellular ATP have been designated P2X receptors. These receptor-channels have recently received much attention due to their potential importance in neuronal function of the central and peripheral nervous systems. Activation of P2X receptors elicits excitatory postsynaptic currents (EPSCs) or excitatory postsynaptic potentials (EPSPs) in both central and peripheral neurons (Edwards et al., 1992, Evans et al., 1992, Silinsky et al., 1992, Galligan and Bertrand, 1994), and excitatory junction potentials in smooth muscle cells (Sneddon et al., 1982). Activation of P2X receptors also mediates excitatory responses in a variety of central and peripheral neurons (Krishtal et al., 1983, Bean, 1990, Fieber and Adams, 1991, Ueno et al., 1992, Li et al., 1993a, Li et al., 1993b, Shen and North, 1993, Khakh et al., 1995). Recent studies have revealed that these receptor-channels are highly sensitive to a number of endogenous agents, including Zn²⁺ (Cloues et al., 1993, Li et al., 1993b, Li et al., 1997c), Cu²⁺ (Li et al., 1996c), H⁺ (Li et al., 1996a, Li et al., 1996b, Li et al., 1997a), Mg²⁺ and Ca²⁺ (Krishtal et al., 1988, Nakazawa and Hess, 1993, Li et al., 1997b). Seven P2X receptor subunits, designated P2X1 to P2X7, have been cloned to date, and these subunits have been found to be widely distributed in the central and peripheral nervous systems, including cerebral cortex, hippocampus, thalamus, hypothalamus, midbrain, cerebellum, spinal cord and sensory ganglia (except for P2X3, which has been found only in sensory ganglia and sensory C-fibers) (Kidd et al., 1995, Buell et al., 1996, Collo et al., 1996, Séguéla). The studies reviewed here on alcohol effects on P2X receptors indicate that alcohols affect the function of these receptor-channels by a unique and specific mechanism that involves a direct alcohol interaction with the receptor protein. In addition, recent evidence suggests that the alcohol sensitivity of P2X receptors may be regulated by physiological mechanisms.