Alcohol effects on the 5-HT₃ ligand-gated ion channel

Abstract

1. Alcohols and volatile anesthetics have both potentiating and channel blocking effects on members of the nicotinic ACh-like subfamily of ligand-gated ion channels. 2. The alcohol and anesthetic sensitivity of the 5-HT₃ receptor, a member of this receptor subfamily, has been examined extensively by several laboratories. 3. Alcohols and volatile anesthetics potentiate receptor function. 4. This potentiation is characterized by an increase in the apparent potency with which agonists activate the receptor, and an apparent stabilization and favoring of the open channel state. 5. Long-chain alcohols and trichloroethanol also inhibit channel function, perhaps through a channel blocking mechanism similar to that seen at the nACh receptor. 6. Studies of the relationship between receptor structure and alcohol and anesthetic effects are just beginning, but some clues from other members of this subfamily of ligand-gated channels may help to pinpoint areas of the receptor that are important for alcohol effects.

Introduction

Alcohols have potent effects on several types of ligand-gated ion channels, and there is strong evidence that these effects contribute to the intoxicating and general anesthetic effects of acute alcohol exposure (Sanna and Harris, 1993, Zhou et al., 1998). Ligand-gated ion channels may also play important roles in alcohol tolerance and dependence in the central nervous system. Three major classes of neurotransmitter-activated ligand gated ion channels have been identified based on comparisons of the primary amino acid sequences and predicted membrane topologies of different receptor types. These are the nicotinic ACh-like receptors, the ionotropic glutamate receptors and the P2x purinergic receptors. Alcohol effects have been examined on members of each of these three receptor subfamilies, and different alcohol actions have been noted for each subfamily (Zhou et al., 1998). The focus of this review will be on the 5-HT₃ receptor, a neurotransmitter-activated ligand-gated ion channel that is a member of the subfamily of nicotinic ACh receptor-like subfamily of receptors.

The 5-HT₃ receptor is the only mammalian ligand-gated ion channel activated by the indolamine 5-HT (Jackson and Yakel, 1995). The receptor forms a cation channel that is predominantly selective for monovalent cations, with some permeability of Ca²⁺ as well. As is the case for most members of the ligand-gated ion channel superfamily of neurotransmitter receptors, the 5-HT₃ receptor appears to be formed by the confluence of several individual protein subunits that coassemble to form a protein that can bind the neurotransmitter and define the transmitter-gated ion pore. In the case of the nAChR-like receptors, including the 5-HT₃R, strong evidence supports a pentameric receptor assembly (Green et al., 1995). Each of the subunits that make up the receptor are integral membrane proteins containing four hydrophobic membrane-spanning domains, a large n-terminal extracellular domain and smaller hydrophilic intra- and extracellular domains linking the putative membrane spanning domains (Maricq et al., 1991). The n-terminal domain is strongly believed to participate in ligand binding and transduction of binding energy into channel gating. The membrane-spanning domains are thought to line and define the channel pore and participate in channel gating on a rapid time scale. Only two subunits that can participate in the formation of 5-HT₃ receptors have been identified to date, and these two subunits appear to be alternatively spliced forms of the same gene product (Hope et al., 1993). Thus, the 5-HT₃R is thought to exist mainly as a homomultimeric receptor channel assembly in neurons.

Alcohols have been shown to have consistent actions on several members of the nAChR-like family of ligand-gated channels. One common action is potentiation of receptor-mediated ion current by EtOH and other alcohols. This action has been observed at GABA_A, glycine and certain types of nACh receptors (Celantano et al., 1988, Aguayo, 1990 Nakahiro et al., 1991, Liu et al., 1994, Peoples and Weight, 1994, Wu et al., 1994, Machu et al., 1996, Mascia et al., 1996, Mihic et al., 1997). Alcohols produce an increase in the apparent potency with which receptor agonists activate these receptors which can be seen as a leftward shift in the agonist dose–response curve. Longer chain alcohols have been reported to inhibit the function of some of these receptors (Forman et al., 1995, Wood et al., 1995). The inhibitory effect of long-chain alkanols on several nACh receptors involves an open-channel blocking mechanism (Forman et al. 1995). The homomultimeric nature, similarity to the nACh receptor and reasonably slow channel gating kinetics of the 5-HT₃ receptor simplify many of the studies of receptor channel kinetics and structure/pharmacology analysis. Thus, we have chosen to examine alcohol effects on the 5-HT₃ receptor with the hope of using this relatively simple ligand-gated ion channel as a model protein to understand alcohol effects on all members of this receptor class. Recent studies have suggested that this receptor is also quite sensitive to the actions of volatile anesthetics (Machu and Harris, 1994, Jenkins et al., 1996), and thus it is worth considering this receptor as a molecular model of a class of volatile anesthetic targets as well.