Short chain and long chain alkanols have different sites of action on nicotinic acetylcholine receptor channels from Torpedo.
At nicotinic acetylcholine receptors, short chain n-alcohols (alkanols) have excitatory actions, whereas long chain alkanols inhibit channel activity. This study tests a previously proposed unitary hypothesis that suggests that these contrasting actions can be accounted for by interaction at just one hydrophobic site within the ion channel lumen. All alkanols bind to this site, but only long chain alkanols are large enough to completely block the channel. Short chain alkanols are too small to cause any channel occlusion, and in binding to the site they stabilize the open state of the receptor and enhance ion flux. In this study, we assay integrated agonist-stimulated ion efflux over 15 msec, as a measure of receptor activity. In nicotinic acetylcholine receptor-rich membrane vesicles from Torpedo, we show that, in contradiction to this elegant model, long chain and short chain alkanols appear to act at different sites. Firstly, ethanol and octanol do not compete for a single site on the receptor. Secondly, alkanol chain length dependencies for inhibition and for flux enhancement are significantly different. Thirdly, intermediate length alkanols do not partially inhibit channels, as required by the model; high concentrations of these alkanols completely inhibit the response. Fourthly, careful measurements, including determination of the free alkanol concentration, of inhibitory potencies of alkanols from propanol to decanol show no evidence for a steric contribution to the ability of an alkanol to inhibit the ion channel. Furthermore, our results suggest that the inhibitory effect of long chain alkanols may be mediated by a discrete site on nicotinic acetylcholine receptors, whereas there is no evidence that a protein site is involved in the excitatory mechanism of short chain alkanols. Indeed, it seems more likely that short chain alkanols may have a nonspecific site of action.
Wood
S C
SC
Department of Biochemistry, University of Oxford, England.
Forman
S A
SA
Miller
K W
KW
D000109:Acetylcholine / Q000494:pharmacology; D000438:Alcohols / Q000494:pharmacology*; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000187:drug effects*; D011978:Receptors, Nicotinic / Q000187:drug effects*; D012413:Rubidium; D013329:Structure-Activity Relationship; D013570:Synaptic Membranes / Q000187:drug effects; D014101:Torpedo
D000438:Alcohols; D007473:Ion Channels; D011978:Receptors, Nicotinic; D002217:Carbachol; D012413:Rubidium; D000109:Acetylcholine