The effects of temperature on the interactions between volatile general anaesthetics and a neuronal nicotinic acetylcholine receptor.
1. Completely isolated identified neurones from the right parietal ganglion of the pond snail Lymnaea stagnalis were investigated under two-electrode voltage clamp. Neuronal nicotinic acetylcholine receptor (AChR) currents were studied at low acetylcholine concentrations (< or = 200 nM). 2. Inhibition of the ACh-induced currents by three volatile general anaesthetics (halothane, isoflurane and methoxyflurane) and the specific inhibitor (+)-tubocurarine was studied as a function of temperature (over the range 4-25 degrees C). 3. The inhibition by the volatile anaesthetics increased (inhibition constants decreased) with decreasing temperature while the inhibition by (+)-tubocurarine did not change significantly near room temperature, but decreased at lower temperatures. The (+)-tubocurarine inhibition appeared to be competitive in nature and showed no significant voltage-dependence. 4. The van't Hoff plots (logarithms of the dissociation constants against reciprocal absolute temperature) were linear for the anaesthetics, but markedly non-linear for (+)-tubocurarine. From these plots, values for the changes in the standard Gibbs free energy delta G degrees water-->AChR, enthalpy delta H degree water-->AChR, entropy delta S degree water-->AChR and heat capacity delta Cp degree water-->AChR were determined. Tubocurarine was found to bind very much tighter to the receptor than the volatile anaesthetics due, entirely, to a favourable increase in entropy on binding. 5. A comparison between the temperature-dependence of the anaesthetic inhibition of the ACh receptor and that of general anaesthetic potencies in animals indicates that the temperature-dependence of animal potencies might be simply accounted for in terms of changes in anaesthetic/receptor binding.
Dickinson
R
R
Biophysics Section, Blackett Laboratory, Imperial College of Science, Technology and Medicine, South Kensington, London.
Lieb
W R
WR
Franks
N P
NP
D000109:Acetylcholine / Q000494:pharmacology; D018685:Anesthetics, Inhalation / Q000378:metabolism*; D000818:Animals; D018118:Chloride Channels / Q000187:drug effects; D008195:Lymnaea; D009474:Neurons / Q000378:metabolism*; D011978:Receptors, Nicotinic / Q000378:metabolism*; D013696:Temperature; D014403:Tubocurarine / Q000378:metabolism
D018685:Anesthetics, Inhalation; D018118:Chloride Channels; D011978:Receptors, Nicotinic; D000109:Acetylcholine; D014403:Tubocurarine