Effects of thio-group modification and calcium(2+) ion on agonist-specific state transitions of a central nicotinic acetylcholine receptor

Publisher Website
Google Scholar
Abstract
Agonist-binding affinities of rat CNS nicotinic acetylcholine receptors (nAcChR) are sensitive to the duration of exposure to agonist. The agonist-induced changes in receptor state may be mimicked by appropriate modification of receptor thio groups and/or by manipulation of solvent ionic composition. In the absence of Ca2+ the concentration of acetylcholine (AcCh) necessary to prevent half of specific 3H-labeled .alpha.-bungarotoxin [Bungarus multicinctus] binding is .apprx. 1 mM for nAcChR treated with dithiothreitol (DTT) or DTT-N-ethylmaleimide (low-affinity states) and .apprx. 40 .mu.M for nAcChR treated with DTT-5,5''-dithiobis(2-nitrobenzoic acid) or for native nAcChR pretreated with AcCh (high-affinity states). Addition of Ca2+ results in an increase in the effectiveness of AcCh toward blocking toxin binding. None of the treatments alters toxin or antagonist binding nor are there differences in Hill numbers for agonist binding. Agonists competitively inhibit toxin binding to low-affinity states; but noncompetitive inhibition is seen for binding to high-affinity states. Values of AcCh Kd estimated for the data fall within the range of values determined physiologically with nAcChR from other systems. The redox state of brain nAcChR thio groups and Ca2+ may mediate important changes in the receptor state during activation and desensitization.