Effects of thio-group modification and Ca2+ on agonist-specific state transitions of a central nicotinic acetylcholine receptor.

Agonist-binding affinities of central nervous system nicotinic acetylcholine receptors (nAcChR) are sensitive to the duration of exposure to agonist. These 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 binding is approximately 1 mM for nAcChR treated with dithiothreitol (DTT) or DTT-N-ethylmaleimide (low-affinity states) and approximately 40 microM 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 these treatments alters toxin or antagonist binding nor are there observed differences in Hill numbers for agonist binding. Agonists competitively inhibit toxin binding to low-affinity states, but noncompetitive inhibition is observed for binding to high-affinity states. Values of AcCh dissociation constants estimated from these data fall within the range of values determined physiologically with nAcChR from other systems. The data indicate that the redox state of brain nAcChR thio groups and Ca2+ may mediate physiologically important changes in the receptor state during activation and desensitization.