Abstract:The complexation of three guests containing 4,4'-bipyridinium redoxactive residues by p-cyclodextrin (p-CD) and its heptakis-(2,6-O-dimethyl) analogue (DM-b-CD) was investigated by means of voltammetric techniques. The three 4,4'-bipyridinium (viologen) derivatives used as guests were designed to be water-soluble in all three accessible oxidation states. The N-substituents chosen to enhance aqueous solubility were: 2-(2-(2-ethoxy)ethoxy)ethanol (guest l2 '), 6-hexanoate (guest 2) , and 3-propanesulfonate (guest 3). Detailed analysis of the voltammetric results by digital simulation

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Neuronal nicotinic receptors composed of the ␣3 and ␤2 subunits are at least 1000-fold more sensitive to blockade by ␣-conotoxin-PnIA than are ␣2␤2 receptors. A series of chimeric subunits, formed from portions of ␣2 and ␣3, were coexpressed with ␤2 in Xenopus oocytes and tested for toxin sensitivity. We found determinants of toxin sensitivity to be widely distributed in the extracellular domain of ␣3. Analysis of receptors formed by a series of mutant ␣3 subunits, in which residues that differ between ␣3 and ␣2 were changed from what occurs in ␣3 to what occurs in ␣2, allowed identification of three determinants of ␣-conotoxin-PnIA sensitivity: proline 182, isoleucine 188, and glutamine 198. Comparison with determinants of ␣-conotoxin-MII and -bungarotoxin sensitivity on the ␣3 subunit revealed overlapping, but distinct, arrays of determinants for each of these three toxins. When tested against an EC 50 concentration of acetylcholine, the IC 50 for ␣-conotoxinPnIA blockade was 25 Ϯ 4 nM for ␣3␤2, 84 Ϯ 7 nM for ␣3P182T␤2, 700 Ϯ 92 nM for ␣3I188K␤2, and 870 Ϯ 61 nM for ␣3Q198P␤2. To examine the location of these residues within the receptor structure, we generated a homology model of the ␣3␤2 receptor extracellular domain using the structure of the acetylcholine binding protein as a template. All three residues are located on the C-loop of the ␣3 subunit, with isoleucine 188 nearest the acetylcholine-binding pocket.Nicotinic acetylcholine receptors (nAChRs) are located at the neuromuscular junction and throughout the central and peripheral nervous systems. The nAChRs, together with GABA-, glycine-, and serotonin-gated ion channels, constitute a superfamily of receptors known as cys-loop receptors. These receptors are pentameric assemblies of subunits, each possessing an extracellular "cys-loop" (two cysteines, separated by 13 residues, forming a disulfide bond) (Corringer et al

Self-complexation of cyclodextrin-viologen conjugates can be controlled electrochemically through changes in the oxidation states of the viologen subunits as well as chemically when competing water-soluble hosts or guests are added to solutions of the conjugates.

Mercuric chloride exerted a biphasic modulatory effect on rat neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes as heteromers of the ␣3 or ␣4 and ␤2 or ␤4 subunits. The degree of modulation was subunit-dependent, with ␤4-containing receptors displaying greater potentiation and ␣4-containing receptors displaying greater inhibition. Thus, ␣4␤4 receptors displayed both robust potentiation and robust inhibition. During prolonged coapplication of HgCl 2 , first potentiation then inhibition of the acetylcholine (ACh) response was observed. Upon coapplication of 1 M HgCl 2 , a 2-fold increase in ACh-induced current was achieved in 55 Ϯ 1 s. With continued HgCl 2 application, the ACh response was slowly inhibited until, after 5 min, less than 10% of the initial response remained. By measuring potentiation at its peak and inhibition 5 min after the start of HgCl 2 coapplication, we obtained EC 50 and IC 50 values of 262 Ϯ 75 and 430 Ϯ 72 nM, respectively. HgCl 2 potentiation was voltage-dependent, increasing at more positive holding potentials. Upon washout of mercury chloride, potentiation reversed with a t 1/2 of 4.6 min. Inhibition reversed more slowly, with less than half the initial response recovered after 15 min of wash. Although free cysteine residues are common targets for mercury, elimination of all free cysteines located in the extracellular domains of the ␣4 and ␤4 subunits did not alter the effects of mercuric chloride. Potentiation and inhibition of neuronal nAChRs may occur through action at a transmembrane or cytoplasmic location after passive diffusion of mercuric chloride across the plasma membrane.