Counting acetylcholine receptors in the electroplax. In: Molecular properties of drug receptors

Discovery of the First Neurotransmitter Receptor: The Acetylcholine Nicotinic Receptor

The concept of pharmacological receptor was proposed at the turn of the 20th century but it took almost 70 years before the first receptor for a neurotransmitter was isolated and identified as a protein. This review retraces the history of the difficulties and successes in the identification of the nicotinic acetylcholine receptor, the first neurotransmitter receptor to be identified.

Affinity labeling of the acetylcholine receptor in the electroplax: electrophoretic separtion in sodium dodecyl sulfate

Electroplax, single cells dissected from electric tissue of Electrophorus, are labeled in a two-step procedure: reduction by dithiothreitol followed by alkylation by the affinity label 4-(N-maleimido)-alpha-benzyltri-[methyl-(3)H]methylammonium iodide, either alone or in combination with [2,3-(14)C]N-ethylmaleimide. Electrophoresis in sodium dodecyl sulfate on polyacrylamide gel of an extract, prepared with this detergent, of single-labeled or of double-labeled cells results in a major peak of (3)H activity, with a mobility corresponding to a polypeptide of molecular weight 42,000. In addition, in the double-labeled samples, there is a unique peak in the ratio of (3)H to (14)C that is coincident with the (3)H peak. The electrophoretic patterns of extracts of cells in which affinity alkylation of the reduced receptor has been suppressed by dithiobischoline, an affinity oxidizing agent, by cobratoxin, an irreversible ligand, or by hexamethonium, a reversible ligand, show a considerably diminished peak of (3)H activity in the region of molecular weight 42,000. This is the predominant difference between the electrophoretic patterns of extracts of unprotected and of protected cells. Furthermore, extracts of cells protected with dithiobischoline before labeling with both tritiated affinity label and [(14)C]N-ethylmaleimide do not show the peak in the (3)H to (14)C ratio seen in the absence of protection. Thus, by several diverse criteria, the peak of (3)H activity corresponding to a molecular weight of 42,000 contains affinity-labeled acetylcholine receptor or receptor subunit.

Chemical events in conducting and synaptic membranes during electrical activity

Evidence has accumulated in recent years for the central role of proteins and enzymes in the function of cell membranes. In the chemical theory proposed for the generation of bioelectricity, i.e., for the control of the ion permeability changes of excitable membranes, the protein assembly associated with the action of acetylcholine plays an essential role. Support of the theory by recent protein studies in which the excitable membranes of the highly specialized electric tissue were used will be discussed. A scheme is presented indicating the possible sequence of chemical reactions that change ion permeability after excitation. A sequence of chemical events within the excitable membranes of the synaptic junctions, i.e., within the pre- and postsynaptic membranes, similar to that proposed for the conducting membranes, is presented in a second scheme as an alternative to the hypothesis of the role of acetylcholine as a transmitter between two cells.

In vitro excitation of purified membrane fragments by cholinergic agonists : III. Comparison of the dose-response curves to decamethonium with the corresponding binding curves of decamethonium to the cholinergic receptor

The reversible binding of(14)C-decamethonium (Deca) to excitable microsacs prepared from the electric tissue ofElectrophorus electricus is followed by an ultracentrifugal assay. α-Bungarotoxin, a snake venom toxin, blocks irreversibly the binding of(14)C-Deca. The displacement is partial. The fraction of(14)C-Deca displaced by α-bungarotoxin corresponds to molecules of Deca bound to the cholinergic receptor site, whereas the fraction of(14)C-Deca bound in the presence of α-bungarotoxin corresponds to molecules bound to the catalytic site of acetylcholinesterase (AcChE). The total number of cholinergic receptor sites is found to be close but not identical to the total number of catalytic sites of AcChE.On the same preparation of microsacs, the binding of(14)C-Deca and the permeability response corresponding to a given concentration of Deca are measured as a function of increased concentration of Deca. The dose-response curve and the binding curve superimpose almost exactly; in other words, the "apparent" affinity of Deca coincides with its "real" affinity. Displacement of(14)C-Deca byd-tubocurarine gives an "apparent" affinity ford-tubocurarine which coincides as well with its "real" affinity.The transport properties of the ionophore controlled by one Deca binding site are estimated.