Cholinergic ligand−induced affinity changes in Torpedo californica acetylcholine receptor

The binding of cholinergic ligands to Torpedo californica acetylcholine receptor has been studied in vitro by inhibition of the time course of 125I−labeled α−bungarotoxin−receptor complex formation. The extent of inhibition was dependent on the duration of exposure to the ligand, the apparent affinity for ligand increasing with time, and was reversible upon removal of ligand. Ligand concentration, temperature, and Ca2+ ions influenced this effect which is reminiscent of receptor desensitization in vivo. Such effects were observed both for a cholinergic agonist, carbamylcholine, and for an antagonist, bis(3−aminopyridinium)−1,10−decane diiodide. A minimal model is discussed which can account for these effects and for receptor ligand association leading to postsynaptic depolarizatio