Inhibition of the acetylcholine receptor by histrionicotoxin

Desensitization of the nicotinic acetylcholine receptor by diisopropylfluorophosphate

The interaction of diisopropylfluorophosphate (DFP) with the nicotinic acetylcholine (ACh) receptor of Torpedo electric organ was studied, using [3H]-phencyclidine ([3H]-PCP) as a reporter probe. Phencyclidine binds with different kinetics to resting, activated, and desensitized receptor conformations. Although DFP did not inhibit binding of [3H]-ACh or 125I-alpha-bungarotoxin (BGT) to the receptor recognition sites and potentiated in a time-dependent manner [3H]-PCP binding to the receptor's high-affinity allosteric site, it inhibited the ACh- or carbamylcholine-stimulated [3H]-PCP binding. This suggested that DFP bound to a third kind of site on the receptor and affected receptor conformation. Preincubation of the membranes with DFP increased the receptor's affinity for carbamylcholine by eightfold and raised the pseudo-first-order rate of [3H]-PCP binding to that of an agonist-desensitized receptor. Accordingly, it is suggested that DFP induces receptor desensitization by binding to a site that is distinct from the recognition or high-affinity noncompetitive sites.

Effects of histrionicotoxin derivatives on ion channels and acetylcholine receptor-channel complexes in bullfrog sympathetic ganglia

1. Four synthetic histrionicotoxin derivatives (H8-, H12-, C4H10- and C5H10-HTX) were applied to bullfrog (Rana catesbeiana) sympathetic ganglia and their effects were compared electrophysiologically. 2. The derivatives (60-100 microM) blocked both acetylcholine receptor-channel complex (ACh RC complex) and Na+ channel to cause a transmission failure. They also blocked K+ and Ca2+ channels. 3. Although all 4 derivatives exhibited similar effects, their potencies on respective ionic channels differed from one another. 4. Two types of (presumably subsynaptic and extrasynaptic) ACh RC complexes in ganglion cells were distinguished based on their differential sensitivities to HTX derivatives.