Cocaine and phencyclidine inhibition of the acetylcholine receptor: analysis of the mechanisms of action based on measurements of ion flux in the millisecond-to-minute time region.

The effects of cocaine and of phencyclidine and procaine on acetylcholine receptor-controlled ion flux were measured in the millisecond-to-minute time region. Chemical kinetic measurements of ion flux were made in membrane vesicles prepared from the electric organ of Electrophorus electricus and in PC-12 cells, a sympathetic neuronal cell line. A quench-flow technique was used to measure ion flux in the millisecond-to-second range in membrane vesicles. Cocaine and phencyclidine both inhibit acetylcholine receptor-controlled ion flux, but by different mechanisms. Both compounds decrease the initial rate of ion flux, an effect observed with the local anesthetic procaine. This inhibition cannot be prevented by saturating concentrations of acetylcholine (1 mM). These results from chemical kinetic experiments are consistent with electrophysiological measurements which indicate that local anesthetics act by interfering with the movement of ions through receptor-formed channels. The chemical kinetic experiments, however, give additional information about the action of phencyclidine. They indicate that phencyclidine also increases the rate of receptor inactivation (desensitization) and changes the equilibrium between active and inactive receptor conformations, effects not observed in the presence of cocaine or procaine.