Voltage- and time-dependent effects of phencyclidines on the endplate current arise from open and closed channel blockade.

The actions of phencyclidine [1-(1-phenylcyclohexyl)piperidine, PCP] and its morpholine analog [1-(1-phenylcyclohexyl)morpholine, PCM] on ionic currents of nicotinic acetylcholine receptors were studied at the neuromuscular junction of frog skeletal muscle and on embryonic rat muscle cells in tissue culture. PCP and PCM reduced the peak amplitude and the decay time constant of the endplate current (EPC). PCP produced a voltage-dependent curvature and a time-dependent hysteresis loop at negative potentials (at potentials from -50 to -150 mV). In contrast, PCM caused a depression of EPC peak amplitude, but the current-voltage relationship (+60 to -150 mV) remained linear. When PCP-modified EPCs were elicited in trains at hyperpolarized potentials the amplitudes of successive events were progressively decreased and the magnitude of the decrease was dependent on the level of hyperpolarization. At positive potentials the process was reversed; the amplitude increased with successive stimulations. The EPC decayed exponentially in the presence of PCP and PCM, with a shortened time constant of decay that was less dependent on membrane potential than control. PCP and PCM caused only a 20% decrease of the amplitude of the iontophoretically evoked acetylcholine potential, which was significantly different from that induced by the desensitizing alkaloid perhydrohistrionicotoxin. Both PCP and PCM reduced by 50% the mean channel open time obtained from rat myoballs, giving a potency ratio for PCP to PCM of 2.5. This relative potency was correlated with that obtained for the reduction in the decay time constant of the EPC (ratio = 2.2). The effects of PCP on the peak amplitude of the EPC seem to be related to a conformational change of the acetylcholine receptor occurring before channel activation and not to a receptor desensitization.