Ionic mechanisms of depolarizing and hyperpolarizing quinine receptor potentials in Paramecium caudatum

The ionic mechanisms of the depolarizing and the hyperpolarizing quinine receptor potentials in the ciliate Paramecium caudatum were examined by using a behavioral mutant strain. The depolarizing receptor potential was induced by stimulating the anterior end of the specimen, and the hyperpolarizing receptor potential by stimulating the posterior end. The amplitude of both the depolarizing and the hyperpolarizing receptor potentials increased linearly with logarithmic increase in quinine concentration applied. Threshold concentration for inducing the depolarizing receptor potential was lower than that for the hyperpolarizing one. The peak level of the depolarizing receptor potential shifted towards the depolarizing direction with increasing external Ca²⁺ concentration while that of the hyperpolarizing receptor potential shifted in the depolarizing direction with increasing external K⁺ concentration. Under voltage-clamp conditions, the specimen produced an inward current in response to anterior stimulation, and an outward current in response to posterior stimulation. Both the peak inward and the peak outward currents showed a linear relationship with membrane potential. Current-voltage relationships of the receptor currents indicated conductance increase during the application of quinine. The depolarizing quinine receptor potential appears to be produced by an activation of Ca²⁺ channels, and the hyperpolarizing quinine receptor potential by an activation of K⁺ channels. Accepted: 3 October 1997