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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.

Noctiluca miliaris, a marine dinoflagellate, exhibits spontaneous flexion of its tentacle. The movement of the tentacle is always accompanied by perturbations of membrane potential, termed the ‘tentacle regulating potentials’ (TRPs) (Eckert and Sibaoka, 1967). Though the waveform of the TRPs is liable to variation, it consists of four successive basic components: (1) a Na+-dependent depolarizing (positive) spike, (2) a depolarizing plateau potential, (3) a Cl−-dependent hyperpolarizing (negative) spike, and (4) a long-lasting hyperpolarization (Eckert and Sibaoka, 1967; Oami et al. 1988). A slow flexion of the tentacle is seen during the plateau, and the flexion is suddenly accelerated when the negative spike occurs. The tentacle then extends during the long-lasting hyperpolarization. There is no correlation between the positive spike and the tentacular movement (Hisada, 1957; Oami et al. 1988).

The membrane potential responses of Paramecium caudatum to the external application of bitter substances were examined by employing conventional electrophysiological techniques. Mutant cells defective in voltage-gated Ca2+ channels were used to record the potential responses in the absence of contamination by Ca2+ action potentials. The cells produced a transient depolarization followed by a transient hyperpolarization in response to a rapid whole-cell application of chloroquine, strychnine nitrate or brucine. Of these chemicals, chloroquine was the most potent. Cells produced a simple depolarization in response to a localized application of test chemicals to the anterior region, whereas they produced a transient hyperpolarization in response to an application to the posterior region. Membrane potential responses to an application of chloroquine declined with repeated application. The presence of chloroquine in the external bathing solution strongly inhibited the membrane potential responses to an application of brucine or strychnine. However, the presence of chloroquine did not affect the membrane potential responses to an application of quinine. It is suggested that chloroquine, strychnine and brucine share a common component of their transduction pathways, but that the transduction pathway for quinine is different.

The topological distribution of the chemoreceptors to quinine in the membrane of a ciliate Paramecium caudatum were examined by conventional electrophysiological techniques. A CNR-mutant specimen defective in voltage-gated Ca channels produced a transient depolarization followed by a transient hyperpolarization and a sustained depolarization when 1 mM quinine-containing solution was applied to its entirety. A Ni2+-paralyzed CNR-mutant specimen produced a simple membrane depolarization in response to a local application of 1 mM quinine-containing solution to its anterior end, whereas it produced a transient membrane hyperpolarization in response to an application to its posterior end. An anterior half fragment of a CNR specimen produced a membrane depolarization whereas a posterior half fragment of the specimen produced a transient hyperpolarization upon application of 1 mM quinine-containing solution. Both anterior depolarization and posterior hyperpolarization took place prior to the contraction of the cell body. It is concluded that Paramecium caudatum possesses two kinds of chemoreceptors or two kinds of coupling of the same receptor to different signal transduction pathways to quinine which are distributed in different locations on the cell surface. Activation of the anterior receptor produces a sustained depolarizing receptor potential while activation of the posterior receptor produces a transient hyperpolarizing receptor potential.

Membrane potential responses of a ciliate protozoan Paramecium caudatum to the external application of quinine were investigated in relation to its motile activities. Wild-type specimens swimming in the reference solution did not enter into a quinine-containing (0.5 mM) test solution due to avoiding responses exhibited at the border between the two solutions, and therefore stayed in the reference solution (chemodispersal). Squirting of a quinine-containing test solution over a wild-type specimen evoked a train of action potentials superimposed on a depolarizing chemoreceptor potential. Squirting of a quinine-containing test solution over a CNR-mutant specimen defective in voltage-gated Ca2+ channel evoked only chemoreceptor potentials, which consisted of an initial transient depolarization, a following transient hyperpolarization and a sustained depolarization. A current-evoked action potential became larger in its amplitude and longer in its duration with the external application of quinine. Under the voltage-clamp condition, the fast inward current did not change whereas the delayed outward current decreased with the external application of quinine. It is concluded that quinine is a potent repellent for Paramecium because it produces a depolarizing chemoreceptor potential which evokes action potentials and prolongs the duration of the action potential.

Effects of the external Ca2+ concentration on the depolarization-induced transient inward Na+ current responsible for the Na+ spike in the dinoflagellate Noctiluca miliaris were examined. The peak value and the duration of the Na+ current increased when lowering the external Ca2+ concentration. The threshold potential level for activation and the reversal potential level of the current were not affected by the external Ca2+ concentration. The inactivation took place even in a solution containing EGTA with very low (

Depolarization-activated and hyperpolarization-activated ion conductances in the membrane of a marine dinoflagellate Noctiluca miliaris were examined under voltage-clamp conditions. Noctiluca exhibited a transient inward current in response to a step depolarization from a holding potential level of −80 mV to a potential level more positive than −50 mV. The I–V relationship for the current exhibited typical N-shaped characteristics similar to those of most excitable membranes. The current was inactivated by a membrane depolarization. The reversal potential of the current shifted in hyperpolarizing direction when the external Na+ concentration was lowered. The transient inward current is assumed to be responsible for the Na+-dependent positive spike in non-clamped specimens of Noctiluca. Noctiluca exhibited a transient outward current in response to a step hyperpolarization from a holding potential level of −20 mV to a potential level more negative than −30 mV. The I–V relationship for the current was a typical N-shape as if it was turned 180° around its origin. The outward current showed a two-step exponential time-decay. The outward current was inactivated by a membrane hyperpolarization. The reversal potential shifted in the depolarizing direction when the external Cl− concentration was lowered. The transient outward current is responsible for the Cl−-dependent negative spike in non-clamped specimens of Noctiluca.

The intracellular conditions required for contraction in the food-gathering tentacle of a marine dinoflagellate Noctiluca miliaris were examined in the isolated tentacle, treated with Triton X-100. The tentacle flexed to its full extent when the pH of the reactivation medium was lowered to 4·0, and extended when it was raised again to its original value. Adenosine triphosphate (ATP) was not needed for the pH-dependent flexion–extension. The flexion was also produced by Ca2+, but in a concentration range more than a thousand times higher than the effective H+ concentration range for producing the flexion. It is concluded that movement of the tentacle in a live specimen of Noctiluca is controlled by membrane potential-regulated cytoplasmic pH.

Membrane potential responses that regulate movement of the food-gathering tentacle ofNoctiluca miliaris (tentacle regulating potentials, TRPs) were examined electrophysiologically under various ionic conditions. These spontaneous TRPs were modified by changing the external ionic conditions. Positive spike appeared as external Ca2+ concentration was lowered. The peak of the spike became more positive with increasing external Na+ concentration. The spike could be evoked by injecting a depolarizing current when the membrane was hyperpolarized. The positive spike is assumed to be caused by regenerative activation of depolarization-sensitive Na channels. The peak of the negative spike, reported by previous workers, became more negative with increasing external Cl− concentration. The spike was evoked by injecting a hyperpolarizing current when the membrane was depolarized. The negative spike is assumed to be caused by regenerative activation of hyperpolarization-sensitive Cl− channels. The waveforms and amplitudes of the TRPs recorded from the nucleus were identical to those recorded from the flotation vacuole. This suggests that the TRPs are generated on the membrane facing the external solution. Possible roles of the TRPs in the control of tentacle movement are discussed.