A heat-induced depolarization of Paramecium and its relationship to thermal avoidance behavior

We have recorded from Paramecium a membrane depolarization in response to heat. This heat-induced depolarization is graded with the magnitude of the temperature change and can trigger action potentials. The mechanism for the Ca-action potential, localized in the cilia, is not needed in generating the heat-induced depolarization, since a ciliary Ca-channel mutant (pwB) and a deciliated wild type both show the same magnitude of depolarization as an intact wild type. The direction and magnitude of change in the membrane potential in response to heat is affected by the initial level of the membrane potential. Conditions which depolarize the wild type decrease both the heat-induced depolarization and thermal avoidance behavior. A mutant with defective thermal avoidance behavior, teaB, is naturally less polarized at rest (Satow and Kung 1981) but can produce heat-induced depolarizations equal to that of the wild type if hyperpolarized to the wild-type levels by injection of constant current. Both the mutant and the wild type have apparent reversal potentials at -5 to -20 mV in 1 mmol/l Ca²⁺, above which the response to heat becomes a hyperpolarization. In both the wild type and teaB, the size of the heat-induced depolarization parallels the strength of the behavioral response to heat as measured by individual or population assays of thermal avoidance.