Rhythmic spontaneous depolarizations determine a slow-and-fast rhythm in walking of the marine hypotrich Euplotes vannus

Abstract

In the marine hypotrich Euplotes vannus, the coupling of behavioural elements to shifts in the freely fluctuating membrane potential was investigated by use of a computerized locomotion track evaluation system. Potential shifts were attributed to bearing of cirri by single-image analysis of simultaneous video-tape recordings. Walking speed rhythmically decreases for 150-200 ms, and increases for 550-670 ms with a frequency of similar to 1.3/s. This pattern is caused by repetitive, spontaneous depolarizations of similar to 200 ms duration from about -25 mV to about -15 mV with a similar frequency. The speed is decreased by the combined reduction of the stroke amplitude of the fronto-ventral cirri and anteriad bending of the transverse cirri. Both the depolarizations and the walking pattern persist in a solution with a reduced Ca2- activity of 50 mu mol/l. Arhythmic spontaneous long depolarizations of 600-1200 ms duration induce a deceleration until movements stop and a fast backward jerking upon reversal of the beat direction of all cirri. This `'side-stepping'' (''avoiding'') reaction requires a high extracellular calcium concentration. Presumably, the depolarization rhythm is based on a pacemaking sequence of conductance changes of potassium.