Sexual behaviour in Euplotes raikovi is accompanied by pheromone-induced modifications of ionic currents

In the marine ciliate Euplotes raikovi, pheromone released by a complementa ry mating type (nonself pheromone) induces typical sexual behaviour, wherea s self pheromone released by the same mating type generally has no effect. Nonself pheromone evokes a reduction of the mean walking speed by 66 %, a t hreefold increase in the frequency and duration of long-lasting rest phases and a doubling in the number of side-stepping reactions. Consequently, tra nslocation is strongly reduced and the cells remain in a small area. This c ould increase the probability of finding a sexual partner for pair formatio n (conjugation), The usual pattern of rhythmic, spontaneous depolarizations controlling the walking rhythm is absent in nonself-pheromone-stimulated c ells. The remaining depolarizations arise from a 4 mV hyperpolarized membra ne potential and do not reach the usual amplitudes of 15-20 mV but only of 6-10 mV, In addition, the amplitudes of K+ currents are increased at depola rizations of more than 20 mV by at least 30 %. Hyperpolarization- and depol arization-activated Na+ current amplitudes are increased, whereas the Ca2current amplitude remains nearly unaffected.