APICALLY ADMINISTERED CYTOCHALASIN-B AND CYTOCHALASIN-D DECREASES SENSITIVITY OF ELECTRORECEPTOR ORGANS IN THE NORTH-AMERICAN CATFISH, ICTALURUS-NEBULOSUS

The receptor cells of the ampullary electroreceptor organs of Ictaluru s nebulosus bear microvilli on the apical membrane. Whereas microvilli in mechanoreceptive hair cells and in chemoreceptor cells have a tran sduction function, the function of these membrane specializations in e lectroreceptor cells is not fully understood. To study the role of the microvilli of the electroreceptor cells, the ampullary electrorecepto r organs were apically exposed to the microfilament-disrupting agents cytochalasin B and D. Electrophysiological measurements showed that cy tochalasin caused a high decrease in sensitivity and a slight decrease in spontaneous activity. Exposure to cytochalasin B resulted in a str iking disorganization of the microvilli on the apical membrane of the electroreceptor cells. The most plausible explanation for the results is that treatment with cytochalasin mainly affects the actin filaments of the microvilli causing an increase of the resistance of the apical membrane. A high apical resistance results in a decrease of the volta ge over the basal membrane, which in turn reduces the sensitivity. The conclusion is that intact apical microvilli are necessary for proper functioning of ampullary electroreceptor organs. Alterations in microv illar properties, like surface area and ion channel conductancy might play a considerable role in the regulation of the sensitivity.