Changes in electric organ discharge after pausing the electromotor system of Gymnotus carapo

During their entire lives, weakly electric fish produce an uninterrupted train of discharges to electrolocate objects and to communicate. In an attempt to learn about activity-dependent processes that might be involved in this ability, the continuous train of discharges of intact Gymnotus carapo was experimentally interrupted to investigate how this pausing affects post-pause electric organ discharges. In particular, an analysis was conducted of how the amplitude and relative timing of the three major deflections of the complex discharge change over the course of the first 1000 post-pause discharges. The dependence of these variables on the duration of the preceding pause and on water temperature is analysed. In addition, pause-induced small reverberations at the end of the discharge are described. Common to all amplitude changes is a fast initial decrease in amplitude with a slow recovery phase; amplitude changes scale with the duration of the preceding pause and are independent of the interdischarge interval. The absence of changes in the postsynaptic-potential-derived first phase of the discharge together with changes in the amplitude ratio of the third and fourth deflections suggest that the amplitude changes are mainly due to pause-induced changes in the inner resistance of the electric organ. A model is formulated that approximates the pattern of amplitude changes. The post-pause changes described here may provide a new way to test current models of complex discharge generation in Gymnotus carapo and illustrate the speed at which changes of an electric organ discharge can take place.