Light–dark-controlled changes in modulations of the electric organ discharge in the teleost Apteronotus leptorhynchus

The brown ghost, Apteronotus leptorhynchus, produces electric organ discharges (EODs) distinguished by their high degree of regularity. Transient modulations of these discharges occur spontaneously in two forms, chirps and gradual frequency rises (GFRs). Fish kept under a 12:12h light:dark cycle showed a pronounced variation in the rate of spontaneous production of these two types of modulatory behaviours. Chirps were 2.4–5.6 times more frequent at night than during the day, whereas the generation of gradual frequency rises was 1.4–2.0 times higher during the day than at night. Fish subjected to 5 days of constant light, followed by 5 days of constant dark, showed similar light–dark-dependent changes in the production of chirps and GFRs as observed during the normal 12:12h light:dark cycle. However, fish subjected to 5 days of constant dark, followed by 5 days of constant light, did not reduce chirp rate, or increase GFRs, during the light phase compared with the dark phase. These results suggest that the rate of production of spontaneous EOD modulations is determined largely by the level of light. Furthermore, exposure of the fish to extended periods of dark induces long-term changes in the propensity to produce spontaneous EOD modulations. We hypothesize that these long-term behavioural changes may be caused by structural alterations in the neural network involved in control of these behaviours.