Gymnotiform fishes are a diverse and widely distributed order of Neotropical fish frequently studied for their ability to generate and detect electric fields. These nocturnal fish use electrolocation as a means of navigating and foraging in dark, turbid waters. The waveforms and frequencies of their electric organ discharges (EODs) have species specific properties, allowing them to signal and identify conspecifics during social interactions. Previous studies have shown that several species are able to regulate different parameters of the EOD. One mechanism regulated by adrenocorticotropic hormone (ACTH) allows some species to increase the amplitude of their EOD in response to circadian rhythms and social stimuli. Amplitude changes involve the regulation of ion channels (primarily sodium channels-NaV1) in the membranes of electrocytes (electric cells). These ion channels are highly conserved throughout the evolutionary history of vertebrates, but may have undergone interesting adaptations in electric fishes in response to environmental pressures and sexual selection. The genus Brachyhypopomus is highly speciose with 28 currently described species. To the best of the authors’ knowledge, the EODs of all species are biphasic with the exception of one, Brachyhypopomus bennetti. We hypothesized that B. bennetti would also display circadian amplitude regulation. We tested the effects of ACTH in two species, B. bennetti and B. brevirostris (Gymnotiformes, Hypopomidae), by injecting fish with 1 µg of ACTH per gram of body weight and recording fish EODs in a recording chamber. We also used current-clamping techniques on single electrocytes, in order to determine the effects of ACTH on membrane potentials and immunolabeling to determine the location and distribution of sodium channels within electrocytes.
Results/Conclusions
We found variable effects of ACTH between species that are due to the unique properties of the waveforms each species produces. B. bennetti does increase its EOD amplitude, however, electrophysiological data suggest that this species is reverting toward an ancestral state.