Fishing for the neuronal origins of aggression

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Gymnotus omarorum, a weakly electric fish, swims in freshwater rivers throughout Uruguay. The fish use electricity to communicate, and they are highly territorial, making them ideal models in which to study the neurobiology of aggressive behavior, says Ana Silva, professor of neuroscience at the Universidad de la República in Montevideo. Because the fish defend their territory year round, Silva’s team has focused on how the hormonal circuits behind that aggression shift with the seasons—in particular, how neuropeptide levels and behavior change over the course of the year.

Nearly 40 years ago, Omar Macadar and Omar Trujillo Cenóz first recorded electric signals from Gymnotus omarorum, named in a combination of Greek and Latin for “naked back of the Omars.” After studying abroad and returning to Uruguay, the pair established the fish as a model species. Their work “opened a new chapter” in neuroethology research in Uruguay, Silva says. Weakly electric fish have since fueled studies into circadian rhythms and electroperception, particularly in Uruguay, but also in Brazil and Chile.

At first, Silva and her team brought electric fish into the lab. But over the past few years, they have also conducted field studies to inform their lab work, engineering a rig that can record the fish’s electric organ discharge in real time, completely noninvasively, in the lakes and rivers outside of Montevideo, where the fish dwell. Developed with collaborators at the University of Tübingen, the University of Berlin and Columbia University, the system relies on carbon rods driven through dense underwater vegetation and wired to a digitizer that logs each fish’s discharge continuously. Because every Gymnotus signal is distinct, the setup lets Silva’s team reconstruct an individual animal’s movements, and track territorial contests, without ever touching the fish. Tracking wild individuals purely by their electric signatures over multiple days, with no intervention at all, has been a “milestone” for the research, Silva says.

The team later collects blood and brain biopsies from field-caught fish to measure circulating hormone levels and quantify neuropeptide receptor expression by brain region. Brain-biopsy transcriptomics have revealed genes whose expression covaries with behavior, data that Silva says would be far weaker had the animals spent time in captivity, which dulls the behavioral and physiological signatures the researchers are after.

Silva describes the platform’s decade-long buildout as characteristic of doing science in the region: It was assembled in small, sometimes uncertain increments, not funded and built in one stroke. Silva is also finishing, with Adriana Migliaro, researcher at the Universidad de la República, and José Luis Peña, professor of neuroscience at the Albert Einstein College of Medicine, what she says will be the first Spanish-language neuroethology textbook, due out for students 1 September.