Until the 1970s, researchers believed that adult animals' brains were pretty much ‘fixed’. But in recent decades, neuroscientists have discovered just how flexible adult brains are. Exposure to complex surroundings can even boost the birth of new brain cells – at least in birds and mammals. Kent Dunlap, a biologist at Trinity College, Hartford, USA, wondered whether the same would be true for fish (p. 794).
To find out, Dunlap decided to take a closer look at electric knifefish brains. Studying electric fish has two advantages, he says. First, electric fish navigate and communicate using specific brain regions that generate and process electric signals; that is, certain brain regions help fish cope with the challenges posed by their physical and social environments. Second, fish take on the temperature of their surroundings – allowing Dunlap to examine whether seasonal changes in environmental temperature influence brain cell birth rates.
Teaming up with Ana Silva at the Instituto de Investigaciones Biológicas Clemente Estable in Uruguay and Mike Chung at Trinity College, Dunlap set out to see how three different environments – natural, semi-natural and isolated – affect brain cell birth rates across different brain regions in electric knifefish. The team first headed to lake Laguna Lavalle in Uruguay to study fish in their natural home during the breeding season. To label newborn brain cells, they caught wild fish and injected them with bromodeoxyuridine (BrdU), an analogue of one of the building blocks of DNA that is incorporated into dividing cells. Then they froze the fish brains, sliced them into thin sections and used anti-BrdU fluorescent antibodies to make the newborn brain cells glow, so that they could count how many there were in different brain sections. To compare brain cell birth rates in wild and captive fish, the team also took fish back to the lab. They housed some fish in groups in small paddling pools and others alone in aquaria. To see how fish brains respond to changing seasons, the team repeated the brain cell labelling process for all three fish populations in the non-breeding season a few months later.
‘After just 2 days of looking at brain sections, it was clear that seasonality had a huge effect,’ Dunlap recalls. During the breeding season, Silva explains, fish had 3–7 times more newborn brain cells than during the non-breeding season. ‘This suggests that warm temperatures and long day lengths not only trigger reproduction in this temperate zone species but may also increase brain cell proliferation,’ says Silva.
But did differences in the physical and social environment also affect brain cell birth rates? Sure enough, just as in birds and mammals, more brain cells are born in fish living in a complex environment; the team found that lake-dwelling fish had higher brain cell birth rates across all brain regions than captive fish. When the team examined the brain sections more closely, they saw that socially housed fish had more newborn brain cells than lonely fish – but only during the breeding season, and only in brain regions involved in electrocommunication. In other words, when they need to woo prospective partners, electric fish pump up brain cell production in those brain regions that boost social signalling prowess. ‘Small changes like living socially have effects on specific brain regions, while big changes like seasonality cause global changes across the whole brain,’ Dunlap concludes and adds, ‘when the global seasonal effect combines with the specific social effect, brains produce cells especially rapidly.’