Young rats that use their brain keep more cells alive

Deep in the young rat brain, a seahorse-shaped structure churns out thousands of new cells destined to die just weeks later. But these new cells can escape death if the brain is kept busy learning a task, scientists report April 23 in Frontiers in Neuroscience.

The results show just how sensitive the young brain is to experience, says study coauthor Tracey Shors of Rutgers University in Piscataway, N.J. The surviving cells might help the fledgling animal learn how to survive on its own, she proposes.

Scientists have known for decades that parts of the brain are capable of producing new neurons throughout life, a process called neurogenesis. More recently, researchers have discovered that many of these newborn cells are destined for a quick death. “It’s kind of curious, actually,” Shors says. “You have all these cells being born, but a significant percentage of them die within a few weeks.” 

In adult rats, which produce fewer neurons than younger rats, these new neurons can be saved by learning, studies have found. But Shors and her colleagues wondered if the same was true for young rats, which produce two to four times as many new neurons as adults do. If learning a task can save most of these cells, the outcome would be a substantial neuron boost in the young brain. Such a boost “could have a pretty big impact on how the brain ends up after puberty,” Shors says.

Shors and her colleagues examined the hippocampus, a brain structure important for learning and memory. In prepubescent rats, the researchers injected a dye that labeled brain cells born within a few hours. A week later, the researchers counted these cells in the rats’ hippocampi . Two parts of the hippocampus, the granule cell layer and the hilus, were teeming with new cells. “It’s really astounding to look under the microscope,” Shors says.

Learning a difficult new task appears to protect most newborn cells against death. Some rats learned to respond to a sound by blinking an eye. Other rats were worse at the task. In the good learners, most of the new cells were still alive three weeks after they were born, the team found.  In poor learners, fewer of the cells survived, suggesting that learning itself may have been responsible for keeping these cells alive.

Neuroscientist Amar Sahay of Harvard Medical School calls the discovery of this cache of newborn cells in the hilus, a locale where new cells aren’t usually found in adults, intriguing. “It begs the question as to what these cells are doing and why.”

The study raises more questions than it answers, he says. It’s not clear whether the new cells are actually neurons, and if so, whether they integrate into the brain properly, he says. And scientists don’t yet know what these cells do, though other studies have also suggested a role in certain aspects of learning. Rats with increased rates of neurogenesis are better able to update memories with new information, for instance.

Human hippocampi also produce new neurons at a surprisingly high rate, recent studies have found. It’s possible, Shors says, that new experiences and learning opportunities during childhood might help these neurons survive.