Every science writer loves a good challenge to dogma. I wish I had been in the working world in the spring of 1992, when one such intellectual overhaul happened in neuroscience. The dogma: Neurons, unlike most of the body’s cells, can’t be replenished. You’re born with just 100 billion of them and you better use them wisely. The challenge: Samuel Weiss and Brent Reynolds reported in Science that brain tissue taken from adult mice could be chemically coaxed into making new neurons.
“It left us speechless,” Weiss told the New York Times. Everybody else was pretty stunned, too. Over the next six years, other researchers confirmed that this so-called neurogenesis happens in the adult hippocampus of many animals, including tree shrews, marmosets, Old World monkeys and people. Today, more than two decades since the splashy Science report, adult neurogenesis is a bona fide subfield, with hundreds of labs studying it around the world.
But after all this time, researchers still don’t really know what it’s for. Studies have uncovered a wide variety of environmental stimuli — what you might think of as inputs — that affect neurogenesis in the dentate gyrus, a part of the hippocampus. Running and antidepressants can ramp up neurogenesis, for example, while stress, social isolation, sleep deprivation and aging can shut it down. Scientists have also looked at the outputs of neurogenesis, showing that a boost of new neurons may be important for exploratory behavior and certain kinds of learning, such as navigating a new space. But how do the inputs lead to the outputs?
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