It’s one of the most controversial issues in modern neuroscience and one that doesn’t have a clear-cut answer. But perhaps, instead of looking to answer this debate, we should start asking better questions, researchers say.
Cultured Rat Hippocampal Neuron. Image credits: ZEISS.
For decades, studies have come up with contradicting results — some have indicated that neurogenesis (the production of new neurons) does take place, at least in some areas of the brain, while other studies have found no evidence of this process.
This debate was beautifully illustrated in 2018 when two studies published just a few weeks apart made very convincing cases for both sides. Now, a new study attempts to reconcile the two camps by addressing methodological differences.
“It’s clear that there is a lot of controversy, which to me seems unwarranted because a yes or no for ‘is there adult neurogenesis’ is a little too simplistic and distracts us from other important questions,” says University of British Columbia professor Jason Snyder, lead author of the new study. “It’s worth asking if methodological differences are the only reason that some people aren’t finding new neurons or if there is some truth to the observations that neurogenesis may be limited with age in humans. I wanted to take a quantitative look at the research and see where it all leads.”
For instance, Snyder argues, the age at which neurogenesis is studied varies between different studies. The studies on mice are typically done on younger mice, whereas the studies on humans are carried out on older adults. In addition, primates and rodents develop most of their neurons at different times in their early development: human neuron populations peak during the first half of gestation, whereas mouse neurogenesis continues through birth or after birth. So again, these disparities could help reconcile the observed difference.
“The literature also indicates that if you look at a middle-aged rodent, it doesn’t have much neurogenesis either,” he says. “If we were to study the same in relative-aged human subjects, I don’t think the story is much different. For much of the adult lifespan, we’re not bursting at the seams with new neurons. While that may be disconcerting for people, it does reconcile the field: it’s not that some studies are right and some are wrong.”
The main takeaway seems to be that if adult neurogenesis happens, it happens at low rates, and only in some parts of the brain such as the hippocampus, where new memories are formed. Even if these neurons do exist, we’re still not exactly sure what these neurons do, if they have any medical significance, and to what extent this process is important for us. However, instead of trying to settle the debate, perhaps researchers should address the problem from a slightly different angle, Snyder says. Science is often not about winning a debate or being right — but rather about asking the right questions.
“The neurogenesis field is a great case study because it may be one of the most dramatic examples of progress ping-ponging back and forth over the course of 50 years,” he says. “This is just how science works, but we shouldn’t let our search for a smoking gun stop us from asking better questions.”
“We’ve gone through phases where people didn’t even study the topic because a paper concluded adult neurogenesis didn’t occur, which meant labs weren’t investigating its potential for human health,” Snyder says. “We need to appreciate the work of others–everyone is producing a lot of really sound and solid stuff. But rather than focus on winning the debate, we need to collaborate more to go after the truth. The field is still finding its way.”
The study “Recalibrating the relevance of adult neurogenesis” has been published in Trends in Neuroscience. https://www.cell.com/trends/neurosciences/fulltext/S0166-2236(18)30314-X
