Unlike in other organs in the body, in the adult brain, new cells form throughout our lifetimes – creating new opportunities to learn. Turns out that the same region of the brain where new nerve cells are generated is the same region of the brain involved in distinguishing events in our memories. Researchers are now studying these new cells to better understand how they change over time and affect our decision-making.
Fred “Rusty” Gage of the Salk Institute is a pioneer in the study of “neurogenesis” – the creation of new nerve cells, or neurons, in the brain. He kicked off the 20th annual meeting of the Cognitive Neuroscience Society (CNS) on April 13 with a presentation on how his team is using techniques from the field of genetics, as well as computer modeling, to uncover how environmental stimuli affect neurogenesis. Gage received the 19th George A. Miller award, a CNS award that celebrates the integration of interdisciplinary approaches in the field.
For years, the idea of the creation of neurons in the adult brain seemed preposterous to many. If adult humans were able to produce new neural cells throughout their lifetimes, rather than only once as embryos, how would they build on their experience and learn over time?
Gage put those questions to rest in 1998 when he and his team demonstrated that neurogenesis happens over the course of a person’s life. He showed that neurons don’t come from existing nerve cells. Rather, they arise from stem cells. And they don’t form new networks. Instead, they find a synapse that already exists and, over time, they gradually replace the neuron involved in that synapse. As a neuron ages, researchers have found that it secretes a chemical called glutamate, which trips sensors in fledgling neurons, spurring them to differentiate. This process explains how new neurons can be formed to replace older ones that might not function as well, and at the same time, how an organism can build up a storehouse of learning over its lifetime. Inevitably, that discovery led to more questions.
“It’s not just that a new neuron is being born. It’s that it’s integrating [into the hippocampus]. Presumably, if it’s becoming a fully fledged neuron, it’s receiving synaptic input,” Gage said during his talk yesterday. “How could it possibly do that, and does it actually do that?”
Gage’s more recent work has shown that these new neurons become fully functioning members of the hippocampus in a remarkably short period of time – as little as two months. The neurons form in a very specific part of the brain called the dentate gyrus. That is the same spot in the brain responsible for “pattern separation,” a process by which we distinguish between two (or more) related events in creating memories. The brain has to process new information, while also comparing it to information gleaned from past experience.
Gage also discussed the role an organism’s environment plays in how many stem cells survive. In rats, research shows that enrichment in the form of unique living environments, such as mazes, can drastically ramp up production of neurons. He cited one study that showed that even the simple act of running can increase production by four to five times. However, if there’s no reason for the stem cells to differentiate – that is, nothing is stimulating the need – as many as half of the stem cells may die before they ever become neurons, depending on the organism and its surroundings.
The current thrust of Gage’s research centers on a sort of learning that happens to each neuron throughout its life. Researchers know that neurons go through a period of “hyper-excitability” early on between 4 to 6 weeks of the cell’s age. Now, Gage’s team is using a computational model to uncover how a neuron responds as it grows older, hypothesizing that cells don’t stop learning.
“So, throughout life, the cells are going through a critical period, acquiring information about the environment and their surroundings, and they encode that as they mature,” Gage told the audience. “And that’s used, in part, to assess future decision making.”