Neurogenesis is a wonderful word that means our brains continue to grow new neurons throughout our lifetimes. Not long ago, the brain was thought of as a static hunk of tissue that stopped growing after a neuronal "pruning" period early in our lives.
With time, neuroscience research uncovered two parts of the brain that evidence neurogenesis: the hippocampus, associated with memory formation, and the olfactory bulb, associated with the sense of smell.
Now, a study has uncovered a third part of the brain that, at least in mice, shows positive signs of neurogenesis: the hypothalamus, associated with body temperature, metabolism, sleep, hunger, thirst and a few other critical functions.
The news about this particular form of neurogenesis, however, isn't so wonderful.
Researchers from the Department of Neuroscience at the Johns Hopkins University School of Medicine injected mice with a chemical that incorporates itself into newly dividing cells. They found that the chemical appeared in rapidly proliferating cells called tanycytes in the hypothalamus, and further tests confirmed that the tanycytes specifically produced new neurons and not other types 0f cells.
The research team then wanted to find out what these neurons do, so they studied the new hypothalamus neurons in mice that had been fed a high fat diet since birth. Since the hypothalamus is associated with hunger and metabolism, the team speculated that the neurons may be linked in some way to weight gain. Turns out, they were right.
At a very young age, the mice fed a high fat diet didn't show a difference in neurogenesis from young mice fed a normal diet. But when they became adults, the mice fed a high fat diet showed four times the neurogenesis of the normal mice, and gained significantly more weight and had much higher fat mass.
To make sure that the new neurons were actually correlating with the weight gain, the researchers killed the neurons in some of the mice with focused X-rays. Those mice showed far lower weight gain and body fat than those fed the same high fat diet, and even lower than mice that were more active.
In other words, it's clear that these neurons have a major impact on weight regulation and fat storage in mice -- and it's altogether possible the same holds true for us.
Further tests will have to be conducted to find out if that's the case, but from an evolutionary standpoint it would make sense. Dr. Seth Blackshaw, the lead researcher, comments that hypothalamic neurogenesis may be a mechanism that evolved to help wild animals survive and probably also our ancestors. "Wild animals that find a rich and abundant source of food typically eat as much as possible as these foods are generally rare to find."
But in a culture with an abundance of food, that formerly life-saving advantage can turn into a distinct disadvantage. Blackshaw explains, "In the case of the lab animals and also in people in developed countries who have an almost unlimited access to food, this neurogenesis is not at all beneficial as it potentially encourages unnecessary excessive weight gain and fat storage." In short, our diets may be training our brains to keep us fat.
On the upside, if these findings are confirmed in humans, they may eventually lead to a drug that blocks neurogenesis in the hypothalamus -- but we're a long way from there.
The study was published in the journal Nature Neuroscience.
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