A study in the May edition of Nature Neuroscience reveals that Johns Hopkins researchers have found, in animal studies, that new nerve cells formed in a particular part of the brain could influence how much people eat and their consequent weight.
Leading researcher Seth Blackshaw’s PhD., states that it has been evident for a few decades that the brain continues to form new nerve cells (neurons) into adulthood, yet it was believed that this process (neurogenesis) only occurs in two areas of the brain, i.e. in the hippocampus and in the olfactory bulb. The hippocampus is associated with memory function, whilst the olfactory bulb is associated with smell.
Blackshaw, who is an associate professor in the Solomon H. Snyder Department of Neuroscience at the Johns Hopkins University School of Medicine, says they now discovered a third location in the brain that also forms new neurons, the hypothalamus. The hypothalamus is linked to various body functions, such as body temperature, sleep, hunger and thirst, although the precise source of the neurogenesis and the function of the new neurons have not yet been established.
Blackshaw and his team conducted a mice model to see if there were any high levels of cell growth in a particular location within the hypothalamus that would indicate neurogenesis after injecting the mice with an easily detectable compound called bromodeoxyuridine (BrdU) that selectively incorporates itself into newly replicating DNA of dividing cells. After a few days they discovered high levels of BrdU in the median eminence of the hypothalamus located at the base of the brain’s fluid-filled third ventricle.
After examining these rapidly proliferating cells the team discovered that they were tanycytes, bipolar cells that could be well suited to produce new neurons due to the fact that they share many characteristics with cells involved in neurogenesis during early development. Blackshaw and his team bred mice with a fluorescent protein that was only visible in the tanycytes in order to establish that it was indeed the tanycytes that produced the new neurons and not other cell types. Their finding that the cells were produced by tanycyte progenitors was confirmed when after a few weeks the new neurons also proved to be fluorescent.
Having established the neurons source the team started investigating its function. Based on earlier studies, which indicate that animals fed a high fat diet have a considerably higher risk of obesity and metabolic syndrome as adults, the team hypothesized whether hypothalamic neurogenesis may be involved in this phenomenon.
From when they were weaned, the team fed the mice on a high fat diet and periodically examined them for evidence of neurogenesis. They found that compared with animals fed on a normal diet, there was no difference to very young animals on a high fat diet, although as adults those fed on a high fat diet since weaning, neurogenesis quadrupled. They gained more weight and had a higher fat mass compared with those on a normal diet.
Blackshaw and his team killed off new neurons in the high-fat group by irradiating just their median eminences with precise X-ray beams and observed a considerable lower weight gain and fat compared with those who were fed the same diet and that were considerably more active. This indicates that these neurons have a critical impact on weight regulation, fat storage and energy expenditure.
Blackshaw explains: “People typically think growing new neurons in the brain is a good thing – but it’s really just another way for the brain to modify behavior.”
He continues saying that hypothalamic neurogenesis may be a mechanism that evolved to help wild animals to 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.
Having unlimited food supplies during youth and therefore prompting the growth of new neurons, which promote food intake and energy storage at a later time would be of a advantage, but Blackshaw explains that 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.
Blackshaw states that should his findings be confirmed in further studies, they may prove important for developing a new treatment for obesity either by blocking hypothalamic neurogenesis through irradiating the median eminence or by designing drugs that can block this process.
Written By Petra Rattue