Scientists have discovered a distinct class of neurons or brain cells that may control eating behaviours. Consuming fatty foods alters the activity of these neurons in mice, essentially “releasing the brake” on eating. And could explain why some people have a tendency to overeat. The findings were published on 28 June in Science (1).
Obesity has become a major health concern, affecting more than 500 million adults worldwide and is associated with type 2 diabetes, cardiovascular disease, autoimmune disorders, and even, cancer. Overeating is the largest determinant of obesity, writes Dr Stephanie Borgland, a neuroscientist at the University of Calgary in Canada, in an accompanying perspective (2). So, why do some people continue to eat even when they’re full?
To shed some light on the topic — and to perhaps, uncover how obesity develops — the team of researchers from the US and the UK, led Prof Garret Stuber at the University of North Carolina, sequenced RNA of neurons located in the lateral hypothalamus, an area in the lower part of the brain known to regulate feeding. Then, they compared gene expression patterns of cells from obese mice on a high-fat diet versus normal mice.
Interestingly, the researchers identified a subset of neurons in obese mice that express a signalling molecule called glutamate. So, the scientists used calcium imaging to observe brain activity in live mice by modifying the so-called glutamatergic cells to fluorescently glow when they take up calcium — which indicates neural firing or activity— and observing them under a fluorescent microscope. The study is the first to use this type of imaging to observe the long-term brain activity of animals.
Just 12 weeks after switching to a high-fat diet, the glutamatergic cells were around 80 per cent less active in response to sugary drinks. Moreover, the cells of lean mice were more active just after eating — which tells the mice to stop eating — compared to mice that had been fasting prior to feeding.
In other words, regularly consuming sugary and fatty foods can make these cells less responsive, which can lead to overeating. However, the authors note that there is still uncertainty surrounding whether changes in brain activity of mice results from the diet itself or the associated weight gain. They also note that the taste of the food — the sugary, fatty food tastes better than the blander alternative — could lead mice to consume more.
Nonetheless, the authors provide a compelling hypothesis. The brain cells certainly merit more attention and in the future, could provide a target for drug therapies for treating eating disorders and preventing obesity.
But this is definitely a long way off. And stimulating these neurons could have other undesirable and unpleasant effects. So, for now, more research is needed to determine exactly how glutamatergic neurons contribute to weight gain and further elucidate the contributions of certain brain cells to eating behaviours.
(1) Rossi, M.A. et al. Obesity remodels activity and transcriptional state of a lateral hypothalamic brake on feeding. Science (2019). DOI: 10.1126/science.aax1184
(2) Borgland, S.L. Releasing the brake on eating. Science (2019). DOI: 10.1126/science.aay0204