Fluorescence microscopy image of the hypothalamus arcuate nucleus with POMC neurons in green
(Image: Alicia G Gómez-Valadés / Institute for Research in Biomedicine Barcelona).
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Does our stock of neurons diminish inevitably in adulthood? No. Over the last few years, public research has revealed continuous production of neurons throughout the life in the brain of most mammals, including humans. This phenomenon is called adult neurogenesis. It puts an end to the belief that governed neurosciences for more than a century, that neurons only reproduce during our brain’s development phase, until adolescence. Adults also manufacture juvenile neurons, using stem cells that have the ability to differentiate into specialist cells. Reservoirs have been located in two regions of the brain: the hippocampus, where new memories are formed, and the ventricle wall, the cavity where cerebrospinal fluid circulates. In rodents, we know that once neurons are produced, they leave the reservoir, migrate and integrate a pre-existing functional neuronal network.
What is the physiological role of this neurogenesis? Juvenile neurons have unique features that suggest they fulfil a specific function in the operation of the brain. In the hippocampus, they seem to play an important role in learning and in spatial awareness. Other studies have shown that stem cells located on the ventricle wall differentiate into neurons in the olfactory system. They migrate towards the olfactory bulb, which in the brain is the first relay for interpreting olfactory stimuli, and are therefore essential in learning and memorising smells. We also know that stress or depression can reduce neurogenesis and that social interactions, physical exercise and learning can all increase the production and survival of these young neurons.
Recently, a team from the University of Columbia in New York, went even further. They showed that in the hypothalamus (the region which controls the production of hormones and the physiological condition of our brain), “proopiomelanocortin” neurons (POMC neurons) connect to the anterior ventral of the ventricle wall, where they release endorphins. This allows them to control the proliferation of a special sub-population of stem cells, which then differentiate into neurons in the olfactory bulb. POMC neurons are activated by food intake and their activity drops during fasting. Thus, reducing food intake reduces the production of a specific group of neurons in the olfactory bulb. It is highly likely that there are numerous other control mechanisms for other categories of juvenile neurons.
What does such neuronal control mean? It could be interpreted as adaptation to the environment. A change in the availability of food could for example require special olfactory methods. By controlling the production of a certain type of juvenile neurons in the olfactory bulb, rodents could be better equipped to detect the smells associated with food, to the detriment of a loss of discrimination in their congeners. On the contrary, during reproduction periods, they may have special olfactory needs to recognise the right partners, requiring fine adjustment of this system’s neuronal network.
This study underlines the idea of “on demand” neurogenesis, which would model our brain according to specific needs. Although our fundamental research is a long way from finding an immediate application, the idea that our brains could potentially regenerate as and when needed is fascinating. If we could one day manipulate and reroute these new neurons, we could envisage new therapies, particularly to repair regions damaged by neurodegenerative diseases or brain injuries.
Mariana Alonso is a neuroscientist at the Laboratory of Perception and Memory at Institut Pasteur. Her work looks into neurogenesis and brain plasticity in adults, the neuronal bases of smell linked to behaviour and the relationships between the microbiota, immunity and the brain.