Have you ever wondered why just seeing food can make
your mouth start to water? By visualizing neuronal activity in specific areas
of the zebrafish brain, scientists at the National Institute of Genetics (NIG)
in Japan have revealed a direct link between visual perception of food and
feeding motivation. The study, published in the April 20, 2017 issue of Nature
Communications, suggests that "eating with the eyes" is deeply rooted
in evolution.
"In vertebrate animals, feeding behavior is
regulated by a brain area called the hypothalamus. The hypothalamic feeding
center integrates information about bodily energy requirements and
environmental food availability. Zebrafish, like humans, mostly use vision for
recognition of food or prey. It was not known how the hypothalamus receives
visual information about prey. We first demonstrated that neurons in the
hypothalamus do indeed respond to the sight of prey. Then we looked for neurons
in the visual system that responded to prey and discovered 'prey detector'
neurons in an area called the pretectum. Furthermore, we found a direct neural
link connecting the prey detector neurons to the hypothalamic feeding
center," Dr. Muto, the leading author of the study, explained.
The key to this discovery has been recent progress
in the development and improvement of the highly sensitive, genetically encoded
calcium indicator GCaMP, which can be used to monitor neuronal activity in the
form of calcium signals. Another important technology is the ability to control
the specific neurons in which GCaMP is expressed. This was critical for
recording distinct calcium signals from identifiable neurons.
Prof. Kawakami, the senior author, showed us his
zebrafish facility where thousands of fish tanks can be seen, each of which
contains genetically different fish that can turn on, or drive the GCaMP
expression in different types of cells in the brain or in the body. This
collection of driver fish lines is being used to study various tissues and cell
types by zebrafish researchers all over the world. Of the nearly 2,000 such
driver fish lines in the lab, two played important roles in the current study:
one for the imaging of the prey detector neurons, and the other for the feeding
center in the hypothalamus.
"Successful brain imaging was made possible
through development of our genetic resources on which I have spent more than
twenty years. This is the power of zebrafish genetics. This work showcases a
successful application of our genetic resources in the study of brain
function," Prof. Kawakami said.
"Our study demonstrates how tightly visual
perception of food is linked to motivational feeding behavior in vertebrate
animals. This is an important step toward understanding how feeding is
regulated and can be modulated in normal conditions as well as in feeding
disorders," Dr. Muto said.
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