Nav: Home

Running away from carbon dioxide: The terminal connection

January 30, 2018

Like us, fish need oxygen, and swimming through a patch of carbon dioxide turns out not to be a pleasant experience. Instead, they prefer to avoid carbon dioxide altogether. In experiments published in Cell Reports on Jan. 30, researchers at the RIKEN Brain Science Institute in Japan have discovered a neuronal pathway that makes this behavior possible.

High levels of carbon dioxide are dangerous. Many animals have built-in avoidance behaviors that take over when necessary and people can even experience fear and panic attacks when too much carbon dioxide is in the air. In efforts to understand the neurobiology behind these types of responses, the team at RIKEN turned to zebrafish-animals that are useful because the larvae have easy to characterize behaviors and because their transparent brains make imaging neuronal activity a breeze.

Larval zebrafish are known to have a fast response when touched on the head; they flee in a stereotyped pattern within 10 milliseconds. "In contrast," notes lead author Tetsuya Koide, "we showed that their avoidance response to carbon dioxide happened after around 4-5 seconds, which is about 400 to 500 times slower." Additionally, the escape routes taken by the fish to avoid carbon dioxide were much more variable than how they responded to being touched, and they swam away at much slower speeds. All these differences pointed to an as yet unknown sensation-response pathway in the brain.

To identify the responsible pathway, the researchers used transgenic zebrafish made specially for calcium imaging. This technique visualizes brain activity by genetically expressing fluorescent protein sensitive to calcium, a key molecule involved in the transmission of neuronal signals. The team was able to see a series of responses to carbon dioxide in the brain, the earliest being in the olfactory bulb-the part of the brain that processes smell in mammals. A few seconds later, they saw responses in trigeminal sensory neurons-the nerve that carries touch and pain sensations from the face. The final response was from the habenula-a part of the brain known to be involved in learning associations with unpleasant experiences.

In order to determine which of these three systems was necessary for the response to carbon dioxide, the team used a laser to remove each one separately. They found that only damage to the trigeminal pathway and to the nose affected the response to carbon dioxide. This was somewhat surprising because damaging the olfactory pathway itself did not change the avoidance behavior. "This meant that a non-olfactory component in the nose is critical for avoiding carbon dioxide," explains Koide.

The team next wanted to determine how carbon dioxide was sensed in the nose. Calcium imaging of the zebrafish nose revealed a cluster of cells that responded to carbon dioxide. Tests indicated that these cells were part of the terminal nerve-also called cranial nerve zero-and their removal blocked the avoidance response to carbon dioxide. Thus, the zebrafish nose contains terminal nerve chemosensors that are unrelated to smell and that can control behavioral responses to noxious chemicals.

"We were surprised to find that the terminal nerve acts as a carbon dioxide sensor in zebrafish," says Koide. "Although it was identified as an additional cranial nerve in humans and other vertebrates more than a century ago, ours is the first to report its function in chemosensation." Indeed, the terminal nerve has been thought to function in reproductive behavior because it produces gonadotropin-releasing hormone, a major hormone which in turn stimulates the production of reproductive hormones.

"As humans and other vertebrates also possess the terminal nerve system," continues Koide, "we next hope to further characterize its chemosensory functions across different species, including humans."
-end-
Reference:

Koide T, Yabuki Y, Yoshihara Y (2018) Terminal nerve GnRH3 neurons mediate slow avoidance of carbon dioxide in larval zebrafish. Cell Reports. doi: 10.1016/j.celrep.2018.01.019

RIKEN

Related Brain Articles:

Study describes changes to structural brain networks after radiotherapy for brain tumors
Researchers compared the thickness of brain cortex in patients with brain tumors before and after radiation therapy was applied and found significant dose-dependent changes in the structural properties of cortical neural networks, at both the local and global level.
Blue Brain team discovers a multi-dimensional universe in brain networks
Using a sophisticated type of mathematics in a way that it has never been used before in neuroscience, a team from the Blue Brain Project has uncovered a universe of multi-dimensional geometrical structures and spaces within the networks of the brain.
New brain mapping tool produces higher resolution data during brain surgery
Researchers have developed a new device to map the brain during surgery and distinguish between healthy and diseased tissues.
Newborn baby brain scans will help scientists track brain development
Scientists have today published ground-breaking scans of newborn babies' brains which researchers from all over the world can download and use to study how the human brain develops.
New test may quickly identify mild traumatic brain injury with underlying brain damage
A new test using peripheral vision reaction time could lead to earlier diagnosis and more effective treatment of mild traumatic brain injury, often referred to as a concussion.
This is your brain on God: Spiritual experiences activate brain reward circuits
Religious and spiritual experiences activate the brain reward circuits in much the same way as love, sex, gambling, drugs and music, report researchers at the University of Utah School of Medicine.
Brain scientists at TU Dresden examine brain networks during short-term task learning
'Practice makes perfect' is a common saying. We all have experienced that the initially effortful implementation of novel tasks is becoming rapidly easier and more fluent after only a few repetitions.
Balancing time & space in the brain: New model holds promise for predicting brain dynamics
A team of scientists has extended the balanced network model to provide deep and testable predictions linking brain circuits to brain activity.
New view of brain development: Striking differences between adult and newborn mouse brain
Spikes in neuronal activity in young mice do not spur corresponding boosts in blood flow -- a discovery that stands in stark contrast to the adult mouse brain.
Map of teenage brain provides evidence of link between antisocial behavior and brain development
The brains of teenagers with serious antisocial behavior problems differ significantly in structure to those of their peers, providing the clearest evidence to date that their behavior stems from changes in brain development in early life, according to new research led by the University of Cambridge and the University of Southampton, in collaboration with the University of Rome Tor Vergata in Italy.

Related Brain Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".