Most living organisms require oxygen for survival. During respiration, oxygen is consumed by the cells of the body and carbon dioxide (CO2) is released. Too much carbon dioxide is not good for the body. It acts like an asphyxiant directly reducing the normal concentration of oxygen in the air thereby making it difficult to breathe. It can also impair consciousness and increase the rate of respiration. Living organisms have evolved different mechanisms to detect carbon dioxide in order to escape from it. For example, humans feel panicked and frightened when there is too much carbon dioxide in the air. However, the nerve networks and regions of the brain involved in this response was unknown.
To understand the mechanism of this response, researchers used zebra fish as a model. They used a genetically modified version of the larvae of this organism so that it can be used to visualise brain activity. A fluorescent protein is expressed (becomes visible) whenever there is a change in calcium, a key molecule involved in neural signalling. This way they were able to identify the exact regions of the brain that were responsible for the ‘carbon dioxide avoidance’ response in fish.
Using a syringe pump, they introduced 60-780 milligrams of carbon dioxide per litre of water in which the fish larvae were swimming. They observed changes in the brain activity based on the expression of the fluorescent protein.
They first observed brain activity changes in the olfactory bulb – the part of the brain that processes smell. Then, a few seconds later, they saw responses in trigeminal sensory neurons – the nerve that carries touch and pain sensations from the face. Finally, they observed brain activity changes in the habenula – a part of the brain known to be involved in learning associations with unpleasant experiences.
To exactly pinpoint which of these regions of the brain was crucial for the response to carbon dioxide, the team used a laser to remove each one separately. They found that only damage to the trigeminal region 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 behaviour. The researchers thought that a non-olfactory component in the nose was critical for avoiding carbon dioxide.
The team next wanted to determine how carbon dioxide was sensed in the nose. Calcium imaging of the zebra fish 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. Removal of this nerve blocked the avoidance response to carbon dioxide. Thus, the zebra fish nose contains terminal nerve chemo-sensors that are unrelated to smell and that can control behavioural responses to carbon dioxide.
The following video demonstrates how the larvae specifically respond to carbon dioxide.