By directing pulses of light onto specific regions of the brain, researchers from Columbia University Medical Center induced feelings of extreme thirst in perfectly well hydrated mice — causing them to drink the equivalent of seven pints of water for a human.
Top image: A mouse brain showing the two neurons, CAMKII (red) that triggers thirst and VGAT (green) that suppresses it. Credit: Lab of Charles Zuker
Scientists have suspected for some time that thirst is regulated by neurons in the subfornical organ (SFO) in the brain's hypothalamus (the part of the brain that links the nervous system to the endocrine system via the pituitary gland). But knowing which neurons in particular are responsible has proven difficult.
A team at CUMC theorized that there are two types of neurons in the SFO — those that instigate feelings of thirst and those that suppress it. Indeed, though thirst is thought of as a physical condition (which it is), it's also perceived psychologically; it's the brain's way of telling us we need to grab a drink.
A release from CUMC explains how the team tested their hypothesis:
[Using] optogenetics, researchers can control specific sets of neurons in the brain after inserting light-activated molecules into them. Shining light onto these molecules turns on the neurons without affecting other types of neurons nearby.
These "mind-control" experiments revealed two types of neurons in the SFO that control thirst: CAMKII neurons, which turn thirst on, and VGAT neurons, which turn it off.
When the researchers turned on CAMK11 neurons, mice immediately began to seek water and to drink intensively. This behavior was as strong in well-hydrated mice as in dehydrated ones. Once the neurons were shut off—by turning off the light—the mice immediately stopped drinking.
This was so effective that the mice drank the equivalent of seven pints of water, or 8% of their body weight.
Importantly, the researchers also showed that the rodents' interest in salt or food were not affected, and that the two neuron-types in question were responsible for inducing and suppressing feelings of thirst.
This research, which now appears in Nature, could help scientists understand what goes wrong in disorders that make people drink too much or too little fluid and to develop therapies for dehydration.
Image: Charles Zuker