Deep in the brain of mice, researchers have identified two distinct groups of nerve cells that govern thirst.
Water accounts for about 70 per cent of a person’s body weight and is vital for survival. The sensation of thirst is the body’s way of signalling the need to replenish it.
Experiments done several decades back indicated that a part of the brain known as the hypothalamus was involved in the generation of this primeval urge. Subsequent work has provided clues about which parts of the hypothalamus could be responsible for producing the sensation.
Using mice, scientists led by Charles Zuker of Columbia University in the U.S. have been able to demonstrate that thirst is controlled by two separate sets of nerve cells in a part of the hypothalamus, the subfornical organ. One set of nerve cells drove the animals to drink water while the other set quenched that urge.
Their results revealed “an innate brain circuit that can turn an animal’s water-drinking behaviour on and off, and probably functions as a centre for thirst control in the mammalian brain,” the scientists noted in a paper published online by Nature earlier this week.
Yuki Oka, the paper’s first author and now at California Institute of Technology, was able to induce the two sets of nerve cells in living mice to produce a light-sensitive protein. The nerve cells could then be stimulated by shining blue light from a laser on them.
A video shot by the scientists showed a dramatic change in mouse behaviour when the excitatory nerve cells were activated with laser light. A mouse, which had been wandering about its cage, started lapping up water as soon as the light was switched on and stopped doing so when the light was switched off.
Under laser stimulation, even mice that had already consumed ample water avidly drank more. The mice could drink nearly eight per cent of their body within 15 minutes, the scientists reported.
The laser-induced thirst was highly specific for water, and the animals would refuse to take other fluids, even honey. Nor would they drink water if it contained a bitter compound or was very salty.
Moreover, the excitatory neurons possessed a receptor for angiotensin, a hormone that stimulates drinking of water.
Laser stimulation of the inhibitory neurons, on the other hand, stopped thirsty animals from consuming water.
“This brain centre is part of a larger circuit that uses thirst signals to guide motivation, actions and behaviours” related to water drinking, remarked Dr. Zuker, a Howard Hughes Medical Institute investigator, in an email.
