thirst switch
This image of a mouse brain shows the two neurons, CAMKII (in red) that triggers thirst and VGAT (in green) that inhibit itCharles Zuker

The on-off switch for thirst has been discovered and used to control the drinking habits of mice.

Scientists from the Howard Hughes Medical Institute found the switch to be so effective that when turned on, the mice drank the equivalent of seven pints of water, or 8% of their body weight.

The thirst-regulating circuit was found in a region of the brain called the subfornical organ (SFO) – an area believed to have two elements that interact with one another to maintain balance, so you do not drink too much or too little.

Published in the journal Nature, researchers had previously shown the nervous system devotes multiple pathways to sensing and responding to salt – which has to be balanced by water intake.

The SFO shows increased activity in dehydrated animals. Researchers analysed genetic markers and identified three distinct cells types in the SFO that mediated key aspects they believe are behind the motivation to drink.

Study leader Charles S. Zuker said: "If these neurons really mediated key aspects in driving the motivation to drink, then their activation should trigger active drinking, irrespective of the degree of fluid satiety. And if you silence these populations, you should suppress the motivation to drink, even if you are extraordinarily thirsty."

The team introduced a light-sensitive protein into the cells that allowed them to activate the thirst switch in mice. Findings showed their predictions were correct.

"There is an animal that is happily wondering around, with zero interest in drinking. You activate this group of excitatory neurons, and it just beelines to the water spout," Zuker said. "As long as the light is on, that mouse keeps on drinking."

Researchers also switched off the thirst switch and found mice reduced their water intake by about 80%, without affecting the creature's interest in food or salt – suggesting the neurons regulate water intake specifically.

Lead author Yuki Oka concluded: "Together, these findings show that the SFO is a dedicated brain system for thirst."