Researchers Identify Brain Circuit That Regulates Thirst

Howard Hughes Medical Institute scientists have identified a circuit in the brains of mice that regulates thirst. When a subset of cells in the circuit is switched on, mice immediately begin drinking water, even if they are fully hydrated. A second set of cells suppresses the urge to drink.

The thirst-regulating circuit is located in a region of the brain called the subfornical organ (SFO). “We view the SFO as a dedicated circuit that has two elements that likely interact with each other to maintain the perfect balance. So you drink when you have to and you don’t drink when you don’t need to,” says Charles Zuker, an HHMI investigator at Columbia University who led the research. By working in this way, the circuit ensures that animals take in the right amount of fluid to maintain blood pressure, electrolyte balance, and cell volume. This work, led by postdoctoral fellow Yuki Oka was published January 26, 2015, in the journal Nature.

Zuker’s lab is primarily interested in the biology of taste. Their studies have identified the receptors for the five basic tastes (sweet, sour, bitter, umami and salt), and shown that the nervous system devotes multiple pathways to sensing and responding to salt. These circuits ensure that salt is appealing to humans at low concentrations, but not at high concentrations. “This is how the taste system regulates salt intake, which is very important for salt homeostasis in the body,” says Oka. “But this is just one side of the coin. Salt intake has to be balanced by water intake.”

The scientists knew a different mechanism must be responsible for controlling an animal’s water intake. “There are no concentration changes for water – water is water,” Oka says. “But when you’re thirsty, water is really attractive.” Zuker and Oka set out to determine how the brain regulated the motivation to drink.

They began their search in the brain region known as the SFO, which shows increased activity in dehydrated animals. The SFO is one of the few regions of the brain located outside the blood-brain barrier, meaning it has direct contact with body fluids. “These cells might then have the opportunity to directly sense electrolyte balance in body fluids,” Zuker points out.

Past experiments in which researchers had electrically stimulated various circumventricular organs in the brain of mice, including the SFO, had yielded inconsistent results. Oka wanted to find out if there were specific cells in the SFO that triggered drinking behavior. By analyzing genetic markers, he identified three distinct cell types in the SFO: one set of excitatory cells, one set of inhibitory cells, and a third population of supporting cells known as astrocytes.

“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,” Zuker says. “And if you silence these populations, you should suppress the motivation to drink, even if you are extraordinarily thirsty.”

To test these predictions, Oka introduced a light-sensitive protein into cells in the SFO, enabling the scientists to selectively activate those cells in the mice. Using blue light from a laser, they then switched on the excitatory cells in the SFO of mice that had already had plenty to drink. The results were dramatic.

“There is an animal that is happily wandering around, with zero interest in drinking. You activate this group of excitatory neurons, and it just beelines to the water spout,” Zuker says. “As long as the light is on, that mouse keeps on drinking.” The animals were uninterested in other fluids, but would avidly drink water for prolonged periods, consuming as much as eight percent of their body weight. For humans, that would amount to about 1.5 gallons of water, Oka points out.

“It’s very exciting,” Zuker says. “This circuit informs and directs the mouse into a complex program of actions and behaviors: ‘I’m thirsty. I need to identify a source of water. I have to go where the water is. I have to begin to consume that water and I have to continue until this signal is suppressed.'”  

The next step involved testing the effect of the inhibitory neurons in the SFO. When Oka switched those cells on in thirsty animals, the mice reduced their water intake by about 80 percent. Activating the inhibitory cells did not affect the animals’ interest in food or salt, indicating that the neurons specifically regulate water consumption.

The two groups of cells appear to work together to respond to changing hydration levels and maintain fluid balance, the scientists say. “You can imagine this must be a very tightly controlled feedback loop,” Zuker says. “As fluid is being consumed, the electrolyte balance is changing, and this is being sensed.”

Zuker also points out “the behavior of the mouse is independent of learning, experience, or context,” indicating that the thirst-regulating circuit is hard-wired into the brain. Mingyu Ye, another postdoctoral fellow in Zuker’s lab, also participated in the study. Yuki Oka recently acccepted a position as assistant professor at the California Institute of Technology.

Hate to Diet? It’s How We’re Wired

If you’re finding it difficult to stick to a weight-loss diet, scientists at the Howard Hughes Medical Institute’s Janelia Research Campus say you can likely blame hunger-sensitive cells in your brain known as AGRP neurons. According to new experiments, these neurons are responsible for the unpleasant feelings of hunger that make snacking irresistible.

The negative emotions associated with hunger can make it hard to maintain a diet and lose weight, and these neurons help explain that struggle, says Scott Sternson, a group leader at Janelia. In an environment where food is readily available, their difficult-to-ignore signal may seem like an annoyance, but from an evolutionary point of view, they make sense. For earlier humans or animals in the wild, pursuing food or water can mean venturing into a risky environment, which might require some encouragement. “We suspect that what these neurons are doing is imposing a cost on not dealing with your physiological needs,” he adds.

AGRP neurons do not directly drive an animal to eat, but rather teach an animal to respond to sensory cues that signal the presence of food. “We suspect that these neurons are a very old motivational system to force an animal to satisfy its physiological needs. Part of the motivation for seeking food is to shut these neurons off,” says Sternson, whose team also demonstrated that a different set of neurons is specialized to generate unpleasant feelings of thirst. Sternson and his colleagues published their findings in the journal Nature on April 27, 2015.

Hunger affects nearly every cell in the body, and several types of neurons are dedicated to making sure an animal eats when energy stores are low. But Sternson says that until now, what scientists had learned about those neurons had not completely matched up to something we already know: hunger is unpleasant.

“There was an early prediction that there would be neurons that make you feel bad when you were hungry or thirsty. This made sense from an intuitive point of view, but all of the neurons that had been looked at seemed to have the opposite effect,” he says. In earlier studies, researchers found that neurons that promoted eating did so by increasing positive feelings associated with food. In other words – not surprisingly – hunger makes food tastes better.

Some scientists had begun to suspect their ideas about a negative signal in the brain motivating hunger might be wrong. But their knowledge of the system was incomplete. AGRP neurons, located in a regulatory area of the brain known as the hypothalamus, were clearly involved in feeding behaviors: When the body lacks energy, AGRP neurons become active, and when AGRP neurons are active, animals eat. But no one had yet investigated those cells’ strategy for generating that motivation.

Postdoctoral researcher Nicholas Betley and graduate student Zhen Fang Huang Cao began to address the question with a series of behavioral experiments. In the first, they offered well-fed mice two flavored gels – one strawberry and the other orange. Neither gel contained any nutrients, but the hungry mice sampled them both. Then the scientists’ manipulated the hunger signals in the animals’ brains by switching AGRP neurons on while they consumed one of the two flavors. In subsequent tests, the animals avoided the flavor associated with the false hunger signal.

In a reverse experiment, the scientists switched AGRP neurons off while hungry animals consumed a particular flavor. The animals developed a preference for the flavor choice that led to silencing of AGRP neurons, suggesting they were motivated to turn off the cells’ unpleasant signal. In further experiments, the scientists found that mice also learn to seek out places in their environment where AGRP neurons had been silenced and avoid places where those cells were active.

Next, postdoctoral researcher Shengjin Xu used a tiny, mobile microscope to peer inside the brains of hungry mice and monitor the activity of AGRP neurons. As expected, the cells were active until the mice found food. What was surprising, Sternson says, is that mice did not actually have to eat to quiet the neurons. Instead, the cells ceased activity as soon as an animal saw food – or even a signal that predicted food. And their activity remained low while the animal was eating.

That wouldn’t make sense if the job of AGRP neurons was to make food taste better or if they directly controlled the individual actions that go into eating, which were two possibilities, Sternson says. But to encourage eating, a negative signal would need to turn off when an animal consumed food. So their imaging experiments further supported what they had learned in their previous experiments.

The team later conducted similar experiments in which they manipulated thirst-sensitive neurons instead of AGRP neurons. Those neurons, found in a part of the brain known as the subfornical organ (SFO) behaved similarly: animals avoided places where the SFO neurons had been active, indicating that the cells generated a negative feeling. Again, the findings were consistent with everyday experience: “There’s a similar motivational quality to hunger and thirst,” Sternson says. “You want them to end.” But although AGRP and SFO neurons motivate similar behaviors, their goals are very specific: AGRP neurons only drive animals to eat and SFO neurons only drive animals to drink. Recent independent work by HHMI researcher Charles Zuker at Columbia University has also shown that a circuit in the SFO regulates thirst.

In further experiments, Sternson’s team will investigate similarities and differences between the two groups of cells. In addition, his group is interested in understanding more about how to interfere with the functions of AGRP neurons, which, in the future, might make it easier to keep those extra pounds off next time you go on a diet.