Most people laugh when they’re tickled.

So do rats.

Casey Cromwell, an associate professor in the psychology department at Bowling Green State University, has been studying rat vocalizations for years. He is especially interested in the high-pitched, 50-kilohertz chirps rats emit when tickled or happy. He is also interested in how the “laughing gene” is passed from one generation to the next through breeding “laughers” with “non-laughers.”

“We might actually obtain animals that are happier and animals that are much less happy,” Cromwell said.

This type of research could help scientists understand what genes contribute to happiness levels in rats, which could help establish new treatments for depression, anxiety and other stress disorders in humans. Cromwell said the psychology department received a grant from the Hope for Depression Research Foundation to continue these studies in selective breeding based on levels of laughter.

Cromwell said the studies so far show that low laughers tend to not show any emotional responses at all, like fear or anger, which corresponds with how people with depression react.

To test other emotional responses in low laughers, Cromwell said they put rats in the same cage as cat hair.

“Rats are naturally afraid of cats,” Cromwell said. “The low [laughers] didn’t show response. When we took the cat hair away, it was like they had never been exposed to it at all.”

Garrett Gilmer, associate director of the Counseling Center at the University, said depression manifests in different ways for different people: too much or not enough sleep, loss of pleasure, difficulty concentrating and irritability.

Gilmer said the best current treatment for depression is counseling with medication, but at best treatment is still at a trial-and-error phase. Individuals react to different medications in different ways, and it often takes time to hash out the right treatment.

Depression has a genetic component, but most of the time there is a trigger, Gilmer said, such as bereavement, puberty, neglect, oppression and traumatic events as well as good changes, like getting a new job in a big city.

“Any major changes could trigger depression,” Gilmer said. “A lot of times there’s a family history of depression because there’s a genetic component.”

With such triggers, Gilmer said people with depression talk a lot about a “downward spiral.”

“There’s a sense of, ‘there’s something wrong with me,'” he said.

Cromwell said these emotional irregularities seem to be an impairment in the amygdala of the brain, in humans and rats. The research now is focused on identifying the genes that could be linked to depression.

Jaak Panksepp, a distinguished research professor in Animal Well-Being Science at Washington State University, began the rat vocalization project while he was a psychology professor at the University.

His original research, and what he studied for 20 years, was breeding rats based on aggression levels. But 40 years ago, Panksepp tried tickling rats to see if they laughed.

“We’ve gradually moved from negative systems to positive systems,” he said.

When rats are scared or apprehensive, they emit 22-kHz chirps; when happy, they chirp at 50-kHz, according to a research paper written by Panksepp, titled “Breeding for 50-kHz Positive Affective Vocalization in Rats.” After breeding five generations of rats based on how much they laugh, descendants of high laughers chirped more 50-kHz than those not descended of high laughers,

Current anti-depression medications don’t work all of the time, Panksepp said. The analysis of this emotional circuitry in the brain is showing the link between genes and depression and, with this research, new medications can be created.

“This is a gateway to developing psychiatric drugs no one has thought of,” he said.

These emotional systems in the brain, Panksepp said, are roughly the same in rats as in humans and as in all animals.

“These systems are in very ancient recesses of the brain,” he said. “Many of the brain patterns we humans have come from our animal past … We learn more about the human body by looking at animals.”

Panksepp said one of the reasons why he started this research years ago was to some day try to understand how to help humans with emotional disorders.

“We’re not interested in rat laughter for its own sake,” he said. “It’s about understanding this in our own brains.”

Adult population with mental disorders in the United States

Major depressive disorder: 6.7 percent, or 14.8 million peopleAnxiety disorder: 18.1 percent, or 40 million peopleAny mental disorder: 26.2 percent, or one in four peopleSource: National Institute of Mental Health