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Researchers reported on Sunday that a hormone produced by the body in response to stress that normally serves to calm adults and younger children instead increases anxiety in adolescents.

They conducted experiments with female mice focusing on the hormone THP that demonstrated this paradoxical effect, and described the brain mechanism that explains it.

If, as the scientists suspect, the same thing happens in people, the phenomenon may help account for the mood swings and anxiety exhibited by many adolescents, they said.

"Teenagers don't go around crazy all the time," lead researcher Sheryl Smith, a professor of physiology and pharmacology at the State University of New York Downstate Medical Center, said in a telephone interview.

"But it really is a mood swing where things seem fine and calm, and then the next thing is someone's crying or angry," she added. "And I think that's why people have used the term 'raging hormones."'

The emotional swings are not always benign, Smith's team reports in Sunday's issue of the journal Nature Neuroscience.

"Responses to stressful events are amplified, and anxiety and panic disorder first emerge at this time, being twice as likely to occur in girls as in boys," they wrote.

"In addition, suicide risk increases in adolescence, despite the use of adult-based medical strategies."

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THP, also called allopregnanolone, generally serves as a natural tranquilizer. It is not produced immediately with stress, but rather several minutes later, and calms neural activity to reduce anxiety and assist the individual in adapting and functioning amid stress.

"It's not the immediate fight-or-flight response," Smith said. "And it's thought to be one way that we all can compensate for stress, so we just stay focused and don't go crazy, sort of focus on our task."

Smith's team examined brain activity and behavior in mice before puberty, during puberty and as adults.

The researchers subjected the mice to a stressful event by suddenly placing them inside a plexiglass container just slightly larger than a mouse's body -- sort of a claustrophobic experience -- and keeping them there for 45 minutes.

"Twenty minutes after stress, both the young mice and the adult mice showed less anxiety. But the pubertal mice showed more anxiety," Smith said.

Further experiments attributed this increased excitability to the effects of THP, the researchers said. THP acts on brain cells via molecular doorways known as receptors.

During adolescence, mice have the usual receptors, but also extra-high levels of a second kind that brings an anxious, rather than calming, response when THP attaches to it.

"The parallel with humans is that in humans there are similar hormonal changes going on in puberty," Smith said.

"So the beginning of puberty is a time when a lot of emotions and responses to stress are increased. It's nothing new that teenagers go through a difficult time. Hopefully this will shed some new light on it."