By Julie Steenhuysen

CHICAGO, June 29 (Reuters) - A new study in mice may help explain some of the rare but strange side effects in people taking the sleep drug Ambien, including sleep walking, midnight binges and even driving while not fully awake.

Ambien, made by Sanofi-Aventis, can shut down powerful brain circuits responsible for inhibiting brain activity under certain circumstances, leaving other brain circuits unchecked, researchers at Georgetown University Medical Center in Washington said.

'You are kind of releasing the brakes,' said Molly Huntsman of Georgetown, who worked on the study that appears in the Proceedings of the National Academy of Sciences.

This may stimulate brain circuits that would normally be silenced. 'In a way, Ambien is awakening other circuits because the brakes are not in place,' Huntsman said.

To study the effects of the drug, known generically as zolpidem, Huntsman and colleagues conducted a series of experiments in mice.

The team wanted to see how mice on the drug would respond when the researchers trimmed their whiskers, which rodents use as their primary sensory system -- much like humans rely on vision to take in information about the world.

The team found that mice that were deprived of this sensory information had changes in their brain that affected the way they responded to the drug Ambien.

'It's a population of neurons that is normally in place to stop activity. We find what Ambien does is inhibit their function to inhibit,' Huntsman said in a telephone interview.

She said normally the drug has no effect on this group of neurons, but trimming the whiskers was enough to change brain receptors, making the mice respond differently to the drug.

'What's happening in the mouse brain is we are seeing the inhibitory neurons that are becoming sensitive to Ambien, where they normally wouldn't be,' she said.

Huntsman said humans and mice share many of the same brain circuits and she thinks it is possible the same thing is going on in some people, whose brains are somehow predisposed to react differently to the drug.

'The thing about humans is that it doesn't happen in every case. But in a few cases, people will start to sleep walk and sleep eat and have these crazy bizarre situations,' she said.

Huntsman said it may be that something has happened to change the brain receptors that affect the way these people respond to the drug.

'A lot of things can change your brain chemistry -- stress, alcohol use,' she said.

Huntsman cautioned that more study was needed to understand these effects but said the findings offer new clues for future research.

(Editing by Maggie Fox and Todd Eastham) Keywords: BRAIN AMBIEN/

(julie.steenhuysen@thomsonreuters.com ; +1 312 408 8131)

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