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MICHELE NORRIS, host: Scientists have identified a substance that can dramatically improve memory in rats, at least. It's a growth hormone that's produced naturally in rodents and humans.

As NPR's Jon Hamilton reports, the discovery could lead to drugs that affect memories in people.

JON HAMILTON: Most memories are fleeting. That phone number you just saw will be gone in seconds unless you work hard to remember it. But some memories don't disappear.

Dr. CRISTINA ALBERINI (Researcher, Mount Sinai School of Medicine): We are talking about long-term memories. Memories that last for days, weeks, years, even a lifetime.

HAMILTON: Cristina Alberini, a researcher at Mount Sinai School of Medicine in New York, studies these sorts of memories.

To understand how they're formed, Alberini and a team of scientists have been studying rats that get an electric shock when they step into a dark area of their cage.

Dr. ALBERINI: And they learn that that's unpleasant, and they're going to avoid it.

HAMILTON: For a while. Then the unpleasant memory gradually fades away.

Alberini says the time it takes them to forget provides a way to measure the persistence of a memory.

Dr. ALBERINI: Which is, for how long did they avoid this place in which they had this unpleasant experience before?

HAMILTON: The scientists realized that soon after rats got a shock, levels of a hormone called IGF2 increased sharply in a part of the brain called the hippocampus（海馬體）. Before this, scientists had known what IGF did in other parts of the body but they hadn't studied it much in the brain. The increase in the hippocampus led them to suspect that the hormone was somehow involved in turning short-term memories into long-term ones. They confirmed this by reducing the amount of hormone in rats' brains. These rats never learned to avoid the dangerous place.

And Alberini says...

Dr. ALBERINI: When we gave IGF2, we saw that they avoided the place for much, much longer.

HAMILTON: More than twice as long. But the extra hormone only made a difference if the rats got it within a few hours of a shock. Alberini says this suggests there is a critical period in which IGF2 helps form long-term memories. And, she says, the hormone probably plays a similar role in many other species.

Dr. ALBERINI: I think there is a, you know, a number of suggestions here that are very encouraging for thinking that it may work in humans.

HAMILTON: Other scientists are pretty intrigued by the new study.

Professor LI-HUEI TSAI (Picower Institute for Learning and Memory, MIT): Really, really exciting.

HAMILTON: Li-Huei Tsai studies Learning and Memory at MIT. Tsai says the new research could help scientists design a drug that enhances memory in people, perhaps even those with Alzheimer's disease. But she says IGF2 itself may not be a good choice because of things it does outside the brain.

Prof. TSAI: It actually can increase the growth of cancer cells. So, I just hope that people wouldn't think about, you know, injecting IGF2 into themselves or something like that.

HAMILTON: Some researchers are more interested in what the study suggests about eliminating bad memories, like the ones associated with post-traumatic stress disorder（創(chuàng)傷后精神緊張性精神障礙）, or PTSD.

Thomas Insel directs the National Institute of Mental Health, which helped fund the research. He says the new study changes the scientific model of how a frightening experience becomes a long-term memory.

Dr. THOMAS INSEL (Director, National Institute of Mental Health): For understanding that, we're going to have a new player that we have to think about. This is a biochemical step that, frankly, no one had identified before.

HAMILTON: Insel says people with PTSD might benefit from a treatment that reduces the amount of IGF2 in the brain, which could help people get rid of a bad memory.

Dr. INSEL: That actually may be more relevant to this particular paper because this is, after all, looking at a fear or emotional memory. And what happens in PTSD is not that you want to remember, you want to learn to forget.

HAMILTON: And manipulating levels of IGF2 might help them do that. The new study appears in the journal Nature.

Jon Hamilton, NPR News.

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