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Growth Hormone Also a Memory Booster

Your ginkgo biloba, rosemary, and other purported brain boosters may soon face competition—from an unexpected source. Researchers have found that insulin-like growth factor 2 (IGF-2), a naturally occurring hormone, can boost memory retention in animals. The discovery offers a rare glimpse into a mysterious stage of the learning process, and it may one day provide scientists with a new way to treat memory impairments, such as those caused by dementia.

For neuroscientists chasing down the molecules that make up memories, IGF-2 isn't the most likely of targets. It's better known for roles in cell growth and repair. But IGF-2 has been spotted in the hippocampus, a region of the brain associated with learning and memory. That made researchers wonder whether it was actually playing a part in cognition.

So neuroscientist Cristina Alberini and her team at the Mount Sinai School of Medicine in New York City devised a memory test for rats that would help pinpoint IGF-2's modus operandi. The team came up with a box that was lit on one side and unlit on the other. Rats that entered the dark side got a mild foot shock. The rats' subsequent hesitation to return to the dark after getting shocked gave the scientists a measure of how well they remembered the traumatic event.

Naturally occurring IGF-2 levels in the rats' hippocampuses peaked after receiving the foot shock, and all of the rodents were fairly good at retaining the unpleasant memory. In fact, for weeks after their shock, some rats hesitated to enter the dark side of the box.

The researchers then ran the experiment again, this time injecting some of the rats' brains with extra IGF-2 at different times after their training, such as immediately after a shock or more than a day later. The IGF-2 boost enhanced their recall dramatically. "Their hesitation, or latency, more than doubled," says Alberini. "It's a very potent effect."

IGF-2 improved a rodent's memory only when administered in a precise window of time—roughly within 24 hours after the foot shock—which coincides with a stage in the learning process called "memory consolidation." That's a poorly understood transition period when a memory is still malleable but becoming more established and robust.

The team's results, published today in Nature, offer a new piece of the consolidation puzzle, says neurobiologist Alcino Silva of the University of California, Los Angeles, who was not involved in the study. "We know a lot about the first few seconds of learning but not so much about the hours and days later," he says. "Looking at this period is a big contribution. I'm envious, actually."

The results may also contribute to developing memory-boosting drugs. The fact that IGF-2 is naturally occurring and can cross the blood-brain barrier makes it a promising candidate for treating memory-impairing diseases or even forgetfulness, Alberini notes. But any clinical applications are a long way off, as one key question remains unanswered—namely, how exactly IGF-2 boosts memory. Preliminary results suggest the growth factor strengthens the connections between nerve cell synapses during consolidation, says Alberini. Her team's next steps will explore the mechanism, in hopes of one day putting IGF-2 to work in humans.