Monday, October 8, 2007

Emotion Hormone Sharpens Memory

It's well known that events that occur during times of intense happiness, anger or fear often remain most vivid in our memories -- far more likely to be recalled than less emotionally charged events. Now researchers think they have uncovered why: A hormone released during high emotion enhances the activity of nerve cells that then form unusually strong memory circuits in the brain.

The chemical that floods the nerve cells is norepinephrine, the "fight or flight" hormone. Researchers at the Johns Hopkins School of Medicine and elsewhere found that the hormone enhances the memory process by adding phosphate molecules to a nerve cell receptor. Bathed in these additional phosphates, the receptors are better able to insert themselves into adjacent synapses, the spaces between nerve cells -- making a new memory circuit more powerful.

The research, published last week in the journal Cell, describes the brain as a complex circuit board where each new experience creates a new circuit.

Richard Huganir, a neuroscientist, and his team tested their theory by injecting mice with two substances known to increase norepinephrine levels in the brain -- adrenaline or fox urine. They then analyzed slices of the mice's brains and saw the expected increases in phosphates on the receptors.

They also gave two sets of caged mice -- those that had received the injections and those that had not -- a mild shock before removing them. When the mice were returned to the cage, those with the added fox urine or adrenaline tended to freeze in fear, an indication that they associated the cage with the shocks. The unexposed mice froze significantly less.

-- Marc Kaufman

Bird Chirps Warn Voiceless Iguanas

Marine iguanas in the Galapagos Islands, which sun themselves in rocky crannies and so have trouble spotting the hungry hawks that swoop down on them several times a day, have a secret emergency warning system.

Although the reptiles do not communicate vocally -- relying primarily on scent and vision -- their ears can tell the difference between standard mockingbird songs and the alarm calls the birds chirp when a hawk approaches. When they pick up those avian alarms, the iguanas tend to perk up, look around and take evasive action.

The new finding is "the first demonstration that a species that lacks vocal communication can associate the auditory alarm signals of another species" with a threat, wrote Maren N. Vitousek of Princeton University, with colleagues from there and the University of Bath in England.

The team recorded mockingbird songs and alarm calls on Santa Fe Island in the Galapagos. Then they played them in random order to hundreds of iguanas at three sites.

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