PET scans of a rat's brain made with the RatCAP scanner (horizontal view superimposed on a rat brain atlas figure, left, and a coronal slice, right). Redder colors indicate higher and violet colors lower levels of a radiotracer that binds to dopamine receptors.
updated 3/16/2011 3:44:39 PM ET 2011-03-16T19:44:39

Rats sporting tiny metal caps are not the latest in rodent fashion but rather an attempt to learn more about their brains and by extension help unravel mysteries about the human brain.

The headgear, appropriately called a RatCAP, is a positron emission tomography (PET) scanner that creates three-dimensional color images of what's happening in the rodent’s brain while it runs around its laboratory home.

"Positron emission tomography is a powerful tool for studying the molecular processes that occur in the brain," lead study researcher Paul Vaska, head of PET physics at Brookhaven National Laboratory (BNL) in Upton, N.Y., said in a statement.

Rats are considered good models for gaining insights into the human brain. But until now, studies of how rats' nervous systems work could be done only while the animal was either anesthetized and sleeping or immobilized and awake. That meant scientists couldn't use such studies to link brain activity with particular animal behaviors.

"Immobilization and anesthesia make it impossible to simultaneously study neurochemistry and the animals' behavior — the actions resulting from what goes on in the brain," said study co-author David Schlyer, a senior scientist at BNL. "Our approach was to eliminate the need for restraint by developing a PET scanner that would move with the animal, thus opening up the possibility of directly correlating the imaging data with behavioral data acquired at the same time."

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Although miniature on human-size scales, the device was still a hefty load for the rats, some 0.6 pounds, which is a substantial fraction of the rat's weight, the researchers said. To stabilize the donut-shaped headgear on the rat's head, the researchers counterbalanced it with springs.

To test out the miniature scanner, the researchers injected a radioactive tracer into the brains of female rats. The tracer specifically binds to dopamine receptors on the outside of brain cells, showing from which parts of the brain the neurotransmitter dopamine was being released as the rats moved about their enclosures.

The researchers discovered that greater activity by the rats resulted in lower levels of dopamine in the brain. These findings went against expectations that more movement entailed more dopamine secretion by brain cells, and suggests that the RatCAP might call into question some long-held assumptions about the correlation of brain function with behavior.

Now that rats can go about their business while simultaneously offering glimpses of their minds at work, one future application of the new BNL method is the study of sexual behavior's influence on the brain, the researchers propose in the April 2011 issue of the journal Nature Methods.

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