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COLOUR-changing frog cells could be used to detect performance-enhancing
drugs and leave cheating athletes red-faced. Sensors based on the cells can pick
up traces of drugs in body fluids, and could even detect new drugs that other
methods fail to pick up.

Some frogs, such as the African clawed frog (Xenopus laevis), change
colour to cope with sunlight and heat and also to improve their camouflage. They
do this by activating cells in their skin that contain granules of melanin, the
dark brown pigment.

These colour-changing cells, called melanophores, are normally dark but can
be triggered by a particular hormone released in the frog. When the hormone
binds to the cell wall, it sets off a reaction that moves the pigment granules
to the centre of the cell, making it look colourless. Once the hormone detaches,
the melanin grains disperse throughout the cell, making it appear dark again.
This colour switch has now been adapted to detect tiny amounts of
opiates鈥攊ncluding heroin, morphine and codeine.

Annika Karlsson and her colleagues at Link枚ping University in Sweden
zapped melanophores with an electric field to tear tiny holes in their
membranes, and then inserted strands of human DNA into the cells. The DNA then
churns out cell-membrane receptors that opiates will bind to. The cells were
cultured and transferred to small plates.

To check whether activating these receptors could trigger the colour change,
Karlsson added drops of naloxone, a drug that blocks opiate receptors by binding
strongly to them. Naloxone is used in rehabilitation programmes because it
reduces the effects of drugs like heroin and morphine.

Karlsson found that the cells lightened significantly when the naloxone was
added. 鈥淭he response was obvious, even with the naked eye,鈥� she says. Although
the team didn鈥檛 test the precise sensitivity of the cells, she says that tests
worked using just tens of micrograms of naloxone per litre.

The advantage of the frog-based sensor is that it doesn鈥檛 need to know what
drug to look for, says Hagan Bayley, head of medical biochemistry and genetics
at Texas A&M University in College Station. 鈥淵ou could have someone who鈥檚
taken something that activates the opiate receptor and you鈥檇 detect it without
even knowing what the drug is,鈥� he says. So if an athlete was using a very new
opiate-type drug, the test would pick it up.

The sensor has other uses too. Since it changes colour with any chemical that
acts like an opiate, it could be used to screen chemical compounds for useful
opiate drugs, says Bayley.