BioWorld International Correspondent

LONDON - Just how the drugs that induce general anesthesia work, causing loss of consciousness and insensitivity to pain, has been a mystery for more than a century.

The fog surrounding the understanding of how and where the drugs have their effects on the central nervous system is starting to become more clear, however, with the knowledge that they increase the activity of a potassium channel in the brain.

A research team at Imperial College, London, has pinpointed the exact amino acid in a particular class of potassium channels that is responsible for conferring anesthetic sensitivity on the channel.

In the long term, the finding could lead to a new generation of anesthetics. Although current anesthetics are very safe, they have many unwanted side effects, including nausea and vomiting and effects on the heart, because they are thought to bind to several different targets in the body. Drugs that are more specific therefore could be safer and have fewer unwanted side effects.

Nicholas Franks, professor of biophysics and anesthetics at Imperial College, told BioWorld International: "We have identified a critical anesthetic determinant that makes a particular class of potassium channel found in the human brain sensitive to anesthetics. We hope that this finding will allow us to test the importance of this channel to anesthesia and, ultimately, that it will provide the information needed to make it possible to design more selective anesthetics that are safer and have fewer side effects."

Franks and his colleagues reported their findings in the July 20 issue of The Journal of Biological Chemistry. The title of their paper is "Determinants of the anesthetic sensitivity of TASK channels: molecular cloning of an anesthetic-activated potassium channel from Lymnaea stagnalis."

The groundwork for the study was laid by Franks' team in 1988, when he and colleagues identified an unusual anesthetic-sensitive neuron in the central nervous system of the great pond snail, Lymnaea stagnalis. The earlier studies found that when anesthetic drugs such as halothane and isoflurane were added to this neuron, they activated the potassium channels.

The effect was for potassium to flood out of the cell, giving its interior a more negative charge, and preventing the cell from firing a nerve impulse. (In contrast, for the cell to generate a nerve impulse, its interior has to have a more positive charge.)

Some years later, other research teams discovered that humans had a similar class of potassium channels, which are known as two-pore-domain potassium channels. More recently, it has been shown that such channels exist in many other mammalian species.

Franks' group also was the first to show that when they separated the two optical isomers of isoflurane, each isomer had different effects on the potassium channels of the L. stagnalis neuron. "This finding demonstrated that isoflurane was binding directly to the channel," Franks said.

The researchers therefore realized that such channels could well be the targets for anesthetic drugs, and set out to prove it, with the help of the channel from the great pond snail.

Franks and his colleagues first established that the DNA sequence encoding the protein that makes the pond snail channel was very similar to that encoding some of the human potassium channels. They then made a series of chimeric channels that were part snail and part human. By mutating the protein that formed the channel one amino acid at a time, the team was able to identify the one amino acid that permits interaction with anesthetic drugs.

"We need to confirm this, but from our results so far, it looks as though this mutation purely affects the anesthetic sensitivity of the channel, and does not affect the channel's function in any other way. In other words, it is a silent mutation," Franks said.

The mutation also had the effect of abolishing the different response of the channel to the two different optical isomers of the anesthetic drug, implying that the mutation lay within the anesthetic binding site.

Franks and his colleagues plan to introduce such a mutation into the genome of a mouse, to produce an animal that is normal in all respects other than the single amino acid change in the potassium channel. "We will then be able to find out if the mouse is relatively insensitive to anesthetics; if it is, then we will know that we have modified the key anesthetic target," Franks said.

Future steps ultimately will include determining the structure of the channel, including high-resolution crystallography studies.