Carbon monoxide, the toxic gas found in car exhaust and industrial emissions, appears also to be a natural brain chemical that plays a key role in relaying messages from one nerve cell to the next.
The discovery by scientists at the Johns Hopkins School of Medicine expands by one the list of neurotransmitters -- chemical messengers -- that regulate the way we feel, move, sense the environment and react to stress. These chemicals number about 50, but carbon monoxide is just the second gas found to play this role.
For many years, scientists have known that carbon monoxide is released throughout the body as a natural but useless byproduct in the breakdown of red blood cells. The gas is produced whenever the cells are destroyed and replaced by new ones.
But the new finding, reported in today's issue of Science, is that carbon monoxide is also produced by nerve cells -- known as neurons -- for the specific purpose of relaying messages along the circuitry of the brain.
"While it's always been thought that the little bit of carbon monoxide you make is a waste product -- you breathe it off -- we're now suggesting that it also may play a role in the biochemistry of the brain," said Dr.Ajay Verma, a neurology resident at the Walter Reed Army Medical Center who carried out the research while a doctoral student at Hopkins.
"We hope to use animal models to see what role carbon monoxide is playing in certain diseases," he said. The Hopkins studies have been done on rat cells grown in the laboratory.
Some diseases occur because the brain produces too little of a particular neurotransmitter, or when it is assaulted by too much of another.
Dr. Solomon Snyder, the Hopkins neuroscientist who supervises where the discovery was made, said he could only speculate about carbon monoxide's specific function. Other neurotransmitters, for instance, are known to regulate muscle contraction, heart beat, blood flow and responses to stress.
Although further study is needed, Dr. Snyder said carbon monoxide may protect people from getting overexcited.
"Our theory -- and it's pure theory -- is that carbon monoxide may be a regulator of excess brain activity," Dr. Snyder said. "It may be what handles brain functioning when you are very excited, when someone's screaming at you. It would modulate things -- protect you from overexcitation."
Along the same lines, he said, it might also protect people from diseases such as epilepsy that are marked by uncontrolled nervous excitement. Another possibility is that it protects the brain from one of the damaging effects of stroke -- the massive release of chemicals that kill cells by overstimulating them.
"Our guess is that the carbon monoxide system is one that doesn't do much normally -- it's one of the things that is activated when you have big trouble," Dr. Snyder said.
Intense exposure to carbon monoxide from exhaust fumes can be deadly because it saps blood cells of their oxygen, choking off tissues that need oxygen to live. It remains to be seen, however, whether breathing too much carbon monoxide also plays havoc with its normal functioning as a neurotransmitter.
Doctors have long known that patients who survive carbon monoxidepoisoning are susceptible to Parkinson's disease, a severe movement disorder. The reason, although unknown, could be brain cells starved of oxygen, or a neurotransmitter system thrown out of balance.
There are billions of neurons in the nervous system. Messages flow from one neuron to the next along a complicated circuitry that passes electric signals from the brain to the body's tissues and back.
Neurotransmitters act like intermediaries, carrying the signal across the tiny space that separates one neuron from the next.
Several years ago, nitric oxide was identified as a neurotransmitter -- establishing gases as a new class of neural messengers.