Scientists have learned how to picture and track the key chemicals that make the brain work or fail.

This should enable them to follow the flow of these chemicals--the neurotransmitters, the substances that carry messages from one brain cell to another--and to observe what happens to living brains in both physical and mental illnesses.

Almost simultaneously, scientists at McMaster University in Hamilton, Ontario, have made the first pictures of a neurotransmitter at work in live human brains, and scientists at Johns Hopkins University in Baltimore have made the first pictures of the neurotransmitter's receptors, the parts of the cell to which it attaches.

The pictures are computer reconstructions of cross-sections of the brain.

They are made by injecting radioactive chemicals into a person, then recording the radioactive emissions--positively-charged particles called positrons--with a million-dollar PET (for Positron Emission Tomography) scanner.

The neurotransmitter caught at work is dopamine, but it could be only the first of several important ones visualized.

Parkinson's disease, a serious nerve disorder, is caused by the brain's failure to make enough dopamine. And schizophrenia, according to much current evidence, may be caused in whole or part by overactivity of the brain's dopamine-making system.

So the new technique could be used to study the mechanisms that go wrong--and perhaps might be cured--in Parkinson's disease and possibly in other movement disorders, like Huntington's Disease.

It could be used to compare the brains of normal individuals and known schizophrenics to see if schizophrenia is a dopamine disorder.

The new studies were called "significant events" by Dr. Frederick Goodwin, director of intramural research at the National Institute of Mental Health here.

"Take schizophrenia," Dr. Michael Kuhar at Johns Hopkins said. "We've always been able to take the brains from dead schizophrenics and study the dopamine receptors.

"Now for the first time we can study them, we can study the physiology of the brain, before death. We can do this during the very course and treatment of a disease."

The Canadian work is described in the British journal Nature by McMaster's Dr. E. Stephen Garnett, Dr. Gunter Firnau and Claude Nahmias.

The Johns Hopkins research is reported in Science by Dr. Henry Wagner Jr. and 12 co-workers.

The Johns Hopkins scientists pictured the dopamine receptors--bright collections of molecules on cell surfaces--in the brains of five subjects. The McMaster scientists showed the dopamine in the brains of three volunteers on the laboratory staff.

"Our work and McMaster's is complementary," Kuhar said. "In some diseases there is a neurotransmitter disorder, and in others a receptor disorder."

McMasters' Garnett said the scientists worked with L-dopa, commonly used to treat Parkinsonism. The chemical is converted into dopamine in the brain. L-dopa was first labeled with radioactive fluorine and then injected into a subject's arm.

It localized in the dopamine pathways, the parts of the brain, mainly the basal ganglia, where many nerve cells communicate with each other by way of dopamine. And the radioactive L-dopa became radioactive dopamine, which can be detected by the PET scanner.

In the photos, the dopamine shows as unmistakable collections of matter, concentrated in the basal ganglia and, to a lesser degree, the cingulate (a bundle of fibers) and the frontal cortex, part of the brain's outer cap.

In a similar way, the Johns Hopkins group injected a radioactive form of a drug called spiperone, which concentrates in the dopamine receptors along the same pathways.

Scientists at three other centers--the National Institute of Neurological and Communicative Disorders and Stroke in Bethesda, Brookhaven National Laboratory on Long Island and Washington University in St. Louis--have begun using PET scanning to examine the dopamine receptors in primates or human volunteers.

Their main obstacle is maintaining a steady supply of the right radioactive chemicals.

"We're trying to get a $2.5 million grant for a medical cyclotron" to convert normal atoms to radioactive ones, Garnett said. "If we had it we could study two or three subjects a day, instead of one every week or 10 days."