After years of little progress, scientists are making headway in the search for a better, faster-acting antidepressant. Experiments with an anesthetic called ketamine have yielded important clues about the biology of depression, leading scientists to attack the mood disorder in new ways.

Improved treatments are sorely needed. Depression affects about one in 10 adult Americans each year, while current drugs work in only 50 percent to 60 percent of patients, can cause sexual problems and take weeks to work. They also carry a small risk of suicidal thoughts and behaviors in adolescents and young adults.

All existing antidepressant drugs work in much the same manner. They act on a handful of neurotransmitters — brain chemicals that pass messages along defined pathways — that belong to a family called monoamines and circulate in a relatively small portion of the brain.

The most recognized of these neurotransmitters is serotonin, low levels of which have been linked to depression. Drugs such as Prozac and Celexa are aimed at raising serotonin levels available to the brain. The other transmitters targeted by antidepressants are dopamine and norepinephrine. Wellbutrin is an example of an antidepressant that boosts levels of these neurotransmitters but not serotonin.

Scientists first discovered these neurotransmitters affect depression in the 1950s when they found that the tuberculosis drug iproniazid also lifted patients' moods. The drug blocks an enzyme that converted serotonin, dopamine and norepinephrine into inactive forms, and after some study the drug was marketed for depression. Pharmaceutical companies have developed drugs targeting the trio of neurotransmitters ever since.

Because it is expensive to discover and develop new drugs, it is safer for pharmaceutical companies to stick with known biological targets than to head in risky new directions, said Dr. Carlos Zarate Jr., a researcher at the National Institute of Mental Health. As a result, "we have not made substantial gains in terms of antidepressants," Zarate said.

But recent studies on ketamine have pushed researchers to change how they think about depression. The anesthetic acts on glutamate, the brain's most plentiful neurotransmitter, which circulates widely in the brain and hasn't been linked to depression in the past.

In a small experiment led by Zarate last year, five of 18 people who received a single intravenous dose of ketamine experienced a dramatic reduction of their depression the first day and were still much better a week later. All patients in the experiment had first tried regular antidepressants but did not improve on them, according to the study published last August in the Archives of General Psychiatry.

Similar fast improvements were found in a study of eight patients conducted at Yale Medical School and published in Biological Psychiatry in 2000.

The rapid response was encouraging, Zarate said, suggesting that a faster-acting antidepressant may be possible. Current antidepressants take two to three weeks to begin working, and until recently, "it was just accepted as fact we couldn't do any better," he said.

'CLOSER TO THE CORE'

Dr. Gerard Sanacora, director of the depression research program at Yale Medical School, said the results also mean that glutamate may have a more direct role in depression than serotonin and other brain chemicals targeted by current antidepressants.

"The fact that it acts so rapidly means that it is getting closer to the core of depression," Sanacora said.

In some ways, it's surprising that researchers studying depression didn't suspect the role of glutamate, an amino acid, in the disorder sooner. Malfunctions in the glutamate system have long been linked to other psychological and neurological disorders, including schizophrenia and Alzheimer's disease. Glutamate has long been associated with learning and memory.

"It is really what makes the brain run," Sanacora said.

Still, scientists haven't yet figured out how the glutamate system goes awry in depression.

One theory is that glutamate leaks from brain cells, perhaps in response to chronic emotional stress, and causes changes in key brain structures.