Decoding Schizophrenia

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Some evidence supports each of these ideas. For instance, postmortem studies of schizophrenic patients reveal not only lower levels of glutamate but also higher levels of two compounds (NAAG and kynurenic acid) that impair the activity of NMDA receptors. Moreover, blood levels of the amino acid homocysteine are elevated; homocysteine, like kynurenic acid, blocks NMDA receptors in the brain. Overall, schizophrenia's pattern of onset and symptoms suggests that chemicals disrupting NMDA receptors may accumulate in sufferers' brains, although the research verdict is not yet in. Entirely different mechanisms may end up explaining why NMDA receptor transmission becomes attenuated.

New Schizophrenia Treatment Possibilities

Regardless of what causes NMDA signaling to go awry in schizophrenia, the new understanding--and preliminary studies in patients--offers hope that drug therapy can correct the problem. Support for this idea comes from studies showing that clozapine, one of the most effective medications for schizophrenia identified to date, can reverse the behavioral effects of PCP in animals, something that older antipsychotics cannot do. Further, short-term trials with agents known to stimulate NMDA receptors have produced encouraging results. Beyond adding support to the glutamate hypothesis, these results have enabled long-term clinical trials to begin. If proved effective in large-scale tests, agents that activate NMDA receptors will become the first entirely new class of medicines developed specifically to target the negative and cognitive symptoms of schizophrenia.

The two of us have conducted some of those studies. When we and our colleagues administered the amino acids glycine and D-serine to patients with their standard medications, the subjects showed a 30 to 40 percent decline in cognitive and negative symptoms and some improvement in positive symptoms. Delivery of a medication, D-cycloserine, that is primarily used for treating tuberculosis but happens to cross-react with the NMDA receptor, produced similar results. Based on such findings, the National Institute of Mental Health has organized multicenter clinical trials at four hospitals to determine the effectiveness of D-cycloserine and glycine as therapies for schizophrenia; results should be available this year. Trials of D-serine, which is not yet approved for use in the U.S., are ongoing elsewhere with encouraging preliminary results as well. These agents have also been helpful when taken with the newest generation of atypical antipsychotics, which raises the hope that therapy can be developed to control all three major classes of symptoms at once.

None of the agents tested to date may have the properties needed for commercialization; for instance, the doses required may be too high. We and others are therefore exploring alternative avenues. Molecules that slow glycine's removal from brain synapses--known as glycine transport inhibitors--might enable glycine to stick around longer than usual, thereby increasing stimulation of NMDA receptors. Agents that directly activate "AMPA-type" glutamate receptors, which work in concert with NMDA receptors, are also under active investigation. And agents that prevent the breakdown of glycine or D-serine in the brain have been proposed.

Many Avenues of Attack

Scientists interested in easing schizophrenia are also looking beyond signaling systems in the brain to other factors that might contribute to, or protect against, the disorder. For example, investigators have applied so-called gene chips to study brain tissue from people who have died, simultaneously comparing the activity of tens of thousands of genes in individuals with and without schizophrenia. So far they have determined that many genes important to signal transmission across synapses are less active in those with schizophrenia--but exactly what this information says about how the disorder develops or how to treat it is unclear.

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new schizophrenia drugs in development ~ steep social costs of schizophrenia

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