The evolution of the serotonin-dopamine antagonist concept.

Before the dopamine hypothesis of schizophrenia became established, a serotonin (5-hydroxy-tryptamine) 5-HT hypothesis was popular. This was based on the hallucinogenic properties of lysergic acid diethlyamide and abnormal serotonin levels in schizophrenics. Suggestions that serotonin might be involved in the cause of schizophrenia or could be a target for antipsychotic drug action began with the discovery that the antipsychotic agent clozapine is a potent serotonin 5-HT2A antagonist, as well as being a dopamine D2 antagonist. This led to the formulation of the serotonin-dopamine antagonist (SDA) concept for antipsychotics, with wider spectrums of activity and lower extrapyramidal side effects (EPS) liability. The principle of the SDAs is that the drug should be a potent serotonin 5-HT2A antagonist, with slightly less potent dopamine D2 receptor-blocking properties. The clinical experience with risperidone, the first member of the new class of antipsychotics, seems to offer the promise that the SDAs have significant advantages over both the conventional dopamine-blocking neuroleptics and the atypical antipsychotic clozapine. Risperidone has efficacy against both the positive and negative symptoms of schizophrenia and has a low tendency to produce EPS. Only time will tell whether other SDAs will have the same advantages.