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Several attempts have been made to relate subtypes of schizophrenia to dopaminergic status. For example, Crow's hypothesis states that Type I schizophrenia is characterised by positive symptoms, increased activity of the dopaminergic system, good response to neuroleptics, and no brain structural changes; and Type II by negative symptoms, reduced dopaminergic activity, poor response to neuroleptics, and brain structural changes. Testing of this hypothesis has, however, been inconclusive, and it seems more likely that both types are inter-related.

• Fourteen schizophrenic subjects were evaluated for degree of psychosis before and after treatment with the antipsychotic drug haloperidol and for plasma homovanillic acid concentration after four and 28 days of treatment. A significant correlation was found between an increase in homovanillic acid concentration on day 4 or a decrease from day 4 to day 28 and the degree of improvement after four weeks of treatment. Thus, those patients who had the greatest change in plasma homovanillic acid in response to neuroleptic blockade showed the greatest improvement. These and other observations have led us to propose that the central dopaminergic system, through adaptive changes in activity, serves as a physiologic buffer system protecting against destabilization of mental function from diverse biologic or psychologic insults.