Background: Serotonergic abnormalities are found in both major depressive disorder (MDD) and schizophrenia. Depressive symptoms commonly occur alongside the negative or defect symptoms in schizophrenia and antiserotonergic drugs may be particularly effective in their treatment. We wished to explore whether these symptoms could be distinguished biologically by directly comparing serotonergic function in these two illnesses.
Method: Fifteen patients with MDD and 13 patients with schizophrenia underwent testing with the specific serotonin releasing agent D-fenfluramine (D-FEN). Prolactin and cortisol responses were measured to ascertain central serotonergic function. Individual patient results were compared with their own carefully matched control to correct for the effect of age, sex, weight and menstrual cycle, before the two patient groups were then compared.
Results: Prolactin responses differed significantly between the two patient groups, being lower in MDD patients and higher in schizophrenia patients than their individually matched controls. Cortisol responses did not differ. Within the schizophrenia group, increased serotonergic function correlated positively with depressive symptoms, but there was no such correlation with defect symptoms. Depressive scores were negatively correlated with the presence of negative symptoms in the schizophrenic group.
Conclusions: Schizophrenia and MDD have distinct and opposite neuroendocrine responses to D-FEN. There is no evidence that depressive symptoms in these two conditions have a common serotonergic basis. Moreover, these responses distinguished between negative and depressive symptoms in our schizophrenic group.