Methods
Patients with bipolar disorder (n=14) either had never received neuroleptic medication or had been neuroleptic-free for more than 6 months, and they met DSM-III criteria for currently symptomatic affective disorder. Patients with bipolar disorder were compared with matched schizophrenic patients and normal controls. All received two positron emission tomographic scans, the second of which was preceded by oral administration of haloperidol lactate, to permit the calculation of D₂ dopamine receptor Bmax.

Results
Diagnostic groups differed in B_max by analysis of variance (P<.0001); post hoc tests showed higher B_max values for psychotic patients with bipolar disorder and schizophrenic patients compared with normal controls and for schizophrenic patients and psychotic patients with bipolar disorder compared with nonpsychotic patients with bipolar disorder. Among patients with bipolar disorder, B_max values correlated significantly with the severity of psychotic symptoms (r=.63) on the Present State Examination but not with the severity of nonpsychotic mood symptoms.

Conclusions
We conclude that, like schizoprenic patients, patients with psychotic bipolar disorder have elevations of D₂ dopamine receptor B_max values and that such elevations in affective disorder are more closely associated with the presence of psychosis than with mood abnormality. Elevations in dopamine receptor values thus may occur in psychiatric states that are characterized by psychotic symptoms rather than being specific to schizophrenia.