Oxidation and nitration in dopaminergic areas of the prefrontal cortex from patients with bipolar disorder and schizophrenia

Background

Increased oxidative stress is strongly implicated in bipolar disorder (BD), where protein oxidation, lipid peroxidation and oxidative damage to DNA have been consistently reported. High levels of dopamine (DA) in mania are also well-recognized in patients with BD, and DA produces reactive oxygen species and electron-deficient quinones that can oxidize proteins when it is metabolized.

Methods

Using immunohistochemistry and acceptor photobleaching Förster resonance energy transfer (FRET), we examined oxidation and nitration of areas immunoreactive for the DA transporter (DAT) and tyrosine hydroxylase (TH) in the postmortem prefrontal cortex from patients with BD, schizophrenia and major depression as well as nonpsychiatric controls.

Results

We found increased oxidation of DAT-immunoreactive regions in patients with BD (F_3,48 = 6.76, p = 0.001; Dunnett post hoc test p = 0.001) and decreased nitration of TH-immunoreactive regions in both patients with BD (F_3,45 = 3.10, p = 0.036; Dunnett post hoc test p = 0.011) and schizophrenia (p = 0.027). On the other hand, we found increased global levels of oxidation in patients with BD (F_3,44 = 6.74, p = 0.001; Dunnett post hoc test p = 0.001) and schizophrenia (p = 0.020), although nitration levels did not differ between the groups (F_3,46 = 1.75; p = 0.17).

Limitations

Limitations of this study include the use of postmortem brain sections, which may have been affected by factors such as postmortem interval and antemortem agonal states, although demographic factors and postmortem interval were accounted for in our statistical analysis.

Conclusion

These findings suggest alterations in levels of protein oxidation and nitration in DA-rich regions of the prefrontal cortex in patients with BD and schizophrenia, but more markedly in those with BD.