Objective: Studies have demonstrated an association between cortisol levels and availability of the serotonin transporter (SERT) in healthy subjects and in those with major depressive disorder (MDD). We evaluated if this association could be observed in bipolar disorder (BD). In addition, a second biomarker, brain-derived neurotropic factor (BDNF), was considered to test the influence of the BDNF level on SERT availability in BD.
Methods: Twenty-eight subjects with euthymic BD type I as well as 28 sex- and age-matched healthy controls (HCs) were recruited. 123I-ADAM with single-photon emission computed tomography (SPECT) was applied for measurement of SERT availability in the brain. Ten milliliters of venous blood were taken when the subject underwent SPECT for measuring plasma levels of cortisol and BDNF. The simple ratio method was used for calculation of SERT availability.
Results: A significant decrease in SERT availability in BD compared with HCs was noted, whereas plasma levels of cortisol and BDNF did not show a significant difference. Linear regression analyses showed that changes in SERT availability could be explained only by plasma levels of cortisol but not by BDNF levels and their interaction in HCs. Notably, this phenomenon was not observed in BD patients. These data suggest that the association of cortisol level and SERT availability seen in HCs appears to be disrupted in BD patients.
Conclusion: Our data demonstrate not only a disruption in the relationship between cortisol level and SERT availability in BD, but also imply a different role of cortisol regulation of the SERT between MDD and BD.