TSPO and major depressive disorder: In a subsequent study, the authors confirmed that the duration of untreated major depression was a strong predictor of TSPO distribution volume (VT), that microglial activation was higher in depressive patients who had not been treated with medication for a longer time than in those with a shorter course of the disease, and that the degree of microglial activation no longer increased yearly when antidepressants were administered [13, 14].