HTR4 and major depressive disorder: There was a trend towards a difference in the direct effect of testosterone (i.e. when disregarding the contribution from estradiol) on the global LV of 5-HT4R (γ1MDD-HC = 0.0076, CI: −0.001; 0.016, p = 0.082) between men with MDD and healthy men (Fig. 2, group stratified partial correlations of regional 5-HT4R and testosterone levels are displayed in Supplementary Fig. 3).