HTR1A and major depressive disorder: Because 5-HT system participates to MDD pathophysiology and can be impacted by inflammation and related modulation of KYN and BH4 pathways, we measured whether UCMS and HFD affected gene expression of key 5-HT elements (5-HT1A, 1B, 2C receptors, 5-HT transporter (5-HTT) and monoamine oxidases (MAO) degradation enzymes; Figs. 4A, 5A).