Inhalation of EO from Citrus sinensis and C. reticulata, rich in d-limonene (90.7% and 76.7%, respectively), significantly improved depression-like behavior in mice, suggesting the involvement of the main compound in key mechanisms such as increased expression of BDNF and 5-HT1A receptor, which results in neurogenesis and serotoninergic pathway improvement, respectively [67,80]. The gene discussed is HTR1A; the disease is major depressive disorder.