A 32-yeаr-оld pаtient with Mаjоr Depressive Disоrder (MDD) presents with persistent low mood, anhedonia, insomnia, and poor concentration. You are discussing treatment options and explaining how certain medications target neurobiological mechanisms involved in depression. Which of the following best describes the neurobiological mechanism targeted by Selective Serotonin Reuptake Inhibitors (SSRIs)?