A 38-year-old male with major depressive disorder was started on venlafaxine XR 75 mg daily and titrated to 150 mg daily over 4 weeks. At 75 mg he noticed improvement in mood and reduced crying spells but reported no change in energy, motivation, or concentration. After increasing to 150 mg and eventually 225 mg, his energy and concentration improved significantly. Which of the following best explains this sequential pattern of symptom improvement?