A 42-year-old woman with recurrent major depressive disorder has been on fluoxetine 40 mg daily for 8 weeks with partial response. The PMHNP considers augmentation and reviews the neuroplasticity hypothesis of antidepressant action. The patient asks why her medication took several weeks to produce any benefit if it increases serotonin almost immediately. Which of the following best explains the delayed therapeutic response to antidepressant medications through the lens of the neuroplasticity hypothesis?