A 51-year-old male with no psychiatric history presents with diabetic peripheral neuropathy causing bilateral burning foot pain rated 6/10, significantly impairing his sleep and daily functioning. His endocrinologist starts duloxetine 60 mg daily specifically for neuropathic pain. The patient asks why an antidepressant is being prescribed for pain when he is not depressed. Which of the following best explains how duloxetine reduces neuropathic pain independent of its antidepressant effect?