A 41-year-old female with generalized anxiety disorder has been taking sertraline 200 mg daily for 14 weeks. She reports meaningful but incomplete improvement: her HAM-A score has decreased from 30 to 18, she no longer has panic-like episodes, but she continues to experience persistent worry, muscle tension, and difficulty relaxing. She tolerates sertraline well with no significant side effects and prefers not to switch antidepressants. The PMHNP considers augmentation with buspirone 7.5 mg twice daily. Which of the following best describes the pharmacologic rationale for adding buspirone to augment an SSRI partial response?