A 68-year-old male with major depressive disorder presents with a 15-pound weight loss over 3 months, severe insomnia with early morning awakening, and persistent nausea that limits his oral intake. He has no history of diabetes or metabolic syndrome. Previous treatment with escitalopram was discontinued due to worsening nausea and sexual dysfunction. The PMHNP selects mirtazapine as the next treatment. Which of the following best explains why mirtazapine's receptor profile makes it particularly suited for this patient's symptom constellation?