A 24-year-old man is brought to the emergency department by police for erratic behavior and complaints of insects crawling under his skin. He scratches his arms vigorously. He also reports chest pain and palpitations. He admits to insufflating an illicit substance to stay awake while studying for his exams and says he has been awake for 48 hours. Physical examination shows a temperature of 37.5°C, blood pressure 170/100 mm Hg, heart rate 135/min, diaphoresis, and dilated pupils. Laboratory studies show a positive urine benzoylecgonine test. Which of the following drug actions is most responsible for his psychotic symptoms?