A 31-year-old male with opioid use disorder is brought to the emergency department after being found unresponsive with a respiratory rate of 4 breaths per minute and pinpoint pupils. Naloxone 0.4 mg is administered intranasally with rapid improvement in consciousness and respiration. Within minutes, he develops severe agitation, profuse diaphoresis, projectile vomiting, diarrhea, piloerection, and diffuse myalgias. He becomes combative and demands to leave. Which of the following best explains the pathophysiology of this presentation?