This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
[{'content': 'Site of action of opioid receptor is: \nSelect and explain. \nA. Area postrema, B. Dorsal horn, C. Injury site, D. Brain', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Opioid receptors subtypes "u" and "k" receptors ;high density of u receptor detected in periaqueductal gray,thalamus,nucleus tract us solitarus ,nucleus ambiguous,area postrema. Ref KD Tripathi 8th ed.', 'role': 'assistant'}]