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
Site of action of opioid receptor is: 
Select and explain. 
A. Area postrema, B. Dorsal horn, C. Injury site, D. Brain
A: Area postrema
Answer: A: Area postrema 
Explanation: 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.