This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Pharmacology
[{'content': 'Mechanism of action of methadone is \nSelect and explain. \nA. Opioid receptor agonist, B. Opioid receptor partial agonist, C. Opioid receptor antagonist, D. Opioid receptor inverse agonist', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Opioid receptor agonist Methadone* Methadone is opioid receptor agonist.* Most important feature is high oral: parenteral activity ratio (1:2) and firm binding to tissue protein.* It relieves cancer and neuropathic pain not responding to morphine.* Abuse potential is less than morphine.* It also blocks NMDA receptors and uptake of monoamines.* It is the drug of choice for substitution therapy of opioid dependence.* 1 mg of methadone can be substituted for 4 mg of morphine.* Methadone is less hypnotic than morphine (sedative action is less intense).", 'role': 'assistant'}]