Mechanism of action of methadone is

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).