Antagonist of morphine is:
**Core Concept**
Morphine is an opioid analgesic that acts on the mu-opioid receptor to produce its analgesic effects. To counteract its effects, an antagonist that can competitively inhibit its binding to the receptor is required.
**Why the Correct Answer is Right**
Naloxone is a pure opioid antagonist that specifically binds to the mu-opioid receptor, reversing the effects of morphine. It does not have any agonist activity, making it an effective antagonist. Naloxone's mechanism of action involves competitively inhibiting the binding of morphine to the mu-opioid receptor, thereby blocking its analgesic and other effects.
**Why Each Wrong Option is Incorrect**
**Option A:** Nalorphine is a partial agonist-antagonist at the mu-opioid receptor, meaning it can produce some analgesic effects while also having antagonist properties. However, its partial agonist activity makes it less effective as an antagonist compared to naloxone.
**Option B:** Methadone is a full opioid agonist that acts on the mu-opioid receptor, producing analgesic effects similar to morphine. It is not an antagonist.
**Option C:** Buprenorphine is a partial agonist at the mu-opioid receptor, with some agonist activity and limited antagonist properties. Its partial agonist activity makes it less effective as an antagonist compared to naloxone.
**Clinical Pearl / High-Yield Fact**
Naloxone is a specific antidote for opioid overdose and should be administered immediately in cases of suspected opioid toxicity. Its rapid onset of action makes it an essential medication in emergency settings.
**Correct Answer:** D. Naloxone