Methacholine acts at which receptor?(AIIMS May 2015. May 2014)

**Core Concept**
Methacholine is a synthetic **cholinergic agonist** that acts on the parasympathetic nervous system, mimicking the action of **acetylcholine**. It is used in bronchial challenge tests to assess airway responsiveness. The underlying pharmacological principle being tested is the mechanism of action of methacholine on **cholinergic receptors**.

**Why the Correct Answer is Right**
Methacholine acts on **muscarinic receptors**, which are a subtype of cholinergic receptors. These receptors are coupled to **G-proteins** and are involved in various physiological processes, including smooth muscle contraction, glandular secretion, and modulation of heart rate. Methacholine's action on muscarinic receptors leads to bronchoconstriction, making it a useful tool for diagnosing and monitoring **asthma**.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because methacholine does not act on **nicotinic receptors**, which are another subtype of cholinergic receptors involved in neuromuscular transmission and neurotransmission in the autonomic nervous system.
**Option B:** and **Option D:** are not relevant to the mechanism of action of methacholine.

**Clinical Pearl / High-Yield Fact**
Methacholine challenge tests are commonly used to diagnose **asthma**, as they can induce bronchoconstriction in susceptible individuals. A positive response to methacholine is indicative of airway hyperresponsiveness, a hallmark of asthma.

**Correct Answer:** D. Muscarinic receptor.