Question

Consider channel activation by agonists:
(A). Nm receptors allow influx of sodium ions
(B). Glycine receptors allow influx of chloride ions
(C). \(\text{GABA}_\text{A}\) receptors allow influx of chloride ions
(D). \(\text{GABA}_\text{B}\) receptors allow influx of chloride ions
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B) and (C) only.
• C. (A), (B), (C) and (D).
• D. (A), (C) and (D) only.

Hint:

To remember GABA receptor differences:
- \(\text{GABA}_\text{A}\) is ionotropic: Directly opens a chloride (\(\text{Cl}^-\)) channel.
- \(\text{GABA}_\text{B}\) is metabotropic: Works through G-proteins to affect potassium (\(\text{K}^+\)) and calcium channels.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Receptors are classified into ionotropic (ligand-gated ion channels) and metabotropic (G-protein coupled) receptors.
Ionotropic receptors open an internal channel upon agonist binding, allowing specific ions to flow across the cell membrane down their electrochemical gradient.

Step 2: Detailed Explanation:
Let us analyze each receptor statement:
- Statement (A): Nicotinic muscular (\(\text{N}_\text{m}\)) receptors located at the neuromuscular junction are ligand-gated ion channels.
Agonist (acetylcholine) binding opens the channel, allowing a rapid influx of sodium ions (\(\text{Na}^+\)) into the muscle cell, which causes depolarization. Thus, statement (A) is correct.
- Statement (B): Glycine receptors are inhibitory ligand-gated ion channels found in the central nervous system.
Activation by glycine opens the channel, allowing an influx of chloride ions (\(\text{Cl}^-\)), which hyperpolarizes the cell. Thus, statement (B) is correct.
- Statement (C): \(\text{GABA}_\text{A}\) receptors are ionotropic ligand-gated ion channels.
GABA binding opens the channel, allowing an influx of chloride ions (\(\text{Cl}^-\)), leading to hyperpolarization and central nervous system depression. Thus, statement (C) is correct.
- Statement (D): \(\text{GABA}_\text{B}\) receptors are metabotropic G-protein coupled receptors.
Their activation does not open a chloride channel; instead, they act through G-proteins to open potassium (\(\text{K}^+\)) channels (causing potassium efflux) and inhibit calcium (\(\text{Ca}^{2+}\)) channels, hyperpolarizing the cell. Thus, statement (D) is incorrect.

Step 3: Final Answer:
The correct statements are (A), (B), and (C). This corresponds to option (B).