What is the mechanism of metabotropic receptors?

• A. Receptors directly gate ion channels to allow immediate ion flow.
• B. Ligand activates G-protein leading to a second messenger cascade.
• C. Receptors open channels via voltage changes.
• D. Receptors terminate signaling through transporter uptake.

Answer: (B)

Metabotropic receptors operate through G-protein signaling that triggers second messenger cascades, rather than directly opening ion channels. When a ligand binds, the receptor activates a heterotrimeric G protein by promoting GDP-to-GTP exchange on the alpha subunit. The alpha and the beta-gamma parts then modulate downstream effectors such as adenylyl cyclase or phospholipase C, producing second messengers like cAMP or IP3/DAG. These messengers activate kinase cascades (like PKA, PKC, or CaMK) and can modulate ion channels indirectly, alter metabolism, or influence gene expression. This setup provides signal amplification and a slower, longer-lasting response compared with ionotropic receptors, which gate ions directly in response to ligand binding.