Which of the following are examples of metabotropic receptors?

• A. GABAA receptors
• B. NMDA receptors
• C. Nicotinic acetylcholine receptors
• D. GABAb receptors

Answer: (D)

The key idea is the distinction between metabotropic receptors, which are G-protein coupled and act through intracellular signaling to modulate cell activity, and ionotropic receptors, which are ligand-gated ion channels that produce fast, direct ion flow.

GABA-B receptors are metabotropic because they are GPCRs. When GABA binds, these receptors activate G proteins (typically Gi/o), which then influence downstream effectors such as inhibiting adenylate cyclase, opening inward-rectifying potassium channels, or reducing voltage-gated calcium channel activity. This leads to slower, longer-lasting inhibitory effects and indirect changes in neuronal excitability.

The others are ionotropic. GABA-A receptors are ligand-gated chloride channels that produce fast inhibitory postsynaptic currents by allowing Cl− to flow directly through the receptor channel. NMDA receptors are ion channels that conduct Ca2+ (and Na+/K+) when activated by glutamate and membrane depolarization. Nicotinic acetylcholine receptors are cation channels that open to allow Na+ and K+ (and sometimes Ca2+) through, generating rapid excitatory signaling.

So, among these options, the metabotropic receptor is the GABA-B receptor because its signaling is mediated by G-proteins and second messengers rather than forming a direct ion channel pore.