Which of the following receptors is coupled to a heterotrimeric G protein?: (A) Glycine receptor, (B) GABA-B  receptor, (C) Nicotinic acetylcholine receptor at myoneural junction, (D) 5-HT3 receptor
Answer is B. The GABA-B receptors are metabotropic and are coupled to heterotrimeric G proteins that increase conductance in K+ channels, inhibit adenylyl cyclase, and inhibit Ca2+ influx.