Which of the following receptors is coupled to a heterotrimeric G protein?
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.
Glycine receptor
GABA-B  receptor
Nicotinic acetylcholine receptor at myoneural junction
5-HT3 receptor