<prompt> Which of the following receptors is coupled to a heterotrimeric G protein?Glycine receptorGABA-B receptorNicotinic acetylcholine receptor at myoneural junction5-HT3 receptor  <response>: 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. 