Serotonin (5-HT) regulates neurotransmission in the nervous system via G protein-coupled receptors, including 5-HT3. 5-HT6 and 5-HT7 receptors, which are prevalent in brain regions such as the hippocampus and neocortex, and which stimulate adenylyl cyclases through G(s), influencing cAMP levels and responding to psychiatric drugs.