HTR4 and Alzheimer disease: We used a combination of in vivo trans-synaptic circuit-dissecting anatomical approaches, chemogenetic manipulations, optogenetic manipulations, pharmacological methods, behavioral testing, and electrophysiological recording to reveal that the DRN–HPC  serotonergic circuit may be a new pathway for AD-associated depression, and that targeting 5-HT1BR/5-HT4R is a potential therapeutic intervention for AD.