Serotonin receptor 4 in the hippocampus modulates mood and anxiety

Serotonin receptor 4 (5‑HT4R) plays an important role in regulating mood, anxiety, and cognition, and drugs that activate this receptor have fast-acting antidepressant (AD)-like effects in preclinical models. However, 5‑HT4R is widely expressed throughout the central nervous system (CNS) and periphery, making it difficult to pinpoint the cell types and circuits underlying its effects. Therefore, we generated a Cre-dependent 5‑HT4R knockout mouse line to dissect the function of 5‑HT4R in specific brain regions and cell types. We show that the loss of functional 5‑HT4R specifically from excitatory neurons of hippocampus led to robust AD-like behavioral responses and an elevation in baseline anxiety. 5‑HT4R was necessary to maintain the proper excitability of dentate gyrus (DG) granule cells and cell type specific molecular profiling revealed a dysregulation of genes necessary for normal neural function and plasticity in cells lacking 5‑HT4R. These adaptations were accompanied by an increase in the number of immature neurons in ventral, but not dorsal, dentate gyrus, indicating a broad impact of 5‑HT4R loss on the local cellular environment. This study is the first to use conditional genetic targeting to demonstrate a direct role for hippocampal 5‑HT4R signaling in modulating mood and anxiety. Our findings also underscore the need for cell type-based approaches to elucidate the complex action of neuromodulatory systems on distinct neural circuits. Polysome-bound mRNAs from TRAP IPs were compared to whole tissue mRNAs (input) or TRAP IP mRNAs were compared between wildtype (WT) and conditional Htr4 knockouts (cKO). Data was collected from neurons in the ventral dentate gyrus using TRAP. We collected three replicates for WT IP and inputs, while cKO data was collected in duplicate.