Role of serotonin 4 receptor in the growth of hippocampal neurons during the embryonic development in mice.

Serotonin (5-HT) homeostasis is critical for the brain development which influences neurogenesis, neuronal migration, and circuit formation. Distinctive distribution patterns of serotonin receptors (5-HTRs) in the brain govern various physiological activities. Amongst the 5-HTRs, serotonin 4 receptor (5-HT4R) is widely expressed in embryonic forebrain and affects neuronal development, synaptogenesis, and behavior, but its specific role in brain development is still not completely understood. Therefore, in the present study, we addressed the roles of 5-HT4R in the growth of hippocampal neurons during the development of mice brain. We cultured hippocampal neurons of the mouse at embryonic day 18 and then treatment of 5-HT4R agonist RS67333 was employed. We found RS67333 significantly increased the axonal length, diameter and branching along with total dendritic length, number of primary dendrites and their branching. In addition, these effects were neutralized by the concomitant treatment of 5-HT4R antagonist GR125487, which confirmed the specific role of the 5-HT4R in the growth of axon and dendrites. Further, the treatment of RS67333 upregulated the mRNA expression of collapsin response mediator protein-2 (CRMP2) and non-phosphorylated CRMP2 (npCRMP2) together with neurotrophic factors (BDNF, NT-3, NGF) and TRK-A. Additionally, the current research findings reveal that the knockdown of CRMP2 inhibited RS67333-induced growth of the axons and dendrites, which indicates that CRMP2 is required for the 5-HT4R-mediated growth of the axons and dendrites. Overall, the findings of the present in vitro study enrich the understanding and provide insight roles of 5-HT4R in embryonic brain development by promoting the growth of hippocampal neurons.
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