Other morphological traits of the studied nerve cells with well-formed synaptic connections (DIV12 and older), such as the length and the number of neurites, the complexity of dendritic branching and spine density were not affected by 5-HT4R. In contrast, our previous study in immature neurons (DIV1) reported that 5-HT4R activation reduced the number of neurites and decreased the neurite length, the effects comparable to those in neuroblastoma cells27. Here, HTR4 is linked to neuroblastoma.