5-HT1A agonist inhibited dendrite formation of cultured hippocampal neurons. 5-HT4 agonist promoted dendrite formation of cultured hippocampal neurons. PKA inhibitor neutralized the effects of 5-HT4 agonist on dendrite formation. 5-HT4 agonist increased the expression of BDNF mRNA. TrkB antagonist neutralized the effects of 5-HT4 agonist on dendrite formation.