Serotonin 5-HT1A and 5-HT1B receptors and the 5-HT transporter are key regulators of the serotoninergic neuronal phenotype. We show here that genetic deletion of any of these elements differentially regulates 5-HT neuronal number in rostral raphe cultures from E14 mice. Serotonin neuronal number was increased by almost four-fold and 1.8-fold in cultures from 5-HT1AR-/- and 5-HT1BR-/- mice, respectively. In contrast, the lack of serotonin transporter expression was associated with a 50% decrease in 5-HT neuronal number. In raphe cultures from the rat, BDNF and cAMP have been shown to up-regulate the neuronal serotoninergic phenotype through TrkB-dependent mechanisms [Rumajogee et al. (2002) J. Neurochem., 83, 1525-1528]. Similar tyrosine kinase-dependent up-regulating effects, in the absence of serotoninergic key-elements are reported here, on both 5-HT neuronal number and neurites length. However, the extents of BDNF-triggered and cAMP-triggered effects on serotoninergic neuritic length were approximately 1.5-fold higher in 5-HT1AR-/- mutants. These findings show that the up-regulatory mechanisms triggered by BDNF on serotoninergic neuronal number and neurite extension are different and that the latter are partially linked to 5-HT, probably through 5-HT1A autoreceptors. Together, these data suggest that serotonin autoreceptors, mainly 5-HT1A but also 5-HT1B, may be responsible for a tonic auto-inhibitory effect of 5-HT itself on the serotoninergic neuronal phenotype during embryonic development, particularly marked in the absence of the 5-HT transporter.