Normal development of serotonergic neurons in mice lacking S100B.

S100B, a glia-derived calcium binding protein, exhibits strong neurite extension activity in cultured serotonergic neurons. Using S100B-knockout mice, we examined whether this protein possesses in vivo serotonergic trophic activity. The distribution of serotonergic fibers, determined by immunohistochemistry, in the brains of S100B-knockout mice was quite similar to that of wild-type mice. Furthermore, the content of serotonin and its metabolite 5-hydroxyindole-3-acetic acid in knockout mice was also indistinguishable from those of wild-type mice. Our findings argue against the hypothesis that S100B has a crucial role in neurite extension of serotonergic neurons.