Figure 6.

Deficient 5-HT_3A-signaling or embryonic SSRI-treatment alters the morphologies layers II–III pyramidal dendrites. Serotonergic afferents (purple) containing large vesicles rich in 5-HT are located in the marginal zone and below the cortical plate that send branches toward MZ. 5-HT could be released from the entirety of the axonal length. In MZ, 5-HT fibers make synaptic contacts with C-R neurons that were initially drawn by Cajal (1891). In control conditions 5-HT activates 5-TH_3A receptors located on C-R neurons. 5-HT and the activation of the 5-HT_3A has been shown to induce reelin secretion that in turn modifies the morphology of apical dendrites by controlling the growth and sprouting of their arborization. In absence of 5-HT_3A receptor that could be observed in 5-HT_3A-knockout mice or ex vivo following pharmacological blockade of the 5-HT_3A the morphology of apical dendrites become exuberant while basal dendrites that are far from the source of reelin are preserved. Following SSRI-treatment during embryogenesis excess extracellular 5-HT leads to an increased reelin secretion and to a reduction in the complexity of apical and basal dendrites.