Development of serotonergic projections to the suprachiasmatic nucleus in the mouse brain

Serotonin (5-HT) and its innervation have been implicated in various neural functions including circadian systems. Although classical studies have examined the 5-HT innervation pattern in the adult suprachiasmatic nucleus (SCN), the fine-grained morphological study of the development of pathway and terminal projections to the SCN remains scarce. Here, we utilize transgenic mice expressing GFP under the serotonin transporter (SERT) promoter to subserve our developmental mapping study. We demonstrate that the morphology of 5-HT pathway fibers decussating over the supraoptic commissure that projects to the SCN exhibits two distinct developmental patterns. The punctate fibers at the fetal stage gradually become smooth and filamentous, especially during postnatal one week and remain constant thereafter. The innervation field in the SCN develops properly only during postnatal two weeks. Its ventromedial area remains one of the highest 5-HT innervated areas in the adult brain, whereas the dorsolateral area is less innervated. Thus, we provide novel and specific insights on the developmental map of 5-HT system into the SCN using transgenic mouse.