Early serotonergic projections to Cajal-Retzius cells: relevance for cortical development.

Although the serotonergic system plays an important role in various neurological disorders, the role of early serotonergic projections to the developing cerebral cortex is not well understood. Because serotonergic fibers enter the marginal zone (MZ) before birth, it has been suggested that they may influence cortical development through synaptic contacts with Cajal-Retzius (CR) cells. We used immunohistochemistry combined with confocal and electron microscopy to show that the earliest serotonergic projections to the MZ form synaptic contacts with the somata and proximal dendrites of CR cells as early as embryonic day 17. To elucidate the functional significance of these early serotonergic contacts with CR cells, we perturbed their normal development by injecting pregnant mice with 5-methoxytryptamine. Lower reelin levels were detected in the brains of newborn pups from the exposed animals. Because reelin plays an important role in the cortical laminar and columnar organization during development, we killed some pups from the same litters on postnatal day 7 and analyzed their presubicular cortex. We found that the supragranular layers of the presubicular cortex (which normally display a visible columnar deployment of neurons) were altered in the treated animals. Our results suggest a mechanism of how serotonergic abnormalities during cortical development may disturb the normal cortical organization; and, therefore, may be relevant for understanding neurological disorders in which abnormalities of the serotonergic system are accompanied by cortical pathology (such as autism).