The development of serotonergic projections to the olfactory bulb of Monodelphis domestica (the grey, short-tailed opossum)

The offspring of Monodelphis domestica (grey, short-tailed opossum) are born in a very immature state after a short (14-day) gestation period. As a result, they provide a useful mammalian model for examining quite early stages of brain maturation. The present study demonstrates that the onset and development of serotonergic (5-HT) projections to the Monodelphis olfactory bulb can be traced entirely after birth. 5-HT was visualized in tissue sections from postnatal day 5 (P5), P10, P20, P30, and adult opossums using avidin-biotin peroxidase immunocytochemistry. 5-HT afferent fibers are rare in the bulb until P10. Maturation occurs slowly, but by P30, 5-HT fibers assume the adult form characterized by preferential glomerular innervation. Due to the postnatal development, slow maturation, and specificity of innervation patterns, the Monodelphis bulb provides a model with which to study the regulation of serotonergic development.