A temporal analysis of the origin and distribution of serotoninergic afferents in the cerebellum of pouch young opossums.

In the present study, a temporal analysis of the pattern of distribution of serotoninergic fibers and varicosities within the cerebellum of pouch young opossums was carried out. Particular attention was focused on animals ranging in age from postnatal day (PD) 21-PD 72, because there is a transient expression of serotonin immunoreactivity in the cerebellar cortex during that interval. Between PD 1-33, there is a progressive increase in serotoninergic immunoreactivity throughout the cerebellar cortex. After PD 33, there is a decrease in the relative number of immunostained fibers followed by a reorganization into the adult pattern of distribution. A double labeling paradigm, in which horseradish peroxidase, used as a retrograde marker, combined with serotonin immunohistochemistry was employed to localize serotoninergic neurons that project to the developing cerebellum. Initially (PD 9), serotoninergic cells in the medullary reticular formation and dorsolateral pontine tegmentum are double labeled. After PD 77, only neurons in the medullary reticular formation were double labeled. The course taken by serotoninergic axons from the brainstem to the cerebellum also was analyzed. Between PD 1 and PD 42, serotoninergic axons enter the cerebellum via four different routes: 1) the inferior cerebellar peduncle; 2) a pathway located lateral and rostral to the inferior cerebellar peduncle; this bundle of serotonin axons contains immunoreactive fibers that also enter the tectum (this tract is referred to as the tecto-cerebellar bundle in this report); 3) the medial aspect of the superior cerebellar peduncle; and 4) the tela choroidea. After PD 40, the latter two pathways are the primary routes by which serotoninergic fibers enter the cerebellum. The loss of serotoninergic fibers in the first two pathways coincides with the decrease in serotoninergic immunoreactivity seen in the cerebellar cortex described above. In summary, the results suggest that the serotoninergic projection to the opossum's cerebellum is remodelled during development. It is proposed that the serotonin fibers present at early stages of development may play a role in regulating specific events in cerebellar maturation. In contrast, the serotoninergic axons which have a more restricted pattern of distribution later in development, and in the adult, likely modulate neuronal activity within the cerebellum.