An Immunohistochemical Study of Serotonin Development in the Opossum Cerebellum

In the present study we have used the indirect antibody peroxidase anti-peroxidase technique to analyze the development of serotonin in the cerebellum of pouch young opossums ranging in age from birth (postnatal day (PD) 1) - to PD 47. The pathways by which serotoninergic axons enter the cerebellum appear to change during development. Between PD 1 and PD 11 varicose serotoninergic fibers course though the continuity between the tectum and the dorsal medial aspect of the cerebellar plate and distribute primarily to the intermediate zone. By PD 11 serotoninergic fibers enter the cerebellum via a connecting band between the cerebellum and the dorsal lateral aspect of the rostral medulla. Fibers entering the cerebellum via this later route course around the lateral recess of the fourth ventricle and initially distribute to ventral and lateral areas of the cerebellum. At later developmental ages (PD 14-PD 47) serotoninergic fibers are present in: 1) the cellular zone of migration between the Purkinje cell layer and deep nuclei; 2) the Purkinje cell layer; 3) the internal granule cell layer; and 4) the deep cerebellar nuclei. The external granule cell and the molecular layers rarely contain serotoninergic fibers. The present study has shown that serotonin is present in the cerebellar anlage on PD 1 (within 13 days of conception). This is prior to the arrival of other major afferent systems (King et al. 1982; Bishop et al. 1983; Martin et al. 1983; Morgan et al. 1983). Further, serotoninergic axons reach the cerebellum via different routes at different stages of development. The presence of serotonin in the intermediate zone early in development supports the hypothesis that it may influence neuronal migration and differentiation (Lauder and Krebs 1976). Axons containing serotonin and arriving later in development may function in synaptic transmission, a role proposed for this indoleamine in the adult (Strahlendorf et al. 1979).