Localization of serotonin immunoreactivity in the opossum cerebellum

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Abstract
We have used the indirect antibody peroxidase‐antiperoxidase technique to analyze (1) the course of serotonin (5‐hydroxytryptamine; 5HT) fibers to the deep cerebellar nuclei; (2) the distribution of serotonin within the nuclei; (3) the continued course of 5HT fibers to the cerebellar cortex; and (4) the lobular and laminar distribution of this indoleamine in the cerebellar cortex. Only rarely are fibers found in either the restiform body or the brachium pontis. However, a distinct bundle of serotoninergic axons is present in the medial aspect of the brachium conjunctivum. Axons arise from this bundle and course dorsally into the neuropil of the deep cerebellar nuclei. The densest immunostaining is present in posterior and ventral regions of all four cerebellar nuclei. Within the nuclei large (24% of total) and small (76% of total) varicosities are present. The average distance between varicosities on individual axons is 3.85 μm (S.D. = 1.2). The innervation of the cerebellar cortex is derived primarily from fibers that course through the deep nuclei. At levels caudal to the deep nuclei a single midsagittal band courses into lobules VIII and IX.In the cerebellar cortex, serotoninergic axons and varicosities are present in all lobules; however, the fiber density is not uniform. The densest distribution is present in vermal lobule VIII and the dorsal folia of lobule IX. Within the granule cell layer of lobules VIII and IX, immunoreactive elements form a midsagittal band, and to a lesser degree, two parasagittal bands. Beaded serotoninergic fibers course through the deep and middle portion of the granule cell layer and give rise to a plexus at the border between the Purkinje cell and granule cell layers. Within this plexus axons extend long distances in the transverse and sagittal planes. Long beaded axons oriented in the transverse plane of the folia are also present in the deep molecular layer. A few radial serotoninergic fibers ascend to the pial surface and give rise to very short tangential branches. In all three cortical layers, both large (19% of total) and small (81% of total) varicosities are present. The average distance between varicosities on individual fibers is 5.3 μm (S.D. = 2.2).In summary, these data have shown that (1) serotoninergic fibers enter the cerebellum via the brachium conjunctivum and form a dense plexus within the deep cerebellar nuclei; (2) there is a differential distribution of serotonin within these nuclei; (3) serotoninergic fibers pass through the deep nuclei and are topographically organized in their course to specific regions of the cerebellar cortex; (4) there is a differential distribution of serotonin to the various cortical lobules; (5) the densest distribution of serotonin within the cortex is to the Purkinje‐cell‐granule‐cell border; little immunoreactiveity is present in the molecular layer; and (6) this distribution is species specifie.