Topographical organization in the origin of serotoninergic projections to different regions of the cat cerebellar cortex

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Abstract
The distribution of serotonin immunoreactivity in the cat cerebellum was studied by using the indirect antibody peroxidase-antiperoxidase (PAP) technique. Furthermore, the origin of these chemically defined afferents was determined by combining the retrograde transport of horseradish peroxidase (HRP) with the PAP technique. In the cerebellar cortex, serotonin immunoreactivity is present in a plexus of beaded fibers that is confined almost exclusively to the granule and Purkinje cell layers; a few fibers are present in the molecular layer. Serotoninergic axons and varicosities have a dense and uniform distribution throughout all lobules of the cerebellum with the exception of lobule X where the fiber density is sparse. Serotonin cell bodies were not found within the cerebellar cortex. However, following pretreatment with pargyline and L-tryptophan, serotonin positive cell bodies were found in all deep cerebellar nuclei as well as the raphe and reticular nuclei in the brainstem. The present study demonstrates that the serotoninergic projection to the cat's cerebellum has some degree of topographical organization. Serotoninergic fibers in the anterior vermis (lobules I–V) were shown to arise from neurons located within the paramedian reticular nucleus, the lateral reticular nucleus, and the lateral tegmental field. Injections of HRP into either the posterior vermis (lobule VI–IX) or the paramedian lobule, labeled serotoninergic neurons exclusively in the lateral reticular nucleus. Lobus simplex, crus I and crus II (the hemisphere) receive a serotoninergic input from cells located in the lateral tegmental field, the peri-olivary reticular formation and the paramedian reticular nucleus. In no cases were neurons in the raphe double-labeled, although there were cells positive for HRP or serotonin alone. The data indicate that there is a topographical organization in the serotoninergic projection from the caudal brainstem to specific regions of the cat's cerebellar cortex. In addition to climbing and mossy fibers, this projection represents a third major source of cerebellar afferents based on its dense and widespread distribution as well as its morphological and chemical characteristics.