Serotoninergic fibres form dense synaptic contacts with Purkinje cells in the mouse cerebellar cortex--an immunohistochemical study.

The distribution of serotonin immunoreactivity in the mouse cerebellar cortex was studied using the indirect antibody peroxidase-antiperoxidase (PAP) technique of Sternberger (1979) on epoxy embedded semithin sections. The great majority of serotonin-positive afferents distribute throughout the Purkinje cell layer and form dense synaptic contacts with the somata of the Purkinje neurons. Only a sparse immunostaining of serotoninergic fibres could be detected at the granular cell and molecular layers. The microanatomical organization of the serotoninergic projections to the mouse cerebellar cortex is quite different from that observed in other animal species. These findings suggest that in the mouse cerebellar cortex, the Purkinje cell population represents the main target for serotoninergic afferents. Our histochemical data provide morphological support for a series of electrophysiological observations which indicate serotonin as a potential modulatory neurotransmitter for Purkinje cell firing activity.