Morphological and functional studies on the serotoninergic innervation of the inferior olive

The inferior olive of the cat has, with fluorescence histochemistry, been shown to receive a rich serotoninergic innervation. The distribution of this innervation agrees with the topography of spinal afferent termination as well as the olivo-cerebellar climbing fiber projection. This indicates that different olivary compartments are under different serotoninergic influence. The serotoninergic innervation of the dorsal accessory nucleus (DAO) of the inferior olive of the rat has been identified with electron microscopic radioautography after labelling with 3H-serotonin. The serotoninergic varicosities contain microcanaliculi, tubular-vesicular organelles and large granular vesicles. Few of the serotoninergic varicosities engage in typical synaptic junctions. However, non-junctional varicosities often display other ultrastructural indications of polarity and directed transmitted release. Electrophysiological results indicate that the harmaline-induced tremor, as well as the tremor component of the "serotonin-syndrome", depends on the serotoninergic innervation of the inferior olive. Thus, the sensitivity of different olivary compartments to the induction of rhythmic, synchronous activity by harmaline parallels the distribution of serotoninergic innervation. Neurotoxic destruction of the serotoninergic innervation leads to decreased sensitivity to harmaline. Further, the serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine, as well as monoamine oxidase inhibition + L-tryptophan loading, leads to rhythmic mass climbing fiber activity in the cerebellum and whole body tremor. A neuromodulatory effect of serotonin on the olivary action potential mechanisms is proposed.