Dopaminergic and non-dopaminergic neurons in the ventral tegmental area of the rat project, respectively, to the cerebellar cortex and deep cerebellar nuclei.

It has been suggested recently that dopamine in the cerebellum not only acts as a precursor for noradrenaline in afferent fibers supplied by locus coeruleus neurons, but also subserves an independent transmitter role in a separate neural system. The present study was initiated to investigate the possible sources for dopaminergic innervation of the cerebellum. Employing anterograde and retrograde axonal tracing with cholera toxin and a combination of fluorescent retrograde axonal tracing with Fluoro-Gold and tyrosine hydroxylase immunofluorescence histochemistry, we found in the rat that the ventral tegmental area, containing the A10 dopaminergic cell group, sends projection fibers to the cerebellum bilaterally with a slight contralateral predominance. The projections from the ventral tegmental area to the cerebellum were segregated into the dopaminergic one to the cerebellar cortex and the non-dopaminergic one to the deep cerebellar nuclei. Dopaminergic fibers projecting from the ventral tegmental area to the cerebellar cortex terminated mainly in the granular layer, additionally in the Purkinje cell layer, but not at all in the molecular layer. They were distributed predominantly in the crus I ansiform lobule and paraflocculus, and to a lesser extent in the crus II ansiform lobule. On the other hand, non-dopaminergic fibers projecting from the ventral tegmental area to the deep cerebellar nuclei were seen to terminate mainly in the lateral nucleus, to a lesser extent in the interpositus nucleus, but not at all in the medial nucleus. The ventral tegmental area was also observed to receive projection fibers from the lateral and interpositus cerebellar nuclei bilaterally with a contralateral predominance. The projections from the ventral tegmental area to the cerebellum revealed in the present study might exert limbic influences upon the cerebro-cerebellar loops subserving the execution and co-ordination of voluntary movements.