The olivocerebellar projection in the rat: an autoradiographic study.

The extent of the olivocerebellar projection was examined in the rat using autoradiographic techniques. In animals in which injections of [3H]leucine encompassed the whole olive unilaterally (4 cases), the vast majority of olive cells was densely labelled and climbing fibres were heavily labelled throughout the contralateral hemicerebellum, except for some small gaps which were not consistently located between cases. The multiple injections required to cover the oliver inevitably labelled cells in the reticular formation surrounding the olive, and it is possible that these neurones might also provide climbing fibres to the cerebellum. To control for this possibility, the inferior olive was pharmacologically destroyed (4 cases) prior to [3H]leucine injections similar in size and placement to those given to normal animals. Examination of the cerebellar cortex of these pretreated animals revealed no molecular layer labelling despite identification of labelled reticular neurones. It was thus demonstrated that all regions of the cerebellar cortex receive afferents from the inferior olive which terminate as climbing fibres. The distribution of these terminations over the entire cortex permits the conclusion that the inferior olive is the major source of climbing fibres in the rat. The same conclusions are reached using [3H]methionine as the tracer (4 cases).