Climbing fibre collaterals contact neurons in the cerebellar nuclei that provide a GABAergic feedback to the inferior olive.

The inferior olive provides climbing fibres to Purkinje cells in the cerebellar cortex and gives off axon collaterals to the cerebellar nuclei. The cerebellar nuclei contain GABAergic neurons that provide an inhibitory projection to the inferior olive and excitatory neurons that influence behaviour through various other premotor nuclei in the brainstem and diencephalon. Whether the olivary axon collaterals innervate the GABAergic neurons in the cerebellar nuclei is unknown. In the present study we investigated this projection in mice at the ultrastructural level using post-embedding GABA immunocytochemistry and anterograde and retrograde tracing of biotinylated dextrane amine and gold-lectin. It is demonstrated that the olivary axon collaterals do not only innervate non-GABAergic neurons in the cerebellar nuclei, but also GABAergic nucleo-olivary cells, thus establishing a direct feedback loop to the inferior olive.