OLIVARY PROJECTING NEURONS IN THE NUCLEUS OF DARKSCHEWITSCH IN THE CAT RECEIVE EXCITATORY MONOSYNAPTIC INPUT FROM THE CEREBELLAR NUCLEI

The direct projection from the cerebellar nuclei to the inferior olive is GABAergic. In the present study, we investigated the projection fr om the cerebellar nuclei to the mesodiencephalic junction which is kno wn to provide an excitatory projection to the inferior olive. The meso diencephalic junction was studied in cat following anterograde transpo rt of wheatgerm agglutinated horseradish peroxidase from the cerebella r nuclei in combination with: (1) retrograde transport of gold-lectin conjugate from the inferior olive; and (2) postembedding GABA-immunocy tochemistry. Light microscopic analysis revealed that overlap of the a nterograde and retrograde labeling was especially prominent in the nuc leus of Darkschewitsch. Electron microscopic examination of this area showed: (1) that many cerebellar terminals made synaptic contacts with neurons that project to the inferior olive; (2) that virtually all ce rebellar terminals were non-GABAergic and displayed an excitatory morp hology; and (3) olivary projecting neurons were non-GABAergic. It is c oncluded that the indirect cerebellar projection to the inferior olive via the nucleus of Darkschewitsch is disynaptic and excitatory.