ENLARGEMENT OF OLIVOCEREBELLAR MICROZONES IN THE AGRANULAR CEREBELLUMOF ADULT-RATS

Previous work on normal adult rat showed that the vibrissae project, t hrough the climbing fiber (CF) system, onto the Purkinje cells (PCs) o f the contralateral cerebellar hemivermis of lobule VII. The highly el aborated CF projections from a given row of vibrissae delimit a narrow parasagittal zone which can be regarded as a functional olive-cerebel lar microzone. Interestingly, the adult one-to-one relationship betwee n PCs and CFs is preceded by a transient phase during which each PC re ceives synaptic inputs from several CF collaterals which will be elimi nated but one, when granule cells begin to establish synapses on PCs. Therefore, the question arose as to whether this synaptic elimination process could participate in the refinement of the topographical organ ization of CF projections and could contribute to the formation of suc h precise peripheral maps onto the cerebellum. In the present study, t he topographical map of the CF-mediated projection of mystacial vibris sae onto the vermal PCs of lobule VII was determined in adult rats who se cerebellar PCs remain polyinnervated by olivary CFs due to degranul ation by postnatal X-irradiation. Using intracellular recordings, we e xamined the responsiveness of PCs in lobule VII during mechanical stim ulation of the 3rd row of contralateral vibrissae, and positioned cell s responding to the stimulation on an averaged planar map of lobule VI I. Comparison of the results to those obtained in our previous work on normal rats showed that the activated cells were more numerous and mo re diffusely distributed. PCs exhibiting the best characteristics of r esponse were distributed evenly from the midline to 700 mu m lateral i n the contralateral hemivermis and not gathered in a parasagittal micr ozone centered on the plane 200 mu m as in normal rats. These results strongly suggest that synaptic elimination in the olive-cerebellar sys tem participates in the segregation of CF-mediated inputs into functio nal microzones on the cerebellar cortex.