Enlargement of olivo-cerebellar microzones in the agranular cerebellum of adult rats.

Previous work on normal adult rat showed that the vibrissae project, through the climbing fiber (CF) system, onto the Purkinje cells (PCs) of the contralateral cerebellar hemivermis of lobule VII. The highly elaborated CF projections from a given row of vibrissae delimit a narrow parasagittal zone which can be regarded as a functional olivo-cerebellar microzone. Interestingly, the adult one-to-one relationship between PCs and CFs is preceded by a transient phase during which each PC receives synaptic inputs from several CF collaterals which will be eliminated 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 organization of CF projections and could contribute to the formation of such precise peripheral maps onto the cerebellum. In the present study, the topographical map of the CF-mediated projection of mystacial vibrissae onto the vermal PCs of lobule VII was determined in adult rats whose cerebellar PCs remain polyinnervated by olivary CFs due to degranulation by postnatal X-irradiation. Using intracellular recordings, we examined the responsiveness of PCs in lobule VII during mechanical stimulation of the 3rd row of contralateral vibrissae, and positioned cells responding to the stimulation on an averaged planar map of lobule VII. Comparison of the results to those obtained in our previous work on normal rats showed that the activated cells were more numerous and more diffusely distributed.(ABSTRACT TRUNCATED AT 250 WORDS)