ABNORMAL IPSILATERAL FUNCTIONAL VIBRISSAE PROJECTION ONTO PURKINJE-CELLS MULTIPLY INNERVATED BY CLIMBING FIBERS IN THE RAT

We have previously shown that synapse elimination occurring in the cli mbing fiber (CF)-Purkinje cell (PC) relationships during normal postna tal development is likely involved in the refinement of vibrissae proj ections onto the cerebellar cortex. In normal adult rats, CF-mediated vibrissae projections onto cerebellar Purkinje cells of the vermis of lobule VII are strictly contralateral and located in a narrow microzon e whereas they are widely distributed in rats whose PCs remained multi ply innervated by CFs due to postnatal irradiation. Given the proximit y of this microzone to the midline, the question arose as to whether t his synapse elimination process could participate in the segregation o f ipsilateral and contralateral projections. In the present study, we compared the topographical map of the ipsilateral and contralateral CF -mediated projections of the third row of vibrissae onto the vermal PC s of lobule VII in adult normal rats and in polyinnervated rats. Using intracellular electrophysiological recordings, we examined the respon siveness of PCs to mechanical stimulation of vibrissae, and positioned responsive cells on an averaged planar map of lobule VII. In normal r ats no ipsilateral responses were found, while in irradiated rats ipsi lateral responses were distributed evenly from the midline to 700 mu m apart. These results suggest that synapse elimination participates in the segregation of ipsi and contralateral mystacial inputs to the ver mis.