EFFECT OF CEREBELLAR GRANULE CELL DEPLETION ON LEARNING OF THE EQUILIBRIUM BEHAVIOR - STUDY IN POSTNATALLY X-IRRADIATED RATS

To assess the role of the mossy fibre-granule cell pathway in learning , the cerebellum of young DA/HAN strain rats was irradiated to make th e cortex completely or partially agranular. The X-rays were delivered according to two different schedules, between 5-14 postnatal days (ear ly group) and between 10-14 postnatal days (late group). Histological controls at 35 days showed a mean loss of granule cells of 96 +/- 1% i n the early group and of 61 +/- 3% in the late group. The irradiated a nimals were subjected, from day 23 to day 35, to daily sensorimotor tr aining on a rotorod. The scores and the strategy used (walking or hang ing) by the rats were noted. The results demonstrate that a partial lo ss of granule cells due to a late X-irradiation schedule induced mild motor disabilities but no learning deficit, the only problem being dif ficulty in elaborating rapidly an efficient strategy to solve a novel problem. A sub-total loss of the granule cells, due to an early X-irra diation schedule, induced gross motor disabilities and the animals use d hanging >90% of the time. Due to the discrepancy between the learnin g abilities, which were preserved at least in part, and the gross moto r impairments, the animals elaborated a novel strategy (jumping from t he beam), allowing them to escape the experimental situation. This avo idance behaviour may be due to a decrease of anxiety, a lack of behavi oural inhibition and/or attentional deficits that have been already ob served in several other examples of cerebellar abnormalities.