Effect of cerebellar granule cell depletion on learning of the equilibrium behaviour: 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 the cortex completely or partially agranular. The X-rays were delivered according to two different schedules, between 5-14 postnatal days (early group) and between 10-14 postnatal days (late group). Histological controls at 35 days showed a mean loss of granule cells of 96 +/- 1% in the early group and of 61 +/- 3% in the late group. The irradiated animals were subjected, from day 23 to day 35, to daily sensorimotor training on a rotorod. The scores and the strategy used (walking or hanging) by the rats were noted. The results demonstrate that a partial loss of granule cells due to a late X-irradiation schedule induced mild motor disabilities but no learning deficit, the only problem being difficulty in elaborating rapidly an efficient strategy to solve a novel problem. A sub-total loss of the granule cells, due to an early X-irradiation schedule, induced gross motor disabilities and the animals used hanging > 90% of the time. Due to the discrepancy between the learning abilities, which were preserved at least in part, and the gross motor impairments, the animals elaborated a novel strategy (jumping from the beam), allowing them to escape the experimental situation. This avoidance behaviour may be due to a decrease of anxiety, a lack of behavioural inhibition and/or attentional deficits that have been already observed in several other examples of cerebellar abnormalities.