Eradication of cerebellar granular cells alters thedevelopmental expression of TRK receptors in the ratinferior olive

Granule cells which relay the mossy fibre afferent system to the cerebellar cortex are generated postnatally in mammals. In their absence, the climbing fibres, i.e. the second afferent system to the cerebellum originating in the inferior olivary nucleus, remain in an immature stage, and substantial elimination of redundant synapses they establish on the Purkinje cells does not occur in the rat between day five (P5) and day fifteen (P15). It is generally assumed that synapse elimination is partly regulated by electrical activity which modulates the competition among afferent fibres for the uptake of a limited amount of trophic factors released by the target. The neurotrophins, whose expression is developmentally regulated in the cerebellum, especially in granule cells, could be this retrograde signal. Using RT-PCR, we studied the expression of their trk receptors in the inferior olivary nucleus of developing and adult rats, and its alteration after eradication of the granule cell precursors by X-irradiation on P5. From P0 to P90, the amount of trkA mRNA is low and remains stable in control rats; the high levels of trkB and C mRNAs detected at P0 markedly decrease in parallel from P5 and reach their minimal values at P15, when the process of synapse elimination is completed in the cerebellum. X-irradiation of the cerebellum decreases the level of expression of the three trks, but a transient upregulation of trkC occurs at P10. The down-regulation of trkB and C expression in the inferior olivary nucleus, contemporary with the altered expression of neurotrophins in the cerebellum, suggest that NT-3 and/or BDNF/NT-4/5 could be involved in the remodelling of olivocerebellar relationships during development. In addition, the transient overexpression of trkC after granule cells eradication is consistent with a paracrin effect exerted on the olivary cells by granule cells release of NT-3, at the time when the climbing fibres invest the growing Purkinje cell dendrites in the molecular layer.