Effect of thyroid deficiency on the synaptic organization of the rat cerebellar cortex

Abstract

The effect of thyroid deficiency on synaptic organization was studied by following the development of the climbing fibre and mossy fibre synapses which are characteristics components of the two main operational circuits in the cerebellum. In the normal rat, somatic spines, which are synaptic sites of the climbing fibres, disappeared from the Purkinje cells between 8 and 16 days after birth; they persisted for a longer time in the thyroid deficient rats although they disappeared by day 27. The mossy fibre terminals form synapses on the dendrites of the granule cells in the cerebellar glomeruli which approached full development in the normal rat by day 27. At that age, the development of the glomeruli was severely retarded in the thyroid deficient animals: in comparison with controls, the glomeruli were small and contained large glial spaces. The dendrites of the granule cells showed all the signs of immaturity: they were electron-dense and the big dendritic trunks, which often did not split into digits, formed multiple synapses with the relatively small mossy fibre ‘rosette’. The results are consistent with the view that thyroid deficiency may lead not only to quantitative but also to qualitative changes in synaptic organization: the ‘wiring’ pattern in the cerebellum persists for a long time in an immature form which could favour the dominance of the climbing fibre-Purkinje cell circuit over the mossy fibre- granule cell-Purkinje circuit.