[Repercussions of hypothyroidism on the synaptogenesis of the rat cerebellar cortex: quantitative study of the thyroid hormone effect according to the age (author's transl)].

The corrective action of Thyroxine on the deficit of the synaptic density of the molecular layer of the cerebellar cortex was studed quantitatively, in the rat made hypothyroid by propylthiouracile (P.T.U.). In the normal as in the hypothyroid rat, the increase of the synaptic density does not extend over the fifth postnatal week. In the PTU treated animals, during the 2 first postnatal weeks, the administration of 0,25 mug/d during the third and 0.50 mug/d during the fourth postnatal week brings the synaptic density back to normal. In the PTU treated animals, during the 5 postnatal weeks, the administration during a week only of 0.50 mug/d of LT4 between 1 and 14 days or, of 1 mug/d between 15 and 28 days, increases, but does not return to normal the synaptic density. On the contrary, the administration of 1 mug/d of LT4 during 1 or even 2 consecutive weeks 28 days later, remains without effect on the synaptic density. After the end of the fourth postnatal week, the deficit of the synaptic density becomes irreversible.