Quantitative study of Purkinje cell perikaryon synaptogenesis. Effects of thyroid hormones and of heat value restriction of food intake on synaptic densities evolution

Abstract

Hyper-and hypothyroidism repercussions on axosomatic synatogenesis of Purkinje cell perikaryon evolution were studied by electron microscopy on 7-,14-, 21-, and 35-day-old rats. Normal adult animals were compared with adult ones made hypothyroid until they were 35 days old. Also, heat value restriction of food intake effects were compared with hypothyroid ones at 14 and 35 days. By 7 days, Basket cell (CC) axons and Purkinje cell (CP) axon collaterals synaptic endings had not acquired entirely their adult morphological characteristics, but it was possible to identify them. In normal animals, general synaptic density (DSG) and corrected general synaptic density (DSGC) increased from 7 to 35 days, at which stage these parameters reached their definitive values. General synaptic density on somatic spines (DSGE), specific density of climbing fibers (DSS.FG) and their relative DSS decreased markedly from 7 to 35 days, at which stage all these values were nullified. DSS.CP increased from 7 to 35 days, at which stage it reached its definitive value, while relative DSS of these endings reached a maximum by day 21, then decreased until it reached its definitive value on day 35. Hyperthyroidism led to an increase of DSG and DSGC by day 14. Then these parameters were markedly decreased on day 35. DSS. FG were increased at 7 days. Afterward, its decrease as well as that of relative DSS was accelerated without advancing markedly the age at which FG did not establish a synapse on the CP soma anymore. It put to 14 days instead of 21, the stage at which DSS.CC and relative DSS if these endings reached their maximum. From this stage it led to a decreased DSS.CC but did not bring a DSS.CP or a relative DSS of these synaptic endings noticeable change. On the contrary, hypothyroidism decreased DSG and DSGC under normal. It slackened the rate of DSS.FG and of their relative DSS decrease. It maintained DSS.CC, relative DSS.CC and DS.CP lower than normal. But after 21 days, hypothyroidism increased the relative DSS.CP value. Furthermore, it led to the temporary establishment of synapses between Basket cell axons and somatic processes still present in hyperthyroid animals. Heat value restriction of food intake did not considerably changes these parameters at the studied stages. These findings were discussed according to the recent data of synaptogenesis in the cerebellum of mutant mice and in the rat Purkinje cells cultivated in the absence of afferent systems.