Role of thyroid hormones in the maturation of interhemispheric connections in rats

Abstract

Hipothyroidism causes mental retardation secondary to changes in the organization of the CNS. These changes affect higher brain functions for which interhemispheric transfer of information is crucial. In present study, the anterior commissure (AC) and corpus callosum (CC) of normal (C) and hypothyroid (H) rats has been examined using quantitative electron microscopy. H rats received an antithyroid treatment with methimazole from embryonic day 14 (E14) and surgical thyroidectomy at postnatal day 6 (P6). In the AC, the number of axons (unmyelinated and myelinated) increased from0.17 × 106 axons at E18 to1.08 × 106 axons at P4 and it was almost the same at P180 (1.01 × 106 axons). In H rats the number of axons between P14 and P180 was similar to that of C rats. In contrast, there were only 0.11 × 106 myelinated axons at P180 resulting in a 66% reduction with respect to C rats (0.36 × 106 axons). In the CC of C rats, the number of myelinated axons increased from 1.76 × 103 axons at P12 to 3.34 × 106 axons at P184. In H rats, there were only 0.84 × 106 axons at P184 resulting in a 76% reduction with respect to C rats. This reduction was more important in the posterior sector of the CC (95%) than in the rest (on average 63%). Therefore these results show that thyroid hormones play an important role in the processes involved in the maturation of commissural axons.