Endocrine Abstracts

Adult-onset hypothyroidism is associated with cognitive dysfunction, and a significant fraction of hypothyroid patients still shows persistent disturbances despite standard T4 replacement therapy. Experimental results showed that tissue levels of T4 metabolites, particularly T1AM, are decreased in hypothyroidism and remain low after T4 replacement. Thyroid hormones are important for the promotion of neurogenesis and neuron migration to different areas of the cerebral cortex, including hippocampus, and it is known that hippocampus-dependent memory is impaired in the hypothyroid mice. We aimed to evaluate the effects of hypothyroidism and different replacement treatments on neurogenesis in the sub-granular zone of the dentate gyrus (SGZ). Six-week-old C57BL/6J male mice were given methimazole and potassium perchlorate (0.20 mg/g/die and 0.30 mg/g/die) in drinking water for 49 days while the control littermates received water. At day 21 mice were implanted with subcutaneous ALZET® osmotic pumps delivering replacement treatments for 28 days. Animals were divided in 4 groups: euthyroid; hypothyroid; hypothyroid treated with T4 (0.04 μg/g BW/die); hypothyroid treated with T4+T1AM (0.04 μg T4 & 0.004 μg T1AM/g BW/die). Specific markers were used to quantitate cell proliferation (Ki67) and the presence of neuroblasts/immature neurons (doublecortin-DCX) through immunofluorescence analysis performed in the SGZ. Then, following hippocampal RNA isolation, we analysed gene targets involved in neurogenesis pathway using a PrimePCR pre disegned 96-well collection panel (Bio-Rad). A 49-day period of adult-onset hypothyroidism induced a reduction of around 20% in the number of DCX positive newly generated cells. Compared to hypothyroidism, T4 treatment increased the number of cells by 45.58% (mean diff=-106.9: P<0.05) while the T4 + T1AM treatment produced a 58.20% increase (mean diff=-141.6: P<0.01). One-way ANOVA revealed a global significant effect among the 4 groups (P=0.003). Real time PCR highlighted significant changes in the expression of genes related to neurogenesis as ANOVA revealed an upregulation in the expression of Ngf, Kdr, Ntf3, Mapk1/3 and Neurog2 genes in T4+T1AM treated mice compared to T4 treated mice (P<0.05). Our results indicate that cognitive dysfunctions related to adult-onset hypothyroidism could be due in part to impaired hippocampal neurogenesis. Cellular markers and gene expression analysis suggest the presence of underlying molecular mechanisms activated only by the combination of T4 and T1AM. Further investigations are required to elucidate the potential pathophysiological and clinical relevance of these findings.