Abstract
Thyroid hormone is essential for hippocampal redox environment and neuronal viability in adulthood, where its deficiency causes hypothyroidism related to oxidative and endoplasmic reticulum stresses in the hippocampus, resulting in neuronal death. One option of treatment is antioxidants; however, they must be transported across the blood-brain barrier. Gallic acid is a polyphenol that meets these criteria. Thus, this study aimed to prove that the neuroprotective mechanism of GA is associated with the prevention of oxidative and endoplasmic reticulum stresses in the hippocampus of adult-onset hypothyroid rats. Male Wistar rats were divided into euthyroid (n = 20) and hypothyroid groups (n = 20). Thyroidectomy with parathyroid gland reimplementation caused hypothyroidism. Each group was subdivided into two: vehicle and 50 mg/kg/d of gallic acid. 3 weeks after thyroidectomy, six animals of each group were euthanized, and the hippocampus was dissected to evaluate oxidative and endoplasmic reticulum stress markers. The rest of the animals were euthanized after 4 weeks of treatment for histological analysis of the hippocampus. The results showed that hypothyroidism increased lipid peroxidation, reactive oxygen species, and nitrites; it also increased endoplasmic reticulum stress by activating the inositol-requiring enzyme-1α (IRE1α) pathway, the protein kinase RNA-like endoplasmic reticulum kinase (PERK) and activated transcription factor 6α (ATF6α) pathways associated with a proapoptotic state that culminates in hippocampal neuronal damage. Meanwhile, the hypothyroid rat treated with gallic acid reduced oxidative stress and increased endoplasmic reticulum-associated degradation (ERAD) through IRE1α and ATF6. Also, the gallic acid treatment prevented the Bax/BCl2 ratio from increasing and the overexpression of p53 and caspase 12. This treatment in hypothyroid animals was associated with the neuronal protection observed in the hippocampus. In conclusion, gallic acid prevents hypothyroidism-induced hippocampal damage associated with oxidative and endoplasmic reticulum stresses.
Highlights
Thyroid hormones are crucial for brain development in the perinatal age
Hypothyroidism increased in the hippocampus the expression of protein kinase RNA-like endoplasmic reticulum kinase (PERK) (134%, panel A), p-eukaryotic initiation factor 2α (eIF2α)−(Ser 52)/eIF2α (592%, panel B), the relation p-eIF2α−(Ser 52)/eIF2α (1331%, panel D)
Hypothyroid animals which received treatment with gallic acid normalized the expression of f PERK and GADD153, as well as, it partially prevented the alteration of peIF2α−(Ser52), eIF2α, GADD34, activating transcription factor 4 (ATF4), and activated transcription factor 6α (ATF6α) expression
Summary
Thyroid hormones are crucial for brain development in the perinatal age. in adulthood, they participate in the steady-stable state of brain function (Carageorgiou et al, 2005; Cano-Europa et al, 2008; Alva-Sánchez et al, 2009).Hypothyroidism is a common pathological condition of thyroid hormone deficiency, and it is not enough to satisfy the metabolic demand of all tissue that depends on the hormone action. Thyroid hormones are crucial for brain development in the perinatal age. In adulthood, they participate in the steady-stable state of brain function (Carageorgiou et al, 2005; Cano-Europa et al, 2008; Alva-Sánchez et al, 2009). One of the brain regions involved in those symptoms is the hippocampus, which is sensitive to thyroid hormone deprivation (Koromilas et al, 2010). In adult animals, this region is susceptible to hypothyroidism-modifies cholinergic, noradrenergic, opioidergic, and glutamatergic neurotransmission (Ortiz-Butron et al, 2003; Carageorgiou et al, 2005; Ghosh and Das, 2007). Several studies have shown that hypothyroidism-induced hippocampal damage is associated with the changes in the glutamatergic neurotransmission producing the reduction in the glutamate release
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