Investigação da ação da crisina e os possíveis mecanismos envolvidos na redução do déficit de memória induzido pelo hipotireoidismo em camundongos

Abstract

The reduction in the secretion of the hormones 3,5,3'-triiodothyronine (T3) and 3,5,3',5'- tetraiodothyronine or thyroxine (T4) by the thyroid gland leads to the development of hypothyroidism, an endocrine dysfunction that is associated with neuropsychiatric disorders and cognitive decline. The flavonoid 5,7-dihydroxy-2-phenyl-4H-chromene-4-one or 5,7- dihydroxyflavone, popularly known as chrysin, presents therapeutic actions already elucidated, such as neuroprotective action in different studies of neurological disorders, including the memory improvement in rodents. In this sense, the aim of this thesis was to investigate whether chrysin is able to reduce the cognitive deficit caused by hypothyroidism and to verify the involvement of the glutamatergic, cholinergic and neurotrophinergic systems. Firstly, C57BL/6 female mice were divided into two large groups: control and hypothyroidism. Hypothyroidism was induced by continuous exposure to the antithyroid drug methimazole (MTZ) 0.1% + 0.475% sucralose for 31 days in drinking water. On day 31, blood was drawn from the tail vein and T3 and T4 levels were determined. Afterwards, the animals were separated into four groups: control, hypothyroidism, chrysin, hypothyroidism + chrysin. Chrysin (20 mg/kg) was administered from day 33 of treatment, daily for 28 days, orally. At the end of the treatment, the animals underwent behavioral tests for locomotion and memory. After behavioral tests, blood was collected to measure plasma levels of T3 and T4. Brain structurals (prefrontal cortex and hippocampus) were also collected for biochemical analyses. We analyzed the effect of chrysin in alleviating these memory deficits by in vivo, ex vivo and in silico analysis. Thus, this thesis involved two main studies separated into two published articles. In the first study, we assessed long-term memory through the Morris water maze and passive avoidance. In both tests, chrysin was able to reverse the memory deficit caused by hypothyroidism. Thus, we analyzed the glutamatergic system, through the levels of glutamate and activity of the enzyme Na+,K+-ATPase, and the cholinergic system through the activity of the enzyme acetylcholinesterase (AChE) in brain structures. The results showed that chrysin acts by increasing glutamate levels in the prefrontal cortex and partially increasing these levels in the hippocampus, which were decreased in animals with hypothyroidism, and increased the activity of Na+,K+-ATPase in both brain structures which were decreased in animals with hypothyroidism. AChE activity was not affected by hypothyroidism. Thus, we highlight the protective action of chrysin on some molecular targets of the glutamatergic system that were altered by hypothyroidism, attenuating cognitive impairment in hypothyroid animals. In the 11 second study, we analyzed the neurotrophinergic system, through the assessment of long-term memory using the Morris water maze; these results corroborate the results of the first article, in which animals with hypothyroidism showed a memory deficit, and the treatment with chrysin was able to reverse this cognitive impairment. In the ex vivo analyses, we verified the levels of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in which hypothyroidism decreased the level of these neurotrophins, and chrysin was able to normalize the levels of hippocampal BDNF and of the NGF in the hippocampus and prefrontal cortex. Finally, we performed the in silico analysis through molecular docking, and verified the high affinity of chrysin with the neurotrophin receptors TrkA, TrkB and p75NTR. Our results show that chrysin was effective in improving the memory deficit caused by hypothyroidism, and we suggest that its therapeutic effect involves the modulation of parameters associated with the glutamatergic and neurotrophinergic systems.