Effects of thyroxin and donepezil on hippocampal acetylcholine content and syntaxin-1 and munc-18 expression in adult rats with hypothyroidism

Abstract

Adult-onset hypothyroidism induces various impairments in hippocampus-dependent cognitive function, in which numerous synaptic proteins and neurotransmitters are involved. Donepezil (DON), an acetylcholinesterase inhibitor, has been shown to be efficient in improving cognitive function. The aim of the present study was to investigate the effects of adult-onset hypothyroidism on the expression levels of the synaptic proteins syntaxin-1 and munc-18, as well as the content of the neurotransmitter acetylcholine (ACh) in the hippocampus. In addition, the study explored the effects of thyroxin (T4) and DON treatment on the altered parameters. The study involved 55 Sprague-Dawley rats that were randomly divided into five groups: Control, hypothyroid (0.05% 6-n-propyl-2-thiouracil; added to the drinking water), hypothyroid treated with T4 (6 μg/100 g body weight once daily; intraperitoneal injection), hypothyroid treated with DON (0.005%; added to the drinking water) and hypothyroid treated with a combination of the two drugs (6 μg/100 g T4 and 0.005% DON). The concentration of ACh was determined in the homogenized hippocampus of each animal by alkaline hydroxylamine colorimetry. The protein levels of syntaxin-1 and munc-18 were determined by immunohistochemistry. The results showed that the content of ACh in the hippocampi of the hypothyroid rats was significantly decreased compared with that in the controls and that T4 monotherapy and DON administration restored the ACh content to normal values. In the hippocampi of the hypothyroid group, munc-18 was expressed at significantly lower levels, while the expression levels of syntaxin-1 were increased compared with the levels in the control group. Treatment with T4 alone restored the expression of syntaxin-1 but failed to normalize munc-18 expression levels. The co-administration of T4 and DON returned the munc-18 levels to normal values. These observations indicate that adult-onset hypothyroidism induces alterations in the levels of munc-18, syntaxin-1 and ACh in the hippocampus. Syntaxin-1 and ACh levels were restored by T4 monotherapy while munc-18 levels were not. In addition, the co-administration of T4 and DON resulted in more effective restoration than either alone. The thyroid hormone has a direct effect on metabolism of hippocampal ACh in adult rats and DON is helpful for treatment of synaptic protein impairment induced by hypothyroidism.

Effect of adult onset hypothyroidism on behavioral parameters and acetylcholinesterase isoforms activity in specific brain regions of male mice

“…Although many neurotransmitter systems like GABAergic, glutaminergic and dopaminergic have been shown to be affected by thyroid hormone imbalance, the cholinergic system is the most strongly affected indicating a very close functional association between TH and cholinergic function [29,31,48]. Acetylcholine is metabolized into Acetyl-CoA and choline by the enzyme AChE, the activity of which has been recently shown to be directly affected by thyroxin [35]. The predominant AChE isoforms in brain are the monomeric, salt soluble (SS) G1 and the tetrameric, detergent soluble (DS) G4.…”

“…In the light of current studies, revealing TH along with AChE inhibitors as a combined pharmacological treatment of hippocampus synaptic protein impairment induced by adult onset hypothyroidism [35], the aim of the present study was to comprehensively investigate the effects of adult onset hypothyroidism on learning/memory and fear/anxiety behavior and on the activity of two different AChE isoforms in five different brain regions of adult male Balb/cJ mice. Although most relevant reports focus on cerebral hemispheres and hippocampus, we expand the current knowledge by investigating concomitantly cerebellum, midbrain and striatum along with cerebral hemispheres and hippocampus since our previous publications strongly suggest that cholinergic system function of all these brain regions affects differentially learning/memory and fear/anxiety behavior [36].…”

“…In addition, increasing experimental evidence support an extensive interrelationship between TH and brain cholinergic system [29][30][31]. However, a comprehensive study of the effects of adult on set hypothyroidism on both G1 and G4 AChE isoforms has never been performed so far and most studies focus on the investigation of developmental hypothyroidism on cholinergic system function with emphasis on total or G1 AChE isoform activity [8,[32][33][34] To this end, recently, it has been reported that TH have a direct effect on total AChE activity and consequently on the metabolism of acetylcholine at least in the hippocampus of adult rodents [35].…”

Concurrent administration of thyroxine and donepezil induces plastic changes in the prefrontal cortex of adult hypothyroid rats

“…However, previous clinical studies have indicated that patients with hypothyroidism who were properly treated with T4 have not always fully recovered from the symptoms affecting the central nervous system, such as the cognitive functioning and well-being ( 16 – 18 ). Previous animal studies of the authors have revealed that hypothyroidism induces significant expression changes of several synaptic proteins in the brain regions related to learning and memory, whereas some of the proteins were not fully restored to control values by T4 treatment ( 13 , 14 , 19 ). Furthermore, in previous morphological studies, Madeira et al ( 20 , 21 ) discovered that hypothyroidism induces reductions in the number of pyramidal cells in the hippocampal CA1 region and granule cells in the dentate gyrus, which were also not restored following the normalization of thyroid hormone levels in adult rats.…”

“…The total treatment time was two weeks. The dose of T4 was selected based on previous studies ( 14 , 19 ), and the dosage was adjusted according to the rats' weights weighed weekly.…”