Oral exposure to aluminum leads to reduced nicotinic acetylcholine receptor gene expression, severe neurodegeneration and impaired hippocampus dependent learning in mice.

Aluminum (Al) is known for its neurotoxicity for over a century and is reported to have specifically high toxicity for cholinergic system. The effect of Al on muscarinic acetylcholine receptors is widely reported, but its effect on nicotinic acetylcholine receptors (nAChRs) is less well known. The aim of this study was to determine the effects of Al on hippocampus dependent learning and memory, function and expression of nAChRs in the hippocampus. Al concentration and neurodegeneration were also measured in the hippocampus following Al treatment. The mice were treated with 250 mg/kg AlCl3.6H2O in drinking water for a period of 42 days. Results show that Al treated animals have significantly reduced spatial reference memory as compared to control animals in Morris water maze test. Similarly, Al treated animals showed reduced contextual memory for Pavlovian fear compared to control animals. Al treated animals show higher anxiety in elevated plus maze as compared to control animals. The analysis of nAChR expression via RT-PCR showed reduced expression of α7, α4 and ß2 nAChR gene expression in the hippocampus of Al treated animals. High Al accumulation was observed in Al-treated animals (688.14 ± 242.82 µg/g) compared to the control group (115.14 ± 18.18 µg/g) that resulted in severe neurodegeneration in the hippocampus. These results demonstrated that Al exposure caused neurotoxicity in mice hippocampus which is manifested by reduced memory and elevated anxiety. The results were further validated by high Al accumulation in the hippocampus, severe neurodegeneration and reduced expression of nAChRs.

Expression of DnMTs and MBDs in AlCl3-Induced Neurotoxicity Mouse Model.

Alteration in DNA methylation after aluminum exposure has been shown to contribute in pathogenesis of Alzheimer's disease (AD). This study is aimed to determine the effect of Al exposure (42 and 60 days) on learning and memory and the expression of proteins involved in DNA methylation (MBD1, MBD2, MBD3, MeCP2 (methyl CpG binding protein 2), DnMT1 and DnMT3a). Male BALB/c mice were treated with AlCl3 for either 42 days or 60 days. After treatment completion, learning and memory were compared to the control group using novel object recognition test, elevated plus maze test, open field test, and Morris water maze test. The treated animals and their respective controls were sacrificed after cognitive testing and samples from their whole cortex and hippocampus were harvested for gene expression analysis. Mice treated with AlCl3 showed significant cognitive deficit with impaired short-term memory, elevated anxiety, and deterioration in spatial and reference memory. The AlCl3 treatment showed significant reduction in the expression of MBDs in the whole cortex at 60 days of treatment as compared to control. AlCl3-treated animals showed decreased expression of MBDs and DnMT3a in the hippocampus for longer treated animals but strikingly, MBD2 showed significantly increased expression in AlCl3-treated animals at 60 days p ≤ 0.001. In conclusion, this study showed that AlCl3-treated animals showed significant memory and cognitive deficits and it is associated with significant changes in the expression of proteins involved in DNA methylation mechanism. Moreover, different Al exposure duration had slightly different effects.

Cortex- and Amygdala-Dependent Learning and Nicotinic Acetylcholine Receptor Gene Expression is Severely Impaired in Mice Orally Treated with AlCl3.

Recent industrialization has increased human exposure to bio-available aluminum (Al). If more Al enters the brain than leaves, Al concentration will rise in the brain leading to neurodegenerative disorders. The aim of the present study was to determine Al concentration, neurodegeneration, and nicotinic acetylcholine receptor (nAChR) gene expression in the cortex and amygdala after oral ingestion of Al salt. The effect of Al on cortex- and amygdala-dependent learning and memory functions was also assessed. Mice were given AlCl3 (250 mg/kg) in drinking water for 42 days. nAChR gene expression was determined in the cortex and amygdala. The mice were subjected to behavior tests (fear conditioning, fear extinction, and open field), to assess memory deficits. The acquisition of fear memory in the fear conditioning test remained unaffected due to the Al administration. However, fear extinction (which is a new learning) was severely impaired. The behavioral analysis in the open field test showed greater anxiety and less adaptability to the new environment in Al-treated animals. High Al concentration and severe neurodegeneration in the cortex were observed following Al treatment while a slight, non-significant elevation in Al concentration was observed in the amygdala of Al-treated animals. The analysis of nAChR gene expression via RT-PCR showed a significant reduction in expression of α7, α4, and ß2 nAChR genes in the cortex of Al-treated animals, while in the amygdala, the level of the α4 nAChR gene remained unaltered. Oral Al ingestion causes neuropathological changes and suppresses expression of nAChR genes that lead to deficits in learning and higher anxiety in Al-treated animals.

Role of cholinergic receptors in memory retrieval depends on gender and age of memory.

The phenomenon of utilizing information acquired in the past to make decision and performance in present depends on memory retrieval, which is affected in retrograde amnesia. Role of cholinergic receptors in memory retrieval is not much explored. In this study we evaluated the gender specific role of cholinergic receptors, i.e. muscarinic and nicotinic receptors, in memory retrieval in young Balb/c mice. Acute (only one injection, 30min before test) and sub-chronic (five days) muscarinic blockade (using scopolamine=1mg/kg) before test impaired retrieval of contextual fear memory in male (31.45±5.39% and 33.36±3.78% respectively) and female mice (22.88±5.73%; P<0.05), except sub-chronically treated female group (33.31±4.90%; P>0.05). Only sub-chronic nicotinic receptor antagonism (using methyllycaconitine MLA=87.5µg/kg and dihydro ß erythroidine DHßE=1mg/kg) in female showed significantly higher freezing response than control during contextual fear memory retrieval (60.85±7.71% and 40.91±7.53% respectively; P<0.001). Acute and sub-chronic muscarinic antagonism (but not nicotinic antagonism) impaired spatial memory retrieval in male (P<0.05) but not in female mice (P>0.05). There was no effect of acute and sub-chronic cholinergic receptor antagonism on discriminating novel object from the familiar one in male and female mice, however, nicotinic receptor blockade affected the working memory of all male and female mice on test day compared to the training sessions. Our results suggested that cholinergic receptors involvement in retrieving spatial and fear memories depends on the age of the memory and gender.

Cholinergic System and Post-translational Modifications: An Insight on the Role in Alzheimer's Disease.

BACKGROUND: Alzheimer's disease (AD) is the most common form of old age dementia. The formation of amyloid plaques (Aß), neurofibrillary tangles and loss of basal forebrain cholinergic neurons are the hallmark events in the pathology of AD. LITERATURE REVIEW: Cholinergic system is one of the most important neurotransmitter system involved in learning and memory which preferentially degenerates in the initial stages of AD. Activation of cholinergic receptors (muscarinic and nicotinic) activates multiple pathways which result in post translational modifications (PTMs) in multiple proteins which bring changes in nervous system. Cholinergic receptors-mediated PTMs "in-part" substantially affect the biosynthesis, proteolysis, degradation and expression of many proteins and in particular, amyloid precursor protein (APP). APP is subjected to several PTMs (proteolytic processing, glycosylation, sulfation, and phosphorylation) during its course of processing, resulting in Aß deposition, leading to AD. Aß also alters the PTMs of tau which is a microtubule associated protein. Therefore, post-translationally modified tau and Aß collectively aggravate the neuronal loss that leads to cholinergic hypofunction. CONCLUSION: Despite the accumulating evidences, the interaction between cholinergic neurotransmission and the physiological significance of PTM events remain speculative and still needs further exploration. This review focuses on the role of cholinergic system and discusses the significance of PTMs in pathological progression of AD and highlights some important future directions.

Aluminum-Induced Cholinergic Deficits in Different Brain Parts and Its Implications on Sociability and Cognitive Functions in Mouse.

Aluminum is associated with etiology of many neurodegenerative diseases specially Alzheimer's disease. Chronic exposure to aluminum via drinking water results in aluminum deposition in the brain that leads to cognitive deficits. The study aimed to determine the effects of aluminum on cholinergic biomarkers, i.e., acetylcholine level, free choline level, and choline acetyltransferase gene expression, and how cholinergic deficit affects novel object recognition and sociability in mice. Mice were treated with AlCl3 (250 mg/kg). Acetylcholine level, free choline level, and choline acetyltransferase gene expression were determined in cortex, hippocampus, and amygdala. The mice were subjected to behavior tests (novel object recognition and social novelty preference) to assess memory deficits. The acetylcholine level in cortex and hippocampus was significantly reduced in aluminum-treated animals, as compared to cortex and hippocampus of control animals. Acetylcholine level in amygdala of aluminum-treated animals remained unchanged. Free choline level in all the three brain parts was found unaltered in aluminum-treated mice. The novel object recognition memory was severely impaired in aluminum-treated mice, as compared to the control group. Similarly, animals treated with aluminum showed reduced sociability compared to the control mice group. Our study demonstrates that aluminum exposure via drinking water causes reduced acetylcholine synthesis in spite of normal free choline availability. This deficit is caused by reduced recycling of acetylcholine due to lower choline acetyltransferase level. This cholinergic hypofunction leads to cognitive and memory deficits. Moreover, hippocampus is the most affected brain part after aluminum intoxication.

Ibuprofen targets neuronal pentraxins expresion and improves cognitive function in mouse model of AlCl3-induced neurotoxicity.

Aluminum is known to exert neurotoxic effects associated with various neurodegenerative disorders, including Alzheimer's disease (AD). Ibuprofen is a well-known non-steroidal anti-inflammatory drug, which has demonstrated potential efficacy in the treatment of numerous inflammatory and neurodegenerative disorders, including AD. The present study aimed to investigate the protective effects of ibuprofen on cognitive function, and the expression levels of neuronal pentraxins (NPs) and interleukin (IL)-1ß in an aluminum chloride (AlCl3)-induced mouse model of neurotoxicity. The effects of ibuprofen (100 mg/kg/day for 12 days) on learning and memory were evaluated in the AlCl3-induced neurotoxic mice using a Morris water maze and open field tests. In addition, ibuprofen was assessed for its effects on the expression levels of NPs and IL-1ß in the hippocampus, cortex and amygdala of the brain. Treatment of the AlCl3-treated mice with ibuprofen decreased anxiety levels (6.90±0.34 min) compared with the AlCl3-treated group (1.80±0.29 min), as indicated by the time spent in the central area in an open field test. Furthermore, the expression levels of NP1 (1.32±0.47) and IL-1ß (0.99±0.21) were significantly decreased in the hippocampus of mice following ibuprofen treatment, as compared with the AlCl3-treated mice (8.62±1.54 and 7.47±0.53, respectively). In the present study, ibuprofen was able to target novel structures in order to attenuate the inflammation associated with an AlCl3-induced mouse model of neurotoxicity; thus suggesting that ibuprofen may be considered a potential therapeutic option for the treatment of neurodegenerative diseases, including AD.

Memory Enhancing Effect of Black Pepper in the AlCl3 Induced Neurotoxicity Mouse Model is Mediated Through Its Active Component Chavicine.

UNLABELLED: Black pepper (Piper nigrum Linn.) has vital pharmacological properties with profound effects on central nervous system. Neurotoxic agents like Aluminum Chloride (AlCl3) cause the oxidative stress and result in improper processing of amyloid proteins leading to accumulation of amyloid ß plaques. AIM: The study aimed to explore the neuroprotective potential of black pepper (BP) extract (12.5mg/kg/day) on memory enhancement and its effect on expression of amyloid precursor protein (APP) isoforms (APP770 and APP695) in AlCl3 induced neurotoxicity (250mg/kg) mouse model. The study included the isolation and identification of pure compound from BP (chavicine) which was found pharmacologically active. METHODS: Morris water maze test, elevated plus maze, fear conditioning, context and cue dependent test and social preference tests were performed to investigate the learning and memory. Gene expression (APP isoforms) and in-vitro and ex-vivo DPPH free radical scavenging activity were performed to evaluate the role of BP. RESULTS: BP significantly improved memory in AlCl3 induced neurotoxicity mouse model along with effectively decreasing the expression of APP770 (amyloidogenic) isoform and improved level of APP695 (non-amyloidogenic) in hippocampus, amygdala and cortex. Fear extinction learning was considerably improved in BP treated group (7.83±2.03) than AlCl3 induced neurotoxicity group (39.75±4.25). In the hippocampus, BP significantly reduced the expression of APP770 (0.37±0.05) as compared to AlCl3 induced neurotoxicity group (0.72±0.06), and effectively increased (34.80±1.39) the percentage inhibition of DPPH free radicals as compared to AlCl3 induced neurotoxicity group (14±2.68). CONCLUSION: The study revealed that BP improves memory and chavicine is a lead compound producing pharmacological effects of BP.

Alpha-lipoic acid-mediated activation of muscarinic receptors improves hippocampus- and amygdala-dependent memory.

Aluminum (Al) is a neurotoxic agent which readily crosses the blood-brain-barrier (BBB) and accumulates in the brain leading to neurodegenerative disorders, characterised by cognitive impairment. Alpha-lipoic acid (ALA) is an antioxidant and has a potential to improve cognitive functions. This study aimed to evaluate the neuroprotective effect of ALA in AlCl3-induced neurotoxicity mouse model. Effect of ALA (25mg/kg/day) was evaluated in the AlCl3-induced neurotoxicity (AlCl3 150 mg/kg/day) mouse model on learning and memory using behaviour tests and on the expression of muscarinic receptor genes (using RT-PCR), in hippocampus and amygdala. Following ALA treatment, the expression of muscarinic receptor genes M1, M2 and choline acetyltransferase (ChaT) were significantly improved (p<0.05) relative to AlCl3-treated group. ALA enhanced fear memory (p<0.01) and social novelty preference (p<0.001) comparative to the AlCl3-treated group. Fear extinction memory was remarkably restored (p<0.001) in ALA-treated group demonstrated by reduced freezing response as compared to the AlCl3-treated group which showed higher freezing. In-silico analysis showed that racemic mixture of ALA has higher binding affinity for M1 and M2 compared to acetylcholine. These novel findings highlight the potential role of ALA in cognitive functions and cholinergic system enhancement thus presenting it an enviable therapeutic candidate for the treatment of neurodegenerative disorders.