Aluminum-induced decreases in choline acetyltransferase, tyrosine hydroxylase, and glutamate decarboxylase in selected regions of rabbit brain

The interaction of aluminum with catecholamine-based neurotransmitters: can the formation of these species be considered a potential risk factor for neurodegenerative diseases?

The potential neurotoxic role of Al(iii) and its proposed link with the insurgence of Alzheimer's Disease (AD) have attracted increasing interest towards the determination of the nature of bioligands that are propitious to interact with aluminum. Among them, catecholamine-based neurotransmitters have been proposed to be sensitive to the presence of this non-essential metal ion in the brain. In the present work, we characterize several aluminum-catecholamine complexes in various stoichiometries, determining their structure and thermodynamics of formation. For this purpose, we apply a recently validated computational protocol with results that show a remarkably good agreement with the available experimental data. In particular, we employ Density Functional Theory (DFT) in conjunction with continuum solvation models to calculate complexation energies of aluminum for a set of four important catecholamines: l-DOPA, dopamine, noradrenaline and adrenaline. In addition, by means of the Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analysis (EDA) we assessed the nature of the Al-ligand interactions, finding mainly ionic bonds with an important degree of covalent character. Our results point at the possibility of the formation of aluminum-catecholamine complexes with favorable formation energies, even when proton/aluminum competition is taken into account. Indeed, we found that these catecholamines are better aluminum binders than catechol at physiological pH, because of the electron withdrawing effect of the positively-charged amine that decreases their deprotonation penalty with respect to catechol. However, overall, our results show that, in an open biological environment, the formation of Al-catecholamine complexes is not thermodynamically competitive when compared with the formation of other aluminum species in solution such as Al-hydroxide, or when considering other endogenous/exogenous Al(iii) ligands such as citrate, deferiprone and EDTA. In summary, we rule out the possibility, suggested by some authors, that the formation of Al-catecholamine complexes in solution might be behind some of the toxic roles attributed to aluminum in the brain. An up-to-date view of the catecholamine biosynthesis pathway with sites of aluminum interference (according to the current literature) is presented. Alternative mechanisms that might explain the deleterious effects of this metal on the catecholamine route are thoroughly discussed, and new hypotheses that should be investigated in future are proposed.

Aluminium toxicosis: a review of toxic actions and effects

Aluminium (Al) is frequently accessible to animal and human populations to the extent that intoxications may occur. Intake of Al is by inhalation of aerosols or particles, ingestion of food, water and medicaments, skin contact, vaccination, dialysis and infusions. Toxic actions of Al induce oxidative stress, immunologic alterations, genotoxicity, pro-inflammatory effect, peptide denaturation or transformation, enzymatic dysfunction, metabolic derangement, amyloidogenesis, membrane perturbation, iron dyshomeostasis, apoptosis, necrosis and dysplasia. The pathological conditions associated with Al toxicosis are desquamative interstitial pneumonia, pulmonary alveolar proteinosis, granulomas, granulomatosis and fibrosis, toxic myocarditis, thrombosis and ischemic stroke, granulomatous enteritis, Crohn's disease, inflammatory bowel diseases, anemia, Alzheimer's disease, dementia, sclerosis, autism, macrophagic myofasciitis, osteomalacia, oligospermia and infertility, hepatorenal disease, breast cancer and cyst, pancreatitis, pancreatic necrosis and diabetes mellitus. The review provides a broad overview of Al toxicosis as a background for sustained investigations of the toxicology of Al compounds of public health importance.

Curcumin prevents the midbrain dopaminergic innervations and locomotor performance deficiencies resulting from chronic aluminum exposure in rat

Aluminum (Al) among the abundant metals on the earth crust, is able to cross the biological barriers via the gastrointestinal and lung tissues. Once in the body, this heavy metal accumulates in different organs, especially the central nervous system. Though its influence is evidently shown in the substantia nigra of Parkinson's disease patients and other brain areas in other neurodegenerative diseases, few studies have demonstrated that Al could trigger profound changes in neurotransmission systems including the dopaminergic (DAergic) system. A variety of medicinal plants may be prescribed in such contamination, including some culinary spices such as Curcumin (Cur). Several studies have proven Cur to exhibit a wide variety of biological and pharmacological activities, especially its antioxidant potential. Using the immunohistochemistry, of tyrosine hydroxylase (TH), in the midbrain substantia nigra pars compact (SNc) and the ventral tegmental area (VTA) and the open field test, we examined the DAergic system together with the locomotor behavior respectively in rats exposed chronically to Al (0,3%) in drinking water during 4 months since the intra-uterine age, as well as the neuroprotective effect of the concomitant administration of Cur I (30 mg/kg B.W) of chronic Al exposed rats. Our results have shown a significant decrease of TH immureactivity in both SNc and VTA associated to a loss of the number of crossed boxes, leading to a difficient locomotor performance in the Al group while Cur I prevents such TH immunoreactivity impairment and maintains a higher locomotor activity in the Al-CurI group. Our findings lead to suppose a powerful and obvious neuroprotective potential of CurI against Al-induced neurotoxicity of the DAergic system involved in the control of the locomotor behavior.

Exogenous phosphatidylcholine supplementation retrieve aluminum-induced toxicity in male albino rats

This study investigated the ameliorative potential of exogenous phosphatidylcholine (PC) against aluminum-induced toxicity in male albino rats. Four groups of rats were used for this study (N = 8): group I served as the control, group II (PC treated) received L-α-phosphatidylcholine (egg yolk-derived) 100 mg/kg bwt/day orally, group III (aluminum treated) received aluminum chloride 100 mg/kg bwt/day orally, and group VI (aluminum + PC treated) received similar oral dose of aluminum and PC (100 mg/kg bwt/day). Treatment was continued for 8 weeks. Results revealed that aluminum chloride treatment leading to a significant elevation in serum aspartate aminotransferase, serum alanine aminotransferase, urea, creatinine, malondialdehyde, serum cytokines (tumor necrosis factor-α, interleukin-6), and brain content of acetylcholine, as well as a significant reduction in serum-reduced glutathione, serum testosterone, and brain content of acetylcholinesterase. Moreover, aluminum administration caused significant histopathological alteration in liver, kidney, brain, testes, and epididymis. Co-treatment with exogenous PC resulted in significant improvement in intensity of histopathologic lesions, serum parameters, testosterone level, proinflammatory cytokines, and oxidative/antioxidative status. However, it does not affect the brain content of acetylcholine and acetylcholinesterase. Conclusively, treatment with exogenous PC can retrieve the adverse effect of aluminum toxicities through its antioxidative and anti-inflammatory properties.

Sesamol, a lipid lowering agent, ameliorates aluminium chloride induced behavioral and biochemical alterations in rats

Background:

Sesame oil from the seeds of Sesamum indicum Linn. (Pedaliaceae) has been used traditionally in Indian medical practice of Ayurveda in the treatment of central nervous system disorders and insomnia. A few published reports favor the anti-dementia effect of sesamol (SML), an active constituent of sesame oil.

Objective:

Thus, the present study was aimed to explore the anti-dementia effect and possible mechanism (s) of SML in aluminium chloride (AlCl₃)-induced cognitive dysfunction model in rodents with special emphasis on memory centers viz., hippocampus and frontal cortex.

Methods:

Male Wistar rats were exposed to AlCl₃ (175 mg/kg p.o.) for 60 days. SML (10 and 20 mg/kg) and rivastigmine (1 mg/kg) were administered orally 45 min before administration of AlCl₃ for 60 days. Spatial memory was assessed using Morris water maze test. After 60 days of treatment animals were sacrificed, hippocampus and frontal cortex were collected and analyzed for acetylcholinesterase (AChE) activity, tumor necrosis factor (TNF-α) level, antioxidant enzymes (Glutathione, catalase), lipid peroxidation, and nitrite level. The circulating triglycerides, total cholesterol, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) levels were also analyzed.

Results:

SML significantly prevented behavioral impairments in aluminium-exposed rats. Treatment with SML reversed the increased cholesterol, triglycerides and LDL while raised the HDL levels. SML significantly corrected the effect of AlCl₃ on AChE activity. Further, SML reversed the elevated nitric oxide, TNF-α and reduced antioxidant enzymes in hippocampus and frontal cortex.

Conclusion:

The present study suggests the neuro-protection by SML against cognitive dysfunction induced by environmental toxin (AlCl₃) in hippocampus and frontal cortex.

Link between Aluminum and the Pathogenesis of Alzheimer's Disease: The Integration of the Aluminum and Amyloid Cascade Hypotheses

Whilst being environmentally abundant, aluminum is not essential for life. On the contrary, aluminum is a widely recognized neurotoxin that inhibits more than 200 biologically important functions and causes various adverse effects in plants, animals, and humans. The relationship between aluminum exposure and neurodegenerative diseases, including dialysis encephalopathy, amyotrophic lateral sclerosis and Parkinsonism dementia in the Kii Peninsula and Guam, and Alzheimer's disease (AD) has been suggested. In particular, the link between aluminum and Alzheimer's disease has been the subject of scientific debate for several decades. However, the complex characteristics of aluminum bioavailability make it difficult to evaluate its toxicity and therefore, the relationship remains to be established. Mounting evidence has suggested that significance of oligomerization of β-amyloid protein and neurotoxicity in the molecular mechanism of AD pathogenesis. Aluminum may play crucial roles as a cross-linker in β-amyloid oligomerization. Here, we review the detailed characteristics of aluminum neurotoxicity based on our own studies and the recent literatures. Our aim is to revisit the link between aluminum and AD and to integrate aluminum and amyloid cascade hypotheses in the context of β-amyloid oligomerization and the interactions with other metals.

Chronic exposure to aluminum reduces tyrosine hydroxylase expression in the substantia nigra and locomotor performance in rats

Aluminum (Al) is a neurotoxic agent that accumulates in the substantia nigra of patients affected by Parkinson's disease and in other cerebral areas of different neurodegenerative pathologies. Al has been associated with neuronal and glial dysfunctions, and neuronal changes have been suggested to affect several neurotransmitter systems including the dopaminergic system. The present study was designed to evaluate by means of immunohistochemistry using antibodies against tyrosine hydroxylase (TH; the rate-limiting enzyme of dopamine synthesis) the effects of chronic Al exposure (0, 3%) in drinking water during 4 months in adulthood or since intra-uterine age in the substantia nigra. Our results show a significant decrease in the number of cells labeled by the antibody against TH in rats treated with Al compared to controls. The TH-immunoreactive decrease following Al treatment is accentuated in the rat group treated since intrauterine age. In both treated groups, Al exposure induced a significant decrease of locomotor performance. Interestingly, as for TH-immunoreactivity, the decreased locomotor activity was also accentuated in the group treated since intrauterine age. The Al-induced TH alterations may be one of the causes of aluminum-induced neurotoxicity.

Does neurotransmission impairment accompany aluminium neurotoxicity?

Neurobehavioral disorders, except their most overt form, tend to lie beyond the reach of clinicians. Presently, the use of molecular data in the decision-making processes is limited. However, as details of the mechanisms of neurotoxic action of aluminium become clearer, a more complete picture of possible molecular targets of aluminium can be anticipated, which promises better prediction of the neurotoxicological potential of aluminium exposure. In practical terms, a critical analysis of current data on the effects of aluminium on neurotransmission can be of great benefit due to the rapidly expanding knowledge of the neurotoxicological potential of aluminium. This review concludes that impairment of neurotransmission is a strong predictor of outcome in neurobehavioral disorders. Key questions and challenges for future research into aluminium neurotoxicity are also identified.

A new insight on Al-maltolate-treated aged rabbit as Alzheimer's animal model

Lack of an adequate animal model for Alzheimer's disease (AD) has limited an understanding of the pathogenesis of the disease and the development of therapeutic agents targeting key pathophysiological processes. There are undoubtedly few satisfactory animal models for exploring therapies targeting at amyloid beta (Abeta) secretion, deposition, aggregation, and probably the inflammatory response. However, an understanding of the complex events--tau, Abeta, oxidative stress, redox active iron, etc.--involved in the neuronal cell loss is still unclear due to the lack of a suitable animal model system. The use of neurotoxic agents particularly aluminum-organic complexes, especially Al-maltolate, expands the scope of AD research by providing new animal models exhibiting neurodegenerative processes relevant to AD neuropathology. Examination of different species of aged animals including the rapidly advancing transgenic mouse models revealed very limited AD-like pathology. Most other animal models have single event expression such as extracellular Abeta deposition, intraneuronal neurofilamentous aggregation of proteins akin to neurofibrillary tangles, oxidative stress or apoptosis. To date, there are no paradigms of any animal in which all the features of AD were evident. However, the intravenous injection of Al-maltolate into aged New zealand white rabbits results in conditions which mimics a number of neuropathological, biochemical and behavioral changes observed in AD. Such neurodegenerative effects include the formation of intraneuronal neurofilamentous aggregates that are tau positive, immunopositivity of Abeta, presence of redox active iron, oxidative stress and apoptosis, adds credence to the value of this animal model system. The use of this animal model should not be confused with the ongoing controversy regarding the possible role of Al in the neuropathogenesis, a debate which by no means has been concluded. Above all this animal model involving neuropathology induced by Al-maltolate provides a new information in understanding the mechanism of neurodegeneration.

Dietary protein restriction causes modification in aluminum-induced alteration in glutamate and GABA system of rat brain

BACKGROUND

Alteration of glutamate and gamma-aminobutyrate system have been reported to be associated with neurodegenerative disorders and have been postulated to be involved in aluminum-induced neurotoxicity as well. Aluminum, an well known and commonly exposed neurotoxin, was found to alter glutamate and gamma-aminobutyrate levels as well as activities of associated enzymes with regional specificity. Protein malnutrition also reported to alter glutamate level and some of its metabolic enzymes. Thus the region-wise study of levels of brain glutamate and gamma-aminobutyrate system in protein adequacy and inadequacy may be worthwhile to understand the mechanism of aluminum-induced neurotoxicity.

RESULTS

Protein restriction does not have any significant impact on regional aluminum and gamma-aminobutyrate contents of rat brain. Significant interaction of dietary protein restriction and aluminum intoxication to alter regional brain glutamate level was observed in the tested brain regions except cerebellum. Alteration in glutamate alpha-decarboxylase and gamma-aminobutyrate transaminase activities were found to be significantly influenced by interaction of aluminum intoxication and dietary protein restriction in all the tested brain regions. In case of regional brain succinic semialdehyde content, this interaction was significant only in cerebrum and thalamic area.

CONCLUSION

The alterations of regional brain glutamate and gamma-aminobutyrate levels by aluminum are region specific as well as dependent on dietary protein intake. The impact of aluminum exposure on the metabolism of these amino acid neurotransmitters are also influenced by dietary protein level. Thus, modification of dietary protein level or manipulation of the brain amino acid homeostasis by any other means may be an useful tool to find out a path to restrict amino acid neurotransmitter alterations in aluminum-associated neurodisorders.

Effects of aluminium exposure on brain glutamate and GABA systems: an experimental study in rats

It has been postulated that the neurotoxic effects of aluminium could be mediated through glutamate, an excitatory amino acid. Hence the effects of aluminium administration (at a dose of 4.2mg/kg body weight daily as aluminium chloride, hexahydrate, intraperitoneally, for 4 weeks) on glutamate and gamma-amino butyrate (GABA), an inhibitory amino acid, and related enzyme activities in different regions of the brain were studied in albino rats. The glutamate level increased significantly in the cerebrum, thalamic area, midbrain-hippocampal region and cerebellum in response to in vivo aluminium exposure. The aluminium insult also caused significant increases in glutamate alpha-decarboxylase activity in all the brain regions. However, on aluminium insult, the GABA content was not significantly changed except in the thalamic area, where it was elevated. On the contrary, the GABA-T activities of all the regions were reduced significantly in all regions except the midbrain-hippocampal region. However, the succinic semi-aldehyde content of all brain regions increased, often significantly. The aluminium-induced modification of the enzyme activities may be either due to the direct impact of aluminium or due to aluminium-induced changes in the cellular environment. The aluminium-induced differential regional accumulation of glutamate or other alterations in enzymes of the glutamate-GABA system may be one of the causes of aluminium-induced neurotoxicity.

Experimental and clinical methods in the development of anti-Alzheimer drugs

Methodology used for the development of anti-Alzheimer's disease (AD) drugs raises specific problems which are rarely examined in the literature. While the general development scheme is similar to that required for most drugs, some specific aspects must be analyzed, highly dominated by the dual goal of pharmacology, i.e., to obtain both symptomatic and etiopathogenic drugs. During preclinical studies, aged or lesioned animals are mainly useful for symptomatic drugs, whereas transgenic models or neurodegeneration-induced techniques would probably lead to etiopathogenic drugs potentially slowing down the process of AD. The first administrations of a new compound to human beings raise the question of the activity measurement techniques. Psychometry remains the most informative procedure to detect and analyze the activity of the drugs on the different components of cognition. Electrophysiology and neuroimaging need some complementary studies before they can be proposed as surrogate criteria in phase III trials. At this stage of development, American and the recently published European guidelines are of great help while insisting on long-term (6 months) placebo controlled trials with the use of the triple efficacy criterion: an objective cognition scale, a global assessment, and the opinion of the caregiver. In the long term, pharmacoepidemiology and pharmacoeconomy will have to confirm the rationale of this recent progress in the methodology of anti-AD drug development.

Hippocampal acetylcholine increases during eyeblink conditioning in the rabbit

The classically conditioned rabbit nictitating membrane reflex (NMR) is modulated by the septohippocampal cholinergic system. Disruption of this system retards NMR acquisition. Aluminium (Al) is a neurotoxin that interferes with hippocampal acetylcholine (ACh) synthesis and release. Using microdialysis, this study tested the hypothesis that NMR acquisition in the rabbit is associated with hippocampal ACh release. This was conducted by measuring ACh release in control and A1-intoxicated rabbits during NMR training. NMR training consisted of four sessions of 100 conditioning trials/session in a delay paradigm. The percentage of conditioned responses (CRs) increased with each conditioning session for both groups, although percent CRs was significantly greater in the control group. Acetylcholine release in the ventral hippocampus increased significantly over baseline in the control group during the second and third conditioning sessions. In the Al-intoxicated group, ACh release did not increase significantly during any conditioning session. A separate group of rabbits was pseudoconditioned, receiving the same conditioning stimuli, although explicitly unpaired. This group did not acquire the CR. Acetylcholine release did not significantly increase during any conditioning session, suggesting that the increase in ACh release observed in the control group was not merely a product of conditioning stimuli presentation. The lack of increased ACh release in the Al-intoxicated rabbits was associated with a CR acquisition deficit. The results of this study are consistent with a role of hippocampal cholinergic function in NMR acquisition in the rabbit.

Effects of aluminum chloride on normal and uremic adult male rats. Tissue distribution, brain choline acetyltransferase activity, and some biological variables

Normal and uremic adult male rats were given a daily ip injection of 20 mg Al (Al chloride)/kg for 14 d. The results indicate that Al induces a significant decrease in food ingestion, weight gain, and total protein concentration in the plasma. Compared with control animals, very high increases in Al levels were found in plasma and hepatic homogenates (about 36 and 19 times, respectively). In the brain homogenates, the Al increases were lower (about 23%). The brain cholineacetyltransferase activity was reduced: 10.6 and 14.9% in normal and uremic rats, respectively. The nephrectomy and the food restriction did not affect the total protein concentrations in plasma and the cerebral cholineacetyltransferase activity. Both were only found to be reduced in the rats treated by Al chloride.

Studies of aluminum neurobehavioral toxicity in the intact mammal

1. Aluminum (Al) has been implicated in neurotoxic syndromes in several conditions, including Alzheimer's disease (AD). The developmental stage of the mammalian brain most susceptible to Al was determined in rabbits systematically exposed to Al during the prenatal, postnatal, or second month or for 1 month as adults or as aged subjects. Eyeblink reflex classical conditioning showed an Al-induced learning deficit only in the adult and aged rabbits. 2. 4-Aminopyridine, which was reported to improve learning in AD subjects, attenuated the Al-induced learning deficit. 3. Conditioned eyeblink acquisition is slower in AD subjects than controls, supporting the Al-loaded rabbit as a model of some AD effects. 4. To determine if the Al-loaded rabbit modeled the AD cholinergic deficit, acetylcholine (Ach) overflow was measured in rabbit hippocampus using microdialysis. Aluminum pretreatment reduced basal and potassium-stimulated Ach overflow compared to controls. 5. Acetylcholine overflow increased as control rabbits acquired the conditioned eyeblink reflex, then subsequently decreased, although conditioned eyeblink performance continued. In contrast, Al-loaded rabbits showed a delay in conditioned eyeblink acquisition and greatly attenuated Ach overflow. The Al-induced attenuation of Ach overflow may contribute to the Al-induced learning deficit. 6. Brain Al entry was studied using microdialysis of blood, brain, and lateral ventricle. Aluminum rapidly entered the brain and lateral ventricle. Frontal cortical Al was greater than lateral ventricular Al, suggesting that Al primarily enters the brain through the cerebral microvasculature. 7. The brain/blood Al ratio was always significantly less than 1. This ratio was influenced by the Al form administered, brain site and animal species. Thus, there appears to be an active process moving Al out of brain extracellular fluid (ECF). 8. Brain and blood dialysate Ach concentrations were not different after cyanide addition to the dialysate, supporting the conclusion that an active process moves Al out of brain ECF.

Chronic aluminum fluoride administration. I. Behavioral observations

This study examined the behavioral effects of chronic ingestion of various monofluoroaluminum complexes (AlF3) in drinking water. Forty young adult male Long-Evans rats were divided into four groups of 10 rats each. The groups received different concentrations of AlF3 in the drinking water from three sample solutions having a total Al concentration of 0.5, 5.0, and 50 ppm, respectively, or double-distilled deionized water on an ad lib. basis for 45 weeks. General decline of bodily appearance was observed in the lowest concentration AlF3 group, and animals in this group succumbed in greater numbers during the course of the study than those in any other group. Examinations of performance in an open field, an analysis of walking patterns, and a balance beam test did not find any difficulties indicative of motor disorder. Indeed, on the initial trial on the balance beam, the AlF3-treated animals exhibited superior performance. No group differences were found in behavior assessed by spontaneous alternation or by a modified Morris water maze test. When retested in the Morris maze after a low dose of scopolamine (0.4 mg/kg), the control animals took longer to reach the platform while the AlF3-treated rats were not affected. In an olfactory preference test, the AlF3-treated animals failed to show preferences exhibited by the controls, indicating a possible olfactory impairment. The level of Al in the brains of the AlF3-exposed rats, as determined by direct current plasma analysis, was almost double that of the control animals. There was a similar trend for the Al content found in the kidneys.

Effects of postnatal aluminum exposure on choline acetyltransferase activity and learning abilities in the rat

Young rats were treated by gastric intubation with aluminum lactate (0, 100, and 200 mg Al/kg/day) from postnatal days 5 to 14 to determine the treatment's influence on brain choline acetyltransferase activity and learning abilities. The results indicated that aluminum concentrations in the cerebral areas increased in parallel to plasma aluminum at the dose of 200 mg. In the same case, choline acetyltransferase activity was reduced. At postnatal days 50 and 100, the treated rats did not show alterations in their learning abilities in the 2 tests which are based on different motivations (avoidance of an aversive light or alimentary motivation) and different ways of achievement (pressing on a lever or running in a maze). A low reduction in the general activity, particularly in the radial maze test, was only observed in rats treated with 200 mg Al/kg/day.

Aluminum and chronic renal failure: sources, absorption, transport, and toxicity

In normal subjects the gastrointestinal tract is a relatively impermeable barrier to aluminum with a low fractional absorption rate for this metal ion. Aluminum absorbed from the gastrointestinal tract is normally excreted by the kidneys; in the presence of impaired renal function aluminum is retained and accumulates in body tissues. Aluminum-containing medications are given, by mouth, to patients with chronic renal failure as phosphate-binding agents for the therapeutic control of hyperphosphatemia. Patients with chronic renal failure are also exposed to aluminum in domestic tap-water supplies used either for drinking or, in those on dialysis treatment, in the preparation of their dialysate. In patients with end-stage chronic renal failure, particularly in those on treatment by hemodialysis, the accumulation of aluminum in bone, brain, and other tissues is associated with toxic sequelae. An increased brain content of aluminum appears to be the major etiological factor in the development of a neurological syndrome called either "dialysis encephalopathy" or "dialysis dementia"; an increased bone content causes a specific form of osteomalacia. An excess of aluminum also appears to be an etiological factor in a microcytic, hypochromic anemia that occurs in some patients with chronic renal failure on long-term treatment with hemodialysis. The various mechanisms involved in the toxic phenomena associated with the accumulation of aluminum in body tissues have not been clearly defined but are the subject of extensive investigations.