A link between aluminum (Al) exposure and age-related neurological disorders has long been proposed. Although the exact mechanism by which the metal may influence disease processes is unknown, there is evidence that exposure to Al causes an increase in both oxidative stress and inflammatory events. These processes have also been suggested to play a role in Alzheimer's disease (AD), and exposure to the metal may contribute to the disorder by potentiating these events. Al lactate (0.01, 0.1, and 1 mM) in drinking water for 10 weeks increased inflammatory processes in the brains of mice. The lowest of these levels is in the range found to increase the prevalence of AD in regions where the concentrations of the metal are elevated in residential drinking water (Flaten [2001] Brain Res. Bull. 55:187-196). Nuclear factor-kappaB as well as tumor necrosis factor-alpha (TNF-alpha) and interleukin 1alpha (IL-1alpha) levels were increased in the brains of treated animals. The mRNA for TNF-alpha was also up-regulated following treatment. Enhancement of glial fibrillary acidic protein levels and reactive microglia was seen in the striatum of Al-treated animals. The level of amyloid beta (Abeta40) was not significantly altered in the brains of exposed animals. Insofar as no parallel changes were observed in the serum or liver of treated animals, the proinflammatory effects of the metal may be selective to the brain. Al exposure may not be sufficient to cause abnormal production of the principal component of senile plaques directly but does exacerbate underlying events associated with brain aging and thus could contribute to progression of neurodegeneration.

The basal levels of expression of mRNA of cytokines, interleukin-6 (IL-6) and tumor necrosis factor (TNF-␣), in the cerebral cortex of 5 and 26 month-old male B6C3F1 mice have been compared. In addition, the responsivity of animals of differing age to an inflammatory stimulus (lipopolysaccharide, LPS) has been studied. Basal levels of both of these cytokine mRNAs were elevated in aged animals relative to the younger group. However LPS administration led to a robust increase in cytokine mRNA levels in the younger animals but in aged mice, there was either an unchanged (IL-6) or a depressed (TNF-␣) response. Administration of dietary melatonin (200 ppm) to aged mice for 6 weeks prior to sacrifice, resulted in reduction of basal levels of cytokine mRNA to values found in the younger animals. Furthermore, following administration of LPS to melatonin fed animals, cerebral cytokine mRNA levels were significantly elevated rather than being unchanged or depressed. Taken together these findings reflect a trend in the cortices of melatonin-treated aged mice, to more closely approximate the status of younger mice. For comparative purposes, parallel studies were carried out using an immunologically active organ (spleen) and a non-neural organ with a low rate of cell turnover (heart muscle). In both these tissues, basal levels of cytokine mRNAs of animals of either age were very low, and there was a marked positive response to LPS. Dietary melatonin had no effect on the responses of TNF-␣ mRNA to LPS but attenuated the reaction of splenic IL-6 mRNA, thus bringing the response closer to that of the younger mice.

Brain cellular functions decline with normal aging, accompanied by a changing profile of gene expression. Gene array analysis was used to quantitatively estimate messenger RNA (mRNA) expression levels in the cerebral cortex of both young (4‐month) and old (27‐month) B6C3F1 male mice. A stringent degree of significance was obtained by using multiple gene chips. Out of 12,423 mRNA levels, only 25 changed significantly with age. Nine of these genes coded for inflammatory proteins, all of which were elevated in aged, relative to younger mice. Melatonin (200 p.p.m.) included in the diet of aged animals for 8 wk elevated serum and cortical melatonin and reversed 13 of the 25 genes altered with age. In no case did melatonin potentiate age‐related changes in gene expression. The restoration of a more youthful gene profile to brains of aged animals by melatonin, to a large extent, involves reversal of age‐induced elevation of basal inflammatory parameters.

The expression levels of a number of genes associated with inflammation and immune function change with advancing age. Melatonin modulates gene expression levels of several of these genes. Therefore the declining levels of melatonin associated with age may play a role in the physiological effects of aging. We used oligonucleotide microarrays to measure age-related changes in mRNA expression in the murine CNS, and to study the effect of prolonged administration of dietary melatonin upon these changes. CB6F1 male mice were fed 40ppm melatonin for 2.1 months prior to sacrifice at age 26.5 months, and compared with both age-matched controls and young, 4.5-monthold untreated controls. Total RNA was extracted from whole brain (excluding cerebellum and brain stem) and individual samples were hybridized to Affymetrix Mouse 430-2.0 arrays. The expression of a substantial number of genes was modulated by melatonin treatment and changes in selected genes were validated by quantitative reverse transcription polymerase chain reaction (qRT-PCR). A subset of these genes did not change with age. Conversely, some genes modulated by age were also modulated by melatonin treatment. In general melatonin treatment drove the expression levels of these genes closer to the expression levels detected in the younger animals. Notably, the abundance of lipocalin 2 (Lcn2) mRNA increased with age and was decreased in old animals treated with melatonin. Lcn2 is a member of the acute phase response family of proteins and its mRNA levels in the brain increase in response to inflammation. Many of the genes with expression reduced by melatonin are involved in inflammation and the immune system. This suggests that melatonin treatment may influence the inflammatory responses of old animals, driving them to resemble more closely those occurring in young animals.

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Melatonin modulates the expression of a number of genes related to inflammation and immunity. Declining levels of melatonin with age may thus relate to some of the changes in immune function that occur with age. mRNA expression levels in murine CNS were measured using oligonucleotide microarrays in order to determine whether a dietary melatonin supplement may modify age-related changes in the response to an inflammatory challenge. CB6F1 male mice were fed 40-ppm melatonin for 9 weeks prior to sacrifice at 26.5 months of age, and compared with age-matched untreated controls and 4.5-month-old controls. A subset of both young and old animals was injected i.p. with lipopolysaccharide (LPS). After 3 h, total RNA was extracted from whole brain (excluding brain stem and cerebellum), and individual samples were hybridized to Affymetrix Mouse 430-2.0 arrays. Data were analyzed in Dchip and GeneSpring. Melatonin treatment markedly altered the response in gene expression of older animals subjected to an LPS challenge. These changes in general, caused the response to more closely resemble that of young animals subjected to the same LPS challenge. Thus melatonin treatment effects a major shift in the response of the CNS to an inflammatory challenge, causing a transition to a more youthful mRNA expression profile.

Groups of younger and aged mice were fed either minimal basal diet or the same diet containing 40 ppm melatonin. After 9.3 weeks half of each of these 4 groups of animals received either an intraperitoneal injection of lipopolysaccharide (LPS) or of saline. Three hours after this treatment, all animals were killed and mRNA from brains extracted. Quantitative PCR was performed on 13 selected mRNA species reflecting various aspects of the inflammatory pathway, the melatonin receptor, and a key glycolytic enzyme. An overall trend observed was that the effect of melatonin in modulating LPS-provoked immune responses differed markedly in old and young animals. Melatonin tended to enhance the reaction of younger animals to LPS but suppressed the inflammatory response of older mice. This difference with aging suggests that key immune processes are markedly altered by aging. It is likely that the ability of the immune system to mount a defense is impaired in older animals.

Senescence is a complex and multifactorial process that may predispose organisms to altered responses to environmental stressors. The cytokine interleukin-6 (IL-6) is expressed by a variety of cells and is one of the earliest mediators of the acute inflammatory response. In this study, the level of IL-6 mRNA in younger (7 months) and old (23 months) mice was determined in the tissue of several organs with or without stimulation with lipopolysaccharide (LPS). Whereas younger animals had a basal expression of IL-6 mRNA in all organs, this was undetectable in the old animals. In contrast, when the mice were injected with LPS, in a majority of the organ tissues there was a robust stimulation of IL-6 mRNA in the old mice whereas the younger animals had a more variable response. These data indicate that the aging process may predispose animals to an exaggerated and potentially harmful inflammatory response.