Dietary enrichment with omega-3 polyunsaturated fatty acids reverses age-related decreases in the GluR2 and NR2B glutamate receptor subunits in rat forebrain
Introduction
Brain ageing is associated with multiple morphological and neurochemical changes. Glutamate mediates most of the excitatory transmission in the brain, and ionotropic receptors, such as the N-methyl-d-aspartate (NMDA) receptors and the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, play an essential role in glutamatergic signaling. Significant changes in NMDA and AMPA receptor complexes have been reported in the brain of aged animals [31], [49], [68].
NMDA receptors are a combination of the NR1 and NR2A–NR2D subunits. These combinations confer unique characteristics of ion permeability and desensitisation kinetics. NR2B has been the focus of much attention, for its possible role in learning and memory [69]. Overexpression of this subunit in mouse forebrain leads to an enhanced activation of NMDA receptors, and an improved performance in learning and memory tests [63]. In both rodents and primates, ageing is accompanied by a decrease in NR2B [1], [59], and this deficit is correlated with a decline in learning [34].
Ageing is also associated with changes in the AMPA receptor, formed by combination of the GluR1–GluR4 subunits. The calcium permeability of AMPA receptors is determined by the GluR2 subunit [43]. This subunit declines during ageing [51], potentially altering the cellular calcium dynamics. The altered calcium flux may contribute to the neurodegeneration associated with ageing [16]. Furthermore, GluR2 is important for the growth and maintenance of dendritic spines [52]. These are thought to have an important role in information processing [23]. Their structure and number can be modified by neural activity, suggesting they are a substrate for neuroplasticity. Decreased spine density has been reported in aged rats, primates and humans [27], [35], [50], [64]. Losses in both NR2B and GluR2 may synergize and become key determinants of the plasticity loss in aged brain, leading to cognitive decline and sensorimotor dysfunction.
Brain ageing is also associated with significant changes in phospholipids and their fatty acid composition [12], [20]. There is a marked decrease in polyunsaturated fatty acids (PUFA) of the omega-3 series, such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and of the omega-6 series, such as arachidonic acid (AA). The brain is highly enriched in DHA, with phosphatidylethanolamine (PE) and phosphatidylserine (PS) containing the highest levels [56]. During ageing, the decrease in DHA causes a decrease in membrane fluidity, and decreases the pool of DHA available for signalling. EPA is a precursor of DHA and also has important roles, such as the regulation of gene expression [28]. A reversal in omega-3 PUFA losses seen in ageing would have beneficial effects both at structural level and for neurotransmission.
Epidemiological studies indicate a link between a high omega-3 fatty acid intake and a decreased risk of cognitive decline and dementia in old age [24]. Furthermore, an omega-3 enriched diet reverses cognitive deterioration in aged animals [17]. This suggests that omega-3 PUFA may have a neuroprotective effect in ageing and could be used prophylactically or therapeutically, after the onset of cognitive impairment. Therefore, it is important to identify the cellular and molecular mechanisms that underlie their ability to maintain neurological function in old age. The aim of the present study was to characterise the effects of ageing on the GluR2 and NR2B receptor subunits in three brain areas, the prefrontal cortex, hippocampus and striatum, and establish whether dietary supplementation with EPA and DHA could rectify age-related changes in these receptors, in parallel with a restoration of PUFA structural losses.
Section snippets
Animals
The studies were conducted on male Wistar rats (Charles River, Ramsgate, UK). There were four groups of animals: adult untreated (N = 12), old untreated (N = 10), old receiving a control diet (N = 7) and old receiving an omega-3 PUFA enriched diet (N = 8). Four adult and four old untreated animals were used for in situ hybridisation and Western blot (one hemisphere for each technique). Four adult and four old untreated animals were used for lipid analysis (one hemisphere). Four old control diet and
Age-related changes in NR2B and GluR2 receptor subunits and effect of omega-3 PUFA supplementation
The effect of age on the NMDA glutamate receptor subunits NR1 and NR2B, and the AMPA glutamate receptor subunits GluR1 and GluR2 was studied at protein level in forebrain homogenates. As shown in Fig. 1, the GluR2 subunit was significantly decreased in old rats (21.3%, p < 0.05) and also in old rats on the control diet (23.9%, p < 0.05), compared to the adult values. Similarly, the level of the NR2B subunit was significantly decreased in the old group (22.5%, p < 0.05) and the old group on the
Discussion
Ageing is associated with impaired cognition, learning and memory, and with sensorimotor deficits. The forebrain circuitry underlying these changes involves the prefrontal cortex, the striatum and the hippocampus. In this study, we show that dietary supplementation with omega-3 PUFA reverses age-related decreases in the GluR2 and NR2B subunits in the prefrontal cortex, striatum and hippocampus. This reversal occurred in parallel with only a partial restoration of phospholipid PUFA composition
Acknowledgements
We would like to express our gratitude to Dr. W. Vas Dias and Seven Seas Ltd. for their generous donation of the Maxepa® oil used in this study. This work was funded by the St. Bartholomew's and The Royal London Charitable Foundation.
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