Abstract

In the general deterioration of physiological functions that takes place in aging, the prevalence of cognitive impairments, and particularly of those related to learning and memory, makes these deficits a major concern of public health. Although the exact nature of cellular and molecular substrates underlying learning and memory still remains an open issue for the neurobiologist, the current hypothesis assumes that it is determined by the capacity of brain neuronal networks to express short- and long-term changes in synaptic strength. Accordingly, the capacity of functional plasticity is impaired in the brain of aged memory-deficient animals. Short-term changes in synaptic transmission closely depend on transmitter release and neuronal excitability while long-term modifications are mainly related to the activation of the N-methyl-D-aspartate receptor (NMDA-R), a subtype of glutamate receptors. Because transmitter release, neuronal excitability and NMDA-R activation are modulated by magnesium (Mg²⁺), a change in brain Mg²⁺ homeostasis could affect synaptic strength and plasticity in neuronal networks and consequently could alter memory capacities. In addition, alteration of brain Mg²⁺ levels could be regarded as a possible mechanism contributing to cognitive aging. According to these postulates, long-term increase in Mg²⁺ levels facilitates the conversion of synapses to a plastic state while learning and memory capacities are enhanced in adult animals fed with a diet enriched in Mg²⁺-L-threonate, a treatment that significantly elevates brain Mg²⁺ levels. Because Mg²⁺-L-threonate also improves learning and memory in aged animals, the regulation of brain Mg²⁺ homeostasis may therefore be regarded as a relevant target for the development of new pharmacological strategies aimed at minimizing cognitive aging.

Introduction

Alterations in brain anatomy and physiology are frequent features that gradually take place in the course of aging, to finally impair cognitive functions, such as learning and memory. In the last decades, extensive behavioural experiments performed in animal models of aging confirm that the efficient learning and memory in young individuals is slowed down with age, while forgetfulness is accelerated (Barnes and McNaughton, 1985; Gallagher and Rapp, 1997; Lanahan et al., 1997; Norris and Foster, 1999; Ward et al., 1999), and, like in humans, deficits concern several forms of memory, including spatial, associative and long-term memory (Clayton et al., 2002; Gruart et al., 2008; Houston et al., 1999; Rosenzweig and Barnes, 2003; Sykova et al., 2002; Winocur and Moscovitch, 1990; Zornetzer et al., 1982). Neuronal or synaptic loss is unlikely to significantly account

for the senescent-associated cognitive deficits (Eriksen et al., 2009; Geinisman et al., 2004; Luebke et al., 2010; Morrison and Hof, 2007; Rapp and Gallagher, 1996) and a wealth of data now rather indicate that changes in functional properties within neuronal networks are mainly concerned (Billard 2006; Burke and Barnes, 2006; Craik and Bialystok, 2006; Disterhoft et al., 1994; Driscoll et al., 2003; Erickson and Barnes, 2003; Foster, 2007; Grady and Craik, 2000; Hsu et al., 2002; Kelly et al., 2006; Lister and Barnes, 2009; Magnusson, 1998; Mora et al., 2007; Sykov et al., 1998; Toescu and Verkhratsky, 2004). In particular, studies of long-lasting modifications of glutamatergic neurotransmission, such as long- term potentiation (LTP) and long-term depression (LTD) of synaptic strength, now considered as functional substrates of memory encoding (Bear and Malenka, 1994; Bliss, 1990; Collingridge and Bliss, 1995; Eichenbaum, 1996; Izquierdo, 1991; Kim and Linden, 2007; Lisman and McIntyre, 2001; Martin et al., 2000; Teyler and DiScenna, 1987), show substantial changes with age (Barnes, 2003; Billard, 2006; Burke and Barnes, 2006; Foster, 2006; Norris et al., 1996). Among the different mechanisms that may account for these age-associated impairments of synaptic plasticity (Foster, 2007; Rosenzweig and Barnes 2003), the activation of the N-methyl-D-aspartate subtype of glutamate receptors (NMDA-R) has received particular attention. Indeed, these receptors are pivotal for the regulation of synaptic strength, by means of their high permeability to calcium (Ca²⁺), which triggers the activation of specific intracellular protein kinases and phosphatases (Wang et al., 1997). Although it is obvious that NMDA-R activation is impaired in aging (Barnes et al., 1997; Junjaud et al., 2006; Potier et al., 2000), all of the underlying mech- anisms have yet to be definitely characterized and it remains to be determined to what extent they are involved in age-related deficits in synaptic plasticity and of memory capacities.

Among the different mechanisms possibly involved in cognitive aging is a change in ion homeostasis (Roberts, 1999) since transmitter release, cellular excitability and expression of synaptic plasticity closely depend on ion flux across neuronal membranes. Although Ca²⁺ regulation has initially gathered the largest interest in aging studies (Thibault et al., 2007; Toescu et al., 2004; Verkhratsky and Toescu, 1998), the role of magnesium (Mg²⁺), which is found in a relative large concentration in the central nervous system (CNS) (Chutkow, 1974; Poenaru et al., 1997), is now much more considered. Indeed, aging is a risk factor for Mg²⁺ deficit (Wakimoto and Block, 2001) [for a review see (Durlach et al., 1998)] and brain Mg²⁺ levels are significantly reduced in age- associated neurodegenerative diseases (Andrasi et al., 2000; Andrasi et al., 2005; Basun et al., 1991). On the other hand, the well-known regulation of NMDA-R activation by Mg²⁺ (Mayer and Westbrook, 1987; Nowak et al., 1984) and the fact that altered Mg²⁺ levels impair memory functions (Bardgett et al., 2005; Bardgett et al., 2007; Landfield and Morgan, 1984), strongly suggest that a change in Mg²⁺ homeostasis could contribute to the physiopathology of cognitive aging.

After reviewing the different roles of Mg²⁺ in the regulation of synaptic mechanisms at glutamatergic synapses, the present report will consider whether Mg²⁺ could be involved in deficits of these mechanisms that occur in the aging brain, and, finally, recent data will be presented suggesting Mg²⁺ as a relevant dietary component that could help in reducing age-associated memory impairments.

The impact of brain Mg2+ levels on synaptic mechanisms contributing to cognitive functions

Although Mg²⁺ is the second most abundant intracellular mineral after potassium and is present at large amount in the cerebrospinal fluid of both rodents (0.8 mM) (Chutkow, 1974) and humans (1.0 to 1.2 mM) (Basun et al., 1991; Kapaki et al., 1989), its role on neural activity and synaptic plasticity has been much less considered compared to other divalent cations such as calcium (Ca²⁺). This is rather surprising considering that Mg²⁺ is a cofactor for more than 300 enzymes and also tightly interacts with phospholipids and nucleic acids (Hofmann et al., 2000; Wolf and Cittadini, 2003), suggesting that the mineral should be able to modulate brain activity on a broad scale. Initially, the role of Mg²⁺ has mainly been evaluated in vitro by lowering extracellular levels ([Mg²⁺]_e), a procedure that increases spontaneous firing rate of neurons through membrane depolarization (Furukawa et al., 2009; Stone et al., 1992). This decrease in [Mg²⁺]_e can lead to paroxysmal events in slice preparations from both animals and humans, which resemble abnormal activities occurring during sustained seizures in vivo (Armand et al., 1998; Jones and Heinemann, 1988; Stanton et al., 1987). Using high performance liquid chroma- tography, quantification of basal efflux of amino acids indicates that only levels of glutamate, the neurotransmitter involved in most of excitatory synapses in the CNS, are significantly enhanced in low [Mg²⁺]_e medium (Furukawa et al., 2009; Smith et al., 1989). These data point out a first contribution of Mg²⁺ regarding the activity of excitatory synapses in the brain, that is, the regulation of the probability of transmitter release (Figure 1, left), as initially characterized at neuromuscular junctions (Kelly and Robbins, 1983; Kuno and Takahashi, 1986). But several lines of evidence show that beside this ubiquitous control on the probability of glutamate release at presynaptic terminals, Mg²⁺is also able to influence synaptic activity by acting at postsynaptic level (Figure 1, left). For instance, frequency potentiation (FP), which represents an increase in synaptic strength rapidly developing during repetitive activation of glutamatergic afferent fibres (Anderson and Lomo, 1966; Andreasen and Lambert, 1998), is greater after elevating [Mg²⁺]_e (Landfield and Morgan, 1984). This is mainly due to the ability of Mg²⁺ to reduce the calcium- dependent post-burst after hyper-polarization (AHP) of membrane potential, which is normally induced in depolarized neurons to limit excessive firing (Hotson and Prince, 1980; Lorenzon and Foehring, 1992; Madison and Nicoll, 1984). AHPs are reversibly reduced by the acquisition of learning-dependent behavioural tasks such as trace eye blink conditioning or spatial water maze (Moyer et al., 2000; Thompson et al., 1996). Thus, by controlling the initial postsynaptic depolarization through the regulation of AHP amplitude and duration, Mg²⁺ is able to modulate synaptic strength and to alter cognitive abilities (Disterhoft and Oh, 2006).

Figure 1.

Schematic representation of the positive and negative effects of short-term (left) and long-term (right) elevation of brain Mg² levels on neurotransmission and synaptic plasticity at an excitatory glutamat- ergic synapse. AMPAr: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (more...)

Although there is no doubt that Mg²⁺ may be considered as firmly involved in the regulation of synaptic activity through the pre- and post- synaptic mechanisms described above, an additional and essential role for the mineral has emerged at the end of the 1980s from electrophysiological studies demonstrating that Mg²⁺ inhibits currents through channels associated with the N-methyl-D-aspartate subtype of gluta- mate receptors (NMDA-R) by directly blocking the ion pore (Bekkers and Stevens, 1993; Jahr and Stevens, 1990; Mayer and Westbrook, 1987; Nowak et al., 1984). A transient rise in [Mg²⁺]_e within physiological range, i.e. from 0.8 to 1.2 mM, rapidly reduces the current amplitude mediated by NMDA-R by more than 60% in cultured neurons clamped at potentials below -50 mV while further increase in [Mg²⁺]_e is significantly less effective (Slutsky et al., 2004). Importantly, this depression effect does not occur at depolarized potentials, revealing a voltage dependency for the block of NMDA-R by Mg²⁺, which underlies the pivotal role of NMDA-R in the induction of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD). Indeed, following high-frequency activation of presynaptic terminals, the glutamate-induced depolarization of membrane potential releases NMDA-R from Mg²⁺ block allowing Ca²⁺ to enter the cells (Figure 1, left). As a consequence, activation of specific kinases or phosphatases is triggered, thus modifying synaptic strength (Wang et al., 1997). Several lines of evidence show effects of transient changes in Mg²⁺ levels on the induction of synaptic plasticity. For instance, a rapid rise in [Mg²⁺]_e selectively antagonizes LTP by reducing the depolarization- induced Ca²⁺ influx (Dunwiddie and Lynch, 1979; Malenka et al., 1992; Malenka and Nicoll, 1993). Interestedly, LTP is also suppressed in slices bathed with a Mg²⁺-free medium (Frankiewicz and Parsons, 1999; Jouvenceau et al., 2002), a loss that is independent of NMDA-R activation but rather due to changes in signalling cascades in post-synaptic neurons that remain to be characterized (Hsu et al., 2000). At intracellular level, Mg²⁺ also regulates the activity of Ca²⁺- dependent protein kinases governing NMDA- dependent LTP. For instance, it controls the subcellular localization of protein kinase C (PKC), which closely determines the function of the enzyme (Tanimura et al., 2002) and stimulates the dephosphorylation and deactivation of Calmodulin Kinase II (CaMKII) (Easom et al., 1998).

Despite much of data underlining that a transient increase in Mg²⁺ levels unables synapses to remain highly plastic, an unexpected opposite result has been reported in vitro after studying the effects of long-term elevation of the mineral (Figure 1, right). Indeed, when [Mg²⁺]_e is increased within physiological range in neuronal cultures for more than several hours, NMDA-R-mediated currents are enhanced and the expression of LTP significantly facilitated (Slutsky et al., 2004). In the same conditions, synaptic strength is not modified following a single action potential but increased after a burst of inputs. These long-term facilitation effects of Mg²⁺ on neurotransmission and synaptic plasticity also occur in vivo since they are found in young rats fed with a diet enriched in Mg²⁺-L-threonate (Mg²⁺-T), a new highly bioavailable compound that enhances loading of Mg²⁺ into the brain (Slutsky et al., 2010). The increase in the density of functional presynaptic boutons (Slutsky et al., 2010) coupled with the weaker glutamate release at synapses found after chronic Mg²⁺ elevation (Slutsky et al., 2004) indicate that long-lasting changes in Mg²⁺ homeostasis are able to modify the pattern of synaptic assemblies within neuronal networks, from a limited number of synapses with high probability of release to a larger density with low probability of release (Figure 1, right). In addition, both the increase in [Mg²⁺]_e in vitro and the elevation of brain Mg²⁺ in vivo up-regulate the expression of NR2B-containing NMDA-R (Slutsky et al., 2004; Slutsky et al., 2010). This increase, proposed to counterbalance the higher blockade of NMDA-R opening associated with chronic elevation of [Mg²⁺]_e, contributes to the greater capacity of synapses to be highly plastic.

Because the pattern and strength of synaptic transmission are widely believed to code memory traces (Bear and Malenka, 1994; Bliss, 1990; Collingridge and Bliss, 1995; Eichenbaum, 1996; Izquierdo, 1991; Kim and Linden, 2007; Lisman and McIntyre, 2001; Martin et al., 2000; Neves et al., 2008; Teyler and DiScenna, 1987), their susceptibility to short- and long-term changes in Mg²⁺ homeostasis described above predict that cognitive abilities would also be modulated by altering Mg²⁺ levels. Accordingly, Mg²⁺ deficiency impairs fear-conditioning (Bardgett et al., 2005; Bardgett et al., 2007) while chronically elevating plasma Mg²⁺ over several days improves reversal learning in the hippocampus-dependent T-maze task (Landfield and Morgan, 1984). However, whether brain Mg²⁺ levels are really altered in these studies has been questioned considering that Mg²⁺ loading into the brain is tightly regulated by active transport processes that maintain a concentration gradient between the cerebrospinal fluid (CSF) and the plasma. In fact, Mg²⁺ levels are poorly affected in the brain after long-lasting increase in plasma Mg²⁺ induced by intravenous injection of MgSO₄ both in animals and humans (Kim et al., 1996; McKee et al., 2005).

Nevertheless, this rigorous control of brain Mg²⁺ levels has recently been overcome using the highly bioavailable compound Mg²⁺-T, which increases Mg²⁺ concentrations in CSF by at least 15% (Slutsky et al., 2010). Behavioural characterization of the Mg²⁺-T-treated rats indicates significant improvements of learning abilities, working memory as well as short- and long-term memory compared to control animals (Slutsky et al., 2010), confirming that even moderate, changes in brain Mg²⁺ homeostasis are capable of altering cognitive performances.

The impact of brain Mg2+ levels on impaired synaptic mechanisms underlying cognitive aging

Although a link between Mg²⁺deficiency and cellular senescence has been proposed for the age- related deterioration of a large range of physiological functions (Killilea and Maier, 2008), a causal effect on deficits of cognitive functions is still questioned. Even aging is thought of as a general risk factor for Mg²⁺ deficit (Durlach et al., 1993; Wakimoto and Block, 2001). In fact, Mg²⁺concentrations determined in various brain structures only slightly decrease in healthy aging (Morita et al., 2001; Takahashi et al., 2001). However, since even a very small disturbance of [Mg²⁺]_e is able to substantially modify synaptic assemblies supporting cognitive performances as reported above, it may be postulated that age- related changes in Mg²⁺ levels, even of very weak amplitude, could contribute to the physio- pathology of cognitive aging.

Regarding the regulation of transmitter release at presynaptic terminals, the increase in basal release of glutamate determined in the brain of aged rats and monkeys in vivo (Massieu and Tapia, 1997; Quintero et al., 2007) suggests a possible role for Mg²⁺. According to this postulate, spontaneous miniature end-plate potentials increase in amplitude at neuro- muscular junctions in aging, that has been shown to reflect an impaired regulation of transmission release by Mg²⁺ (Kelly and Robbins, 1983). Nevertheless, whether a similar effect of Mg²⁺ also occurs at central synapses still remains to be demonstrated.

Age-related decrease in neuronal excitability is well documented, indicating that the amplitude and duration of AHPs are enhanced with age (Disterhoft and Oh, 2006; Disterhoft and Oh, 2007; Power et al., 2002; Thibault et al., 2007). AHP amplitude inversely correlates with both acquisition and probe performance in learning behaviours among aged animals (Tombaugh et al., 2005) and pharmacological treatments that rescue the age-related alteration of AHPs, also minimize memory deficits (Oh et al., 1999; Weiss et al., 2000). These data strongly suggest that a decrease in neuronal excitability is a potent mechanism contributing to the physiopathology of cognitive aging (Disterhoft and Oh, 2006). Although the involvement of Mg²⁺ in this functional deficit has not yet been formally demonstrated, some indirect experimental data suggest that this is probably the case. Frequency potentiation, which is negatively correlated to AHP magnitude (Thibault et al., 2001), is reduced by age (Diana et al., 1994; Landfield and Lynch, 1977; Landfield et al., 1986; Thibault et al., 2001) and this alteration is prevented by elevating [Mg²⁺]_e (Landfield and Morgan, 1984; Landfield et al., 1986). From these results, it may be hypothesized that the age-related facilitation of Ca²⁺ conductances supporting the increase in AHPs is not only due to a greater density of Ca²⁺ channels on neuronal membranes (Campbell et al., 1996; Thibault and Landfield, 1996; Thibault et al., 2001; Veng et al., 2003) but also to some extent, to a weaker competition between cations following Mg²⁺ depletion.

Regarding the expression of long-lasting synaptic plasticity, extensive electrophysiological studies report age-related deficits of both LTP and LTD in aged memory-impaired animals that reflects a shift in Ca²⁺ sources, with a weaker role for NMDA-R, and an increased contribution of voltage- gated Ca²⁺ channels and intracellular stores with different kinetic properties (Foster and Kumar, 2002; Gant et al., 2006; Junjaud et al., 2006; Kumar and Foster, 2005; Shankar et al., 1998; Thibault et al., 2007). NMDA-R activation is impaired in aged animals (Barnes et al., 1997; Burke and Barnes, 2010; Clayton et al., 2002; Eckles-Smith et al., 2000; Fontan-Lozano et al., 2007; Kollen et al., 2010; Magnusson, 1998; Ontl et al., 2004; Potier et al., 2000), not because of a reduced receptor density but rather to changes in pharmacological properties of the glutamatergic receptor (Billard and Rouaud, 2007; Junjaud et al., 2006; Kollen et al., 2010; Kuehl-Kovarik et al., 2003; Mothet et al., 2006; Turpin et al., 2009).

Among the possible mechanisms affecting NMDA-R activation in aging, a role for Mg²⁺ has been evaluated. No significant alteration of NMDA-R susceptibility to Mg²⁺ block occurs during aging since the percent decrease in NMDA-R- mediated synaptic potentials is comparable in young and aged animals after transient [Mg²⁺]_e elevation (Barnes et al., 1997; Potier et al., 2000). Also altering [Mg²⁺]_e affects NMDA-R-dependent LTP as well as short-term potentiation (STP) in a similar way in the two groups of animals indicating that a change in Mg²⁺ block of NMDA-R is unlikely to contribute to the age-related impairment of synaptic plasticity (Potier et al., 2000). However, a role for Mg²⁺ in LTP deficits occurring in aged animals does not definitively be discarded. For instance, a weaker activation of NMDA-R by its co-agonist D-serine has been shown to underlie LTP impairment in aged rodents, that is due to a weaker production of the amino acid by its synthesizing enzyme serine racemase (SR) (Junjaud et al., 2006; Mothet et al., 2006; Potier et al., 2010; Turpin et al., 2009). Because SR activity is potently stimulated by Mg²⁺, which increases 5- to 10-fold the rate of racemization of L- to D-serine, an impaired activation of SR by Mg²⁺ may therefore be possibly regarded as a potent mechanism linking lower brain Mg²⁺ levels to deficits of NMDA-R activation and related synaptic plasticity in aging.

Regarding the effects of long-lasting elevation of [Mg²⁺]_e, functional improvements also occur at synapses of the aging brain (Slutsky et al., 2010). The density of synaptophysin- and synaptobrevin- immunostained puncta is decreased in the hippocampal dentate gyrus of aged animals whereas synaptic loss in the stratum radiatum of CA1 subfield is less pronounced and even remains controversial (Burke and Barnes, 2006; Geinisman et al., 2004; Smith et al., 2000). However, in aged Mg²⁺-T treated rats, the number of functional synaptic connections is significantly increased in both hippocampal areas compared to aged controls (Slutsky et al., 2010). As in young animals, a treatment with Mg²⁺-T also up-regulates NR2B- containing NMDA-R, thus improving LTP expression in slices from aged animals (Slutsky et al., 2010). Interestingly, both the increase in functional synaptic connections and the facilitated induction of synaptic plasticity (Figure 2A) (see (Slutsky et al., 2010)). In addition, it is worth noting in these aged treated animals that the control level after the end of Mg²⁺-T supplementation and the time course of this decrease matches that of the reinstallation of impairment in memory scores (Figure 2B).

Figure 2.

Elevation of brain Mg² levels by Mg²⁺-T improves memory in aged rats. (A). Mg²⁺-T aged treated rats show shorter escape time in the water maze task indicating faster learning. (B). Time course of the reversible benefit effect of Mg²⁺-T treatment on spatial (more...)

Other possible routes involving brain Mg²⁺ in cognitive aging

The basal forebrain cholinergic complex including the medial septum/diagonal band, the substantia innominata, and the nucleus basalis of Meynert of Broca, represents a second major excitatory pathway of the CNS (Dutar et al., 1995; McKinney et al., 1983; Mesulam et al., 1983; Mesulam et al., 1984; Woolf et al., 1984). During the past fifty years, many experiments were directed at testing whether this cholinergic pathway is involved in aspects of cognition and a significant role in learning and memory has been firmly debated (see (McKinney and Jacksonville, 2005) for a review). Electrically-induced release of acetyl- choline generates slow and long-lasting excitatory postsynaptic potentials involving the activation of muscarinic receptors (Dutar and Nicoll, 1988; Misgeld et al., 1989; Muller and Misgeld, 1986). These responses are impaired in aging (Lippa et al., 1985; Potier et al., 1993; Segal, 1982; Shen and Barnes, 1996; Taylor and Griffith, 1993), that reflects a decreased transmitter release from cholinergic terminals (Aubert et al., 1995; Birthelmer et al., 2003; Scali et al., 1994; Takei et al., 1989; Vannucchi et al., 1997) but also a weaker activation of postsynaptic receptors (Calhoun et al., 2004; Lippa et al., 1985; Potier et al., 1992; Potier et al., 1993; Segal, 1982; Vannucchi et al., 1997). However, it is worth noting that the degree of memory deficit does not appear to closely correlate with changes in cholinergic synaptic activity (Calhoun et al., 2004; Potier et al., 1993; Shen and Barnes, 1996), suggesting that acetylcholine should not play a critical role in cognitive aging as initially proposed (Bartus et al., 1982; Bartus et al., 1985; Pepeu, 1988).

In addition to reducing acetylcholine release (Jope, 1981; Vickroy and Schneider, 1991), Mg²⁺ also acts on postsynaptic muscarinic receptors through several distinct mechanisms. First, Mg²⁺ is able to bind to the allosteric region of the M2 subtype of receptors, decreasing the inhibitory effects of some modulators on the dissociation of orthosteric ligands (Burgmer et al., 1998). Second, Mg²⁺ reduces a non-selective cationic conductance activated by muscarinic agonists (Guerineau et al., 1995), and, finally, the cation is necessary for activating G-proteins coupled to muscarinic receptors (Cladman and Chidiac, 2002; Shiozaki and Haga, 1992; Zhang et al., 2004). Regarding these multiple interactions, it is obvious that changes in Mg²⁺ levels are likely to contribute to the alteration of cholinergic neurotransmission that occurs in aging. Although direct evidence is still lacking, several data of the literature argue for the possibility that changes in brain Mg²⁺ may contribute to the age-associated impairment of acetylcholine-dependent synaptic activity. For instance, hypomagnesia significantly weakens responses of cortical neurons to ion- tophoretically applied acetylcholine (El-Beheiry and Puil, 1990) while the high-affinity binding at the muscarinic receptor in Alzheimer’s disease is closely regulated by Mg²⁺ (Ladner and Lee, 1999).

Conclusion

Although it is obvious that additional studies are necessary to unravel the mechanisms connecting brain Mg²⁺ homeostasis and cognitive aging, experimental data has progressively accumulated showing that even moderate changes in the concentration of the mineral are able to significantly affect the assembly and functionality of neuronal networks involved in cognition. Recently, the World Health Organization reached consensus that in a majority of the world's population, the dietary Mg²⁺ intake is lower than recommended, especially in the aging population (see also (Ford and Mokdad, 2003; Galan, 1997). Based on the results summarized in the present review, there is no doubt that elevating Mg²⁺ levels in the brain of the elderly could represent a promising strategy to minimize or even prevent cognitive deficits that take place with age.

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