Introduction

Lead (Pb) is a ubiquitous pollutant in the ecosystem. It is one of the most useful metals and is detectable in nearly all phases of the inert environment and in all biological systems. Environmental levels of Pb have increased more than 1,000-fold over the past 3 centuries as the result of human activity; the greatest increase occurred between the years 1950 and 2000 (ATSDR 2007). Despite efforts to reduce exposure through regulation, excessive Pb exposure still persists. The general population is primarily exposed to Pb from air and food; in contrast, occupational exposure to Pb occurs in workers employed in Pb refining, welding of lead painted metal, mines, and in battery plants. Over the last few decades, however, Pb emissions in developed countries have decreased markedly due to the introduction of unleaded gasoline (Järup 2003). Blood Pb levels were revised in the general population over the past 3 decades and the results demonstrated the adverse effects of Pb on child neurodevelopment; consequently, the levels of Pb considered hazardous were reduced to 40 μg/dl in 1971, 30 μg/dl in 1975, 10 μg/dl in 1991 (Bellinger and Bellinger 2006) and 1–2 μg/dl in 2004 (EPA 2006); however, Pb is still a significant public health concern.

The biological half-life of Pb may be considerably longer in children than in adults. Pb in blood has an estimated half-life of 35 days, in soft tissue 40 days and in bones 20–30 years (Papanikolaou et al. 2005). Pb has well-characterized effects on each bodily system, Pb toxicity targets are the haematological and cardiovascular systems and the kidneys, bones and teeth (Klein and Snodgrass 2003; ATSDR 2007) but the nervous system is especially susceptible to its effects. Children are particularly sensitive to the effects of the Pb and it is thus considered a primary environmental hazard (Papanikolaou et al. 2005; White et al. 2007); Pb intoxication in young children produces dramatic effects; they may be affected by behavioural disturbances and difficulties in learning and mental concentration, as well as cognitive deficits that include reduced IQ scores even at low levels of exposure. The increased vulnerability to Pb in children is due, in part, to the different exposure pathway and differences in toxicokinetics (Goyer 1993; Goyer and Clarkson 2001; Järup 2003; Bellinger and Bellinger 2006; White et al. 2007). The effects on memory and learning are mainly caused by the action of Pb on several brain structures; however, Pb primarily affects the hippocampus, where Pb neurotoxicity alters molecular mechanisms involving the nitric oxide synthases.

Several reviews have been published about the molecular mechanisms of Pb neurotoxicity, detailing the effect of Pb on different neuronal processes including metal-transporting proteins, ionic channels, glutamatergic synapses, synaptic plasticity, and signalling molecules, among others. The primary purpose of this paper is to describe the mechanisms of Pb neurotoxicity, showing how lead impairs processes involving nitric oxide synthase and how these processes produce serious repercussions on learning and memory.

Neurotoxicity of Pb

Lead is a heavy, low-melting, bluish-gray metal that occurs naturally in the Earth’s crust. In human, about 50 % of Pb is absorbed by inhalation route, while that only 10–15 % is absorbed by oral route; in each case, 90 % of the Pb that enters is retained in the body and distributed to the bones (Links et al. 2001). Once Pb accumulates in other organs, Pb crosses the blood–brain barrier (BBB) and concentrates in the gray matter of the brain (Goyer and Clarkson 2001; Gwalteney-Brant 2002). In humans, acute Pb toxicity is less common that chronic exposure and usually manifests as headache, irritability, abdominal pain and neurological signs; chronic Pb encephalopathy is often characterized by sleeplessness, poor attention span, vomiting, convulsions and coma (Bellinger et al. 1992). In children, Pb encephalopathy is characterized by lethargy, mental dullness, vomiting, irritability and anorexia; in severe cases, prolonged exposure to Pb can decrease cognitive function and increases behaviour disorders, especially aggression and hyperactivity (Bellinger et al. 1992; Gwalteney-Brant 2002; Järup 2003; ATSDR 2007). The neuropathological effects of chronic Pb intoxication include prominence of cerebral and cerebellar capillaries with endothelial cell swelling and necrosis, resulting in severe cerebral oedema caused by enhanced capillary leakage, loss of neuronal cells (Gwalteney-Brant 2002), cytoplasmic vacuolization, hyperchromatic cells, chromatolysis, interstitial oedema (Hirano and Iwata 1989) and demyelination of nerve fibers (Soltaninejad et al. 2003). Ultrastructurally, Pb causes alterations in mitochondria, Golgi apparatus and increment of gliofilaments in astrocytes (Strużyñska et al. 2001). Apoptotic cells with nuclear fragmentation and apoptotic bodies in hippocampal cells have also been described (Sharifi et al. 2002).

The endothelial cells are the first to be exposed to the Pb passage into the brain, and this exposure alters the maturation or differentiation of astrocytes; the endothelial cells show a marked affinity for Pb, resulting in high accumulations especially in mitochondria (Costa et al. 2004). Pb interferes with the phosphorylation of protein kinase C (PKC), which is activated as a consequence of receptor-dependent increases in intracellular Ca2+ concentrations and diacylglycerol (DAG), this activation increase transendothelial permeability (Zengh et al. 2003), leading enters Pb, ions, and water, and producing oedema and brain damage (Bressler and Goldstein 1991). The Pb enters the astrocytes from receptor-operated and voltage-dependent Ca2+ channels that mediate the uptake of Pb ions in these cells; it is recruited and deposited in the lysosome, the nucleus and other organelles of astroglia, a process that presumably occurs via Pb-binding proteins (Tiffany-Castiglioni and Qian 2001), astrocytes thus play a critical role in the brain, protecting neurons against Pb toxicity (Harry et al. 1996). A major intermediate filament protein found in astrocytes, glial protein fibrillary acidic protein (GFAP), has been suggested as an early indicator of neurotoxic insult (O’Callaghan and Jensen 1992), commonly, exposure of astrocytes to neurotoxic insults results in astrogliosis, which increases the synthesis and concentration of GFAP (Van Den Berg et al. 1996; Strużyñska et al. 2001). Several studies in Pb-exposed rats have shown that the expression, synthesis, and concentration of GFAP, as well as the number of astrocytes, were increased in specific brain regions (Fig. 1), this increase is associated with the formation of reactive gliosis in the rat brain, suggesting a primary response of astrocytes to Pb (Selvin-Testa et al. 1997; Stoltenburg-Didinger et al. 1996; Strużyñska et al. 2001; Tiffany-Castiglioni 1993; Tiffany-Castiglioni and Qian 2001; Harry et al. 1996; Strużyñska et al. 2001).

Fig. 1
figure 1

Cells exposed to Pb can alter the modulation of cell signals through different mechanisms such as: a interfering with processes regulate by Ca2+ that subsequently change transcription factors expression and/or activity; b affecting the LPO leading to oxidative stress altered; c interfering with enzymatic function; d induced alterations of the major nueurotransmitter systems and e interfering with protein components and/or activity. PKC Protein Kinase C, GFAP Glial Protein Fibrillary Acidic, NOS Nitric Oxide Synthase, GPx Glutathione Peroxidase, SOD Super Oxide Dismutase, CAT Catalase, Ca 2+ Calcium, LPO Lipid Peroxidation

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As previously mentioned, Pb can affect the nervous system through multiple pathways. Pb acts by mimicking Ca2+ action and/or disrupting Ca2+ homeostasis. Pb2+ is a divalent cation, and its neurotoxic action might be related to its ability to substitute for Ca2+ and, therefore, inhibit or increase the action of Ca2+ (Audesirk 1993). The interaction between Pb2+ and Ca2+ suggests that Pb2+ may gain entry into cells through one or more of the different types Ca2+ channels expressed in various cells (Bridges and Zalups 2005; ATSDR 2007). Likewise several neurotransmitters are also affected by Pb interaction, in animals exposed to low doses of Pb during development, high levels of dopamine and enhanced catecholaminergic neurotransmission have been observed in the cerebral cortex, hippocampus and cerebellum (Leret et al. 2002; Devi et al. 2005). Rats exposed to high concentrations of Pb have a decrease in norepinephrine, epinephrine and dopamine levels in the cerebral cortex, hippocampus and cerebellum, demonstrating that Pb exposure affects the neurotransmitter system in the developing rat brain (Fig. 1) (Devi et al. 2005). Glutamate is the major excitatory neurotransmitter in the brain ionotropic and metabotropic receptors mediate the action of glutamate via activation of the N-methyl-D-aspartate receptor (NMDAr) and play a central role in learning and memory. The NMDAr, which is mediated by Ca2+, activates protein kinase A (PKA), mitogen-activated protein kinase (MAPK) and calcium/calmodulin-dependent protein kinase (CAMK) pathways, which converge at the cyclic-AMP-response element-binding protein (CREB) (Fig. 2) (Toscano and Guilarte 2005).

Fig. 2
figure 2

In neurons, NO synthesis is initiated by a glutamate release leads Ca2+-influx into the cell, and binds to calmodulin. The binding of Ca2+/calmodulin complex to the catalytic site of NOS induce the oxidation of l-arginine to synthetize citruline and NO. Pb also affects the processes for NOS expression. It is possible that the Pb2+ is negatively acting on Ca2+-dependent transcription elements for CREB and AP-1 for nNOS and eNOS respectivelly. NO nitric oxide, Ca 2+ Calcium, Ca 2+ /CaM Calcium-calmodulin, NOS Nitric Oxide Synthase structure, CREB cAMP response element binding protein, AP-1 activator protein 1

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The two pathways that have received more attention are calmodulin and PKC. Ca2+ induces a conformational change in calmodulin that converts the protein to its active form. Pb2+ acts by displacing Ca2+ ions bound to calmodulin; the activation of calmodulin by Pb2+ results in protein phosphorylation altering cAMP messenger pathways (Goyer 1997; Goyer and Clarkson 2001). Normally, Ca2+ is responsible for activating PKC, which is a serine/threonine protein kinase involved in many processes such as synaptic transmission, neurotransmitter synthesis, dendritic branching, as well as the transduction of signals generated by a wide variety of effectors through specific receptors (Bressler et al. 1999). Pb is better activator of PKC than Ca2+ (Goyer 1997), and the activation of PKC induce genes that regulate the formation of the AP-1 transcriptional regulatory complex (Bressler et al. 1999), this activation disturbs signalling mechanisms in the hippocampus, causing impairment on long-term potentiation and memory in adult rats (Fig. 2), (Altmann et al. 1993; Gu et al. 2005).

Pb and oxidative stress

Oxidative damage is considered an important factor in Pb neurotoxicity, experimental evidence suggests that Pb induces oxidative stress and exerts some of its toxic effects through the disruption of pro-oxidant/anti-oxidant balance, which can lead to brain injury via oxidative damage (Oteiza et al. 1995; Adonaylo and Oteiza 1999; Antonio et al. 2003; Daniel et al. 2004; Villeda-Hernández et al. 2001; Villeda-Hernández et al. 2006). Pb exposure might induce changes in the activities of antioxidant enzymes that can be primarily attributed to the high affinity of Pb for sulphhydryl groups or metal cofactors in antioxidant enzymes (Gurer and Ercal 2000). Several authors have reported that Pb can induce significative decrease in the activity of superoxide dismutase (SOD) and catalase (CAT) in adult mouse and rat brains exposed to Pb (Skoczynska et al. 1993; Nehru and Kanwar 2004; Moreira et al. 2001a). Similar results were obtained by Wang et al. (2006), who found that the activity of SOD, Glutathione peroxidase (GPx) and reduced glutathione (GSH) were decreased significantly in 21-day-old rat brains exposed to Pb during pregnancy. However, contrary reports have indicated a gradual increase in the activity of antioxidant enzymes such as GPx, glutathione-reductase, CAT and SOD, mainly in the cerebellum and hippocampus (Antonio et al. 2003; Bennet et al. 2007). These studies support that Pb enhances lipoperoxidation (LPO) induced by H2O2 and iron and increases fluxes of O−2 and H2O2 (Gurer et al. 1988) resulting from a region-specific oxidative stress response in the brain (Bennet et al. 2007). It is unclear whether oxidative stress is the cause or the consequence of the toxic effect of Pb.

Nitric oxide (NO)

Nitric oxide is a signalling molecule with a variety of physiological functions. It is an unstable molecule with a molecular weight of 30 kDa and a short half-life of approximately 5–30 s. NO is a lipophilic gas that passes through the cellular membrane easily and can reach a diffusion distance of 150–300 μm in 4–15 s. It is produced by the stoichiometric conversion of l-arginine to l-citrulline via different isoforms of nitric oxide synthases (NOS) (Moncada et al. 1989; Kröncke et al. 1997; Bruckdorfer 2005). Molecular cloning experiments have led to the identification of three isoforms of NOS: neuronal NOS (nNOS or NOS-I), endothelial NOS (eNOS or NOS-III) and inducible NOS (iNOS or NOS-II) (Doyle and Slater 1997). NOS is widely distributed; it has been localized and studied in the cerebellum, hippocampus, cerebral cortex, corpus striatum, thalamus, amygdala, and olfactory bulb, among other regions of the central nervous system (Dawson and Dawson 1996; García-Arenas et al. 1999).

Nitric oxide is considered a free radical because it has an unpaired electron. NO reacts rapidly with O−2· to produce the peroxynitrite anion (ONOO−), which protonates at relevant pH to form peroxynitrous acid (ONOOH). Both ONOO− and ONOOH are potent oxidizers; ONOOH exhibits hydroxyl radical (·OH)—like activity, which can initiate a chain reaction generating numerous toxic metabolites (Aschner 1996). Nitrosative stress may lead to nitrosylation reactions that can alter the structure of proteins and so inhibit their normal function. In pathological conditions, NO is synthesized in excess and interacts with superoxide (·O2 −) to give rise to reactive oxygen and nitrogen species (ROS and RNS respectively) cascades (Valko et al. 2006).

Nitric oxide plays an important role in the function of several peripheral organs, including the digestive, respiratory and urogenital tracts (Toda and Okamura 2003). Likewise, is associated with brain processes that involve synaptogenesis (Estrada and Murillo-Carretero 2005), cerebral blood flow (Santizo et al. 2000), neuroendocrine secretion and neurotransmission (Chrousos 1995). NO is implicated in the regulation of the neurotransmitter release pathway that inhibits respiratory complexes contributing to glutamate excitotoxicity or stimulation by activation of guanylyl cyclase, which augment the phosphorylation of synaptic vesicles (Knott and Bossy 2009), is important its participation in immune responses involving cytokines and endotoxins, by produce high NO concentrations with cytotoxic effects on target cells (Simonian and Coyle 1996; Kröncke et al. 1997; Sun et al. 2005; Garthwaite 2008a, b). NO play an important role in Long Term Potentiation (LTP), LTP is a form of synaptic plasticity that is believed to form the cellular basis for learning and memory. It is well know that activation the NMDAr in brain are important in LTP induction, because the overstimulation of NMDAr by glutamate leads to Ca2+ overload in the cell and consequently activation of Ca2+ sensitive enzymes such as cNOS that induces NO biosynthesis (Zhang et al. 1998; García-Arenas et al. 2004; Hawkins et al. 1998).

Likewise, recent findings shown an association between increased concentrations of NO and diseases such as: migraine and epilepsy where the NO can exert inhibitory and excitatory effects on GABA-ergic transmission (Talarek and Fidecka 2003). Studies in humans and rodents shown that exist an increase of NO production in Parkinsons’s Disease, which is associated with a progressive loss of dopaminergic neurons in substance nigra (Burns et al. 1983; Kühn et al. 2003); Alzheimer disease show an increase on the three isoforms of NOS indicating an important role for NO in the pathomechanism of disease (Thorns et al. 1998; Lüth et al. 2001). Several pathological processes how ischemia, hypoxia, stroke, infectious agents, tumours and autoimmune diseases (Zhou and Zhu 2009) can involve NO alterations.

Nitric oxide synthase (NOS)

Neuronal nitric oxide synthase (nNOS)

Neuronal nitric oxide synthase is an enzyme that consists of 1,434 amino acids and has a molecular weight of 160.8 kDa. It is localized in human chromosome number 12 (Boissel and Schwarz 1998; Bruckdorfer 2005). NO synthesis is initiated by the presynaptic release of glutamate, which binds to NMDA, AMPA/Kainate and metabotropic receptors and allows Ca2+ to enter into the cell. The binding of Ca2+/calmodulin to nNOS induces the conformational change of the dimer and allows electrons to flow from the reductase segment of the enzyme, which contains the flavoprotein- (FAD- and FMN-) binding domains, to the catalytic site. This electron flow facilitates the oxidation of l-arginine and the synthesis of l-citrulline and NO (Alderton et al. 2001; Bruckdorfer 2005; Zhou and Zhu 2009). NO quickly spreads in the retrograde direction to the presynaptic neuron, where it united to guanylate cyclase activates the synthesis of cGMP or modifies the release of neurotransmitters such as acetylcholine, aspartate and glutamate (Fig. 2). NO can also return to the site of its original synthesis and influence the release of neurotransmitters in adjacent synapses (Talavera et al. 2003; García and Baltrons 2004; Bruckdorfer 2005). NO is released after stimulation and diffuses from the site of its production (either a neuronal or glial cell) to affect neurons up to 10-400 μm away (Talavera et al. 2003). nNOS is localized in subsets of neurons and astrocytes belonging to different anatomical and functional regions (Dawson and Dawson 1996). Although nNOS-derived NO is a critical molecule in mediating synaptic plasticity and neuronal signalling, it change from a physiological neuromodulator to a neurotoxic factor when an excessive amount of NO is produced (Zhou and Zhu 2009).

Endothelial nitric oxide synthase (eNOS)

The eNOS isoform is constitutively expressed in endothelial cells and cardiac myocytes; it has 1,203 amino acids and a molecular weight of 133 kDa, and it is located in human chromosome number 7 (Chatterjee and Catravas 2008). Although also has been found in astrocytes (Wiencken and Casagrande 1999) and hippocampal neurons (Dinerman et al. 1994); have a similar structure to nNOS, contains an amino terminal that differs from other NOS isoforms due to is especially proline-rich and has a consensus target sequence for myristoylation via acyltransferases. eNOS is a Ca2+-dependent enzyme (Marsden et al. 1993; Mac-Micking et al. 1997).

Inducible nitric oxide synthase (iNOS)

Inducible NOS (iNOS) is regulated predominantly at the transcriptional level and is likely involved in immune reactions. It has a molecular weight of 131 kDa, is composed of 1,153 amino acids and is located in human chromosome number 17 (Alderton et al. 2001). This enzyme is Ca2+ independent because calmodulin remains tightly bound to the protein; after induction, iNOS continuously produces NO until the enzyme is degraded (Mac-Micking et al. 1997). The iNOS isoform has a structure similar to nNOS and eNOS; it is composed of one oxygenase domain and one reductase domain with a catalytic site, it contains binding sites for FAD, FMN and NADPH (Bruckdorfer 2005). The iNOS isoform is not expressed in the healthy brain but is found primarily in immune cells or glial cells (astrocytes and microglia), as it is activated in response to pathogen recognition and cytokine release (Simonian and Coyle 1996; Kröncke et al. 1997).

Regulation and expression of NOS

The Ca2+ then binds to calmodulin to form the Ca2+/calmodulin complex that, in turn, activates Ca2+-responsive signalling pathways that converge on specific transcription factor such as CREB (Lonze and Ginty 2002); this pathways are PKA, CAMK and MAPK, activate CREB by phosphorylation at serine-133; CREB belongs to the bZip superfamily of transcription factors that contain a C-terminal basic domain that mediates DNA binding, and a leucine zipper domain that facilitates dimerization (Toscano and Guilarte 2005; Riccio et al. 2006).

The structure of nNOS gene is composed by 29 exons and its expression is regulated by membrane depolarization, as well the subsequent interaction of CREB and/or CREB-related family members with the calcium response elements (CRE) sites of the nNOS, which is contained on the promoter region within exon 2 immediately upstream of the transcription start site (Sasaki et al. 2000). Many of these processes are calcium-dependent and due to nNOS catalytic activity is also Ca2+-dependent, there is a unique potential for both simultaneous transcriptional and catalytic activity-mediated regulation of nNOS/NO (Wang et al. 1999). Regulation of nNOS transcription may be controlled by several alternatives of splicing of exon 1 to exon 2; the importance of exon 2 is mainly due to able regulating nNOS expression levels in several tissues and different physiological conditions (Boissel and Schwarz 1998; Dawson et al. 1998). The regulation of the expression of nNOS is a Ca2+-regulated gene through the interactions with CREB and these interactions are involved on diverse pathological and physiological processes such as learning and memory (Lonze and Ginty 2002; Toscano and Guilarte 2005).

The eNOS is encoded by 26 exons spanning 21–22 kb of genomic DNA, the promoter region of eNOS contains numerous binding sites for transcription factor such as AP-1, AP-2, NF-IL6 between others, as well as sites of the estrogen- and glucocorticoid-responsive element (Li et al. 2002). The expression of eNOS and posttranscriptional regulation gene occurs in the form of alternative mRNA splicing, and can be altered by different compounds (TNFα, thrombin, amphotericin B, glucocorticoids) or pathophysiological conditions (hypercholesterolemia, artherosclerosis, diabetes, hypertension) (Li et al. 2002).

The iNOS gene contains 27 exons showing a sequence of 16 kb, in cloned iNOS promoter that exhibits homologies to binding sites for numerous transcription factors involved, (AP-1, CAR, NRF, Nuclear Factor-κB lipopolisacharides/cytokines) that mediate induction and regulation of promoter activity (Akar and Feinstein 2009). The regulation of iNOS expression by transcriptional and posttranscriptional mechanisms is the main regulatory step to control iNOS activity. Usually, iNOS synthesizes NO continuously until the enzyme is degraded; relatively little information exist on iNOS splice variants (Pautz et al. 2010).

Effect of Pb on brain NOS

The primary mechanism of Pb toxicity is related to its ability to establish interactions with coordinated divalent cations such as Ca2+ and Zn. Pb can alter the activity and expression of nNOS and eNOS in different brain regions primarily because Pb can mimic Ca2+ at binding sites, this binding may prevent the accessibility of Ca2+ to NOS, thus leading to the decreased activity of nNOS/eNOS and resulting in reduced NO production in different brain regions (Selvin-Testa et al. 1997). Several studies have reported a decrease in nNOS activity in animals exposed to Pb, Chen et al. (2000) founded that rats exposed to different concentrations of Pb displayed a decrease in NO production as indicated by decreased nitrite and nitrate levels. Others studies have also shown that NOS activity in the hippocampus, cerebral cortex and cerebellum is inhibited significantly by Pb exposure; the degree of this inhibitory effect depends on the time span of exposure and the concentration of Pb; however, this accumulation does not necessarily account for the inhibition of NOS calcium-dependent activity in specific regions (Zhu et al. 2005). García-Arenas et al. (1999) measured the activity of calcium-dependent NOS and iNOS in both the synaptosomes and capillaries of mice exposed to Pb and founded that NOS was inhibited in synaptosomal fractions throughout the brain. In another study, the same authors founded a specific dose-dependent decrease of NOS activity in the hippocampus and cerebellum, but not in the cortex or brain stem, of adult rats exposed to Pb (García-Arenas et al. 2004); It also seems that the presence of high Pb concentrations enhances iNOS activity (García-Arenas et al. 1999).

Pb also affects the processes involved in NOS expression. It is possible that the cation negatively acts on Ca2+-dependent transcription elements for nNOS (CREB, cAMP) and eNOS (AP-1, activator protein 1) to decrease protein expression (Fig. 2) (Lonze and Ginty 2002; Riccio et al. 2006; Sasaki et al. 2000; Wang et al. 1999; Dawson et al. 1998; Li et al. 2002). Immunohistochemical studies have shown changes in the pattern of NOS expression in the cortex and hippocampus of adult and developing rats exposed to Pb (Selvin-Testa et al. 1997; Chung et al. 2004). More recent reports have confirmed these results; Nava-Ruíz et al. (2010) showed that the expression patterns of nNOS and eNOS were diminished in the hippocampus of adult rats exposed to Pb using a sub-acute model. In addition, in vitro studies have shown that under the stated experimental conditions, Pb-acetate may decrease the expression of the iNOS protein in C6 glia cells; (Tiffany-Castiglioni et al. 1990).

Effect of Pb on learning and memory

Learning is defined as the processes of acquiring new information or skills, whereas memory refers to the persistence of learning that can be revealed/recalled at a later time. Memory is the usual consequence of learning and reflects enduring changes in the nervous system that result from transient experiences. The hippocampus is an anatomical structure responsible for diverse memory processes that include the spatial and temporal separation of events; this separation is associated with several types of learning and memory formation through long-term potentiation (LTP). LTP results from an increase in the strength of synaptic transmission, which can last from hours to days. Moreover, the LTP found in the hippocampus has been detected in other brain areas such as the amygdala and cortex, as well as their related limbic structures in the mammalian brain (Squire and Knowlton 1995). The CA1 region of the hippocampus has shown both NOS activity and LTP, a type of synaptic plasticity in the hippocampus that is believed to contribute to declarative forms of learning such as spatial learning (Xu et al. 1998; White et al. 2007).

Experimental studies indicate that Pb is accumulated in several brain regions and is highly concentrated in the hippocampus (Villeda-Hernández et al. 2001; García-Arenas et al. 2004; Nava-Ruíz et al. 2010). The possible deleterious effect of Pb might be related to its interference with Ca2+-dependent cellular processes, an interference that significantly affects the induction of LTP that is mediated by NO regulation. This effect on LTP would explain why learning and memory are affected by chronic Pb exposure (Chetty et al. 2000). Other authors have reported changes in LTP induction in learning and memory pathways in developing and adult animals exposed to Pb; Carpenter et al. (1994) founded that Pb can block LTP at micromolar concentrations in rats exposed to Pb. Similar results were reported by Gilbert and Mack (1990) and Robinson and Reed (1992), who both reported that LTP in the CA1 area of the hippocampus and in the dentate gyrus can be readily blocked by NMDAr antagonists (Fig. 2). García-Arenas et al. (2004) showed that LTP induction was affected in a dose-dependent manner in adult rats exposed to 250 and 500 ppm of acetate of Pb; likewise, it has been reported that Pb alters the amplitude of LTP in hippocampal slices, demonstrating that low-level lead exposure can reduce structural plasticity in adults via neurogenesis (Zhao et al. 1999). Moreover, Altmann et al. (1993) founded a decrease in population spike activity in the hippocampal CA1 region; similarly, other studies have observed an increase in the sensitivity threshold of the LTP blocking response of NMDAr in Pb-exposed rodent hippocampus, indicating that Pb induces a decrease in the magnitude and retention time of synaptic plasticity (Gilbert et al. 1996; White et al. 2007).

Weiss et al. (1998) also founded a neuronal phenotype that was particularly vulnerable to Pb in the hippocampus of rats exposed to Pb during the development, this phenotype was positive for nNOS and displayed Pb-induced changes in NMDAr expression. This discovery indicates that Pb can affect the production of NO, which is a messenger essential for hippocampal LTP (Toscano and Guilarte 2005).

Several studies of cognitive deficits related with NOS activity and NO production have been documented in rats and nonhuman primates models after exposition to Pb (O’Dell et al. 1991; Jett et al. 1997; García-Arenas et al. 2004); when the hippocampal NO levels of mice exposed to Pb were decreased, the mice displayed a reduced ability to learn and memorise on a water T-maze test, suggesting the presence of Pb-induced deficits in learning and memory processes (Sun et al. 2005). Kuhlmann et al. (1997) showed that rats exposed to Pb during different developmental periods were tested as adults in a water maze test, presenting long-term changes in cognitive functions. Likewise, recent results have shown that Pb exposure during gestation can cause pups hyperactivity, decrease exploratory behaviour and prolonged learning/memory deficits in young adult rats (Moreira et al. 2001b; Yang et al. 2003; Song et al. 2006). Experiments performed in monkeys treated with Pb revealed deficits in function a variety of behavioural task, showing to display learning and/or memory impairment (Rice 1993, 1996). In summary these studies shown that in animal models the cognitive functions are altered to Pb exposure at different doses, for different time periods affecting the nNOS activity and reflecting a decrease in the production of NO, mainly in hippocampus producing a cognitive (learning and memory) disrupt, likewise these dates are agreement with epidemiological studies related in humans that support the association between blood Pb levels and intellectual impairment in children with deficit and cognitive development impairments (Canfield et al. 2003; Al-Saleh et al. 2001; Bellinger and Needleman 2003; Mendola et al. 2002; Emory et al. 2003; Gomaa et al. 2002).

Final commentary

Pb is a neurotoxic metal that causes health problems in developed and developing countries. The central nervous system is especially sensitive to Pb, displaying functional alterations mainly during development, in humans and experimental animals. These alterations are due to time and pathways exposure as well as the differences toxicokinetics of Pb. In this review, we summarized the findings related to Pb-induced neuronal damage and the possible mechanism involved in NO synthesis mediating activation and/or inhibition of NOS generating disruption on the behaviour and their effect over NOS gene expression. These results indicate that impaired NO production in animals exposed to Pb can affect memory and learning processes. Future investigations must address those neuronal mechanisms in detail in order to understand Pb-induced damage over NOS function, responsible of NO synthesis.