Issues of lead toxicity

Behavioral and neurochemical consequences of perinatal exposure to lead in adult male Wistar rats: protective effect by Centella asiatica

The present study evaluated the protective effects of Centella asiatica (CA) leaf extract on behavioral deficits and neurotoxicity in adult rat exposed to lead during perinatal period. Adult Wistar rats were exposed to 0.15% lead acetate (Pb) from gestation day 6 through drinking water and the pups were exposed lactationally to Pb till weaning. Significant perturbations in locomotor activity and exploratory behavior were observed in rats exposed to Pb during perinatal period. The levels of lipid peroxidation increased significantly with a reduction in levels of glutathione and activity levels of acetylcholinesterase and antioxidant enzymes in hippocampus, cerebrum, cerebellum, and medulla of brains excised from Pb-exposed rats. Oral supplementation of CA during postweaning period provided significant protection against Pb-induced behavioral impairments and neurotoxicity, without chelating tissue Pb levels. The possible neuroprotective efficacy of CA may be due to its antioxidant potential but not by lowering effects of brain Pb content.

Eating everything except food (PICA): A rare case report and review

PICA is an act or habit of eating non-food items such as stone, bricks, chalk, soap, paper, soil etc., It occurs in children who actually start seeing the world through the oral cavity. There are many theories behind it such as iron and zinc deficiency etc., We as dentists should be able to diagnose and treat such conditions, as they may cause ill-effects to the developing dentition. This case report attempt to highlights the importance of proper diagnosis and treatment of pica.

Developmental neuropathology of environmental agents

The developing central nervous system (CNS) is more vulnerable to injury than the adult one. Although a great deal of research has been devoted to subtle effects of developmental exposure, such as neurobehavioral changes, this review instead focuses on a number of chemicals that have been shown, in several experimental models as well as humans, to cause morphological changes in the developing nervous system. Chemicals that are discussed include methylmercury (MeHg), lead (Pb), antiepileptic drugs, and ethanol. Additionally, the issue of silent neurotoxicity, i.e., persistent morphological and/or biochemical injury that remains clinically unapparent until later in life, is discussed.

Lead-induced developmental perturbations in hippocampal Sp1 DNA-binding are prevented by zinc supplementation: in vivo evidence for Pb and Zn competition

Zinc finger protein (ZFP) transcription factors are essential for regulation of gene expression in the developing brain. We previously reported that Pb exposure perturbed the DNA-binding of ZFP such as Sp1 and Egr-1 in the cerebellum, which play critical role in CNS development. In this study, we focused on hippocampal Sp1 DNA-binding and mRNA expression in neonatal Pb-exposed animals. The expression pattern of an Sp1 target (NMDAR1) gene was also monitored. To study in vivo and in vitro competition between Pb and Zn, we supplemented animals with Zn, and examined the effects of both metals on hippocampal Sp1 DNA-binding and the DNA-binding of a recombinant Sp1 protein (rhSp1). Tissue metal analysis revealed that only the disposition of Pb in the brain but not its distribution in the blood was influenced by the presence of Zn. The developmental profile of Sp1 DNA-binding exhibited a peak on PND 15 which subsequently declined to adult levels. Consistent with earlier studies, Pb exposure produced premature peaks of Sp1 DNA-binding on PND 5 which later returned to adult levels. The basal and Pb-induced developmental patterns of Sp1 mRNA departed from its DNA-binding profiles. However, the expression patterns of the NMDAR1 gene were relative to Sp1 DNA-binding. Supplementation with zinc provided a protective effect on Pb-induced changes in Sp1 DNA-binding. Moreover, Pb and Zn directly interfered with the DNA-binding of rhSp1 in vitro. These data suggest that Pb and Zn can compete both in vivo and in vitro at the zinc finger domain of Sp1 with a consequential effect on Sp1 DNA-binding, subsequent gene expression and brain development.

Perinatal lead exposure alters the expression of neuronal nitric oxide synthase in rat brain

Environmental exposure to lead (Pb) is known to affect the developing nervous system causing cognitive deficits in children. The diffusible nitric oxide (NO) is a biological messenger known to be involved in brain development. We examined the developmental changes of neuronal nitric oxide synthase (nNOS) in cerebellum and hippocampus of developing rat brain by radiometric assay, Western blot analysis and immunohistochemistry. Pb-exposure (0.2% Pb acetate) was initiated on gestation day 6 through the drinking water of the dam and continued through birth and postnatal days (PNDs) 1 to 21. The pups were never exposed to Pb directly. Pb exposure was stopped on weaning of pups from mothers on PND 21. The changes in nNOS were measured in the offspring on PNDs 7, 14, 21, and 35. The nNOS activity was increased gradually from PNDs 7 to 35 in both cerebellum and hippocampus of control rats when the enzyme activity was determined in the presence of either 0.5 or 6 microM calcium (Ca2+) in the reaction mixture. However, Pb exposure decreased the nNOS activity significantly at PNDs 21 to 35 as compared to respective controls when the enzyme activity was determined in the presence of 6 microM Ca2+. The decrease of nNOS was even greater and evident at all PNDs tested when the enzyme activity was assayed in the presence of physiological concentration of Ca2+ (0.5 microM). These findings were further strengthened by the in vitro studies. The cerebellar nNOS activity was inhibited much more at low Ca2+ (0.5 microM) as compared to 6 microM Ca2+, with IC50 values of 35 and 50 nM Pb, respectively. The nNOS protein levels and immunoreactivity in the cerebellum and hippocampus of rats perinatally exposed to Pb were decreased as compared to controls at PNDs 21 and 35. These data suggest perinatal Pb exposure decreases the nNOS in the developing brain. The decrease of nNOS activity and protein may explain the Pb-mediated cognitive deficits because NO regulates long-term potentiation (LTP) and other neurophysiological events in the developing nervous system.

Lead exposure in pheochromocytoma (PC12) cells alters neural differentiation and Sp1 DNA-binding

Previous studies have revealed that lead modulates the DNA-binding profile of the transcription factor Sp1 both in vivo and in vitro (Dev Brain Res 1998;107:291). Sp1 is a zinc finger protein, that is selectively up-regulated in certain developing cell types and plays a regulatory role during development and differentiation (Mol Cell Biol 1991;11:2189). In NGF-stimulated PC12 cells, Sp1 DNA-binding activity was induced within 48 h of exposure of NGF naïve cells. Exposure of undifferentiated PC12 cells to lead alone (0.1 microM) also produced a similar increase in Sp1 DNA-binding. Since lead altered the DNA-binding profile of Sp1 in newly differentiating cells, neurite outgrowth was assessed as a morphological marker of differentiation to determine whether or not the effects of lead on differentiation were restricted to the initiation phase (unprimed) or the elaboration phase of this process (NGF-primed). NGF-primed and unprimed PC12 cells were prepared for bioassay following exposure to various concentrations of NGF and/or lead. Neurite outgrowth was measured at 48 and 72 h during early stages of NGF-induced differentiation and at 14 h in NGF primed/replated cells. In the absence of NGF, exposure to lead alone (0.025, 0.05, 0.1 microM) promoted measurable neurite outgrowth in unprimed PC12 cells at 48 and 72 h. A similar phenomenon was also observed in primed/replated PC12 cells at 14 h. However, this effect was two to five times greater than unprimed control cells. In the presence of NGF, a similar trend was apparent at lower concentrations, although the magnitude and temporal nature was different from lead alone. In most cases, the administration of higher lead concentrations (1 and 10 microM), in both the absence or presence of NGF, was less effective than the lower concentrations in potentiating neurite outgrowth. These results suggest that lead alone at low doses may initiate premature stimulation of morphological differentiation that may be related to lead-induced alterations in Sp1 binding to DNA.

Correlation between lead-induced changes in cerebral ornithine decarboxylase and protein kinase C activities during development and in cultured PC 12 cells

Exposure to lead (Pb) interferes with neurodevelopment and disturbs ornithine decarboxylase (ODC) activity. ODC the key regulatory enzyme of the polyamine pathway, is a potential substrate for protein kinase C (PKC). Therefore, we examined developmental changes in PKC activity and its relationship to ODC activity. Male rats were lactationally exposed to 0.2% Pb-acetate from birth to weaning. PKC and ODC activity were measured on postnatal days (PND) 3, 5, 10, 20 and 30. We found that the basal patterns of ODC and PKC activities resembled each other in both the neocortex and cerebellum and Pb-exposure attenuated both enzymes in a similar manner. To determine whether any link existed between these enzymes, ODC and PKC activities were induced to increase using nerve growth factor (NGF) in the presence and/or absence of ODC (difluoromethylornithine, DFMO) and PKC (staurosporine) inhibitors, in control and Pb-exposed Pheochromocytoma (PC-12) cells. Staurosporine decreased both ODC activity and PKC activity, while DFMO had no effect on PKC activity. These data suggest that ODC may be regulated by PKC and that Pb-induced developmental alterations in ODC activity may be secondary to changes in the integrity of PKC.

Sp1 as a target site for metal-induced perturbations of transcriptional regulation of developmental brain gene expression

Differential gene expression is partially regulated by zinc finger proteins (ZFP) such as Sp1, which may be potential targets for perturbations by environmental metals. In this paper, we discuss the selective effects of lead (Pb) and other heavy metals on the in vitro and in vivo DNA-binding of Sp1, and the developmental expression of its target genes. We have found that the presence of Pb, Zn and Cd in a DNA-binding assay differentially modulated the binding of Sp1 to its specific DNA sequence, while Ca, Mg and Ba, did not. In PC12 cells, cultured in the presence of low concentrations of Pb, a premature enhancement of Sp1 DNA-binding was observed. Similarly, Sp1 DNA-binding in the cerebellum of Pb-exposed animals was shifted to the first week after birth, while the developmental profile of a non-ZFP, NFkB, was not. Furthermore, selective premature peaks of myelin basic protein and proteolipid protein mRNA expression were observed to occur in a manner relative to the changes in Sp1 DNA-binding. Since these genes are high targets for Sp1, these data suggest that exposure to heavy metals may alter developmental gene expression and brain development through selective modulation of the transcriptional activity of Sp1.

Exposure to lead-acetate modulates the developmental expression of myelin genes in the rat frontal lobe

Postnatal exposure to high levels (4%) of lead (Pb) have been shown to disrupt myelin formation and result in abnormal conduction of nerve impulses, components necessary for information processing in the CNS. To investigate whether the pathological changes in myelin, due to Pb exposure, might be partially mediated by modulations of the expression of genes involved in CNS myelin, we have examined the developmental profiles of the proteolipid protein (PLP) and myelin basic protein (MBP), two major structural constituents of CNS myelin and 2',3'-cyclic nucleotide 3' phosphodiesterase (CNP), a non-structural enzyme associated with myelin formation. Rat pups were postnatally exposed, from birth to weaning, to moderate amounts of Pb (0.2%), in the drinking water of the dam, and their frontal cortices were assayed for changes in the expression profile of the above genes by Northern Analysis. On PND 20, Pb resulted in a dramatic stimulation of the mRNA levels of PLP and a small increase in MBP mRNA levels, but had no effect on the CNP message. These data suggest that moderate levels of Pb selectively interfere with the gene expression of structural proteins of CNS myelin and may thus influence the composition of myelin in this way.

Developmental profiles of ornithine decarboxylase activity in the hippocampus, neocortex and cerebellum: modulation following lead exposure

Ornithine decarboxylase (ODC) is a growth-associated enzyme which is critical for cell growth and transformation. ODC activity follows a specific ontogenetic pattern of activity in distinct brain regions according to their developmental stage. Perturbations in the pattern of ODC activity have been associated with brain damage including arrested cerebral growth. Modulations in the pattern of ODC activity were examined in the hippocampus, neocortex and cerebellum of neonatal rats (PND 3, 6, 9, 15) exposed via the dam to 0.2% lead-acetate (Pb2+ prenatally (gestational day 13 to birth), postnatally (PND 1-15) or perinatally (gestational day 13 to PND 15). Prenatal exposure to Pb2+ perturbed the profile of ODC activity in all three brain regions examined, while postnatal exposure to Pb2+ resulted in prolonged stimulations of ODC activity in the cerebellum. Following prenatal exposure, these effects were manifested as a stimulation of ODC activity in the hippocampus, a repression of activity in the neocortex and a combination of these effects in the cerebellum. Perinatal exposure to Pb2+ transiently modulated the pattern of ODC activity similarly in all three brain regions, in a characteristic manner irrespective of their developmental stage. These Pb(2+)-induced modulations of ODC activity suggest that polyamine-dependent processes may play a significant role in the manifestation of Pb(2+)-induced neurotoxicity dependent upon developmental factors at specific exposure periods.