Effects of lead salts on the uptake, release, and binding of gamma-aminobutyric acid: the importance of buffer composition

Heavy Metal Exposure: Molecular Pathways, Clinical Implications, and Protective Strategies

Heavy metals are often found in soil and can contaminate drinking water, posing a serious threat to human health. Molecular pathways and curation therapies for mitigating heavy metal toxicity have been studied for a long time. Recent studies on oxidative stress and aging have shown that the molecular foundation of cellular damage caused by heavy metals, namely, apoptosis, endoplasmic reticulum stress, and mitochondrial stress, share the same pathways as those involved in cellular senescence and aging. In recent aging studies, many types of heavy metal exposures have been used in both cellular and animal aging models. Chelation therapy is a traditional treatment for heavy metal toxicity. However, recently, various antioxidants have been found to be effective in treating heavy metal-induced damage, shifting the research focus to investigating the interplay between antioxidants and heavy metals. In this review, we introduce the molecular basis of heavy metal-induced cellular damage and its relationship with aging, summarize its clinical implications, and discuss antioxidants and other agents with protective effects against heavy metal damage.

Endoplasmic reticulum stress associated with lead (Pb)-induced olfactory epithelium toxicity in an olfactory dark basal cell line

Lead (Pb) can damage organs and also have undesirable effects on neural development. To explore the effects of Pb on olfactory cells, we investigated Pb-induced cell toxicity in the DBC1.2 olfactory cell line, with a focus on endoplasmic reticulum (ER) stress, apoptosis, and necroptosis. Representative markers of ER stress, apoptosis, and necroptosis were analyzed by quantitative PCR. The mRNA expression levels of GRP94, GRP78, spliced XBP1, PERK, and ATF6 increased significantly after Pb exposure in a dose-dependent manner. The expression of Caspase 3 and Caspase 12 did not increase after Pb exposure, which suggested that apoptosis-induced cell death was not activated after Pb exposure. However, the mRNA of RIPK3 and MLKL showed increases in expression, which indicated that necroptosis-induced cell death was activated after Pb exposure. These results indicate that Pb exposure induced dose-dependent cytotoxicity through ER stress and necroptosis pathways in DBC1.2 cells, whereas the apoptosis pathway was not significantly stimulated. HEPES buffer showed a partial protective effect in terms of ER stress, apoptosis, and necroptosis. In summary, the necroptosis pathway plays a crucial rule in Pb exposure-induced cytotoxicity in olfactory cells.

Pb2+ impairs GABAergic synaptic transmission in rat hippocampal slices: a possible involvement of presynaptic calcium channels

Pb2+ is a common pollutant that causes a wide variety of detrimental effects on developing central nervous system, including cognitive deficit. However, the mechanisms of Pb2+ neurotoxicity remain to be elucidated. GABAergic synaptic transmission in hippocampus is implicated in learning and memory. In the present study, we examined the effects of Pb2+ on GABA(A)-receptor-mediated inhibitory postsynaptic currents (IPSCs), recorded on CA1 pyramidal neurons in rat hippocampal slices, using whole-cell patch clamp recording. Pb2+ significantly inhibited the peak amplitude of evoked IPSCs and increased paired pulse ratio. In addition, Pb2+ (2-50 microM) significantly diminished the frequency of spontaneous IPSCs in a concentration-dependent manner with an IC(50) of 7.56 microM, without changing the amplitude of spontaneous IPSCs. However, Pb2+ (10 microM) did not alter the frequency and amplitude of miniature IPSCs. It was indicated that Pb2+ impaired GABAergic synaptic transmission via a presynaptic mechanism, inhibiting action potential-dependent GABA release. Interestingly, the inhibition of spontaneous IPSC frequency induced by 10 microM Pb2+ was significantly attenuated either in the presence of 100 muM Cd2+ or in a low-calcium (0.5 mM) bath. It suggested the involvement of voltage-gated calcium channels (VGCC) in Pb2+'s inhibition of GABA release. This study provided electrophysiological evidence from developing hippocampal slices to support that Pb2+ inhibited action potential-dependent GABA release by inhibiting presynaptic VGCC, which might be a mechanism for Pb2+ -induced cognitive deficit.

The importance of glutamate, glycine, and γ-aminobutyric acid transport and regulation in manganese, mercury and lead neurotoxicity

Historically, amino acids were studied in the context of their importance in protein synthesis. In the 1950s, the focus of research shifted as amino acids were recognized as putative neurotransmitters. Today, many amino acids are considered important neurochemicals. Although many amino acids play a role in neurotransmission, glutamate (Glu), glycine (Gly), and gamma-aminobutyric acid (GABA) are among the more prevalent and better understood. Glu, the major excitatory neurotransmitter, and Gly and GABA, the major inhibitory neurotransmitters, in the central nervous system, are known to be tightly regulated. Prolonged exposure to environmental toxicants, such as manganese (Mn), mercury (Hg), or lead (Pb), however, can lead to dysregulation of these neurochemicals and subsequent neurotoxicity. While the ability of these metals to disrupt the regulation of Glu, Gly and GABA have been studied, few articles have examined the collective role of these amino acids in the respective metal's mechanism of toxicity. For each of the neurotransmitters above, we will provide a brief synopsis of their regulatory function, including the importance of transport and re-uptake in maintaining their optimal function. Additionally, the review will address the hypothesis that aberrant homeostasis of any of these amino acids, or a combination of the three, plays a role in the neurotoxicity of Mn, Hg, or Pb.

The effects of lead on transient outward currents of acutely dissociated rat dorsal root ganglia

The effects of Pb2+ on transient outward currents (TOCs) were investigated on rat dorsal root ganglia (DRG) neurons at postnatal days of 15 approximately 21, using the conventional whole-cell patch-clamp technique. In media-sized (35 approximately 40 microm) neurons and in the presence of 50 mM TEA, TOCs that preliminarly included an A-current (IA) and a D-current (ID), were clearly present and dominant. Application of Pb2+ lengthened the initial delay of TOCs and increased the onset-peak time in a concentration-dependent manner. The amplitudes of initial outward current peak were reduced with increasing Pb2+ concentrations. The inhibitory effects of Pb2+ on TOCs were reversible with 80 approximately 90% of current reversed in 2 approximately 10 min at 1 approximately 400 microM Pb2+. For the normalized activation curves fitted by a single Boltzmann equation under each condition, there was a shift to more depolarized voltages with increasing concentrations of Pb2+. The V1/2 and the slope factor (k) increased from 12.76+/-1.49 mV and 15.31+/-1.66 mV (n=10) under control condition to 39.91+/-5.44 mV (n=10, P<0.01) and 21.39+/-3.13 mV (n=10, P<0.05) at 400 microM Pb2+, respectively, indicating that Pb2+ decreased the activation of TOCs. For the normalized steady-state inactivation curves, the V1/2 and the k increased from -92.31+/-2.72 and 8.59+/-1.36 mV (n=10) to -55.65+/-3.67 (n=10, P<0.01) and 23.02+/-2.98 mV (n=10, P<0.01) at 400 microM Pb2+, respectively. The curves were shifted to more depolarized voltages by Pb2+, indicating that channels were less likely to be inactivated at higher concentrations of Pb2+ at any given potential. The fast (tf) and slow (ts) decay time-constants were both significantly increased by increasing concentrations of Pb2+ (n=10, P<0.05), indicating that Pb2+ increased the decay time-course of TOCs. These effects were concentration-dependent and partly reversible following washing. Ca2+ modulated the TOCs gating and might share same binding site with Pb2+, for which Ca2+ had very low affinity. In summary, the results demonstrated that Pb2+ was a dose- and voltage-dependent, and reversible blocker of TOCs in rat DRG neurons. After Pb2+ application, normal sensory physiology of DRG neurons was affected, and these neurons might display aberrant firing properties that resulted in abnormal sensations. This variation caused by Pb2+ could underlie the toxical modulation of sensory input to the central nervous system.

Lead transport in IEC-6 intestinal epithelial cells

This study evaluated the use of IEC-6 cells as a model for studying lead (Pb) transport by intestinal epithelial cells (IECs) and examined potential transport mechanisms for Pb uptake and extrusion. Pb accumulation in IEC-6 cells exposed to 5 and 10 microM Pb for up to 60 min was time- and dose-dependent. Reduction of incubation temperature significantly reduced the total cellular Pb content of IEC-6 cells. Simultaneous exposed of cells to zinc (Zn) and Pb resulted in decreased total cellular Pb contents compared to total cellular Pb contents of cells exposed to Pb only. IEC-6 cells treated with ouabain (1 mM) or sodium azide (1 mM) and 5 microM Pb accumulated more Pb than cells exposed to Pb only. Cells treated with p-chloromercuribenzensulfonic acid (50 microM), p-chloromercuribenzoic acid (50 microM), or iodoacetimide (50 microM) accumulated less Pb than cells treated with Pb only. We conclude that Pb uptake by IEC-6 cells depends on the extracellular Pb concentration. Our data suggest that the mechanism of Pb uptake by IECs is complex, and that Pb transport in IEC-6 cells is time- and temperature-dependent, involves sulfhydryl groups, and is decreased by the presence of Zn. Extrusion of Pb is at least partially dependent on metabolic energy.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Potent blocking action of lead on voltage-activated calcium channels in human neuroblastoma cells SH-SY5Y

Human neuroblastoma cells (SH-SY5Y) have two types of voltage-activated calcium channels, which are equivalent to the N and L types. Both types of calcium channels were equally blocked by lead in a concentration-dependent and reversible manner, with Ki congruent to 1 microM. This lead concentration is in the same order of magnitude as that found in the blood of children exhibiting neuropsychological disorders. Sodium and potassium channel currents were not significantly affected by lead at a concentration of 10 microM.