Long-term potentiation in the piriform cortex is blocked by lead

Lead neurotoxicity: effects on brain nitric oxide synthase

Lead (Pb), a ubiquitous and potent neurotoxicant, induces several neurophysiological and behavioural changes, while Pb alters the function of multiple organs and systems, it primarily affects the central nervous system. In human adults, encephalopathy resulting from Pb intoxication is often characterized by sleeplessness, poor attention span, vomiting, convulsions and coma; in children, Pb-induced encephalopathy is associated with mental dullness, vomiting, irritability and anorexia; diminished cognitive function resulting in a mental deficit has been also observed during Prolonged exposure to Pb. Pb can produce oxidative stress, disrupt the blood-brain barrier and alter several Ca(2+)-dependent processes, including physiological processes that involve nitric oxide synthesis on central nervous system in development and adult animals. This review summarizes recent evidence showing that Pb can interfere with the production of nitric oxide and can disrupt the function of nitric oxide synthase. Lead interferes with nitric oxide-related physiological mechanisms, and Pb neurotoxicity may affect processes involved in learning and memory.

Low-level lead exposure triggers neuronal apoptosis in the developing mouse brain

While the toxic effects of lead have been recognized for millennia, it has remained a significant public health concern due to its continued use and toxicological potential. Of particular interest is the increased susceptibility of young children to the toxic effects of lead. Although the exact mechanism(s) for lead toxicity is currently not well understood, research has established that it can be a potent NMDA antagonist. Previous research has established that exposure to NMDA antagonists during the brain growth spurt period (first 2 weeks of life in mice) can produce apoptotic neurodegeneration throughout the brain. Based on this information, the ability of lead exposure (two injections of 350 mg/kg lead 4h apart) to produce apoptosis in the neonatal mouse brain was assessed histologically 8-24h after treatment using activated caspase-3 immunohistochemistry, De Olmos silver technique, Nissl staining, and electron microscopy. Lead exposure produced significant neurodegeneration in the caudate/putamen, hippocampus, subiculum, and superficial and deep cortical layers of the frontal cortical regions. Further ultrastructural examination revealed cellular profiles consistent with apoptotic cell death. Statistical results showed that lead exposure significantly increased apoptotic neurodegeneration above that seen in normal controls in animals treated at postnatal day 7, but not on day 14. The results of this study may provide a basis for further elucidation of mechanisms through which the immature nervous system may be particularly susceptible to lead exposure.

Increased tau phosphorylation and beta amyloid in the hipocampus of mouse pups by early life lead exposure

The aim of this study was to investigate the effects of maternal lead exposure on the learning and memory ability and expression of tau protein phosphorylation (P-tau) and beta amyloid protein (Abeta) in hippocampus of mice offspring. Pb exposure initiated from beginning of gestation to weaning. Pb acetate administered in drinking solutions was dissolved in distilled deionized water at the concentrations of 0.1%, 0.5% and 1% groups. On the 21 th of postnatal day, the learning and memory ability of the mouse pups was tested by Water Maze test and the Pb levels in blood and hippocampus of the offspring were also determined. The expression of P-tau and Abeta in hippocampus was measured by immunohistochemistry and Western blotting. The Pb levels in blood and hippocampus of all exposure groups were significantly higher than that of the control group ( P < 0.05). In Water Maze test, the performances of 0.5% and 1% groups were worse than that of the control group ( P < 0.05). The expression of P-tau and Abeta was increased in Pb exposed groups than that of the control group ( P < 0.05). Tau hyper-phosphorylation and Abeta increase in the hippocampus of pups may contribute to the impairment of learning and memory associated with maternal Pb exposure.

Molecular targets of lead in brain neurotoxicity

The detrimental effects of lead poisoning have been well known since ancient times, but some of the most severe consequences of exposure to this metal have only been described recently. Lead [Pb(II)] affects the higher functions of the central nervous system and undermines brain growth, preventing the correct development of cognitive and behavioral functions. As an established neurotoxin, Pb(II) crosses the blood-brain barrier rapidly and concentrates in the brain. The mechanisms of lead neurotoxicity are complex and still not fully understood, but recent findings recognized that both Ca(II) dependent proteins and neurotransmitters receptors represent significant targets for Pb(II). In particular, acute and chronic exposure to lead would predominantly affect two specific protein complexes: protein kinase C and the N-methyl-D-aspartate subtype of glutamate receptor. These protein complexes are deeply involved in learning and cognitive functions and are also thought to interact significantly with each other to mediate these functions. This review outlines the most recent hypotheses and evidences that link lead poisoning to impairment of these protein functions, as well as the in vitro experimental approaches that are most likely to provide information on basic mechanicistic processes.

Long-term consequences of developmental exposure to lead or polychlorinated biphenyls: Synaptic transmission and plasticity in the rodent CNS

Exposure to lead (Pb) or polychlorinated biphenyls (PCBs) during early development has been associated with deficits in cognitive function in children (Pediatrics 87 (1991) 219; N. Engl. J. Med. 335 (1996) 783). These effects persist in the child long after exposure has ceased and body burdens have diminished. Despite intensive research, no consensus on the mechanisms of neurotoxicity of these chemicals has resulted. As the primary neurotoxic action of these agents is to impair cognitive ability, a number of laboratories have examined and reported on the detrimental the effects of Pb or PCBs on hippocampal synaptic transmission and long-term potentiation (LTP) in animals exposed during the perinatal period. Use-dependent synaptic plasticity, of which hippocampal LTP is the primary model system, is a fundamental property of neuronal function. In forebrain structures such as amygdala and hippocampus, LTP and related processes are purported to represent a physiological substrate for memory. During brain ontogeny, this type of plasticity guides the establishment and maintenance of synaptic connections in cortical structures based on sensory input. We postulate that the actions of PCBs and Pb in the developing nervous system perturb activity-dependent plasticity and promote organizational changes in brain. Aberrant connectivity derived from perturbations in activity-dependent plasticity during development may manifest as impaired LTP and cognitive ability in the adult organism.

Chronic exposure to low-level lead impairs learning ability during aging and energy metabolism in aged rat brain

The neurotoxic effect of chronic exposure to low-level lead (Pb) with advancing age is becoming an important social issue of public health. To examine the effects of low-level Pb treatment on behavior, cognition and brain energy metabolism in aging, we administered 200 ppm Pb acetate to adult (10-month-old) male Wistar rats for 12.5 months. After 12.5 months' exposure, the mean Pb levels in blood and brain had increased to 17.5 µg/dl and 0.07 µg/g, respectively, and the rats showed impaired learning and memory functions in a holeboard spatial memory test. No significant difference was found between experimental and control groups in locomotor activity and passive avoidance tests. By HPLC analysis of energy-rich phosphate concentrations, mild abnormalities were found in parietotemporal cortex and hippocampus, but only the 4.4% decrease of ATP in the parietotemporal cortex was statistically significant. These results suggest that chronic exposure to Pb during aging stage may selectively impair learning and memory functions and may cause slight cerebral energy impairment.

Nanomolar concentrations of lead inhibit glutamatergic and GABAergic transmission in hippocampal neurons

To investigate whether lead (Pb2+) affects the tetrodotoxin (TTX)-sensitive release of neurotransmitters, the whole-cell mode of the patch-clamp technique was applied to cultured hippocampal neurons. Pb2+ (>/=10 nM) reversibly blocked the TTX-sensitive release of glutamate and gamma-aminobutyric acid (GABA), as evidenced by the reduction of the amplitude and frequency of glutamate- and GABA-mediated postsynaptic currents (PSCs) evoked by spontaneous neuronal firing. This effect of Pb2+, which occurred 2-3 s after exposure of the neurons to Pb2+-containing external solution, was not related to changes in Na+-channel activity, and was quantified by measurements of changes in the amplitude of PSCs evoked when a 50-micros, 5-V stimulus was applied via a bipolar electrode to a neuron synaptically connected to the neuron under study. With an IC50 of approximately 68 nM, Pb2+ blocked the evoked release of glutamate and GABA. This effect was most likely mediated by Pb2+'s actions on extracellular targets, because there was a very short delay (<3 s) for its onset, and it could be completely reversed by the chelator ethylene diaminetetraacetic acid (EDTA). Given that Pb2+-induced blockade of evoked transmitter release could be reversed by 4-aminopyridine, it is suggested that the effect on release was mediated via the binding of Pb2+ to voltage-gated Ca2+ channels. Thus, it is most likely that the neurotoxic effects of Pb2+ in the mammalian brain involve a decrease of the TTX-sensitive, Ca2+-dependent release of neurotransmitters.

Duration of NMDA-dependent synaptic potentiation in piriform cortex in vivo is increased after epileptiform bursting

Stimulation of afferent fibers with current pulse trains has been reported to induce long-term potentiation (LTP) in piriform cortex in vitro but not in vivo. LTP has been observed in vivo only when trains are paired with behavioral reinforcement and as a consequence of kindled epileptogenesis. This study was undertaken in the urethan-anesthetized rat to determine if the reported failures to observe pulse-train evoked LTP in vivo may be related to a lesser persistence rather than lack of occurrence, if disinhibition might facilitate induction, and to examine the nature of the relationship between seizure activity and LTP. Stimulation of afferent fibers in the lateral olfactory tract with theta-burst trains under control conditions potentiated the monosynaptic field excitatory postsynaptic potential (EPSP) by approximately the same extent (20.3 +/- 2%; n = 12) as reported for the slice. However, in contrast to the slice, potentiation in vivo decayed to a low level within 1-2 h after induction (70% loss in 1.5 h, on average). The N-methyl--aspartate (NMDA)-receptor antagonists -APV and MK-801 blocked the induction of this decremental potentiation. Pharmacological reduction of gamma-aminobutyric acid-mediated inhibition at the recording site did not increase the duration of potentiation. In contrast, theta-burst stimulation applied after recovery from a period of epileptiform bursting induced stable NMDA-dependent potentiation. Mean increase in the population EPSP was approximately the same as under control conditions (21 +/- 2%; n = 6), but in five of six experiments there was little or no decay in potentiation for the duration of the monitoring period (</=6 h). It is concluded that seizure activity has an enabling action on the induction of persistent synaptic potentiation by stimulus trains that bypasses the need for behavioral reinforcement.

Effects of chronic lead exposure on [3H]MK-801 binding in the brain of rat

We have used quantitative autoradiographic methods to determine the effects of chronic lead exposure on N-methyl-D-aspartate (NMDA) receptors in the brain of female rat. Rats were exposed pre- and post-natally from day 4 +/- 1 post conception with 1000 ppm lead in their drinking water. This treatment continued after weaning. No effects of lead on [3H]MK-801 binding were found at PN 28. However, lead caused a significant increase in [3H]MK-801 binding in the hippocampus including CA1 and CA2, and in the occipital and temporal cortical areas at PN 56 and at PN 112. An increase in binding was also found in the entorhinal cortex and the dentate gyrus at PN 112. Because the NMDA receptor is involved in learning and memory, the lead-induced disruption of NMDA receptors in the hippocampus and cortex may be associated with the cation-induced cognition deficits.

Chronic developmental lead exposure increases the threshold for long-term potentiation in rat dentate gyrus in vivo

Chronic developmental lead (Pb) exposure has been long associated with cognitive dysfunction in children and animals. In an attempt to more directly relate the behavioral observations of impaired cognitive ability to Pb-induced effects on neuronal activity, we utilized the long-term potentiation (LTP) model of neural plasticity to assess synaptic function. Male rats were chronically exposed to 0.2% Pb(2+)-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. LTP threshold was determined by applying a series of stimulus trains of increasing intensities. Baseline testing of dentate gyrus field potentials indicated that input/output functions, maximal response amplitudes, and threshold currents required to evoke a population spike (PS) did not differ for control and Pb-exposed animals. Despite similarities in baseline synaptic transmission, Pb-exposed animals required a higher train intensity to evoke LTP than controls. With maximal train stimulation, however, control and Pb animals exhibited comparable levels of potentiation. These findings suggest that the mechanisms of LTP induction are preferentially impaired by Pb exposure. Although baseline synaptic transmission was not altered in Pb-exposed animals, decreases in glutamate release following high K+ perfusion and reductions in paired pulse facilitation have been reported in the intact animal. Pb-induced reductions in calcium influx through voltage-sensitive or N-methyl-D-aspartate (NMDA) receptor-dependent channels may mediate increases in LTP threshold. It is possible that the threshold changes in the induction of LTP reported here contribute to cognitive impairments associated with Pb exposure.