Effects Of Rats' Exposure To Low-level Lead On Offspring's Learning, Memory And The Expression Of MGlur1, NMDA Recipients In Different Developmental Stages

With the acceleration of the industrialization and modernization of community, continuous discharge and disposal of industrial waste products containing hazardous materials have resulted in serious contamination of the soil and water bodies, but Pb has grabbed more attention due to its longer persistence in soil and highly toxic effects on both crop production and human health. Lead is a kind of common toxic heavy metal that has obvious and sure damage to apparatus and tissue of individual, especially strong neurotoxicity, which can damage the developmental central nervous system (CNS). It caused major public health problem because of the documented effects on cognitive development. Lead can be transported from mother to fetus through placenta easily and it also can come into the CNS of fetus through the incomplete blood-brain barrier and do harm to the developmental CNS. As a result, the offspring's ability of learning and memory would be affected; it also can transport into newborn by sucking lacto, which induced to the increase of lead burden in baby body; during the childhood, it can cause destruction of child's IQ exposed to lead. Furthermore, these destroy will last during the whole life.During neuronal development and regeneration, hippocampus is an important encephalic region that charge for learning and memory, especially for short-term memory and recent memory. Long-term potentiation (LTP) is a cellular model of the synaptic plasticity that may underlie memory acquisition, which may be representative of learning and memory processes in mammalian brain. LTP is triggered by certain receptors for the excitatory neurotransmitter, glutamate.The N-methyl-D-aspartate (NMDA) receptor receptors are a subclass of excitatory amino acid receptors which play an important role in brain development learning and memory processes. NMDAR on the synaptosome acts as a co-incidence receptor, detecting presynaptically released glutamate, and postsynaptic depolarization, which removes the Mg2+ block leading to Ca2+ influx and the activation of various enzymes involved in LTP. The gene expression of NMDAR subunits differs neuroanatomically and temporally. NMDAR2R play an important role during the course of neuron cell growth and development, synaptogenesis, LTP, neuro signal transduction and the release of neurotransmitter.The NMDAR in developing brain is an important target of lead. Lead is a potent noncompetitive antagonist of the NMDA receptor. In vitro electro-physiological studies showed that lead could inhibit the NMDA activated whole cell currents, and that lead could also inhibit the opening of the single gate of NMDAR. Immature neurons are more sensitive to lead poison than mature neurons. But it is not known clearly whether Pb impairs the expression level of mRNA of NMDAR subunits. And the relationship between the effects and the exposure level of lead also isn't known yet.Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) with eight cloned subtypes classified into three groups based on sequence homology, pharmacology and transduction mechanism. Metabotropic glutamate receptors 1(mGluR1) belonged Group I is an important member of family of GPCRs. The signal transduction induced by mGluR1 plays an important role in the regulation of the central nervous system functions, and is involved in many neurodegenerative diseases participates in LTP; the role of these receptors remains unclear. A pathway linking class I-mGluRs with PKC is sure to enhance the open probability of the NMDAR channel. The mGluR-mediated potentiation of NMDAR activity may play a role in synaptic transmission and plasticity including LTP.Pb toxicity is also known to induce oxidative stress through over production of reactive oxygen species (ROS) including superoxide radicals (O2?), hydroxyl radicals (·OH) and hydrogen peroxide (H2O2). These free radicals and hydrogen peroxides cause membrane damage (which is often related to lipid peroxidation); which effect the development of central nerve system of rats exposed to lead. We have noted enhanced lipid peroxidation and disorders in the activity of antioxidative enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in the brain of rats exposed to Pb.Nitric oxide (NO) is regarded as typical retrograde messenger, important in learning and memory. Its biosynthesis is modulated by nitric oxide synthase (NOS). At Present, the level of NO is reflected mainly by NOS activity. Lead can inhibit NOS activity and therefore reduce NO.Objectives:We established a series of rat models exposure to low-level Lead in different developmental stages, and then measured the concentration of lead in the blood and brain,δ-ALAD in blood, determinate the changes of mRNA expression of mGlur1,NMDAR-2R in different brain regions(hippocampus, cortex), evaluate SOD, GSH-Px, NOS activity. From a widespread domain, research the relationship among the change of each index and lead level, time of exposure, and provid experiment evidences on how lead harms learning and memory function, explore the likely mechanism for the lead neurotoxicology.Methods:1. The rat models were established by exposing rats to 0.2% mg/ml lead acetate solution via drinking water respectively in different developing stages and divided into control group, lead exposure during pregnancy group (GroupA), lead exposure lactation period group (GroupB), and lead exposure after ablactation (GroupC). The samples were taken on 60th～68th day, the body, brain, hippocampus and cerebral cortex weights were measured respectively.2. The contents of lead in blood, hippocampus and cerebral cortex were measured by graphite furnace atomic absorption spectrometry; the activities of ALAD in blood were measured by spectra photometer.3. The ability of learning and memory was tested by Morris water maze.4. Expression of NMDA receptor 2A, 2B, 2C subunits mRNA and mGlur1 mRNA in rat cerebral cortex and hippocampus were measured by semi-quantitative reverse transcript polymerase-chain reaction (RT-PCR), withβ-actin as standard.5. Hippocampus and cerebral cortex tissue respectively were detected activity of SOD, GSH-Px and NOS by commercial kits.Results:1. Compare with the control, no differences in body weights, brain/body of dams were measured between any two groups (P>0.05). Compare with the control, hippocampus weights, brain weights of GroupA, brain weights of GroupB were decrease (P<0.01), Cortex weights of GroupA were higher than those of GroupB and GroupC (P<0.01).2. The blood and brain lead level of Pb-exposed dams were higher than those of control group; the activity of ALAD were lower than those of control group, especially the GroupC (P<0.01, P<0.05). Compare with the control, lead concentra- tion in both hippocampus and cerebral cortex is elevated significantly in GroupA (P<0.01); lead concentration in blood is increase (P<0.01) and the activity of ALAD in blood is decrease (P<0.05).3. The results of Morris water maze showed that the escape latencies in all groups were not significantly different in the first day of the test. In the last three days of the test, the escape latencies in treatment groups were significantly longer than the control group, particularly the GroupC, the average escape latency was significantly elevated compared with the control, while it is different from any other group in any quadrant (P<0.05); Compared with the control, the escape latencies is long in I quadrant of GroupA, and in I, IV quadrant of GroupB (P<0.05). But there was no significant difference between GroupA and the control group (P>0.05) in II and IV quadrant.4. The expression levels of NR2A mRNA in treatment groups were significantly higher than the control group, particularly the GroupA were highest of groups; The expression levels of NR2B mRNA in hippocampus were significantly decreased compared the control (P<0.05), contrary to the control, the expression levels of NR2B mRNA of the GroupB in cerebral cortex were decrease (P<0.05); The expression levels of NR2C mRNA in hippocampus of GroupA were higher than the control (P<0.01).Contrary to the control, the expression of mGlur1 mRNA were significantly lower in both cerebral cortex and hippocampus of GroupA and cortex of GroupB (P<0.01). But contrary to the control, expression of mGlur1 mRNA in cerebral cortex and hippocampus of GroupC were significantly elevated (P<0.01).5. The activity of SOD in GroupA and B increased compared with the control, especially the GroupB is higher than the control and GroupC (P<0.05); Compared with the control, the activity of GSH-Px is elevated, GroupB is higher than the control and GroupA (P<0.05, P<0.01). The activity of NOS is decrease, GroupC is lower than the control (P<0.05).Conclusions:1. The contents of lead in blood, cerebral cortex and hippocampus were significantly increased in treatment groups. Lead acetate could inhabit the concentration of ALAD. Concentration of Pb in brain would decrease with time went by once without Pb exposure.2. The low level Pb do exposure would result in impairment of learning and memory ability for life long.3. The lead in blood in the pregnant rat can come into the body of fetus through the placenta. It also can come across the incomplete blood-brain barrier and reach to nervous system. Lead poisoning can also through latex accumulate in offspring during sucking period. By this way, the abilities of learning and memory would be affected. Further more, the effects would last to the period of maturation.4. Lead could change the expression of mGlur1and NMDAR mRNA in brain. Developmental low level lead exposure of rats can decrease the expression level of NR2BmRNA in brain. Meantime, the abilities of learning and memory would be affected. So, effects on NMDAR, mGlur1may be one of the neurotoxic mechanisms of lead.5. The activity of anti-oxidative enzymes increase after Pb exposure, which would attribute to oxidative injury in brain; the activity of NOS depress indicated that there would be a decrease production of NO. It may be because that lead as a kind of heave mental can inhibit the production of NO by combining with the centre activity of NOS, or it would affect glutamic acid and NMDA receptor as messenger leading to excitatory neurotoxicity. Changes in cognitive ability were perhaps dependent on these results in some degree.