Effects Of Prenatal Exposure To Low Level Lead On Offspring's Learing, Memory And The Expression Of NOV Protein And MRNA In Rat Hippocampus

ObjectivesLead is one kind of toxic metal and harm to many organs and tissues of human. The occupational acute lead poison cases from high level lead expose have reduced with the improvement of occupational condition. The long time and low level expose to environmental lead which is more dangerous to human health have been concerned by society day by day. The nerve toxicity of lead correlate with age, the developing central nervous system is very sensitive to lead. It is reported that the effect of lead to high neural activities especially to children is very serious and it seems that there is no lower limit. The prenatal expose to lead is another dangerous way to children beside environmental pollution. Lead from mother can permeate into the nervous system of fetus easily through the incomplete blood-brain barrier and cumulate in fetus especially in cortex and hippocampus, which is recognized as a key region for learning and memory. So it is very important to learn the effects of low-level expose of lead on children's intelligent and neural activities.Nephroblastoma over-expressed gene( NOV) is one kind of proto-oncogene and belongs to immediately early genes. Encoding NOV protein, which is a kind of IGF-binding protein. NOV have closely relationship with the development and differentiation of central nerve system, and its expression increase with the process. It is reported that NOV takes part in the learning and memory. NOV can promote the neural stem cell to nerve cell and the excretion of aminoglutaminic acid, forming and extending of synaptic, and so on. It is already confirmed that lead can affect many processes which we mentioned above. It clues us that lead may influence the development of nerve system by reducing the composition and expression of NOV, thereby interfere the mechanism of learning and memory.We establish models of pregnant rats expose to low level lead and then determinate the content of lead in blood and hippocampus of offspring, explore the poisoning effect on their learning and memory, investigate the changes of NOV, NOV mRNA and the apoptosis in hippocampus. Then, reveal the relationship between memory and NOV. Finding the possible mechanism of lead neurotoxicology.Methods1 To establish the animal model: The pregnant rats were randomly divided into 4 groups: 3 exposure groups (125, 250,500mg/L lead acetate in the drinking water) and 1 water control group (double evaporated water). On pregnancy day 0, the pregnant rats were assigned to expose and stopped after birth. The samples of descendants were taken on embryo 18th day, postnatal 1st day, 21st day, 60th day. The contents of lead in blood and hippocampus were determined by hydride generation atomic absorption spectrometry method.2 The ability of learning and memory was tested by Morris water maze and Y-maze.3 The expression of NOV protein in hippocampus was tested by immunohistochemistry. The expression of NOV mRNA in hippocampus was tested by in situ hybridization.4 The apoptosis in hippocampus was tested by TUNEL.Results1 Blood and hippocampus lead levels in pups: For the lead exposed groups, the contents on embryo 18th day, 1st day and 21st day are significantly elevated compared with control group(P<0.05), while the period of 60th day there are no significantly differences between them.2 Morris water maze: The time to find the platform for 21st day and 60th day of lead expose groups significantly increased compared with the control group during the last 3 days'training(P<0.05, P<0.01). And there is no significant diffierences between 4 groups on the fist day's training.Y-maze: On 21st day and 60th day period, the abilities of learning(A) and memory(B) and the memory retaining rate(C) were significantly decreased in the lead expose groups compared with the control group(P<0.05, P<0.01).3 Immunohistochemistry: There is no expression of NOV on embryo 18th day; On 1st day, the expression of NOV of all the lead exposed groups are significantly decreased compared with control group(P<0.05); On 21st day, the expression of NOV in middle and high dose groups are significantly decreased compared with control group(P<0.05); On 60th day, there is no significantly difference between lead expose and control groups.In Situ hybridization: On embryo 18th day and 1st day, the expression of NOV mRNA of all the lead exposed groups are significantly decreased compared with control group (P<0.01); On 21st day, the expression of NOV mRNA in middle and high dose groups are significantly decreased compared with control group(P<0.05, P<0.01); On 60th day, only the high dose group have significantly difference compared with control group(P<0.01).4 Apoptosis: On embryo 18th day and 1st day, the Apoptosis Index of all the lead exposed groups are significantly decreased compared with control group (P<0.05); On 21st day, the Apoptosis Index in middle and high dose groups are significantly decreased compared with control group (P<0.05); On 60th day, there is no significantly difference between lead expose and control groups.Conclusions1 Pregnant expose to low-level lead make the lead in blood and hippocampus of the offspring significantly increased compared with control group.2 Pregnant expose to low-level lead impair the spatial learning and memory abilities of offspring, and the effects could last to the maturation period.3 Pregnant expose to low-level lead decrease the expression of NOV protein and mRNA of offspring, this reveals that lead impairs learning and memory through interfere the normal function of NOV gene.4 Pregnant expose to low-level lead increase the cell apoptosis in hippocampus, and recover to normal in maturation period. It demonstrates that expose to lead in prenatal can make nerve cell apoptosis last to childhood.