| ObjectivesLead is a kind of common toxic heavy metal that do harm to apparatus and tissue of individual, especially to the CNS of children and the teen-age. Children are very susceptible to the toxicity of lead and the ratio of lead poisoning has been great raised in recent years. There are researches which proved that in cities of China, about 50% children are threatened by lead poisoning . Besides environmental lead pollution, the prenatal exposure to lead is a main approach to damage on children's development. It is approved that lead have very strong developmental neurotoxicity, even low-level lead exposure can damage the developmental CNS. There is distinguished positive correlation between infant blood lead level and mother blood lead level. 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.Growth-associated protein (GAP-43) is a special phosphor protein on the nerve cell membrance of amniote, and it was shown to be present in neuro cell widely. During neuronal development and regeneration, GAP-43 is synthesized at an increase rate in growth cones and may serve as a marker of neurite growth. GAP-43 play an important role during the course of neuron cell growth and development,synaptogenesis,long-term potentiation,neuro signal transduction and the release of neurotransmitter.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 one of the neuromechanism of learning and memory. It is approved that in the course of inducing LTP, the expression of GAP-43 is increased and GAP-43 was phosphorylationed. The phosphorylation state of GAP-43 is relation to the forming of LTP. In the course of inducing LTP, the strength of EPSP is correlation conspicuously with the increase of GAP-43. The excessive expression of GAP-43 and PKC gene will strengthen the transgenic mice's ability of learning and memory, this indicates that GAP-43 is relation to learning and memory.We establish models of rat's prenantal exposure to low level lead and then detection the concentration of lead in the blood and hippocampus of offspring, explore the poisoning effects on their learning and memory, the change of hippocampus GAP-43 at protein and gene level, and to find the relationship of GAP-43 and the ability of learning and memory. At last, to find the likely mechanism for the lead neurotoxicology.Methods1 The pregnant rats were randomly divided into 4 groups provided with double evaporated water, 125, 250, and 500mg/L lead acetate solution via drinking water respectively during the pregnancy. The samples were taken on embryo 18th day, postnatal 1st day, 21stday and 60thday.2 The contents of lead in blood and hippocampus were measured by hydrogenide generation atomic absorption spectrometry.3 The ability of learning and memory was tested by Y-maze and Morris water maze.4 The expression of GAP-43 protein in hippocampus was detectioned by immunohistochemistry. The level of GAP-43mRNA expression in hippocampus was observed by In Situ Hybridization. This part is to study the effects of lead acetate on the expression of GAP-43 in rat hippocampus.Results1 Result of the measuration on the content of lead: In the embryo rats, compared with the control group, the contents of lead were significantly increased in blood and hippocampus in the treatment groups respectively(P<0.01); In the neonatal rats, compared with the control group, the contents of lead were significantly increased in blood and hippocampus in the treatment groups, respectively (P<0.01, P<0.05); In the weaning rats, compared with the control group, the contents of lead were significantly increased in blood and hippocampus in the treatment groups, respectively (P<0.01, P<0.05); In the adult rats, the contents of lead in blood and hippocampus in all groups had no significant difference (P>0.05).2 Results of the test by Y-maze: In the weaning rats and the adult rats, compared with the control groups, the abilities of learning(A) and memory(B) and the memory retaining rate(C) were significantly decreased in the treatment groups, respectively (P<0.01, P<0.05).Results of the test by Morris water maze: In the weaning rats, 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(P<0.05, P<0.01); In the adult rats, 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(P<0.05, P<0.01).3 Results of immunohistochemistry: The results of this part showed that: in the embryo rats, compared with the control group, the expression of GAP-43 protein were significantly decreased in the treatment groups respectively (P<0.01, P<0.05); in the neonatal rats, compared with the control group, the expression of GAP-43 protein were significantly decreased in the treatment groups respectively(P<0.01, P<0.05); In the weaning rats, the expression of GAP-43 protein were significantly decreased in the treatment groups respectively(P<0.01, P<0.05); In the adult rats, the expression of GAP-43 protein was no significantly changes in each group.4 Results of In Situ Hybridization: The results of this part indicated that: the expression of GAP-43 mRNA were decreased significantly in the treatment groups respectively compared with the control group(P<0.01,P<0.05)in the embryo rats; In the neonatal rats, compared with control group, the expression of GAP-43mRNA were decreased significantly in the treatment groups respectively (P<0.01,P<0.05) ; In the weaning rats, the expression of GAP-43mRNA in control group and low dose group had no significant difference, while the expression of GAP-43mRNA in middle and high dose groups were significantly decreased, compared with the control group respectively (P<0.01,P<0.05). In the adult rats, the expression of GAP-43mRNA was no significantly changes in each group.Conclusions1 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. As a result, the development of brain was affected.2 Prenatal exposure to lead would negatively affect the development of brain in the descendant. By this way, the abilities of learning and memory would be affected. Further more, the effects would last to the period of maturation.3 Lead could reduce the expression of GAP-43 protein and mRNA in hippocampus. Therefore, the abilities of learning and memory would be affected. So, effects on GAP-43 may be one of the neurotoxic mechanisms of lead.
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