| Organophosphorus pesticides(OPs),such as dimethoate(DM),is a major type of pesticides being used on fruit,vegetable,tobacco plant,cotton,and omamental plant in China.In recent years,the cases of acute posioning due to pesticides exposure have been reduced dramatically.However,the adverse effects in human caused by the long-term exposure to low doses of pesticides have been of increasing interest in the literature.Previous studies have shown that long-term low level OPs exposure could affect neurobehavioral outcomes such as impaired reaction time,motor coordination, cognition,emotion,and memory.The main mechanism of OPs poisoning is the excessive accumulation of acetylcholine,which is caused by the inhibition of acetylcholinesterase(AChE) activity.Neurobehavioral dysfunction may also be caused by other mechanisms besides the inhibition of AChE alone.A series of relevant studies have shown that the catecholamine system may play an important role in neurobehavioral changes besides cholinergic toxicity caused by the long-term exposure to low doses of OPs.Monoamine consists of catecholamine and indoleamine,catecholamine includes E, NE,DA,and their products,and the indoleamine mainly serotonin.Contents of these monoamines in the tissue reflect the process of biosynthesis,release,uptake,and deactivation etc..Therefore it also could be the biomarker in the neural tissue.Study of sub-chronic toxicity of DM affecting the levels of monoamine in rat brain and serum may help understand the risk and mechanisms of neurobehavioral changes caused by long-term low level DM exposure.Forty eight male Sprague-Dawley rats,weighing 200-250 g at the beginning of the experiment,obtained from the Animal Research Center of Fudan University were used for this study.The animals were housed in groups of 6 and maintained on a 12/12-hr light/dark cycle under maintained temperature at 21±2℃.They were allowed free access to water and food for a one-week acclimation period and then exposed to DM.Body weight gain and food intake were measured everyday.All procedures were in adherence to the principles stated in the Guidelines for the Care and Use of Laboratory Animals as outlined by the National Institutes of Health(USA).The rats were randomly divided into four groups which were treated with 0,5,10,and 20mg/kg bw DM dissolved in saline via gavage 5 times a week(except weekends) for 90 days.All animal experimental procedures were performed between 9:00 and 14:00. Both the body weights and food consumption of the rats were recorded every 3 days. Venous blood was taken at tails and the activities of AChE in the whole blood were determined with UV spectrophotometry regularly during the period of experiment. The locomotor activities were evaluated in openfield test at the end of the study. Following decapitation,brains were removed from the rats quickly,and cerebral cortex,hippocampus and myelencephalon were dissected on the top of an ice box immediately.Blood was obtained at the time of sacrifice and serum was prepared from it.The Monoamine neurotransmitters in the brain region and serum were detected by HPLC-ECD.AChE activities in myelencephalon were also determined after decapitation of the rats with UV spectrophotometry.The activities of whole blood AChEs decreased significantly during the experimental period(P＜0.05).Similarly,significant depression of AChEs was observed in rat myelencephalon as compared to the control group at the end of the test(P＜0.01).The total course,activity time and mean velocity of the dosed groups were decreased. The rest time of the dosed groups were increased.Significant changes were found in high dose group as compared to that of control.Furthermore,the behavioral alterations mentioned above were statistically correlated with the brain AChE activities in rats.Persistent inhibition of brain AChEs activity may be the mechanism of behavioral changes in rats exposed to DM.From low dose group to high dose group,the contents of NE in the whole cerebral cortex decreased significantly as compared to control group(P＜0.01,P＜0.01,P＜0.05, respectlively);levels of 5-HT in the whole cerebral cortex also decreased significantly at the doses of 10,20 mg/kg bw compared to that of the control group(P＜0.01, P＜0.05,respectively).There were no significant differences in E and DA contents in the whole cerebral cortex between exposed groups and the control except a statistical significant decrease of E was observed in the medium group(P＜0.05).With the increasement of exposure,monoamine neurotransmitters levels were decreased in hippocampus,there is a dose-effect relationship.The contents of NE,E,DA were shown to be significantly lower in the high group as compared to that of control(P＜0.05),and 5-HT in all dosed groups were also significantly lower in comparison with control group(P＜0.01 ).There were no significant differences in NE and DA contents in the serum between exposure groups and control group,but the E and 5-HT levels in the exposure groups were significantly lower when compared with the control group(P＜0.01 ).Statistically significant correlation was found between NE level in hippocampus and activity time(F=0.670,P＜0.05),rest time(F=-0.670,P＜0.05);5-HT level in hippocampus was also correlated with total course(F=0.704,P＜0.05,mean velocity (F=0.705,P＜0.05 ).Decrease of monoamine neurotransmitters in the brain besides cholinergic system may also be the mechanism of neurobehavioral disfuntion in rats after subchronical DM expsure.This research was based on the model of rat subchronically exposed to DM,and investigated the effect of subchronic dimethoate exposure on the locomotor activity in rat,measured AchE activities in whole blood and brain tissue,detected monoamine levels in the brain regions and serum,discussed the relationship of neurochemical substance(AchE and monoamine transmitters system) and locomotor activity, providing evidence on neurotransmitters to explore the mechanism of neurobehavioral disfunciton caused by long-term low level DM exposure.In summary,results from this study indicated that non-cholinergic mechanisms expressed as an altered monoamine levels in rat brain and serum may be an important mechanism by which subchronic exposure to DM may lead to neurobehavioral abnormality.Further investigations are recommended to explore this mechanism in order to minimize adverse health effects caused by long-term low level OPs exposure on human.
|
PDF Full Text Request