Effect Of Postweaning Chronic Aluminum Exposure On LTP And The Content Of Glutamate In Hippocampus Of Rats

IntroductionAluminum is one of the most abundant element on earth. . It has neural tox-icity if much of that accumulate in human's body. Evidence of epidemiology has shown that Al can impair neuron easily and cause deficiency of learning and memory. In animal models of experimentally induced dementia it has been demonstrated that there are a reduced capacity of learning and memory and the pathological change which is similar with Alzheimer's disease after Al exposure.The hippocampus is the most important encephalic region relative to function of learning and memory. Hippocampal long - term potentiation (LTP) is NMDA receptor - dependent persistent enhancement of efficacy in synaptic transmission, it is reputed that LTP represents the most intensively studied synaptic model and neural basis of learning and memory in the mammalian brain. So investigating the effect of aluminum exposure on LTP and the biochemical indicators relative to the LTP synaptic mechanism will help to elucidate the mechanism of aluminum damaging the learning and memory. Although Al has been reported to impair LTP following administration in vivo and in vitro, the underlying mechanisms of Al action on LTP are still unknown. The stage of postweaning is a very important period during postnatal development. The present study has observed the effect of aluminum exposure at low and mid dose on the induction and persistence of LTP and the content of glutamate in postweaning rats, hope to elucidate the mechanism of the effect of Al on LTP at this stage in the level of transmitter release.Materials and methods1. group and exposureHealthy Wistar postweaning rats, male and female in each half, weighing about 80g could be devided to three group, 20 in each group. Which drank distilled water^0. 2% A1C13 solution and 0. 4% A1C13 solution respectively. Persistent exposure for 80 ~ 90d, then drank distilled water. And the room temperature is 18 ~ 23°C , relative humidity is 45 ~ 55%.2. behavioral testingStep - down test was used for evaluating the effect of Al on behavior. Recording the time of the first step down (latency) and the number of shock in 5 min ( fault number) .3. electrophysiological recordingsrats anesthetized by intraperitoneal injection of 20% urethane, (6. 5ml/kg) and mounted in a stereotaxic apparatus. A concentric bipolar stimulating electrode was placed in the Schaffer lateral region (coordinates: 3. 8 mm posterior to bregma ,3.8 mm right lateral to the midline ,3.8 mm under cortex ) and a recording glass microelectrode (coordinates: 3. 3 mm posterior to bregma ,1.5 mm right lateral to the midline) was lowered into the CA, region until the population spike (PS) was induced by single pulse stimulus. When PS was steady , first the PS was recorded for 30 min, then high frequency stimuli ( HFS ) were applied to induce LTP. Single test stimuli were delivered onece every min to assess resulting changes in the PSs over time.4. glutamate content assay in hippocampusThe rats were fixed by heart perfusion with 4% paraformaldehyde after electrophysiological recording. Manufacture routine paraffin section. Determine the content of glutamate in hippocampus and cortex by immunohistochemical method. Analyze the slice by Metamorph/C - 5050/BX41 colour pathological figure analytical system.5. brain and blood Al determinationsWeigh 0. 1 -0. 5g brain tissue or take suction 0. 2 ~0. 5 ml whole blood,add 5 ~ 8 ml violet acid, blank together. Heat at low temperature to dissolve totally, and continue to heat to almost dry. Add 0.2% nitric acid to dissolve the residue, and metered volume to 50 ml in volumetric flask, the brain and blood aluminum concentrations were determined by atomic absorption plumbago.6. statistical analysisExperimental values were indicated as the mean ± SD. Interclass data were tested by analysis of variance. If it is not mormal distribution, which could be change to logarithm, then to test by ANOVA.Result1. The behavioural dada showed: the latency of the first step down reduced, the fault number at 5min increased in the exposure group. These differences of the latency and the fault number were significant as compared to the control group.2. With the increase of exposure dose, the enhancement rate of PS amplitude in exposure group reduced evidently after HFS. The two exposed group as compared to the control group, this reduction was significant, but the difference between the two exposed group was statistically non - significant.3. The exposure dose elevated, the value of optical density(Integrated OD Average, IA ) reduced gradually in two exposed group. The difference of IA value only in 0.4% exposed group was significant as compared to the control group. Although the IA value of 0.2% exposed group was also decreased, this decrese was statistically non - significant as compared to the control group.4. Blood and brain aluminum concentration increased with exposure dose, Both the changes were synchronous.DiscussionThe research on brain impairment in the postweaning period is an important direction relative to intelligent development of infant. The investigating its mechanism is very important to protect the infant from neurogenic disease induced byAl. In this study, the results showed that postweaning chronic aluminum exposure impair the induction and maintainance of LTP in hippocampus, and the degree of impairment was concerned in the exposure dose. The result of behavioral test also reflect this relationship.The present results also showed that postweaning chronic Al exposure could decrease the glutamate content in hippocampus, which was dose - dependent. Some reports showed that Al maight interfere with Glu - Gin cycle. Combining our study, we could presume that the changes of the activity of GS and PAG by Al in hippocampus and cortex might cause total level of glutamate decrease. Another article reported that the glutamate may be transformed redundantly into glutamine in the CNS.The decreases of total level of glutamate could effect on the normal learning and memory. This decrease confined neurotransmitter accumulation in the pres-ynaptic vesicle , it makes neurotransmitter release reduce per pulse stimulation. That Al affect glutamate accumulation in presynaptic vesicle was due to inhibit-ied activity of the Na+ -K+ - ATPase.Al may regulated indirectly glutamate by many path to influence on LTP. Such as affect glutamate release presynaptic by inhibiting VACCs, so that affect the induction of LTP.In a word, Al affects glutamate by many means which play an important role in LTP. Its need to be further investigations.Conclusion1. Postweaning chronic Al exposure impaired the behaviour of the learning and memory in rats.2. Postweaning chronic Al exposure impaired the induction and maintenance of LTP in rats.3. One of the probable mechanisms was that glutamate content was reduced by Al in hippocampus.