| Stress is a kind of general adaptive response during the body is stimulated by a stressor from internal and external environments,and it can be divided into acute and chronic stresses.A chronic stressor(CS)occurs when the body exposure to a long-term stressor,and is ubiquitous in the life.Learning and memory is a foundation for cognitive function,and mammalian hippocampus is a key structure for learning and memory processes which influenced by various stressors.Hippocampus can be structurally divided into CA1-4 and DG regions,and these sub-regions play different roles in learning and memory.Among the subregions of hippocampus,DG is the main entry point of information into the hippocampus and plays an important role in learning and memory.The long-term potentiation(LTP)is widely accepted as an essential neurophysiological mechanism for learning and memory,and LTP in hippocampal DG is typically dependent on the activation of N-methyl-D-aspartate(NMDA)-type glutamate receptors.Although studies have shown that CS affect learning and memory by altering neuronal morphology,neurogenesis,synaptic plasticity and stress-related hormone in the DG,the neurochemical mechanism of CS affecting learning and memory in the DG is incompletely understood.Dopamine(DA)is an important neurotransmitter/neuromodulator in the brain,and it has been shown that DA in the hippocampus may act via D1 receptor(D1R)to modulate learning and memory function and the related synaptic plasticity.Our previous study also showed that DA activation of D1R in the hippocampal DG promotes spatial learning and memory,in part by modulating the responses of excitatory amino acids(EAAs),including glutamate(Glu)and aspartate(Asp),during spatial learning.The activation of locus coeruleus(LC)-sympathetic-adrenal medulla system(LSAM)is an important neuroendocrineical response of stress.Recent evidences have shown that DA may also be released via noradrenergic terminals arising from LC,and the projections from LC are densest at the DG region.This raises the possibility that LC activation during CS is involved in learning and memory partly via regulation of dopaminergic system in the hippocampal DG.There are a number of evidence showing that DA and D1R in the hippocampal DG play a modulatory role in several learning and memory processes and related synaptic plasticity,but the roles of DA and D1R in the hippocampal DG in CS-induced alteration of spatial learning and memory have not been reported.Therefore,in the present study,we measured DA concentration and synaptic efficiency in the hippocampal DG during Morris water maze(MWM)test in CS rats,by using in vivo brain microdialysis,electrophysiological recording and HPLC techniques in freely-moving conscious rats.Next,we examined the effects and mechanism of D1R in the DG on CS-induced alteration of spatial learning and memory by using microinjection of the antagonists into the DGFollowing experiment were carried out:(1)The rat model of CS was prepared by random application of varied stressors and the concentrations of corticosterone(CORT)and epinephrine(E)in serum were examined by Elisa assay in CS rats.(2)MWM was used to observe the spatial learning and memory function of CS rats,Brain microdialysis,high performance liquid chromatography,electrochemical detection and in vivo bioelectrical recording were used to observe DA content and fEPSP amplitude in hippocampus DG region of rats during MWM.(3)The ultrastructure of DG region in hippocampus of CS rats was observed by transmission electron microscopy,and the expressions of D1 receptors(D1R)and doublecortin(DCX,a maker of immature neuron)in the DG were measured by Western blot methods in CS rats.(4)The SCH23390(an antagonist of D1R)was microinjected into the DG region of rat hippocampus by intracerebral microinjection,and then the extracellular concentrations of EAAs(including Glu and Asp)and fEPSP amplitude in the hippocampal DG were measured in CS rats during MWM test.(5)The expressions of PKA,PI3K/AKT and CREB(the downstream signals of D1R)in the DG were measured by Western blot method.The experimental results are as follows:(1)The levels of corticosterone and adrenaline in serum of rats in CS group were significantly higher than those in control group(both P<0.05),suggesting that the rats in CS group were in a state of stress.(2)In the place navigation trail of MWM test,the escape latency was decreased with the increase in training days in both groups,but the escape latency was significantly longer in CS group than control group(P<0.05);in the spatial probe trail of MWM test,the number of platform crossings and proportion of swimming distance in target quadrant were markedly lessened in CS group compared with the control group(P<0.05,respectively).(3)In both groups,the DA concentrations in the DG region were significantly increased during MWM test,however,the increased response of DA was markedly enhanced in CS group compared with control group(n<0.05,on the 2nd and 3rd days);compared with control group,the expression of D1R in the DG was significantly increased in CS group(P<0.05).(4)In control group,the fEPSP amplitude in the DG was significantly increased on the 3rd day of MWM test(P<0.05),in contrast,fEPSP amplitude in the DG was not changed during MWM test in CS group.(5)The changes in ultrastructure of hippocampal DG in CS rats were as follows:the synaptic cleft was blurred and the postsynaptic membrane density was reduced;the membrane and cristae of mitochondria in the synaptosoma were blurred,and the cristae was fractured;the rough endoplasmic reticulum was reduced and arranged in disorder,and free ribosome was increased.(6)The expression of DCX in DG area of hippocampus in CS rats was significantly lower than that in control group(P<0.05),suggesting that CS inhibits neurogenesis in DG area of hippocampus in rats.(7)The microinjection of SCH23390 into the DG induced a significant decrease in mean escape latency and the significant increases in number of platform crossings and proportion of swimming distance in target quadrant during MWM test in CS rats.(8)The microinjection of SCH23390 into the DG reversed the inhibitory effects of CS in changes of fEPSP amplitude in the DG during MWM test.(9)Glu and Asp in DG area of hippocampus in control group increased significantly during water maze training(P<0.05,on 3rd days),and the increases were attenuated in CS rats;the microinjection of SCH23390 into the DG partly reversed the inhibitory effects of CS in changes of Glu and Asp in the DG during MWM test.(10)The microinjection of SCH23390 into the DG partly reversed the inhibitory effects of CS in the expressions of p-PKA,PI3K,p-AKT and p-CREB in the DG resion.Conclusions:(1)Spatial learning and memory impairment of CS rats is related to disorder of ultrastructure,inhibition of neurogenesisi,and over-activation of DA-D1R system in the hippocampal DG.(2)Over-activation of D1R in the DG involves in CS-induced spatial learning and memory impairment via inhibition of learning-dependent LTP(LD-LTP).(3)Glu and Asp in the hippocampal DG were involved in the spatial learning and memory process and LD-LTP,and over-activation of D1R in the DG induces spatial learning and memory impairment in CS rats by means of inhibition of the changes in these amino acids during MWM test.(4)Over-activation of D1R in the DG involves in CS-induced spatial learning and memory impairment via inhibition of its downstream protein expression,including PKA,PI3K/AKT and CREB. |
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