Mechanistic Study Of Dserine Regulation Of Rat Hippocampal Homeostatic Plasticity

Until the last decade,it was widely accepted that Damino acids had no functional role in higher organisms,but that they were restricted to lower organisms,such as bacteria, where they are integrated into the proteoglycans of the cell wall.Over the past decade, a growing body of evidence has emerged on the existence in the brain of a close bidirectional communication system between neurons and astrocytes,which is called tripartite synapse structure.Thus,the cases that glia not only listens but also talks to neurons through the release of neuroactive substances have been gained increasing attention as key players of higher functions in healthy brain,but also in diseases.In the signaling pathway,the nontypical amino acid,D-serine,is widely concerned.It has now been well established that D-serine,a coagonist for the N-methyl-Daspartate (NMDA)glutamate receptors(NR1/NR2 type),is maintained at a high concentration in mammalian brains for life.D-serine in the brain is contained in both the glia and neurons and has specific processes of biosynthesis,extracellular release,uptake,and degradation.Together with the anti-psychotic and anti-ataxic property of D-serine and the pivotal roles of the NMDA receptor in divergent higher brain functions,these observations support the view that the Damino acid may be involved as an endogenous modulator for the NMDA receptor in various neuropsychiatric functions and their pathological conditions.Recently reports show that D-serine participates not only in LTP but also in learning and memory.Generally,it is believed that activity-dependent synaptic plasticity serves as a cellular mechanism underlying learning and memory.Thus,we are interested that how D-serine regulates the higher brain functions,such as synaptic plasticity and learning and memory? Using acute prepared hippocampal slices,electrophysiological recording, biochemistry and behavioural testing methods,we investigated the following 3 questions:1.How dose D-serine affect hippocampal long-term synaptic plasticity? 2. How dose endogenous D-serine regulate the homeostasis of neural network? 3.How dose D-serine affect animal learning and memory?1.Exogenous D-serine affects hippocampal CA1 regional LTD and LTP in a concentration-dependent mannerIn slices incubated in 5μM D-serine,the magnitude of LTD was enhanced significantly compared with that in the control group.However,in slices incubated in 100μM D-serine,the expression of LTD was not affected.According to these results,we wondered whether D-serine regulated LTD in a concentration-dependent manner.Thus,LTD was induced in the slices incubated by D-serine at different concentrations(3μM,5μM,10μM,50μM,100μM). The magnitudes of LTD had a bell-shape:5μM D-serine enhanced the magnitude of LTD to maximum;with the concentration of D-serine increasing,LTD returned to control level in the slices incubated in 100μM D-serine.Having established these data,we further revealed that the LTD was NMDAR dependent,being blocked by AP5 or 7-chlorokynurenic,both are special antagonists of NMDAR.It is the case to LTP.The data presented so far indicate that D-serine regulates long-term synaptic plasticity via NMDAR activation in a concentration-dependent manner.2.Gila release D-serine through sensing presynaptic activities to regulate LTDSince different magnitudes of LTD were induced by different stimulus protocol,we hypothesized that different stimulus intensities during LFS induced the release of corresponding D-serine.After Damino acid oxidase(DAAO,the enzyme which degrades D-serine)eliminated the physiological D-serine,LTD induction was blocked. However,exogenous D-serine rescued the DAAO-inhibited LTD to different degrees. Furthermore,a strong stimulus intensity during rescued the glial impairment-inhibited LTD.in addition,the LTD induced under these condition was blocked by AP5. Interestingly,Enzyme-Linked Immunosorbent Assay(ELISA)showed that the D-serine level altered significantly after LFS compared with that during baseline recording.3.D-serine affected memory in dose-dependent mannerIt is widely believed that long-term synaptic plasticity serves as a cellular mechanism underlying learning and memory[1-3].Thus,we investigated the effects of glial cell impairment and exogenous D-serine on spatial memory retrieval in Morris water maze.Intraperitoneal injection with D-serine(1000 mg/kg) increased memory retrieval significantly compared with the control group while injection with a lower(100 mg/kg)or higher(3000 mg/kg)dose of D-serine did not affect memory retrieval.In contrast,injection with NaFAC(3 mg/kg)impaired memory retrieval and D-serine(1000 mg/kg)restored it partially(Fig.4B).This result provides strong evidence that glial cells are crucial to memory retrieval through releasing D-serine.In brief,these findings suggest that the patterns of presynaptic activity can be sensed by glia leading to corresponding concentrations of D-serine release.And then,as a crucial neuromodulator,D-serine may modify activity-dependent long-term synaptic plasticity and thus maintain neural network homeostasis and memory storage.