Mechanism of developing "deprivation" potentiation of population responses of neurons in the CA1 field in hippocampal slices]

In rat hippocampal slices, synaptic transmission from Schaffer collaterals to CA1 neurons was studied by assessment of the effects of cessation of low-frequency testing stimulation to the population spike amplitude. A positive correlation between the duration of the stimulation interruption period (from 10 to 120 min) and the increment of the population spike amplitude (from 30 to 100% above baseline level). The "deprivational" potentiation (DP) offers a property of summation and is able to persist for long (testing was continued up to one hour). Prior induction of the long-term posttetanic potentiation (LPTP) that for 1-3 hours prevents the development of the late rather than the short-term LPTP phase, in our experiments also suppressed the DP induction after 60-min stimulation interruption interval. The same results were obtained in the presence of polymixin B (20 mcM), which is an inhibitor of protein kinase C (PKC) and PKC-dependent phase of the LPTP. This finding provides support for the earlier assumption about the similarity of the mechanisms responsible for the development of the DP and PKC-dependent LPTP phase related with protein phosphorylation.     

The effect of forelimb deafferentation in early postnatal ontogeny on the development of synaptic transmission in the hippocampus]

Hippocampal slices from 15-20-day-old Wistar rats were used to study the development of some features of synaptic transmission in hippocampus and the influence of partial limitation of the sensory inflow in the early ontogeny of this transmission. The dynamics of population spike changes was observed in the CA1 hippocampal field in response to stimulation of Schaffer collaterals. The early ontogenetic limitation of the sensory inflow was accomplished by cutting n. medianus on the 13th day. Between the 15th and 20th days, the dynamics of the population spike amplitude increase in the control and experimental animals was similar, however, the response amplitude of the control rats remained higher than in the experimental animals throughout the whole period of observation. It is suggested that the partial limitation of sensory inflow from a forelimb at the early stages of the ontogeny alters the formation of synaptic transmission in hippocampus.     

液压打击损伤后海马CA1区神经元兴奋性变化的研究

为考察脑损伤对海马CA1区锥体神经元电活动的影响并研究大黄素对神经元的超兴奋性和突触传递的作用,应用液压打击大鼠脑损伤模型和细胞外记录方法提取诱发的海马CA1区场兴奋性突触后电位(fPSP)和群峰电位(PS),进行相关的数据处理和分析。发现损伤侧比非损伤侧的fPSP斜率明显升高,PS波峰个教显著增加,而PS潜伏期明显减小;在灌流液中施加大黄素,CA1区诱发场电位明显减弱。研究结果表明：颅脑损伤可造成海马CA1区锥体神经元的迟发性过度兴奋;大黄素对神经元的兴奋性有抑制作用,可能对颅脑损伤后的中枢神经系统具有保护功能。     

外侧隔—海马CA1通路电生理分析

电刺激外侧隔区可在海马 CA1区锥体细胞层记录到群锋电位,在 CA1辐射层顶树突记录到兴奋性突触后电位(EPSP)。侧脑室注射微量海人酸损毁海马 CA3-CA4区锥体细胞后,电刺激外侧隔区在 CA1顶树突不再诱发 EPSP,由此认为外侧隔-CA1顶树突的神经联系是通过同侧海马 CA3锥体细胞侧支实现的。但 CA3-CA4损毁后,电刺激外侧隔区在海马 CA1起层锥体细胞底树突仍可记录到 EPSP。这一在正常情况下被掩盖的外侧隔-CA1底树突神经联系及其来源尚有待探讨。     

The excitation wave returning to the hippocampus through the entorhinal cortex can reactivate the populations of "trained" CA1 neurons during deep sleep

It is suggested that the information about a new stimulus from the neocortex is transferred to the hippocampus and forms there a transient trace in the form of a distributed pattern of modified synapses. During sleep, the neuronal populations which store this trace are reactivated and return to the neocortex the information necessary for consolidation of the permanent memory trace. A possible mechanism of the reactivation of the "learned" hippocampal neurons during memory consolidation is the reverberation of excitation in the neuronal circuits connecting the hippocampus and the entorhinal cortex. In rats, we recorded responses in hippocampal field CA1 to stimulation of the Schaffer collaterals with potentiated synapses during wakefulness and sleep. We showed that in the periods of deep sleep, after the discharge of CA1 neurons, the wave of excitation passes through the entorhinal cortex and via the perforant path fibers enters the hippocampus and the dentate gyrus, causing in the latter the discharge of neurons. The repeated discharge of the CA1 neurons develops as the result of interaction of the early wave which is returned directly via the perforant path fibers and the late wave which is returned via the Schaffer collaterals, but not through the dentate gyrus and hippocampal field CA3 (trisynaptic pathway), but, probably, through the field CA2.     

Effects of localized applications of L-glutamate on the population spike in the hippocampal slice preparation

L-Glutamate applied exclusively to the CA3 cell body region of the rat hippocampal slice resulted in long-term potentiation (LTP) of the CA1 population spike evoked by Schaffer collateral stimulation. The same localized glutamate application to the CA1 area caused a prolonged depression of the response. These observations are consistent with the possibility that LTP is at least partly presynaptic and that the depression is due to an effect of glutamate on the postsynaptic elements and/or the presynaptic terminals.     

Long-lasting potentiation in hippocampus is not due to an increase in glutamate receptors

In transversely sectioned rat hippocampal slices, the effects of low (20 Hz, 600 pulses) and high (400 Hz, 200 pulses) frequency tetani of Schaffer collaterals were examined on the CA1 population spike as well as on the binding of 3H-glutamate. The population spike was suppressed while 3H-glutamate binding greatly enhanced following a low frequency tetanus. Verapamil (1 micron), which does not block long-lasting potentiation (LLP), counteracted the depression of the population spike as well as the associated increase in 3H-glutamate binding. The high frequency tetanus induced LLP of the population spike but caused no change in the amino acid binding. These results indicate that the increase in the number of glutamate receptors is not a requirement for LLP.     

Reactions of hippocampal neurons at different stages of rat avoidance conditioning

The electrophysiological responses of neurons were compared in hippocampal slices from rats acquired and not acquired the passive avoidance reaction after the same conditioning procedure. Associative conditioning was accompanied by a gradual increase in the amplitude of the population spike evoked in CA1 area by stimulation of Schaffer collaterals. However, after reaching the learning criterion, the population spike significantly decreased. These phenomena were observed only at low (not maximal) intensities of test stimuli. After reaching the learning criterion, the paired-pulse facilitation was significantly higher in the slices prepared from the well-learned animals as compared with other groups (those having not reached the learning criterion, passive and active control). The obtained evidence validates the hypothesis that the observed intergroup differences stem from modifications of synaptic efficacy and suggests that after behavioral acquisition, plasticity induced by associative learning was substituted by other mechanisms probably related with declarative memory formation.     

Changes in the neuronal excitability of rat hippocampal slices isolated after destruction of the medial septal area]

To determine if electrophysiological properties of hippocampal pathways are altered after medial septal area (MSA) destruction, extracellular recordings were made from hippocampal slices of rats 30 days following lesion and compared with those from unoperated controls. The preparation of slices, data accumulation and data analyses were done under the same conditions. The electrophysiological parameters of interest were the population spike (PS) and the field EPSP, produced in the CA1 pyramidal layer by stimulation of the Schaffer collaterals. The principal finding of this study was that neuronal excitability in slices from MSA-lesioned rats was altered. The most striking abnormalities were an epileptiform activity, which consisted of multiple PSs, and multiple seizure-like after discharges with a delayed onset to low stimulation intensities. In the CA1 region of the slices collected from lesioned rats the input-output curve of field EPSP versus PS showed a leftward shift as compared with their counterparts in normal slices. These changes may be related to relative reduction of inhibitory processes in interneuronal circuits of CA1 region.     

Preservation of plastic properties of synaptic transmission in long-lasting hippocampal slices under the effects of a peptide analog of piracetam, L-pGlu-D-Ala-NH2

The tetanic stimulation of the Schaffer collaterals (SC) in rat hippocamp slices after 6 hrs in vitro conditions did not produce long-term potentiation (LTP) of the field response amplitude in the CA1 pyramidal cell layer. In contrast, LTP after the late tetanization was well preserved in the slices that were perfused for 20 minutes with 0.5 mkM L-pGlu-D-Ala-NH2 (PGAA) after 4-4.5 hrs in vitro. There were no significant reactivity changes during the perfusion of the slices with this drug concentration. Two other drugs with nootropic activity, piracetam (100 mkM) and gamma-hydroxybutyrate (100 mkM, Na-salt) did not prevent the disappearance of LTP in the late period in vitro, while enhanced the reactivity during perfusion period. The maintenance of the plastic properties of the SC-CA1 synaptic transmission under the influence of PGAA is thought to be the result of some specific interaction of the drug with LTP induction mechanisms. LTP damaged in the late period in vitro might be a new model of memory disturbances and this model can turn out to be useful for the comparative estimation of the effectiveness of the drugs with proposed nootropic activity and for the analysis of the possible mechanisms of their action.     

羟基马桑毒素所致的大鼠海马锥体细胞痫样放电活动

本研究采用离体海马脑片电生理研究技术,细胞外记录海马锥体细胞群体锋电位(population spike,PS),观察羟基马桑毒素(tutin)对大鼠海马脑片CA1区锥体细胞电活动的影响,探讨tutin是否具有致痛作用及其致痫机制。结果如下:(1)用40、30和20μg/ml浓度的tutin灌流海马脑片,可显著增高由顺向刺激Schaffer侧支所诱发的PS的幅度,灌流tutin 30min时,PS第一个波的幅度分别为对照的(388.7±20.1)%、(317.2±19.1)%和(180.9±11.6)%(各组n=5,P<0.05)。(2)伴随PS波幅的增高,可出现成串痫样放电波,波数4～11个不等。(3)灌流tutin后的部分脑片(n=9/34),在未刺激Schaffer侧支时也出现自发的成串、高幅痫样放电。(4)灌流CNQX阻断非NMDA受体后,再灌流tutin,PS幅度和放电波数均无显著性变化,即CNQX可完全抑制tutin所致的痫样放电;灌流AP-5阻断NMDA受体后,tutin仍可使PS幅度增高但放电波数无显著性增加,即AP-5可部分抑制tutin所致的痫样放电。上述结果表明,tutin可使海马脑片锥体细胞兴奋活动增强,具有致痫作用;兴奋性谷氨酸受体尤其是非NMDA受体可能介导tutin的致痫作用。     

Neuronal synchrony mediated by astrocytic glutamate through activation of extrasynaptic NMDA receptors

Fast excitatory neurotransmission is mediated by activation of synaptic ionotropic glutamate receptors. In hippocampal slices, we report that stimulation of Schaffer collaterals evokes in CA1 neurons delayed inward currents with slow kinetics, in addition to fast excitatory postsynaptic currents. Similar slow events also occur spontaneously, can still be observed when neuronal activity and synaptic glutamate release are blocked, and are found to be mediated by glutamate released from astrocytes acting preferentially on extrasynaptic NMDA receptors. The slow currents can be triggered by stimuli that evoke Ca2+ oscillations in astrocytes, including photolysis of caged Ca2+ in single astrocytes. As revealed by paired recording and Ca2+ imaging, a striking feature of this NMDA receptor response is that it occurs synchronously in multiple CA1 neurons. Our results reveal a distinct mechanism for neuronal excitation and synchrony and highlight a functional link between astrocytic glutamate and extrasynaptic NMDA receptors.     

Verapamil counteracts depression but not long-lasting potentiation of the hippocampal population spike

In transversely sectioned rat hippocampal slices, population spikes and population "EPSPs" were recorded from CA1 neurones in response to the stimulation of Schaffer collateral and commissural inputs. High frequency tetanic stimulation (400 Hz, 200 pulses) of an input induced LLP of the homosynaptic response without significantly changing the heterosynaptic response. This LLP was not interrupted by either a 400 Hz tetanus given to the heterosynaptic input or by verapamil (0.33 microM) which blocks Ca++ channels, but not transmitter release. A low frequency tetanus (20 Hz, 200 pulses) given to an input induces co-occurring homosynaptic and heterosynaptic depressions of about 20 min duration. This tetanus could also mask an established LLP in homosynaptic or heterosynaptic pathway. Verapamil counteracts homo- and heterosynaptic depressions. The population spike as well as the population "EPSP" were depressed following iontophoretic application of Ca++ (2-100 nA) at the CA1 cell body area. These results indicate that homosynaptic and heterosynaptic depressions are at least partly due to an accumulation of Ca++ into CA1 neurones. An established LLP is not interrupted by LLP of another input. Homo- and heterosynaptic depressions mask, but not reverse, LLP.     

神经元对于高频电刺激的动态响应

深部脑刺激(deep brain stimulation,DBS)在许多神经系统疾病的临床治疗上都展现出良好的应用前景,然而,其作用机制尚不明确.常规DBS采用高频刺激(high frequency stimulation,HFS)的脉冲序列,这种窄脉冲最容易激活神经元结构中的轴突部分,通过轴突的投射,将HFS的作用传播至下游神经元.因此,为了探讨DBS的作用机制,并鉴于海马脑区是治疗癫痫和痴呆症等疾病的重要靶点,我们研究了海马区轴突HFS对于下游神经元的作用.对麻醉大鼠的海马CA1区传入神经通路Schaffer侧支施加1 min的100 Hz高频刺激,记录并提取下游CA1区锥体神经元和中间神经元的单元锋电位.计算锋电位的发放率,以及它们与刺激脉冲之间的锁相值(phase-locking value,PLV)和潜伏期,以定量分析HFS期间神经元动作电位发放的变化趋势.结果显示,在传入轴突上施加HFS时,初期会诱发下游神经元群体同步产生动作电位(即群峰电位).在HFS后期(群峰电位消失之后),两类神经元的单元锋电位发放仍然持续,并且发放率较稳定.但是,锋电位与刺激脉冲之间的锁相性逐渐减弱、潜伏期逐渐延长.而且,与中间神经元相比较,锥体神经元锋电位的锁相性更弱、潜伏期更长.这些结果表明,持续的轴突HFS可以诱导下游神经元产生非同步的活动,高频脉冲刺激引起的不完全轴突传导阻滞可能是导致该现象产生的主要原因.本文的研究为揭示脑刺激的作用机制提供了重要信息.     

Effects of Chronic Stress and Phenytoin on the Long-term Potentiation （LTP） in Rat Hippocampal CA1 Region

Stress is the response to stimulation from inside andoutside with complicated effects on organisms. Appropri-ate stressful reactions are helpful in resisting diseases byactivating unspecific modulation system, while severe orprolonged stresses are harmful and even induce mentaland physical disorders such as recurrent depression, post-traumatic stress disorder (PTSD), Alzheimer’s disease andepilepsy [1]. Hippocampus, a main brain region of keyimportance for learning, memory and emotion, is t…     

The generation of action potentials in the terminals of the Schaffer collaterals during long-term potentiation in the CA1 region of the hippocampus]

Experiments were performed in rat hippocampal slices. Activity of individual CA3 pyramidal neurons and field potentials in the CA1 areas were recorded extracellularly. The collision technique was applied to detect the antidromic origin of the background action potentials in the somata of CA3 neurons. Threshold stimulation of terminals of the Schaffer collaterals in the stratum radiatum of the CA1 area was applied to study their excitability during the CA1 long-term potentiation. During the long-term potentiation, antidromic action potentials appeared in the somata of the CA3 neurons. The obtained evidence suggests that the synaptic potentiation is accompanied by an enhancement of axon terminal excitability resulting in generation of the action potentials.     

Effect of isatin on the electrical activity in the rat hippocampus cells

In vitro superfusion of rat hippocampal slices with isatin changed the population spikes. Isatin perfusion produced two clear effects. 50 microM isatin it increased the amplitude of the population spike in the CA1 evoked by stimulation of stratum radiatum. This effect was readily reversible. 100 microM isatin decreased the population spike amplitude with minimal effect on its latency. High initial response were more suppressed. This effect on the population spike amplitude was not eliminated even after 1 h of washing with saline. The data obtained suggest that isatin-induced electrophysiological changes are involved into the anticonvulsant effect of isatin.     

Chemical preconditioning: A cytoprotective strategy

Brief ischemic or hypoxic episodes may increase or decrease tolerance towards subsequent severe ischemia in heart and brain. A similar phenomenon is observed after mild chemical inhibition of oxidative phosphorylation – chemical preconditioning. We have shown that chemical preconditioning can be induced by chemical inhibition of mitochondrial complex I and mitochondrial complex II. With a time interval of three hours between chemical pretreatment and massive inhibition of oxidative phosphorylation, recovery of population spike amplitude in hippocampal region CA1 after stimulation of the Schaffer collaterals was 31 ± 9% in controls, 98 ± 14% after i.p. treatment with 1 mg/kg body weight haloperidol, an inhibitor of mitochondrial complex I and 90 ± 7% with pretreatment with 3-np, an inhibitor of mitochondrial complex II. Activation of ATP regulated potassium channels partakes in mediating the preconditioning effect. We conclude that chemical preconditioning is a practical prophylactic ph armacologic strategy to increase hypoxic tolerance. (Mol Cell Biochem 174: 249–254, 1997)     

Computational simulation of the input-output relationship in hippocampal pyramidal cells

The precise mapping of how complex patterns of synaptic inputs are integrated into specific patterns of spiking output is an essential step in the characterization of the cellular basis of network dynamics and function. Relative to other principal neurons of the hippocampus, the electrophysiology of CA1 pyramidal cells has been extensively investigated. Yet, the precise input-output relationship is to date unknown even for this neuronal class. CA1 pyramidal neurons receive laminated excitatory inputs from three distinct pathways: recurrent CA1 collaterals on basal dendrites, CA3 Schaffer collaterals, mostly on oblique and proximal apical dendrites, and entorhinal perforant pathway on distal apical dendrites. We implemented detailed computer simulations of pyramidal cell electrophysiology based on three-dimensional anatomical reconstructions and compartmental models of available biophysical properties from the experimental literature. To investigate the effect of synaptic input on axosomatic firing, we stochastically distributed a realistic number of excitatory synapses in each of the three dendritic layers. We then recorded the spiking response to different stimulation patterns. For all dendritic layers, synchronous stimuli resulted in trains of spiking output and a linear relationship between input and output firing frequencies. In contrast, asynchronous stimuli evoked non-bursting spike patterns and the corresponding firing frequency input-output function was logarithmic. The regular/irregular nature of the input synaptic intervals was only reflected in the regularity of output inter-burst intervals in response to synchronous stimulation, and never affected firing frequency. Synaptic stimulations in the basal and proximal apical trees across individual neuronal morphologies yielded remarkably similar input-output relationships. Results were also robust with respect to the detailed distributions of dendritic and synaptic conductances within a plausible range constrained by experimental evidence. In contrast, the input-output relationship in response to distal apical stimuli showed dramatic differences from the other dendritic locations as well as among neurons, and was more sensible to the exact channel densities. Action Editor: Alain Destexhe