Long term potentiation is accompanied by a reduction in dendritic responsiveness to glutamic acid

Emerging Memristive Devices for Brain-Inspired Computing and Artificial Perception

Brain-inspired computing is an emerging field that aims at building a compact and massively parallel architecture, to reduce power consumption in conventional Von Neumann Architecture. Recently, memristive devices have gained great attention due to their immense potential in implementing brain-inspired computing and perception. The conductance of a memristor can be modulated by a voltage pulse, enabling emulations of both essential synaptic and neuronal functions, which are considered as the important building blocks for artificial neural networks. As a result, it is critical to review recent developments of memristive devices in terms of neuromorphic computing and perception applications, waiting for new thoughts and breakthroughs. The device structures, operation mechanisms, and materials are introduced sequentially in this review; additionally, late advances in emergent neuromorphic computing and perception based on memristive devices are summed up. Finally, the challenges that memristive devices toward high-performance brain-inspired computing and perception are also briefly discussed. We believe that the advances and challenges will lead to significant advancements in artificial neural networks and intelligent humanoid robots.

Molecular mechanism of hippocampal long-term potentiation - Towards multiscale understanding of learning and memory

Long-term potentiation (LTP) of synaptic transmission is considered to be a cellular counterpart of learning and memory. Activation of postsynaptic NMDA type glutamate receptor (NMDA-R) induces trafficking of AMPA type glutamate receptors (AMPA-R) and other proteins to the synapse in sequential fashion. At the same time, the dendritic spine expands for long-term and modulation of actin underlies this (structural LTP or sLTP). How these changes persist despite constant diffusion and turnover of the component proteins have been the central focus of the current LTP research. Signaling triggered by Ca²⁺-influx via NMDA-R triggers kinase including Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). CaMKII can sustain longer-term biochemical signaling by forming a reciprocally-activating kinase-effector complex with its substrate proteins including Tiam1, thereby regulating persistence of the downstream signaling. Furthermore, activated CaMKII can condense at the synapse through the mechanism of liquid-liquid phase separation (LLPS). This increases the binding capacity at the synapse, thereby contributing to the maintenance of enlarged protein complexes. It may also serve as the synapse tag, which captures newly synthesized proteins.

Expression of NMDA receptor-dependent LTP in the hippocampus: bridging the divide

A consensus has famously yet to emerge on the locus and mechanisms underlying the expression of the canonical NMDA receptor-dependent form of LTP. An objective assessment of the evidence leads us to conclude that both presynaptic and postsynaptic expression mechanisms contribute to this type of synaptic plasticity.

Long-lasting facilitation of synaptic transmission

After tetanization of several hippocampal pathways (10--50 Hz for 5--15 seconds) there is an increased synaptic transmission of long duration (long-lasting facilitation). The present investigation was undertaken on isolated hippocampal slices to study the mechanism of the effect. The transverse hippocampal slice preparation in vitro allows the simultaneous testing of several afferent fibre systems on the same cell or population of cells. Tetanization of one group of afferent fibres to CA1 pyramids was followed by a long-lasting increase of synaptic transmission along the same fibres, whereas a control input line gave unchanged responses. Using the presynaptic volley as an indicator of the number of afferent impulses, the increased synaptic transmission appeared as an increased excitatory postsynaptic potential (EPSP), increased amplitude and reduced latency of the population spike, and an increased probability of firing of single units. Intracellular recording showed increased EPSPs to afferents of the tetanized line, but no lasting change in membrane resistance or in the response to a depolarizing current pulse. Thus, the effect cannot be ascribed to a general postsynaptic excitability increase. The specific changes in the synaptic transmission may be due either to an increased amount of liberated transmitter or to a local postsynaptic change near the tetanized synapses.

Long-term potentiation in the Eocene

The first ten years of long-term potentiation (LTP) research are reviewed. Surprisingly, given the intensity of current interest, the discovery paper did not trigger a wave of follow-on experiments. Despite this, the initial work laid out what ultimately became standard questions and paradigms. The application of the then still novel hippocampal slice technique oriented LTP towards basic neuroscience, perhaps somewhat at the cost of lesser attention to its functional significance. The use of slices led to the discovery of the events that trigger the formation of LTP and provided some first clues about its extraordinary persistence. Signs of the intense controversy over the nature of LTP expression (release vs receptors) emerged towards the end of the first decade of work. What appears to be lacking in the literature of that time is a widespread concern about LTP and memory. This may reflect a somewhat different attitude that neurobiologists then had towards memory research and a perceived need to integrate the new potentiation phenomenon into the web of established science before advancing extended arguments about its contributions to behaviour.

Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice

Previous work has described the apparent desensitisation of neuronal networks in the rat neocortex to amino acid agonists, following prior exposure several minutes earlier. Since long-term potentiation is believed to involve activation of amino acid receptors, we have now sought to determine whether long-term potentiation can modify the sensitivity of neurones to glutamate receptor agonists in rat hippocampal slices. Responses were measured as the change in population spike or postsynaptic potential (e.p.s.p.) size. Two applications of N-methyl-D-aspartate (NMDA), quinolinic acid, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainate, 45 min apart, did not exhibit any apparent desensitisation. However, the induction of long-term potentiation produced a marked loss of sensitivity to quinolinic acid, with smaller effects on NMDA, AMPA and kainate responses. No marked changes were obtained of e.p. s.p. size. In order to localise the cellular sites of these changes, agonists were also applied by microiontophoresis to the cell bodies or dendritic regions of CA1 neurones. Responses to quinolinic acid showed apparent desensitisation at both sites, whereas no decrease was observed in responses to NMDA or AMPA application. The induction of long-term potentiation again produced a decrease in the size of responses to NMDA and AMPA. Inhibition of nitric oxide (NO) synthase prevented the long-term potentiation-induced loss of responsiveness to NMDA, but not AMPA, implying a role for NO in the loss of NMDA sensitivity. Recordings of single cell activity during the iontophoretic application of agonists and induction of long-term potentiation showed that responses to NMDA were often suppressed to a greater extent than to quinolinic acid. The results indicate that long-term potentiation can modify the sensitivity of hippocampal neurones to glutamate receptor agonists, and that differences exist in the pharmacology of NMDA and quinolinic acid.

Precisely localized LTD in the neocortex revealed by infrared-guided laser stimulation

In a direct approach to elucidate the origin of long-term depression (LTD), glutamate was applied onto dendrites of neurons in rat neocortical slices. An infrared-guided laser stimulation was used to release glutamate from caged glutamate in the focal spot of an ultraviolet laser. A burst of light flashes caused an LTD-like depression of glutamate receptor responses, which was highly confined to the region of "tetanic" stimulation (<10 micrometers). A similar depression of glutamate receptor responses was observed during LTD of synaptic transmission. A spatially highly specific postsynaptic mechanism can account for the LTD induced by glutamate release.

Novel expression mechanism for synaptic potentiation: alignment of presynaptic release site and postsynaptic receptor

A combination of experimental and modeling approaches was used to study cellular-molecular mechanisms underlying the expression of short-term potentiation (STP) and long-term potentiation (LTP) of glutamatergic synaptic transmission in the hippocampal slice. Electrophysiological recordings from dentate granule cells revealed that high-frequency stimulation of perforant path afferents induced a robust STP and LTP of both (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated synaptic responses. However, the decay time constant for STP of the AMPA receptor-mediated excitatory postsynaptic potential was approximately 6 min, whereas the decay time constant for STP of the NMDA receptor-mediated excitatory postsynaptic potential was only 1 min. In addition, focal application of agonists during the expression of STP revealed that the magnitude of conductance change elicited by NMDA application was significantly enhanced, whereas the magnitude of conductance change elicited by application of AMPA remained constant. These findings are most consistent with a postsynaptic mechanism of STP and LTP. Different putative mechanisms were evaluated formally using a computational model that included diffusion of glutamate within the synaptic cleft, different kinetic properties of AMPA and NMDA receptor/channels, and geometric relations between presynaptic release sites and postsynaptic receptor/channels. Simulation results revealed that the only hypothesis consistent with experimental data is that STP and LTP reflect a relocation of AMPA receptor/channels in the postsynaptic membrane such that they become more closely "aligned" with presynaptic release sites. The same mechanism cannot account for STP or LTP of NMDA receptor-mediated responses; instead, potentiation of the NMDA receptor subtype is most consistent with an increase in receptor sensitivity or number.

Flip side of synaptic plasticity: long-term depression mechanisms in the hippocampus

There is growing interest in the phenomenon of long-term depression (LTD) of synaptic efficacy that, together with long-term potentiation (LTP), is a putative information storage mechanism in mammalian brain. In neural network models, multiple learning rules have been used for LTD induction. Similarly, in neurophysiological studies of hippocampal synaptic plasticity, a variety of activity patterns have been effective at inducing LTD, although experimental paradigms are still being optimized. In this review the authors summarize the major experimental paradigms and compare what is known about the mechanisms of LTD induction. Although all paradigms appear to initiate a cascade of events leading to an elevated level of Ca2+ postsynaptically, the extent to which these paradigms involve common expression mechanisms has not yet been tested. The authors discuss several critical experiments that would address this latter issue. Numerous questions about the properties and mechanisms of LTD(s) in the hippocampus remain to be answered, but it is clear that LTD has finally arrived, and will soon be attracting attention equal to its flip side, LTP.

A rise in postsynaptic Ca2+ potentiates miniature excitatory postsynaptic currents and AMPA responses in hippocampal neurons

We have investigated the site of expression of the potentiation of excitatory postsynaptic currents (EPSCs) induced by the activation of postsynaptic voltage-sensitive Ca2+ channels, by examining the effect of depolarizing pulses on miniature (m) EPSCs and responses to AMPA. Application of voltage pulses caused a approximately 2.5-fold increase in the mean amplitude of mEPSCs. This NMDA receptor-independent potentiation of mEPSC amplitudes was transient, returning to control values within 30-40 min. The potentiation was associated with a decrease in the number of small amplitude events and an increase in the number, as well as the maximum amplitude, of the larger events, with no apparent change in mEPSC kinetics. Accompanying the increase in mEPSC amplitudes, there was a 1.6-fold increase in the apparent frequency of events. Voltage pulse-induced potentiation was completely blocked by the inclusion of the Ca2+ chelator BAPTA in the recording pipette. Responses to repeated applications of AMPA were also potentiated following the application of voltage pulses, and the time course of this potentiation was similar to that observed with the mEPSCs. Our data indicate that rises in intracellular Ca2+ that occur independently of NMDA receptor activation can result in a potentiation of quantal size, which is due to an increase in the postsynaptic sensitivity of non-NMDA receptors.

Long-term depression of excitatory synaptic transmission and its relationship to long-term potentiation

In many brain areas, including the cerebellar cortex, neocortex, hippocampus, striatum and nucleus accumbens, brief activation of an excitatory pathway can produce long-term depression (LTD) of synaptic transmission. In most preparations, induction of LTD has been shown to require a minimum level of postsynaptic depolarization and a rise in the intracellular Ca2+ concentration [Ca2+]i in the postsynaptic neurone. Thus, induction conditions resemble those described for the initiation of associative long-term potentiation (LTP). However, data from structures susceptible to both LTD and LTP suggest that a stronger depolarization and a greater increase in [Ca2+]i are required to induce LTP than to initiate LTD. The source of Ca2+ appears to be less critical for the differential induction of LTP and LTD than the amplitude of the Ca2+ surge, since the activation of voltage- and ligand-gated Ca2+ conductances as well as the release from intracellular stores have all been shown to contribute to both LTD and LTP induction. LTD is induceable even at inactive synapses if [Ca2+]i is raised to the appropriate level by antidromic or heterosynaptic activation, or by raising the extracellular Ca2+ concentration [Ca2+]o. These conditions suggest a rule (called here the ABS rule) for activity-dependent synaptic modifications that differs from the classical Hebb rule and that can account for both homosynaptic LTD and LTP as well as for heterosynaptic competition and associativity.

The role of Ca2+ entry via synaptically activated NMDA receptors in the induction of long-term potentiation

Influx of Ca2+ through the NMDA subtype of glutamate receptor is widely accepted as a trigger for many forms of neural plasticity. However, direct support for this model has been elusive, since indirect activation of dendritic voltage-sensitive Ca2+ channels is difficult to exclude. We have optically measured synaptically induced changes in cytoplasmic free Ca2+ concentration in pyramidal cell dendrites in hippocampal slices. Steady postsynaptic depolarization to the synaptic reversal potential eliminated the effect of voltage-sensitive Ca2+ channels. Under these conditions, synaptically induced Ca2+ transients were observed, which were blocked by the NMDA receptor antagonist APV. In addition, the magnitude of LTP was diminished when induced with the postsynaptic membrane held at progressively more positive potentials. LTP could be completely suppressed at potentials near +100 mV. These results provide important experimental support for a role for Ca2+ influx through NMDA receptors in synaptic plasticity.

A synaptic model of memory: long-term potentiation in the hippocampus

Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

Quantal analysis of long-term potentiation of "minimal" excitatory postsynaptic potentials in guinea pig hippocampal slices: binomial approach

"Minimal" excitatory postsynaptic potentials (EPSPs) were recorded from 13 neurones in area CA1 of guinea pig hippocampal slices after double-pulse stimulation of stratum radiatum (str. rad.) and stratum oriens (str. or.). Amplitudes of EPSPs significantly increased in 8 neurones 5 to 55 min after 9 tetanizations in str. rad.. The increase was considered to represent long-term potentiation (LTP). Altogether 26 EPSPs (42 post-tetanic regions) were statistically analysed by four methods of the quantum hypothesis assuming the binomial model of transmitter release: the deconvolution (histogram), the variance, the failures, and the combined (variance-failures) methods. The mean quantal content (m) significantly increased after LTP induction according to all methods used. Quantal size (v) also tended to increase but according to some methods, the increase was not statistically significant and it did not correlate with LTP magnitude. However, for an EPSP subset with a LTP magnitude of less than 1.55, the increase in v correlated with LTP magnitude, whereas the increase in m did not. The relative contribution of the increase in v to LTP magnitude was larger for cases with small LTP than for the whole EPSP set. In general, the increase in m corresponds to previous studies and favours the presynaptic location of major mechanisms of LTP maintenance, i.e. an increase in the average number of transmitter quanta released by each presynaptic volley. The post-tetanic increase in v might reflect some additional mechanisms which presumably include an increase in the amount of transmitter in one quantum.

The NO hypothesis: possible effects of a short-lived, rapidly diffusible signal in the development and function of the nervous system

Several observations suggest that the Ca2(+)-dependent postsynaptic release of nitric oxide (NO) may be important in the formation and function of the vertebrate nervous system. We explore here the hypothesis that the release of NO and its subsequent diffusion may be critically related to three aspects of nervous system function: (i) synaptic plasticity and long-term potentiation in certain regions of the adult nervous system, (ii) the control of cerebral blood flow in such regions, and (iii) the establishment and activity-dependent refinement of axonal projections during the later stages of development. In this paper, we detail and analyze the basic assumptions underlying this NO hypothesis and describe a computer simulation of a minimal version of the hypothesis. In the simulation, a 3-dimensional volume of neuropil is presented with patterned afferent input; NO is produced, diffuses, and is destroyed; and synaptic strengths are determined by a set of synaptic rules based on the correlation of synaptic depolarization and NO levels. According to the hypothesis, voltage-dependent postsynaptic release of this rapidly diffusing substance links the activities of neurons in a local volume of tissue, regardless of whether the neurons are directly connected by synapses. This property is demonstrated in the simulation, and it is this property that is exploited in the hypothesis to account for certain aspects of long-term potentiation and activity-dependent sharpening of axonal arbors.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

The impact of postsynaptic calcium on synaptic transmission--its role in long-term potentiation

Recent studies have gone a long way to explain the steps involved in generating long-term potentiation (LTP). This review focuses on the triggering role of postsynaptic calcium, the sequence of events which might be initiated by calcium, and where the persistent change may ultimately occur during LTP.

Synaptic plasticity in rat hippocampal slice cultures: local "Hebbian" conjunction of pre- and postsynaptic stimulation leads to distributed synaptic enhancement

A central theme in neurobiology is the search for the mechanisms underlying learning and memory. Since the seminal work, first of Cajal and later of Hebb, the synapse is thought to be the basic "storing unit." Hebb proposed that information is stored by correlation: synapses between neurons, which are often coactive, are enhanced. Several recent findings suggest that such a mechanism is indeed operative in the central nervous system. Pairing of activity on presynaptic fibers with strong postsynaptic depolarization results in synaptic enhancement. While there is substantial evidence in favor of a postsynaptic locus for detection of the synchronous pre- and postsynaptic event and subsequent initiation of synaptic enhancement, the locus of this enhancement and its ensuing persistence is still disputed: both pre- and postsynaptic contributions have been suggested. In all previous studies, the enhancement was presumed to be specific to the synapses where synchronous pre- and postsynaptic stimulation was applied. We report here that two recording techniques--optical recording, using voltage-sensitive dyes, and double intracellular recordings--reveal that synaptic enhancement is not restricted to the stimulated cell. Although we paired single afferent volleys with intracellular stimulation confined to one postsynaptic cell, we found that strengthening also occurred on synapses between the stimulated presynaptic fibers and neighboring cells. This suggests that synaptic enhancement by the "paired-stimulation paradigm" is not local on the presynaptic axons and that, in fact, the synapses of many neighboring postsynaptic cells are enhanced.

The role of protein kinase C in long-term potentiation: a testable model

With the use of appropriate reagents, LTP may be divided into at least two stages, induction and maintenance. Induction of LTP is dependent upon the activation of the NMDA receptor, and the consequent influx of calcium into the postsynaptic cell. Both correlational evidence (measures of PKC activity, protein F1 phosphorylation, and PI turnover) and interventive evidence (application of PKC inhibitors and activators) indicate that PKC activation is necessary for maintenance of the LTP response. An important regulatory pathway for PKC activation is the liberation of c-FAs from membrane phospholipids by PLA2. In LTP, activation of this pathway may stabilize PKC in an activated state, and thus contribute to maintenance of the potentiated response. LTP maintenance could result from presynaptic alteration (increased neurotransmitter release), postsynaptic alteration (increases in receptor number or sensitivity, or alterations of postsynaptic morphology), synapse addition, or any of these processes in combination. If LTP maintenance is mediated by presynaptic alteration, as has been indicated by measurement of glutamate release, then one must posit a signal that travels from the postsynaptic to the presynaptic membrane to activate presynaptic PKC. Alternatively, if LTP maintenance is mediated by postsynaptic alteration, a signal contained within the dendritic spine would suffice to activate postsynaptic PKC-mediated maintenance processes. We suggest that the contributions of presynaptic and postsynaptic processes to LTP maintenance may be determined by the differential distribution of PKC subtypes and substrates among hippocampal synaptic zones.

Long-term potentiation: studies in the hippocampal slice

Long-term potentiation (LTP) is an example of activity-dependent plasticity that was discovered in the hippocampal formation. There is growing evidence that LTP not only is a useful model for mnemonic processes, but also may represent the cellular substrate for at least some kinds of learning and memory. The hippocampal slice preparation has proven exceptionally useful in pharmacological studies of possible mechanisms of LTP. A slice remains viable and stable for several hours, and known concentrations of drugs in the bathing medium can be added and then washed out. Drugs can also be applied under visual guidance from micropipettes to discrete neuronal regions, an accomplishment that is aided by the lamellar organization of the hippocampus. Electrical stimulation of the perforant path (PP) in the molecular layer of the dentate gyrus produces a monosynaptic excitatory postsynaptic potential (EPSP) and action potential, which can be recorded extracellularly as a population EPSP and population spike, respectively. Presentation of a high-frequency train (HFT; 100 Hz X 1 s) to the PP results in a long-lasting (greater than 30 min) potentiation of the maximal EPSP slope and of the population spike amplitude. Similarly, exposure of the slice to norepinephrine (e.g. 20 microM for 30 min) results in a long-lasting potentiation (LLP) of both EPSP and population spike (Stanton and Sarvey (1987) Brain Res. Bull., 18: 115). No such LLP was seen in field CA1 following NE application (Stanton and Sarvey (1985) Brain Res., 361: 276). beta-Adrenergic antagonists, such as propranolol, inhibit both LTP and NE-induced LLP in dentate (Stanton and Sarvey, J. Neurosci., 5: 2169 (1985); Stanton and Sarvey (1985) Brain Res., 361: 276). Cyclic AMP levels are increased by either an HFT or NE (Stanton and Sarvey (1985) Brain Res., 358: 343). Thus, NE, acting through a beta-receptor, appears to be both necessary and sufficient to produce long-lasting enhancement of synaptic responses. Finally, inhibitors of protein synthesis, such as emetine, also block both LTP and NE-induced LLP (Stanton and Sarvey, J. Neurosci., (1984) 4: 3080; Stanton and Sarvey (1985) Brain Res., 361: 276). The N-methyl-D-aspartate (NMDA) excitatory amino acid receptor subtype appears to play a role in a number of forms of neuronal plasticity. Bath-application of a 1 microM concentration of the NMDA antagonists D-2-amino-5-phosphonavaleric acid (AVP) or 3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked both LTP and NE-induced LLP in the dentate gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

Temporally distinct pre- and post-synaptic mechanisms maintain long-term potentiation

Long-term potentiation (LTP) in the hippocampus is widely studied as the mechanisms involved in its induction and maintenance are believed to underlie fundamental properties of learning and memory in vertebrates. Most synapses that exhibit LTP use an excitatory amino-acid neurotransmitter that acts on two types of receptor, the N-methyl-D-aspartate (NMDA) and quisqualate receptors. The quisqualate receptor mediates the fast synaptic response evoked by low-frequency stimulation, whereas the NMDA receptor system is activated transiently by tetanic stimulation, leading to the induction of LTP. The events responsible for maintaining LTP once it is established are not known. We now demonstrate that the sensitivity of CA1 neurons in hippocampal slices to ionophoretically-applied quisqualate receptor ligands slowly increases following the induction of LTP. This provides direct evidence for a functional post-synaptic change and suggests that pre-synaptic mechanisms also contribute, but in a temporally distinct manner, to the maintenance of LTP.

A persistent postsynaptic modification mediates long-term potentiation in the hippocampus

Long-term potentiation (LTP) is a long-lasting enhancement of synaptic transmission that can be induced by brief repetitive stimulation of excitatory pathways in the hippocampus. One of the most controversial points is whether the process underlying the enhanced synaptic transmission occurs pre- or postsynaptically. To examine this question, we have taken advantage of the novel physiological properties of excitatory synaptic transmission in the CA1 region of the hippocampus. Synaptically released glutamate activates both NMDA and non-NMDA receptors on pyramidal cells, resulting in an excitatory postsynaptic potential (EPSP) with two distinct components. A selective increase in the non-NMDA component of the EPSP was observed with LTP. This result suggests that the enhancement of synaptic transmission during LTP is caused by an increased sensitivity of the postsynaptic neuron to synaptically released glutamate.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Calcium and unit response decrement to locally applied glutamate on rat hippocampal CA1 neurons

In rat hippocampal slices, sensitivity of CA1 neuronal somata to iontophoretically applied glutamate pulses was reduced subsequent to: (1) a 20 Hz tetanic stimulation of stratum radiatum; (2) an application of N-methyl-DL-aspartate or glutamate or Ca2+. This decreased responsiveness was sometimes followed by an increase. Ca2+ antagonists, verapamil and Mn2+, interfered with the induction of the above changes in sensitivity to glutamate pulses. No correlation was observed between alterations in sensitivity of these neurons to glutamate and changes in synaptically-induced responses. These results suggest that a build-up of CA1 cell Ca2+ could be involved in decreasing the responsiveness (desensitization of receptors?) of the neuron to applied glutamate leading to a compensatory increase in the sensitivity.

Cholinergic kindling: what has it taught us about epilepsy?

We reviewed recent evidence that chemical kindling of epileptic seizures can be induced by injection into the amygdala of multiple cholinergic muscarinic agonists, and blocked by multiple muscarinic antagonists. The stereospecific induction of kindling by (+) but not by (-) acetyl-beta-methylcholine shows that some types of repeated synaptic activation can produce epilepsy, in the absence of specific brain damage. The failure of bicuculline (but not of carbachol) to produce kindling with amygdaloid injections, and its ability to produce a limited seizure spread in neocortex, suggest that repetitive seizure activity alone is not sufficient to produce kindling. A review of some recent neurochemical changes in the synaptic apparatus associated with some types of kindling suggests potential areas for future investigation, but no cause-and-effect relationship between neurochemical and behavioral changes can be inferred so far.

Influences of sinusoidal electric fields on excitability in the rat hippocampal slice

The influence of extracellular sinusoidal electric fields on the amplitude of population spikes evoked by single test pulses in excitatory pathways to CA1 pyramidal neurons was studied in rat hippocampal slices. The fields in the tissue were of the order of EEG gradients. Stimulation at 5 Hz, a frequency representative of hippocampal theta activity, was compared with 60 Hz, which is often used in kindling procedures. Brief stimulation (5-30 s) with both 5 and 60 Hz fields (20-70 mV/cmp-p in the perfusing solution) often produced a long-term increase (longer than 10 min) of the population spike. Fields at 60 Hz, but not at 5 Hz, also induced short-term depression (1-6 min) or transient post-field excitation (15-30 s). Prolonged stimulation (3 min) emphasized this frequency dependent response: fields at 5 Hz induced long-lasting potentiation while fields at 60 Hz always resulted in progressive depression persisting for a few minutes after the end of stimulation. These effects appeared as a global response of CA1 neurons. Antidromic responses studied during blockade of synaptic transmission (0.2 mM Ca2+, 4 mM Mg2+) were depressed during and following 3 min field stimulation at either frequency, which could reflect failing calcium mechanisms in the tissue. The field influence on the potential evoked by synaptic or antidromic stimulation was independent of the phase of the sine wave at which the test pulse was delivered, arguing against a direct polarization of the cell membrane by the fields. The experimental evidence suggests a functional role for EEG-like fields in hippocampal excitability.

Long-term potentiation of hippocampal synaptic transmission affects rate of behavioral learning

Electrical stimulation techniques were used to produce a long-lasting potentiation of synaptic transmission in the hippocampus of naïve rabbits. Animals were then classically conditioned. Long-term potentiation of the hippocampus before training increased the rate at which animals subsequently learned the conditioning task. This result has significance for potential cellular mechanisms of associative learning.

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.

Long-term potentiation in the hippocampus

Long-term potentiation of field and single neuronal responses recorded in various hippocampal fields is described on the basis of author's and literary data. Most of intrahippocampal and extrinsic connections in both in vivo and in vitro hippocampal preparations show this phenomenon after one or several conditioning trains of comparatively short duration (20 s or less) at various frequencies (from 10 to 400 Hz). Properties of hippocampal potentiation are described. The properties include long term persistence (hours and days) of the potentiated response, its low frequency depression, self-restoration after the depression, specificity of the potentiation for the tetanized pathway, necessity of activation of a sufficient number of neuronal elements ('cooperativity') to produce the potentiation, possible involvement of 'reinforcing' brain structures during conditioning tetanization. These properties are distinct from those of 'usual' short-term post-tetanic potentiation and lead to the suggestion that the neuronal mechanisms underlying long-term post-tetanic are similar to those underlying memory and behavioral-conditioned reflex. Neurophysiological mechanisms of long-term potentiation are discussed. The main mechanism consists in an increase in efficacy of excitatory synapses as shown by various methods including intracellular recording and quantal analysis. The latter favours presynaptic localization of changes of synaptic efficacy showing increase in the number of transmitter quanta released per presynaptic impulse. However, changes in the number of subsynaptic receptors or localized changes in dendritic postsynaptic membrane are not excluded. Biochemical studies indicate the increase in transmitter release and calcium-dependent phosphorylation of pyruvate dehydrogenase after tetanization. Instances of persistent response facilitations at other levels of the vertebrate central nervous system (especially at neocortical level) are considered and compared with hippocampal long-term potentiation. It is suggested that modifiable excitatory synapses necessary for learning have been identified in studies of long-term potentiation. These synapses are presumably modified as a result of close sequential activation of the following three structures: excitatory presynaptic fibers, the postsynaptic neuron and a 'reinforcing' brain system.

Asymmetric relationships between homosynaptic long-term potentiation and heterosynaptic long-term depression

All synaptically-based neuropsychological theories of learning postulate that there are changes resulting from neural activity which are long-lasting and confined to specific sets of synapses. In the past decade a form of synaptic strengthening known as long-term potentiation (LTP) has been found which results from high-frequency neural activity and is of sufficient duration to model as a learning mechanism. Some early tests of the synaptic specificity of LTP in area CA1 of the hippocampus indicated that although LTP was specific to the tetanized pathway, in a converging untetanized pathway it was associated with depression of synaptic transmission lasting for at least 30 min. However, others have found that this heterosynaptic depression more usually decays within 5-15 min post-tetanus despite the maintenance of LTP in the tetanized pathway. Similarly, in the dentate gyrus (DG), LTP of either the lateral (LPP) or medial (MPP) components of the perforant path afferents has been associated with only short-lasting reciprocal heterosynaptic depression. Here, using more detailed measurement of stimulus intensity curves, we report that tetanization of either MPP or LPP reliably depresses synaptic transmission in the other pathway for at least 3 h. This heterosynaptic depression, considerably smaller than the usual magnitude of LTP, was obtained regardless of whether LTP had been produced in the tetanized homosynaptic pathway. Heterosynaptic long-term depression was not observed if the test pathway had been previously tetanized.

Increased resting and evoked release of transmitter following repetitive electrical tetanization in hippocampus: a biochemical correlate to long-lasting synaptic potentiation

In vitro stimulation of two axonal branches from hippocampal CA3 pyramids, the CA1 afferent Schaffer collaterals and the CA3 efferents to septum through fimbria, released D-[3H]aspartate as a measure for endogenous L-glutamate. Following bursts of repetitive electrical stimuli to the Schaffer collaterals, a long-lasting and significantly increased resting efflux, as well as an increased stimulus evoked release of D-aspartate, appeared. No such persistent increase in D-aspartate efflux was recorded from the septal terminals. We propose that increased transmitter liberation may account for long-term synaptic potentiation in hippocampus.