Quantal analysis of hippocampal long-term potentiation

Optical Quantal Analysis

Understanding the mechanisms by which long-term synaptic plasticity is expressed remains an important objective in neuroscience. From a physiological perspective, the strength of a synapse can be considered a consequence of several parameters including the probability that a presynaptic action potential (AP) evokes the release of neurotransmitter, the mean number of quanta of transmitter released when release is evoked, and the mean amplitude of a postsynaptic response to a single quantum. Various methods have been employed to estimate these quantal parameters from electrophysiological recordings; such "quantal analysis" has been used to support competing accounts of mechanisms of expression of long-term plasticity. Because electrophysiological recordings, even with minimal presynaptic stimulation, can reflect responses arising at multiple synaptic sites, these methods are open to alternative interpretations. By combining intracellular electrical recording with optical detection of transmission at individual synapses, however, it is possible to eliminate such ambiguity. Here, we describe methods for such combined optical and electrical monitoring of synaptic transmission in brain slice preparations and illustrate how quantal analyses thereby obtained permit more definitive conclusions about the physiological changes that underlie long-term synaptic plasticity.

Alzheimer's disease risk factor lymphocyte-specific protein tyrosine kinase regulates long-term synaptic strengthening, spatial learning and memory

The lymphocyte-specific protein tyrosine kinase (Lck), which belongs to the Src kinase-family, is expressed in neurons of the hippocampus, a structure critical for learning and memory. Recent evidence demonstrated a significant downregulation of Lck in Alzheimer's disease. Lck has additionally been proposed to be a risk factor for Alzheimer's disease, thus suggesting the involvement of Lck in memory function. The neuronal role of Lck, however, and its involvement in learning and memory remain largely unexplored. Here, in vitro electrophysiology, confocal microscopy, and molecular, pharmacological, genetic and biochemical techniques were combined with in vivo behavioral approaches to examine the role of Lck in the mouse hippocampus. Specific pharmacological inhibition and genetic silencing indicated the involvement of Lck in the regulation of neuritic outgrowth. In the functional pre-established synaptic networks that were examined electrophysiologically, specific Lck-inhibition also selectively impaired the long-term hippocampal synaptic plasticity without affecting spontaneous excitatory synaptic transmission or short-term synaptic potentiation. The selective inhibition of Lck also significantly altered hippocampus-dependent spatial learning and memory in vivo. These data provide the basis for the functional characterization of brain Lck, describing the importance of Lck as a critical regulator of both neuronal morphology and in vivo long-term memory.

Presynaptic LTP and LTD of excitatory and inhibitory synapses

Ubiquitous forms of long-term potentiation (LTP) and depression (LTD) are caused by enduring increases or decreases in neurotransmitter release. Such forms or presynaptic plasticity are equally observed at excitatory and inhibitory synapses and the list of locations expressing presynaptic LTP and LTD continues to grow. In addition to the mechanistically distinct forms of postsynaptic plasticity, presynaptic plasticity offers a powerful means to modify neural circuits. A wide range of induction mechanisms has been identified, some of which occur entirely in the presynaptic terminal, whereas others require retrograde signaling from the postsynaptic to presynaptic terminals. In spite of this diversity of induction mechanisms, some common induction rules can be identified across synapses. Although the precise molecular mechanism underlying long-term changes in transmitter release in most cases remains unclear, increasing evidence indicates that presynaptic LTP and LTD can occur in vivo and likely mediate some forms of learning.

Activation and desensitization of neuronal nicotinic receptors modulate glutamatergic transmission on neonatal rat hypoglossal motoneurons

In the neonate the muscles of the tongue, which are exclusively innervated by the XII cranial nerves originating from the brainstem nucleus hypoglossus, must contract rhythmically in coincidence with breathing, suckling and swallowing. These motor commands are generated by hypoglossal motoneurons excited by glutamatergic inputs. Because in forebrain areas the efficiency of glutamatergic transmission is modulated by neuronal nicotinic receptors (nAChRs), the role and identity of nAChRs within the nucleus hypoglossus of the neonatal rat were explored using an in vitro brainstem slice preparation. This area expressed immunoreactivity for alpha4, alpha7 and beta2 nAChR subunits. Whole-cell patch-clamp recording from hypoglossal motoneurons showed lack of spontaneous cholinergic events mediated by nAChRs even in the presence of a cholinesterase inhibitor. However, pharmacological antagonism of alpha7- or beta2-containing receptors depressed glutamatergic currents arising either spontaneously or by electrical stimulation of the reticular formation. Hypoglossal motoneurons expressed functional nAChRs with characteristics of alpha4beta2 and alpha7 receptor subunits. Such receptors underwent fast desensitization (time constant of 200 ms) with full recovery within 1 min. Low (0.5 microm) concentration of nicotine first facilitated glutamatergic transmission on motoneurons and later depressed it through receptor desensitization. When 0.1 microm nicotine was used, only depression of synaptic transmission occurred, in keeping with the suggestion that nAChRs can be desensitized without prior activation. These results highlight the role of tonic nAChR activity in shaping excitatory inputs to hypoglossal motoneurons, and suggest that nAChR desensitization by ambient nicotine could contribute to disorders of tongue muscle movements.

Optical quantal analysis indicates that long-term potentiation at single hippocampal mossy fiber synapses is expressed through increased release probability, recruitment of new release sites, and activation of silent synapses

It is generally believed that long-term potentiation (LTP) at hippocampal mossy fiber synapses between dentate granule and CA3 pyramidal cells is expressed through presynaptic mechanisms leading to an increase in quantal content. The source of this increase has remained undefined but could include enhanced probability of transmitter release at existing functional release sites or increases in the number of active release sites. We performed optical quantal analyses of transmission at individual mossy fiber synapses in cultured hippocampal slices, using confocal microscopy and intracellular fluorescent Ca(2+) indicators. Our results indicate that LTP is expressed at functional synapses by both increased probability of transmitter release and recruitment of new release sites, including the activation of previously silent synapses here visualized for the first time.

Combining principal component and spectral analyses with the method of moments in studies of quantal transmission

This chapter considers methods for measurements of postsynaptic responses and simple approaches to the estimation of parameters of quantal release in synapses of the central nervous system of vertebrates. The use of these methods is illustrated by the analysis of single-fibre and "minimal" monosynaptic postsynaptic potentials (PSPs) or currents (PSCs) recorded from neurons of the frog spinal cord and rat hippocampus. First, we briefly discuss traditional methods of the response measurements using peak amplitudes or areas, further focusing on a novel method based on multivariate statistical techniques of the principal component analysis (PCA). This approach provides typically better signal-to-noise ratios and is able to separate two or more response components, which can arise due to activation of more than one presynaptic fibre, axon collaterals, receptor subtypes or spatially separated transmission sites. Second, spectral analysis is introduced as the method of choice to verify whether the amplitude fluctuations of the postsynaptic responses have a quantal nature and to obtain estimations of the "basic" quantal parameters, i.e. the quantal size (Q) and mean quantal content (m), without introducing assumptions on release statistics. Third, we show how the method of moments could be applied in the framework of the Poisson and binomial models to estimate the basic quantal parameters and parameters p and n, which reflect the release probability and maximum number of quanta released (or the number of effective release sites), respectively. Fourth, we show that the analysis of the moments can also be instrumental to reveal non-uniformity of release probabilities and compare how several competing models of neurotransmitter release fit to multiple experimental data sets.

Silent synapses: what are they telling us about long-term potentiation?

At several cortical synapses glutamate release events can be mediated exclusively by NMDA receptors, with no detectable contribution from AMPA receptors. This observation was originally made by comparing the trial-to-trial variability of the two components of synaptic signals evoked in hippocampal neurons, and was subsequently confirmed by recording apparently pure NMDA receptor-mediated EPSCs with stimulation of small numbers of axons. It has come to be known as the 'silent synapse' phenomenon, and is widely assumed to be caused by the absence of functional AMPA receptors, which can, however, be recruited into the postsynaptic density by long-term potentiation (LTP) induction. Thus, it provides an important impetus for relating AMPA receptor trafficking mechanisms to the expression of LTP, a theme that is taken up elsewhere in this issue. This article draws attention to several findings that call for caution in identifying silent synapses exclusively with synapses without AMPA receptors. In addition, it attempts to identify several missing pieces of evidence that are required to show that unsilencing of such synapses is entirely accounted for by insertion of AMPA receptors into the postsynaptic density. Some aspects of the early stages of LTP expression remain open to alternative explanations.

Quantal analysis suggests strong involvement of presynaptic mechanisms during the initial 3 h maintenance of long-term potentiation in rat hippocampal CA1 area in vitro

Long-term potentiation (LTP) is the most prominent model to study neuronal plasticity. Previous studies using quantal analysis of an early stage of LTP in the CA1 hippocampal region (<1 h after induction) suggested increases in both the mean number of transmitter quanta released by each presynaptic pulse (m, quantal content) and postsynaptic effect of a single quantum (v, quantal size). When LTP was large, it was m that increased predominantly suggesting prevailing presynaptic contribution. However, LTP consists of several temporary phases with presumably different mechanisms. Here we recorded excitatory postsynaptic potentials from CA1 hippocampal slices before and up to 3.5 h after LTP induction. A new version of the noise deconvolution revealed significant increases in m with smaller and often not statistically significant changes in v. The changes in m were similar for both early (<1 h) and later (1-3 h) post-tetanic periods and correlated with LTP magnitude. The coefficient of variation of the response amplitude and the number of failures decreased during both early and late post-tetanic periods. The results suggest that both early (<0.5 h) and later LTP components (0.5-3 h) are maintained by presynaptic changes, which include increases in release probabilities and the number of effective release sites. In addition initially silent synapses can be converted into effective ones due to either pre- or postsynaptic rearrangements. If this occurs, our data indicate that the number and the efficacy of the receptors in the new transmission sites are approximately similar to those in the previously effective sites.

Long-term potentiation of the AMPA and NMDA components of minimal postsynaptic currents in rat hippocampal field Ca1

The aim of the present work was to study the potentiation of the AMPA and NMDA components of minimal excitatory postsynaptic currents (EPSC) evoked by activation of restricted numbers of synapses. EPSC of neurons in field CA1 in hippocampal slices were recorded in whole-call patch-clamp conditions selected such that both (AMPA and NMDA) components were present, and these were measured in parallel using computational methods in combination with pharmacological receptor blockade. There was a quite strong correlation between the amplitudes of the AMPA and NMDA components and this was regarded as evidence that they were generated by the same synapses. In cases producing this correlation, both components showed essentially equal long-term potentiation lasting from 5 min to 2 h after afferent tetanization. The data did not support the postsynaptic hypothesis and were in better agreement with the concept that the major mechanism for the persistence of the initial phase of long-term potentiation (up to 1-2 h) is based on increases in the quantity of transmitter released presynaptically.

St John's wort (Hypericum perforatum) modulates evoked potentials in guinea pig hippocampal slices via AMPA and GABA receptors

The effects of an ethanolic extract of the plant Hypericum perforatum L. (St John's wort) (HYP) and its hydrosoluble fraction (HYPWS) on electrically evoked population spikes and fEPSP were investigated in this study. Concentration dependent (10(-6) to 10(-4) g/l) excitatory effects were found. Above concentrations of 10(-3) g/l, HYP reduced the evoked responses, whereas HYPWS further increased them. Paired pulse facilitation was unaffected with HYPWS (10(-4) to 10(-2) g/l). The excitatory effects of HYPWS were amplified by the GABA(A) and GABA(B) receptor antagonists bicuculline and phaclofen, respectively. These excitations were antagonised by the AMPA receptor antagonist CNQX. Excitations caused by hypericum were not antagonised by the NMDA receptor antagonists D-APV and MK801, the metabotropic glutamate receptor (type I and II) antagonist MCPG, or the L-type calcium channel blocker verapamil. Hypericin and hyperforin, two components of H. perforatum, were found not to be responsible for the excitatory effects of the extract.

Unveiling synaptic plasticity: a new graphical and analytical approach

Short-term synaptic plasticity has a key role in information processing in the CNS, whereas memories can be formed through long-lasting changes in synaptic strength. Despite the importance of these phenomena, it remains difficult to determine whether a synaptic modulation is expressed at a presynaptic or postsynaptic site. This article describes a new approach that, in its simplest form, can identify the site of expression by direct graphical means. A more-sophisticated form of the technique can quantify functional synaptic properties and determine which of these properties is altered following a modulation of synaptic strength.

Interaction between paired-pulse facilitation and long-term potentiation of minimal excitatory postsynaptic potentials in rat hippocampal slices: a patch-clamp study

Long-term potentiation is an experimental paradigm used to study synaptic plasticity and memory mechanisms. One similarity between long-term potentiation and memory is the existence of several distinct phases. However, our preliminary quantal analysis did not reveal essential differences in expression mechanisms of the early (< 1 h) and later (up to 3 h) phases of long-term potentiation. The data were compatible with presynaptic mechanisms of both phases. Another approach to distinguish between presynaptic and postsynaptic mechanisms is analysis of interaction between long-term potentiation and presynaptic paired-pulse facilitation. Such analysis had been previously done mainly with recordings of field potentials reflecting the activity of large neuronal populations. Only the early potentiation phase had been previously analysed with recordings from single neurons. The results from different groups were contradictory. In the present study, minimal excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons of rat hippocampal slices. Paired-pulse facilitation ratios were calculated for various periods (up to 2-3 h) following induction of long-term potentiation. The ratio persistently decreased in the majority of neurons following long-term potentiation induction. The decrease in the paired-pulse facilitation ratio correlated with the magnitude of long-term potentiation and with the initial (pretetanic) facilitation ratio. Therefore, the general results of the present analysis was similar with the results of the quantal analysis: it is consistent with a strong involvement of presynaptic mechanisms in maintenance of both early and late phases of long-term potentiation. However, individual neurons could show variable changes in the paired-pulse facilitation, e.g., increases at late (> 0.5-1 h) periods after tetanus. Calculations of partial correlations and regression analysis indicated that positive correlation between potentiation magnitude and initial (pretetanic) paired-pulse facilitation tended to increase in the late potentiation phase (1.5-2.5 h post-tetanus) indicating that different mechanisms are involved in the early (0.5 h post-tetanus) and the late phase of long-term potentiation. The findings are compatible with involvement of presynaptic mechanisms in both the early and late phases of long-term potentiation. However, the results suggest that contribution of changes in release probability and in effective number of transmitter release sites may differ during the two phases. It is suggested that activation of silent synapses and increases in the number of transmission zones due to pre- and postsynaptic structural rearrangements represent important mechanisms of the late phase of long-term potentiation.

Dynamics of excitatory transmitter release: analysis of synaptic responses in CA3 hippocampal neurons after repetitive stimulation of afferent fibers

The patch-clamp technique (whole cell configuration) was used to record excitatory postsynaptic currents (EPSCs) evoked by repetitive stimulation (4 pulses at 50-ms intervals) of afferent fibers in the stratum lucidum-radiatum. Different synaptic behaviors (EPSC patterns) were classified in terms of facilitation or depression of the mean amplitude of the second, third, and fourth EPSC with respect to the previous one. A large variety of EPSC patterns was observed by stimulating different afferent fibers. Experiments with the mGluR2/mGluR3 agonist 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) (1 microM), a compound that reduces release at mossy but not at associative commissural fibers and therefore allows to identify the origin of synaptic responses, showed that particular EPSC patterns could not be associated to the activation of a specific type of synaptic input. To investigate the role of the probability of release in the dynamics of synaptic activity, the extracellular calcium concentration was varied from 0.8 to 4 mM in several experiments. EPSC patterns dominated by depression, characteristics of high release probability conditions, could be observed in the majority of the cases in the presence of higher calcium concentrations. A quantitative model for dynamics of transmitter release has been developed. Experimental results were compared with data computed with the model taking into account the probability of release and the time course of reavailability. This work indicates that short-term changes of presynaptic conditions occurring during a train of action potentials can account for the high variability of EPSC responses. The model that is proposed also suggests a general method of experimental data analysis to investigate the possible presynaptic mechanisms underlying long-lasting changes in synaptic efficacy.

Potential for multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses

Recent experimental evidence indicates that in the neocortex, the manner in which each synapse releases neurotransmitter in response to trains of presynaptic action potentials is potentially unique. These unique transmission characteristics arise because of a large heterogeneity in various synaptic properties that determine frequency dependence of transmission such as those governing the rates of synaptic depression and facilitation. A theoretical analysis was therefore undertaken to explore the phenomenologies of changes in the values of these synaptic parameters. The results illustrate how the change in any one of several synaptic parameters produces a distinctive effect on synaptic transmission and how these distinctive effects can point to the most likely biophysical mechanisms. These results could therefore be useful in studies of synaptic plasticity in order to obtain a full characterization of the phenomenologies of synaptic modifications and to isolate potential biophysical mechanisms. Based on this theoretical analysis and experimental data, it is proposed that there exists multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses. Finally, it is shown that the impact of changing the values of synaptic parameters depends on the values of the other parameters. This may indicate that the various mechanisms, phenomena and algorithms are interlinked in a 'synaptic plasticity code'.

Synaptic refractory period provides a measure of probability of release in the hippocampus

Despite extensive research, much controversy remains regarding the locus of expression of long-term potentiation (LTP) in area CA1 of the hippocampus, specifically, whether LTP is accompanied by an increase in the probability of release (p(r)) of synaptic vesicles. We have developed a novel method for assaying p(r), which utilizes the synaptic refractory period--a brief 5-6 ms period following release during which the synapse is incapable of transmission (Stevens and Wang, 1995). We show that this assay is sensitive to a battery of manipulations that affect p(r) but find no change following either NMDA receptor-dependent LTP or long-term depression (LTD).

Relations between long-term synaptic modifications and paired-pulse interactions in the rat neocortex

The phenomenon of paired-pulse facilitation (PPF) was exploited to investigate the role of presynaptic mechanisms in the induction and maintenance of long-term synaptic plasticity in the neocortex. Long-term potentiation (LTP) and depression (LTD) were induced without afferent activation by applying tetani of intracellular pulses. Our results show that synaptic modifications closely resembling LTP and LTD can be induced by postsynaptic activation alone. The polarity of these synaptic modifications depends on initial properties of the input, as indicated by a correlation between initial PPF ratio and post-tetanic amplitude changes: inputs exhibiting strong PPF, which might be associated with low release probability tend to be potentiated, while inputs with small PPF are more likely to show depression. Maintenance of both LTP and LTD involve presynaptic mechanisms, as indicated by changes in PPF ratios and in failure rate after LTP or LTD induction. Presynaptic mechanisms could include changes in release probability and/or in the number of active release sites. Because induction was postsynaptic, this supports the notion of a retrograde signal. The relative contribution of pre- and postsynaptic mechanisms in the maintenance of long-term synaptic modifications depends on the initial state of the synaptic input and on LTP magnitude. PPF changes were especially pronounced in inputs which had initially high PPF and underwent strong potentiation. Since LTP and LTD are associated with changes of PPF ratios these synaptic modifications do not only alter the gain but also the temporal properties of synaptic transmission. Because of the LTP associated reduction of PPF, potentiated inputs profit less from temporal summation, favouring transmission of synchronized, low frequency activity.

Noise deconvolution based on the L1-metric and decomposition of discrete distributions of postsynaptic responses

A statistical approach to analysis of amplitude fluctuations of postsynaptic responses is described. This includes (1) using a L1-metric in the space of distribution functions for minimisation with application of linear programming methods to decompose amplitude distributions into a convolution of Gaussian and discrete distributions; (2) deconvolution of the resulting discrete distribution with determination of the release probabilities and the quantal amplitude for cases with a small number (< 5) of discrete components. The methods were tested against simulated data over a range of sample sizes and signal-to-noise ratios which mimicked those observed in physiological experiments. In computer simulation experiments, comparisons were made with other methods of 'unconstrained' (generalized) and constrained reconstruction of discrete components from convolutions. The simulation results provided additional criteria for improving the solutions to overcome 'over-fitting phenomena' and to constrain the number of components with small probabilities. Application of the programme to recordings from hippocampal neurones demonstrated its usefulness for the analysis of amplitude distributions of postsynaptic responses.

Changes in paired-pulse facilitation correlate with induction of long-term potentiation in area CA1 of rat hippocampal slices

The phenomenon of long-term potentiation is widely used as an experimental model of memory. An approach that has been used to study its underlying mechanisms is to analyse its interaction with presynaptic paired-pulse facilitation. Several studies found no evidence for an interaction in the CA1 hippocampal area, whereas other data, for example from quantal analysis, suggested that presynaptic mechanisms contribute to the maintenance of long-term potentiation. In the present study, initial slopes of field potentials in area CA1 were measured in rat hippocampal slices. "Conventional" long-term potentiation was induced by high-frequency (100 Hz) afferent tetanization of the testing input. "Associative" long-term potentiation was induced by combining lower frequency (40 Hz) tetanization of a testing input with high-frequency tetanization of a second input. The paired-pulse facilitation ratio decreased in the majority of experiments in which long-term potentiation was induced conventionally, but it decreased, increased or did not change after inducing associative potentiation. Decreases in the paired-pulse facilitation correlated inversely with the initial (pre-tetanic) facilitation ratio. A more detailed regression analysis suggests that this correlation results from two other correlations: (i) that between changes in paired-pulse facilitation and the magnitude of long-term potentiation, and (ii) that between initial paired-pulse facilitation and the magnitude of long-term potentiation. The first correlation prevailed during the initial 10 min following tetanization, while the second prevailed 40-60 min later. A post-tetanic decrease in paired-pulse facilitation is evidence for an involvement of presynaptic mechanisms in the maintenance of long-term potentiation. The lack of significant changes in some studies could be due to the inclusion in the analyses of experiments with long-term potentiation of small magnitude, in which changes in paired-pulse facilitation ratios would have been inconsistent. The present study suggests that the early (10-20 min) and late (40-50 min) phases of long-term potentiation were mediated by different mechanisms, with a mixture of these mechanisms during the intermediate period. On the basis of the present and previous studies, the following scheme of involvement of several mechanisms in long-term potentiation maintenance is proposed. The early phase includes two major mechanisms: an increase in the probability of transmitter release, leading to an apparent increase in the number of effective release sites, and an increase in efficacy of one transmitter quantum, probably due to an increased number of postsynaptic receptors. The later phase of long-term potentiation is attributed to an increase in the number of transmitter zones, presumably due to structural modifications.