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

Tunicamycin impairs olfactory learning and synaptic plasticity in the olfactory bulb

Tunicamycin (TM) induces endoplasmic reticulum (ER) stress and inhibits N-glycosylation in cells. ER stress is associated with neuronal death in neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease, and most patients complain of the impairment of olfactory recognition. Here we examined the effects of TM on aversive olfactory learning and the underlying synaptic plasticity in the main olfactory bulb (MOB). Behavioral experiments demonstrated that the intrabulbar infusion of TM disabled aversive olfactory learning without affecting short-term memory. Histological analyses revealed that TM infusion upregulated C/EBP homologous protein (CHOP), a marker of ER stress, in the mitral and granule cell layers of MOB. Electrophysiological data indicated that TM inhibited tetanus-induced long-term potentiation (LTP) at the dendrodendritic excitatory synapse from mitral to granule cells. A low dose of TM (250nM) abolished the late phase of LTP, and a high dose (1μM) inhibited the early and late phases of LTP. Further, high-dose, but not low-dose, TM reduced the paired-pulse facilitation ratio, suggesting that the inhibitory effects of TM on LTP are partially mediated through the presynaptic machinery. Thus, our results support the hypothesis that TM-induced ER stress impairs olfactory learning by inhibiting synaptic plasticity via presynaptic and postsynaptic mechanisms in MOB.

Berberine chloride improved synaptic plasticity in STZ induced diabetic rats

Previous studies indicated that diabetes affects synaptic transmission in the hippocampus, leading to impairments of synaptic plasticity and defects in learning and memory. Although berberine treatment ameliorates memory impairment and improves synaptic plasticity in streptozotocin (STZ) induced diabetic rats, it is not clear if the effects are pre- or post-synaptic or both. The aim of this study was to evaluate the effects of berberine chloride on short-term plasticity in inhibitory interneurons in the dentate gyrus of STZ-induced diabetic rats. Experimental groups included: The control, control berberine treated (100 mg/kg), diabetic and diabetic berberine treated (50,100 mg/kg/day for 12 weeks) groups. The paired pulse paradigm was used to stimulate the perforant pathway and field excitatory post-synaptic potentials (fEPSP) were recorded in dentate gyrus (DG). In comparison with control, paired pulse facilitation in the diabetic group was significantly increased (P < 0.01) and this effect prevented by chronic berberine treatment (50,100 mg/kg). However, there were no differences between responses of the control berberine 100 mg/kg treated and diabetes berberine treated (50 and 100 mg/kg) groups as compared to the control group. The present results suggest that the pre-synaptic component of synaptic plasticity in the dentate gyrus is affected under diabetic conditions and that berberine prevents this effect.

Postsynaptic protein synthesis is required for presynaptic enhancement in persistent forms of long-term potentiation

Long-term potentiation (LTP) in the hippocampus is a fundamental process underlying learning and memory in the brain. At CA3-CA1 synapses, three discrete forms of LTP (LTP1, 2, and 3) have been differentiated on the basis of their persistence, maintenance mechanisms, Ca²⁺ signaling pathways, expression loci, and electrophysiological requirements. We previously showed that LTP2 and LTP3 involve a presynaptic expression component that is established in a translation-dependent manner. Here we investigate the locus of translation required for presynaptic expression. Neurotransmitter release rate was estimated via FM 1-43 destaining from CA3 terminals in hippocampal slices from male Wistar rats (6–8 weeks). Destaining was measured at sites making putative contact with CA1 dendritic processes in stratum radiatum that had been filled with a membrane impermeable translation inhibitor and a fluorescent indicator. Our results suggest that inhibition of postsynaptic translation eliminates the enhanced release ordinarily observed at 160 min post-LTP induction, and that this effect is limited to sites closely apposed to the filled postsynaptic cell. We conclude that postsynaptic translation is required for the presynaptic component of LTP2 and LTP3 expression. These data considerably strengthen the mechanistic separation of LTP1, 2, and 3 and provide evidence for an expanded repertoire of communication between synaptic elements.

Role of reuniens nucleus projections to the medial prefrontal cortex and to the hippocampal pyramidal CA1 area in associative learning

We studied the interactions between short- and long-term plastic changes taking place during the acquisition of a classical eyeblink conditioning and following high-frequency stimulation (HFS) of the reuniens nucleus in behaving mice. Synaptic changes in strength were studied at the reuniens-medial prefrontal cortex (mPFC) and the reuniens-CA1 synapses. Input/output curves and a paired-pulse study enabled determining the functional capabilities of the two synapses and the optimal intensities to be applied at the reuniens nucleus during classical eyeblink conditioning and for HFS applied to the reuniens nucleus. Animals were conditioned using a trace paradigm, with a tone as conditioned stimulus (CS) and an electric shock to the trigeminal nerve as unconditioned stimulus (US). A single pulse was presented to the reuniens nucleus to evoke field EPSPs (fEPSPs) in mPFC and CA1 areas during the CS-US interval. No significant changes in synaptic strength were observed at the reuniens-mPFC and reuniens-CA1 synapses during the acquisition of eyelid conditioned responses (CRs). Two successive HFS sessions carried out during the first two conditioning days decreased the percentage of CRs, without evoking any long-term potentiation (LTP) at the recording sites. HFS of the reuniens nucleus also prevented the proper acquisition of an object discrimination task. A subsequent study revealed that HFS of the reuniens nucleus evoked a significant decrease of paired-pulse facilitation. In conclusion, reuniens nucleus projections to prefrontal and hippocampal circuits seem to participate in the acquisition of associative learning through a mechanism that does not required the development of LTP.

Regulation of GABA(A) and glutamate receptor expression, synaptic facilitation and long-term potentiation in the hippocampus of prion mutant mice

Background

Prionopathies are characterized by spongiform brain degeneration, myoclonia, dementia, and periodic electroencephalographic (EEG) disturbances. The hallmark of prioniopathies is the presence of an abnormal conformational isoform (PrP^sc) of the natural cellular prion protein (PrP^c) encoded by the Prnp gene. Although several roles have been attributed to PrP^c, its putative functions in neuronal excitability are unknown. Although early studies of the behavior of Prnp knockout mice described minor changes, later studies report altered behavior. To date, most functional PrP^c studies on synaptic plasticity have been performed in vitro. To our knowledge, only one electrophysiological study has been performed in vivo in anesthetized mice, by Curtis and coworkers. They reported no significant differences in paired-pulse facilitation or LTP in the CA1 region after Schaffer collateral/commissural pathway stimulation.

Methodology/Principal Findings

Here we explore the role of PrP^c expression in neurotransmission and neural excitability using wild-type, Prnp −/− and PrP^c-overexpressing mice (Tg20 strain). By correlating histopathology with electrophysiology in living behaving mice, we demonstrate that both Prnp −/− mice but, more relevantly Tg20 mice show increased susceptibility to KA, leading to significant cell death in the hippocampus. This finding correlates with enhanced synaptic facilitation in paired-pulse experiments and hippocampal LTP in living behaving mutant mice. Gene expression profiling using Illumina™ microarrays and Ingenuity pathways analysis showed that 129 genes involved in canonical pathways such as Ubiquitination or Neurotransmission were co-regulated in Prnp −/− and Tg20 mice. Lastly, RT-qPCR of neurotransmission-related genes indicated that subunits of GABA_A and AMPA-kainate receptors are co-regulated in both Prnp −/− and Tg20 mice.

Conclusions/Significance

Present results demonstrate that PrP^c is necessary for the proper homeostatic functioning of hippocampal circuits, because of its relationships with GABA_A and AMPA-Kainate neurotransmission. New PrP^c functions have recently been described, which point to PrP^c as a target for putative therapies in Alzheimer's disease. However, our results indicate that a “gain of function” strategy in Alzheimer's disease, or a “loss of function” in prionopathies, may impair PrP^c function, with devastating effects. In conclusion, we believe that present data should be taken into account in the development of future therapies.

Differing presynaptic contributions to LTP and associative learning in behaving mice

The hippocampal CA3-CA1 synapse is an excellent experimental model for studying the interactions between short- and long-term plastic changes taking place following high-frequency stimulation (HFS) of Schaffer collaterals and during the acquisition and extinction of a classical eyeblink conditioning in behaving mice. Input/output curves and a full-range paired-pulse study enabled determining the optimal intensities and inter-stimulus intervals for evoking paired-pulse facilitation (PPF) or depression (PPD) at the CA3-CA1 synapse. Long-term potentiation (LTP) induced by HFS lasted ≈10 days. HFS-induced LTP evoked an initial depression of basal PPF. Recovery of PPF baseline values was a steady and progressive process lasting ≈20 days, i.e., longer than the total duration of the LTP. In a subsequent series of experiments, we checked whether PPF was affected similarly during activity-dependent synaptic changes. Animals were conditioned using a trace paradigm, with a tone as a conditioned stimulus (CS) and an electrical shock to the trigeminal nerve as an unconditioned stimulus (US). A pair of pulses (40 ms interval) was presented to the Schaffer collateral-commissural pathway to evoke field EPSPs (fEPSPs) during the CS-US interval. Basal PPF decreased steadily across conditioning sessions (i.e., in the opposite direction to that during LTP), reaching a minimum value during the 10th conditioning session. Thus, LTP and classical eyeblink conditioning share some presynaptic mechanisms, but with an opposite evolution. Furthermore, PPF and PPD might play a homeostatic role during long-term plastic changes at the CA3-CA1 synapse.

A brief, but repeated, swimming protocol is sufficient to overcome amyloid beta-protein inhibition of hippocampal long-term potentiation

Alzheimer's disease starts as an almost imperceptible malady, first observed clinically as a mild memory problem. Accumulating genetic and biochemical data have suggested that amyloid beta-protein (Abeta) plays an important role in this memory loss, and Abeta has been shown to suppress long-term potentiation (LTP), a cellular model for memory and learning. Here we show that a very brief (3 min) swimming, twice daily for 2 weeks, rescues LTP inhibition in the CA1 region of hippocampal slices caused by Abeta(42) or Abeta(40) carrying the Arctic mutation using a theta burst stimulation (TBS) protocol. Whereas the input-output curve was not affected, the paired-pulse ratio was reduced in mice receiving our repeated swimming protocol, suggesting a possible involvement of presynaptic facilitation. Similar to swimming, Abeta's inhibition of LTP could be rescued with the adenylyl cyclase, forskolin. Interestingly, this swimming protocol produced conditions in which a weak-TBS could invoke LTP not observed in naïve mice, which again was mimicked by forskolin. In contrast, the protein kinase A (PKA) inhibitor, H89, blocked both the forskolin and swimming potentiation of LTP; these data implicate cAMP/PKA signaling in the protective effect of swimming and mediating Abeta' detrimental effects. Our data add a new simple behavior paradigm that shows the importance of an environmental factor in reversing the pathophysiological effects of Abeta, and suggest new therapeutic avenues.

Postsynaptic depolarisation enhances transmitter release and causes the appearance of responses at “silent” synapses in rat hippocampus

Recent data indicate that most "silent" synapses in the hippocampus are "presynaptically silent" due to low transmitter release rather than "postsynaptically silent" due to "latent" receptors of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid type (AMPARs). That synapses bearing only N-methyl-d-aspartate (NMDAR) receptors do exist is suggested by the decreased number of transmission failures during postsynaptic depolarisation and by the presence of NMDA-mediated excitatory postsynaptic currents (EPSCs) in synapses silent at rest. We tested whether these effects could be due to potentiated transmitter release at depolarised postsynaptic potentials rather than removal of Mg(2+) block from NMDARs. Using whole-cell recordings of minimal EPSCs from CA1 and CA3 neurones of hippocampal slices we confirmed decreased incidence of failures at +40 mV as compared with -60 mV. This effect was associated with a gradual increase of EPSC amplitude after switching to +40 mV and with a decrease of paired-pulse facilitation. In initially silent synapses, potentiation of pharmacologically isolated AMPAR-mediated EPSCs was still observed at +40 mV and this persisted after stepping back to -60 mV. All above effects were blocked when the cell was dialysed with the Ca(2+) chelator BAPTA (20 mM). These observations are difficult to reconcile with the "latent AMPAR" hypothesis and suggest an alternative explanation, namely that the reduction in failure rates at positive potentials is due to potentiation of transmitter release following Ca(2+) influx through NMDARs. Our results suggest that silent synapses can be mainly "presynaptically" rather than "postsynaptically silent" and thus increased transmitter release rather than insertion of AMPARs is a major mechanism of early long-term potentiation maintenance.

Synapse-specific mGluR1-dependent long-term potentiation in interneurones regulates mouse hippocampal inhibition

Hippocampal CA1 inhibitory interneurones control the excitability and synchronization of pyramidal cells, and participate in hippocampal synaptic plasticity. Pairing theta-burst stimulation (TBS) with postsynaptic depolarization, we induced long-term potentiation (LTP) of putative single-fibre excitatory postsynaptic currents (EPSCs) in stratum oriens/alveus (O/A) interneurones of mouse hippocampal slices. LTP induction was absent in metabotropic glutamate receptor 1 (mGluR1) knockout mice, was correlated with the postsynaptic presence of mGluR1a, and required a postsynaptic Ca2+ rise. Changes in paired-pulse facilitation and coefficient of variation indicated that LTP expression involved presynaptic mechanisms. LTP was synapse specific, occurring selectively at synapses modulated by presynaptic group II, but not group III, mGluRs. Furthermore, the TBS protocol applied in O/A induced a long-term increase of polysynaptic inhibitory responses in CA1 pyramidal cells, that was absent in mGluR1 knockout mice. These results uncover the mechanisms of a novel form of interneurone synaptic plasticity that can adaptively regulate inhibition of hippocampal pyramidal cells.

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.

Long-term potentiation recruits a trisynaptic excitatory associative network within the mouse dentate gyrus

Granule cells of the hippocampal dentate gyrus receive two powerful excitatory inputs: the perforant path, originating from the entorhinal cortex, and the associational pathway, originating from mossy cells, the principal neurons of the dentate gyrus hilus. We examined the electrophysiological properties of the less well-studied associational pathway and its interaction with the perforant path in the intact mouse hippocampus and then tested homosynaptic, trans-synaptic and associative long-term potentiation of these pathways. The associational pathway was either monosynaptically activated by stimulation within the inner molecular layer or trisynaptically activated after stimulation of the perforant path. Laminar profiles of extracellularly recorded associational pathway field potentials demonstrated a bell-shaped curve with a peak in the inner molecular layer. Tetanization of the perforant path induced not only homosynaptic potentiation of the perforant path (162.4 +/- 6.7% at 0.5-1.5 h after tetanus) but also heterosynaptic potentiation of the associational pathway (115.7 +/- 4.9%). Direct tetanization of the associational pathway within the inner molecular layer was ineffective in either the septo-temporal (97.2 +/- 4.5%) or temporal-septal (104.4 +/- 4.6%) direction. In contrast, conjoint tetanization of the associational pathway with the perforant path potentiated the associational pathway responses in both the septo-temporal (123.4 +/- 5.8%) and the temporal-septal (124.8 +/- 7.3%) directions. Paired-pulse facilitation was attenuated by long-term potentiation in the perforant path and the associational pathway, suggesting pre-synaptic involvement. These results demonstrate that long-term potentiation of the associational pathway and the perforant path is a product of the network properties of the dentate gyrus rather than of each monosynaptic input alone. The architecture of this neural network may be designed for flexible dynamic associations of the afferent perforant path inputs to configure encoded information within hippocampal neuronal ensembles.

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.

Ryanodine receptors contribute to cGMP-induced late-phase LTP and CREB phosphorylation in the hippocampus

We previously found that the nitric oxide (NO)-cGMP-cGMP-dependent protein kinase (PKG) signaling pathway acts in parallel with the cAMP-cAMP-dependent protein kinase (PKA) pathway to produce protein and RNA synthesis-dependent late-phase long-term potentiation (L-LTP) and cAMP response element-binding protein (CREB) phosphorylation in the CA1 region of mouse hippocampus. We have now investigated the possible involvement of a downstream target of PKG, ryanodine receptors. L-LTP can be induced by either multiple-train tetanization, NO or 8-Br-cGMP paired with one-train tetanization, or the cAMP activator forskolin, and all three types of potentiation are accompanied by an increase in phospho-CREB immunofluorescence in the CA1 cell body area. Both the potentiation and the increase in phospho-CREB immunofluorescence induced by multiple-train tetanization or 8-Br-cGMP paired with one-train tetanization are reduced by prolonged perfusion with ryanodine, which blocks Ca(2+) release from ryanodine-sensitive Ca(2+) stores. By contrast, neither the potentiation nor the increase in immunofluorescence induced by forskolin are reduced by depletion of ryanodine and inositol-1,4,5-triphosphate (IP3)-sensitive Ca(2+) stores. These results suggest that NO, cGMP, and PKG cause release of Ca(2+) from ryanodine-sensitive stores, which in turn causes phosphorylation of CREB in parallel with PKA during the induction of L-LTP.

Comparison of paired-pulse facilitation of AMPA and NMDA synaptic currents in the lateral amygdala

Stimulating thalamic fibers exiting from the internal capsule evokes a glutamatergic excitatory postsynaptic current (EPSC) recorded in vitro with patch electrodes in neurons of the rat lateral amygdala (LA). The purpose of this study is to compare paired-pulse facilitation (PPF), a form of short-term synaptic plasticity, of AMPA and NMDA receptor-mediated EPSCs. Analysis of PPF at this synapse is important since, in fear-conditioned animals, PPF reflects an enhanced transmitter release but the amplitude of only AMPA EPSCs is facilitated. PPF magnitude of the composite EPSC is a result of both AMPA and NMDA receptor activation; however, the characteristics of AMPA and NMDA PPF are dissimilar. Specifically, the NMDA EPSC shows greater PPF (NMDA PPF) than does the AMPA EPSC whether measuring the NMDA PPF magnitude in an AMPA antagonist/Mg(2+)-free solution or by subtracting the AMPA EPSC from the composite EPSC in normal Mg(2+). Presynaptic NMDA receptors neither influence AMPA PPF nor account for the difference between the NMDA and AMPA PPF. Another difference was that removal of inhibitory tone enhanced AMPA PPF, while it had mixed effects on NMDA PPF. Furthermore, AMPA PPF was independent of stimulus intensity and postsynaptic voltage, unlike the NMDA PPF. Another dissimilarity was that the amplitudes of pairs of AMPA EPSCs were not correlated, suggesting presynaptic mechanisms. In contrast, NMDA PPF was dependent on stimulus intensity and postsynaptic voltage and the amplitudes of paired NMDA EPSCs had a positive correlation, suggesting a postsynaptic influence. Both AMPA and NMDA PPF were influenced by GABA inhibition and this could be a factor in the magnitude disparity. These data show that AMPA and NMDA PPF have different characteristics and contribute to the composite PPF in the thalamic to lateral amygdala pathway.

A persistent reduction in short-term facilitation accompanies long-term potentiation in the CA1 area in the intact hippocampus

Exploration of the nature of the relationship between short-term and long-term synaptic plasticity should aid our understanding of their roles in brain function. The effects of inducing long-term potentiation on short-term facilitation at CA1 synapses in the stratum radiatum of the intact hippocampus were examined by recording the slope of the field excitatory postsynaptic potential in both urethane and freely behaving adult rats. Facilitation of the second synaptic response to paired-pulse stimulation (40ms interstimulus interval) was monitored before and after the induction of long-term potentiation by high-frequency stimulation (10 trains of 20 pulses at 200Hz). The tetanus triggered a rapid overall reduction in paired-pulse facilitation that persisted for at least 2h. In the anaesthetized animals a detailed correlation analysis revealed that initial paired-pulse facilitation level correlated strongly with the subsequent reduction in paired-pulse facilitation and the magnitude of long-term potentiation. The reduction in paired-pulse facilitation also correlated with long-term potentiation magnitude. These relationships were not observed in animals with low initial degrees of paired-pulse facilitation. It was concluded that the relative contribution of different expression mechanisms of long-term potentiation varies depending on the initial facilitation characteristics of the synapses. Furthermore, the temporal selectivity and gain control of synapses can be altered persistently in the intact hippocampus. This suggests that there is considerable variation in the fidelity of temporal information storage at different synapses during learning and memory in the CA1 area.

Differences in amplitude-voltage relations between minimal and composite mossy fibre responses of rat CA3 hippocampal neurons support the existence of intrasynaptic ephaptic feedback in large synapses

Computer simulations and electrophysiological experiments have been performed to test the hypothesis on the existence of an ephaptic interaction in purely chemical synapses. According to this hypothesis, the excitatory postsynaptic current would depolarize the presynaptic release site and further increase transmitter release, thus creating an intrasynaptic positive feedback. For synapses with the ephaptic feedback, computer simulations predicted non-linear amplitude-voltage relations and voltage dependence of paired-pulse facilitation. The deviation from linearity depended on the strength of the feedback determined by the value of the synaptic cleft resistance. The simulations showed that, in the presence of the intrasynaptic feedback, recruitment of imperfectly clamped synapses and synapses with linear amplitude-voltage relations tended to reduce the non-linearity and voltage dependence of paired-pulse facilitation. Therefore, the simulations predicted that the intrasynaptic feedback would particularly affect small excitatory postsynaptic currents induced by activation of electrotonically close synapses with long synaptic clefts. In electrophysiological experiments performed on hippocampal slices, the whole-cell configuration of the patch-clamp technique was used to record excitatory postsynaptic currents evoked in CA3 pyramidal cells by activation of large mossy fibre synapses. In accordance with the simulation results, minimal excitatory postsynaptic currents exhibited "supralinear" amplitude-voltage relations at hyperpolarized membrane potentials, decreases in the failure rate and voltage-dependent paired-pulse facilitation. Composite excitatory postsynaptic currents evoked by activation of a large amount of presynaptic fibres typically bear linear amplitude-voltage relationships and voltage-independent paired-pulse facilitation. These data are consistent with the hypothesis on a strong ephaptic feedback in large mossy fibre synapses. The feedback would provide a mechanism whereby signals from large synapses would be amplified. The ephaptic feedback would be more effective on synapses activated in isolation or together with electrotonically remote inputs. During synchronous activation of a large number of neighbouring inputs, suppression of the positive intrasynaptic feedback would prevent abnormal boosting of potent signals.

Functional state of corticostriatal synapses determines their expression of short- and long-term plasticity

Relationships between presynaptic function and short- and long-term plasticity were investigated at adult corticostriatal synapses. Wide variability was observed in the expression of short- and long-term synaptic plasticity. Intracellular records from 47 cells produced 17 examples of LTD (<90% of control), 10 examples of no long-term change (between 90-110% of control), and 20 examples of LTP (>110% of control). Similar variation existed in paired-pulse and posttetanic plasticities. The variability expressed in all three forms of plasticity appears to be related, based on correlations found between the paired-pulse ratio (PPR) and tetanus-induced short- (3 min posttetanus) and long-term plasticities (16-20 min posttetanus). These data suggest that tetanus-induced changes in synaptic strength are related to the intrinsic, preconditioned behavior of synapses. Every cell showing paired-pulse depression also expressed LTD in response to high-frequency activation of its afferents. Those synapses showing paired-pulse potentiation tended to express LTP, although exceptions did exist. Similar relationships were found in a parallel analysis of population spikes. PPR also changed in association with the expression of posttetanic and long-term depression. Greater paired-pulse potentiation was observed in medial intracellular recordings, but no medial to lateral differences were seen in posttetanic plasticities. Field recordings also showed a medial bias toward paired-pulse and posttetanic potentiation, but not in long-term plasticity. Block of postsynaptic L-type Ca(2+) channels with nifedipine eliminated LTD expression, but overall no differences were found between nifedipine and control cells.

Postsynaptic hyperpolarization increases the strength of AMPA-mediated synaptic transmission at large synapses between mossy fibers and CA3 pyramidal cells

In chemical synapses information flow is polarized. However, the postsynaptic cells can affect transmitter release via retrograde chemical signaling. Here we explored the hypothesis that, in large synapses, having large synaptic cleft resistance, transmitter release can be enhanced by electrical (ephaptic) signaling due to depolarization of the presynaptic release site induced by the excitatory postsynaptic current itself. The hypothesis predicts that, in such synapses, postsynaptic hyperpolarization would increase response amplitudes "supralinearly", i.e. stronger than predicted from the driving force shift. We found supralinear increases in the amplitude of minimal excitatory postsynaptic potential (EPSP) during hyperpolarization of CA3 pyramidal neurons. Failure rate, paired-pulse facilitation, coefficient of variation of the EPSP amplitude and EPSP quantal content were also modified. The effects were especially strong on mossy fiber EPSPs (MF-EPSPs) mediated by the activation of large synapses and identified pharmacologically or by their kinetics. The effects were weaker on commissural fiber EPSPs mediated by smaller and more remote synapses. Even spontaneous membrane potential fluctuations were associated with supralinear MF-EPSP increases and failure rate reduction. The results suggest the existence of a novel mechanism for retrograde control of synaptic efficacy from postsynaptic membrane potential and are consistent with the ephaptic feedback hypothesis.

Intrasynaptic ephaptic feedback in central synapses

Electrophysiological laboratory studies on rat visual cortex and hippocampus slices are reviewed. The aim was to confirm the existence of positive feedback in central synapses operating by an electrical (ephaptic) mechanism, as suggested by Byzov. Byzov's hypothesis holds that artificial hyperpolarization of the postsynaptic membrane potential should increase the amplitude of the excitatory postsynaptic current (EPSC) and potential (EPSP) in some central synapses not only by means of increases in the electromotive force (EMF). but also by means of increases in transmitter release from the presynaptic apparatus. Some experiments showed that hyperpolarization altered the parameters of presynaptic transmitter release, i.e., the quantity of "failed" responses N0, the coefficient of variation CV, and the quantum composition m of minimal EPSC and EPSP. The effect was particularly marked for EPSP in giant synapses formed by mossy fibers on neurons in field CA3. "Supralinear" functions were observed for these synapses in the relationship between EPSC amplitude and membrane potential in conditions of hyperpolarization of membrane potentials and in the relationship between presynaptic paired-stimulus facilitation and membrane potential. All of these "non-classical" effects disappeared when summed rather than minimal EPSC were evoked. The results are in agreement with computer experiments based on the Byzov model and are regarded as support for Byzov's hypothesis. Regardless of their explanation, the data obtained here demonstrate a new feedback mechanism for central synapses, which allows the postsynaptic neuron to control the efficiency of some synapses via changes in membrane potential. This mechanism can significantly increase the efficiency of large ("perforated") synapses and explains the increase in the number of this type of synapse after various experimental manipulations, such as those inducing long-term potentiation or forming conditioned reflexes.

Long-term synaptic changes induced by intracellular tetanization of CA3 pyramidal neurons in hippocampal slices from juvenile rats

Minimal excitatory postsynaptic potentials were evoked in CA3 pyramidal neurons by activation of the mossy fibres in hippocampal slices from seven- to 16-day-old rats. Conditioning intracellular depolarizing pulses were delivered as 50- or 100-Hz bursts. A statistically significant depression and potentiation was induced in four and five of 13 cases, respectively. The initial state of the synapses influenced the effect: the amplitude changes correlated with the pretetanic paired-pulse facilitation ratio. Afferent (mossy fibre) tetanization produced a significant depression in four of six inputs, and no significant changes in two inputs. Quantal content decreased or increased following induction of the depression or potentiation, respectively, whereas no significant changes in quantal size were observed. Compatible with presynaptic maintenance mechanisms of both depression and potentiation, changes in the mean quantal content were associated with modifications in the paired-pulse facilitation ratios, coefficient of variation of response amplitudes and number of response failures. Cases were encountered when apparently "presynaptically silent" synapses were converted into functional synapses during potentiation or when effective synapses became "presynaptically silent" when depression was induced, suggesting respective changes in the probability of transmitter release. It is concluded that, in juvenile rats, it is possible to induce lasting potentiation at the mossy fibre-CA3 synapses by purely postsynaptic stimulation, while afferent tetanization is accompanied by long-lasting depression. The data support the existence not only of a presynaptically induced, but also a postsynaptically induced form of long-term potentiation in the mossy fibre-CA3 synapse. Despite a postsynaptic induction mechanism, maintenance of both potentiation and depression is likely to occur presynaptically.

Evidence for an ephaptic feedback in cortical synapses: postsynaptic hyperpolarization alters the number of response failures and quantal content

The amplitude of excitatory postsynaptic potentials and currents increases with membrane potential hyperpolarization. This has been attributed to an increase in the driving force when the membrane potential deviates from the equilibrium potential of the respective ions. Here we report that in a subset of neocortical and hippocampal synapses, postsynaptic hyperpolarization affects traditional measures of transmitter release: the number of failures, coefficient of variation of response amplitudes, and quantal content, suggesting increased presynaptic release. The result is compatible with the hypothesis of Byzov on the existence of electrical (or "ephaptic") linking in purely chemical synapses. The linking, although negligible at neuromuscular junctions, could be functionally significant in influencing transmitter release at synapses with high resistance along the synaptic cleft. Our findings necessitate reconsideration of classical amplitude-voltage relations for such synapses. Thus, synaptic strength may be enhanced by hyperpolarization of the postsynaptic membrane potential. The positive ephaptic feedback could account for "all-or-none" excitatory postsynaptic potentials at some cortical synapses, large evoked and spontaneous multiquantal events and a high efficacy of large "perforated" synapses whose number increases following behavioural learning or the induction of long-term potentiation.

Hippocampal long-term potentiation preserves the fidelity of postsynaptic responses to presynaptic bursts

Hippocampal cells often fire prolonged bursts of action potentials, resulting in dynamic modulation of postsynaptic responses; yet long-term potentiation (LTP) has routinely been studied using only single presynaptic stimuli given at low frequency. Recent work on neocortical synapses has suggested that LTP may cause a "redistribution of synaptic strength" in which synaptic responses to the first stimulus of a presynaptic burst of action potentials are potentiated with later responses depressed. We have examined whether this redistribution occurs at hippocampal synapses during LTP. Using prolonged bursts that result in maximal short-term depression of later responses within the burst, we found that LTP resulted in a uniform potentiation of individual responses throughout the burst rather than a redistribution of synaptic strength. This occurred both at Schaffer collateral-CA1 synapses and at CA3-CA3 synapses, the latter being activated and monitored using paired recordings. Thus in the hippocampus, LTP preserves the fidelity of postsynaptic responses to presynaptic bursts by a uniform increase rather than a redistribution of synaptic strength, a finding that suggests there are important differences between neocortex and hippocampus in how long-term changes in synaptic strength are used to encode new information.

On the Respective Roles of Nitric Oxide and Carbon Monoxide in Long-Term Potentiation in the Hippocampus

Perfusion of hippocampal slices with an inhibitor nitric oxide (NO) synthase blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

On the Respective Roles of Nitric Oxide and Carbon Monoxide in Long-Term Potentiation in the Hippocampus

Perfusion of hippocampal slices with an inhibitor of nitric oxide (NO) synthase-blocked induction of long-term potentiation (LTP) produced by a one-train tetanus and significantly reduced LTP by a two-train tetanus, but only slightly reduced LTP by a four-train tetanus. Inhibitors of heme oxygenase, the synthetic enzyme for carbon monoxide (CO), significantly reduced LTP by either a two-train or four-train tetanus. These results suggest that NO and CO are both involved in LTP but may play somewhat different roles. One possibility is that NO serves a phasic, signaling role, whereas CO provides tonic, background stimulation. Another possibility is that NO and CO are phasically activated under somewhat different circumstances, perhaps involving different receptors and second messengers. Because NO is known to be activated by stimulation of NMDA receptors during tetanus, we investigated the possibility that CO might be activated by stimulation of metabotropic glutamate receptors (mGluRs). Consistent with this idea, long-lasting potentiation by the mGluR agonist tACPD was blocked by inhibitors of heme oxygenase but not NO synthase. Potentiation by tACPD was also blocked by inhibitors of soluble guanylyl cyclase (a target of both NO and CO) or cGMP-dependent protein kinase, and guanylyl cyclase was activated by tACPD in hippocampal slices. However, biochemical assays indicate that whereas heme oxygenase is constitutively active in hippocampus, it does not appear to be stimulated by either tetanus or tACPD. These results are most consistent with the possibility that constitutive (tonic) rather than stimulated (phasic) heme oxygenase activity is necessary for potentiation by tetanus or tACPD, and suggest that mGluR activation stimulates guanylyl cyclase phasically through some other pathway.

Principal component analysis of minimal excitatory postsynaptic potentials

'Minimal' excitatory postsynaptic potentials (EPSPs) are often recorded from central neurones, specifically for quantal analysis. However the EPSPs may emerge from activation of several fibres or transmission sites so that formal quantal analysis may give false results. Here we extended application of the principal component analysis (PCA) to minimal EPSPs. We tested a PCA algorithm and a new graphical 'alignment' procedure against both simulated data and hippocampal EPSPs. Minimal EPSPs were recorded before and up to 3.5 h following induction of long-term potentiation (LTP) in CA1 neurones. In 29 out of 45 EPSPs, two (N=22) or three (N=7) components were detected which differed in latencies, rise time (Trise) or both. The detected differences ranged from 0.6 to 7.8 ms for the latency and from 1.6-9 ms for Trise. Different components behaved differently following LTP induction. Cases were found when one component was potentiated immediately after tetanus whereas the other with a delay of 15-60 min. The immediately potentiated component could decline in 1-2 h so that the two components contributed differently into early (< 1 h) LTP1 and later (1-4 h) LTP2 phases. The noise deconvolution techniques was applied to both conventional EPSP amplitudes and scores of separate components. Cases are illustrated when quantal size (upsilon) estimated from the EPSP amplitudes increased whereas upsilon estimated from the component scores was stable during LTP1. Analysis of component scores could show apparent double-fold increases in upsilon which are interpreted as reflections of synchronized quantal releases. In general, the results demonstrate PCA applicability to separate EPSPs into different components and its usefulness for precise analysis of synaptic transmission.