Synaptic release rather than failure in the conditioning pulse results in paired-pulse facilitation during minimal synaptic stimulation in the rat hippocampal CA1 neurones

Early life GABAA blockade alters the synaptic plasticity and cognitive functions in male and female rats

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in adults, has a critical contribution to balanced excitatory-inhibitory networks in the brain. Alteration in depolarizing action of GABA during early life is connected to a wide variety of neurodevelopmental disorders. Additionally, the effects of postnatal GABA blockade on neuronal synaptic plasticity are not known and therefore, we set out to determine whether postnatal exposure to bicuculline, a competitive antagonist of GABA_A receptors, affects electrophysiologic changes in hippocampal CA1 neurons later on. To this end, male and female Wistar rats received vehicle or bicuculline (300 μg/kg) on postnatal days (PNDs) 7, 9 and 11, and then underwent different behavioral and electrophysiological examinations in adulthood. Postnatal exposure to bicuculline did not affect basic synaptic transmission but led to a pronounced decrease in paired-pulse facilitation (PPF) in CA1 pyramidal neurons. Bicuculline treatment also attenuated the long-term potentiation (LTP) and long-term depression (LTD) of CA1 neurons accompanied by decreased theta-burst responses in male and female adult rats. These electrophysiology findings together with the reduced brain-derived neurotrophic factor (BDNF) levels in the hippocampus and prefrontal cortex reliably explain the disturbance in spatial reference and working memories of bicuculline-treated animals. This study suggests that postnatal GABA_A blockade deteriorates short- and long-term synaptic plasticity of hippocampal CA1 neurons and related encoding of spatial memory in adulthood.

Neonatal NMDA blockade alters the LTP, LTD and cognitive functions in male and female Wistar rats

There is compelling evidence that neonatal blockade of NMDA receptors by phencyclidine (PCP) is associated with cognitive impairment in adulthood but little is known about the effects of early life PCP treatment on synaptic function later in life. Here, we sought to determine whether early life exposure to PCP alters the electrophysiologic function of hippocampal CA1 neurons in adult rats. To this end, male and female Wistar rats received either saline or PCP (10 mg/kg) on postnatal days (PND) 7, 9, and 11, and then underwent separate behavioral and electrophysiology tests in adulthood. Neonatal PCP treatment did not alter basic synaptic transmission and had only a modest effect on frequency following (FF) capacity but significantly decreased the paired-pulse facilitation (PPF) in the Schaffer collateral (SC)-CA1 pathway. We found that PCP treatment significantly attenuated the long-term potentiation (LTP) and long-term depression (LTD) in CA1 neurons accompanied by pronounced alteration in complex response profile in adult rats. The electrophysiology data were comparable in male and female rats and reliably associated with impaired spatial reference and working memories in these animals. Overall, this study suggests that blockade of NMDA receptors during early life deteriorates the short-term and long-term synaptic plasticity and complex response profile of CA1 neurons in adulthood.

Neuronal hibernation following hippocampal demyelination

Cognitive dysfunction occurs in greater than 50% of individuals with multiple sclerosis (MS). Hippocampal demyelination is a prominent feature of postmortem MS brains and hippocampal atrophy correlates with cognitive decline in MS patients. Cellular and molecular mechanisms responsible for neuronal dysfunction in demyelinated hippocampi are not fully understood. Here we investigate a mouse model of hippocampal demyelination where twelve weeks of treatment with the oligodendrocyte toxin, cuprizone, demyelinates over 90% of the hippocampus and causes decreased memory/learning. Long-term potentiation (LTP) of hippocampal CA1 pyramidal neurons is considered to be a major cellular readout of learning and memory in the mammalian brain. In acute slices, we establish that hippocampal demyelination abolishes LTP and excitatory post-synaptic potentials of CA1 neurons, while pre-synaptic function of Schaeffer collateral fibers is preserved. Demyelination also reduced Ca²⁺-mediated firing of hippocampal neurons in vivo. Using three-dimensional electron microscopy, we investigated the number, shape (mushroom, stubby, thin), and post-synaptic densities (PSDs) of dendritic spines that facilitate LTP. Hippocampal demyelination did not alter the number of dendritic spines. Surprisingly, dendritic spines appeared to be more mature in demyelinated hippocampi, with a significant increase in mushroom-shaped spines, more perforated PSDs, and more astrocyte participation in the tripartite synapse. RNA sequencing experiments identified 400 altered transcripts in demyelinated hippocampi. Gene transcripts that regulate myelination, synaptic signaling, astrocyte function, and innate immunity were altered in demyelinated hippocampi. Hippocampal remyelination rescued synaptic transmission, LTP, and the majority of gene transcript changes. We establish that CA1 neurons projecting demyelinated axons silence their dendritic spines and hibernate in a state that may protect the demyelinated axon and facilitates functional recovery following remyelination.

Prophylactic liraglutide treatment prevents amyloid plaque deposition, chronic inflammation and memory impairment in APP/PS1 mice

Type 2 diabetes is a risk factor for Alzheimer's disease (AD). Previously, we have shown that the diabetes drug liraglutide is protective in middle aged and in old APP/PS1 mice. Here, we show that liraglutide has prophylactic properties. When injecting liraglutide once-daily ip. in two months old mice for 8 months, the main hallmarks of AD were much reduced. Memory formation in object recognition and Morris water maze were normalised and synapse loss and the loss of synaptic plasticity was prevented. In addition, amyloid plaque load, including dense core congophilic plaques, was much reduced. Chronic inflammation (activated microglia) was also reduced in the cortex, and neurogenesis was enhanced in the dentate gyrus. The results demonstrate that liraglutide may protect from progressive neurodegeneration that develops in AD. The drug is currently in clinical trials in patients with AD.

Ketones prevent oxidative impairment of hippocampal synaptic integrity through KATP channels

Dietary and metabolic therapies are increasingly being considered for a variety of neurological disorders, based in part on growing evidence for the neuroprotective properties of the ketogenic diet (KD) and ketones. Earlier, we demonstrated that ketones afford hippocampal synaptic protection against exogenous oxidative stress, but the mechanisms underlying these actions remain unclear. Recent studies have shown that ketones may modulate neuronal firing through interactions with ATP-sensitive potassium (K_ATP) channels. Here, we used a combination of electrophysiological, pharmacological, and biochemical assays to determine whether hippocampal synaptic protection by ketones is a consequence of K_ATP channel activation. Ketones dose-dependently reversed oxidative impairment of hippocampal synaptic integrity, neuronal viability, and bioenergetic capacity, and this action was mirrored by the K_ATP channel activator diazoxide. Inhibition of K_ATP channels reversed ketone-evoked hippocampal protection, and genetic ablation of the inwardly rectifying K⁺ channel subunit Kir6.2, a critical component of K_ATP channels, partially negated the synaptic protection afforded by ketones. This partial protection was completely reversed by co-application of the K_ATP blocker, 5-hydoxydecanoate (5HD). We conclude that, under conditions of oxidative injury, ketones induce synaptic protection in part through activation of K_ATP channels.

Lixisenatide, a drug developed to treat type 2 diabetes, shows neuroprotective effects in a mouse model of Alzheimer's disease

Type 2 diabetes is a risk factor for developing Alzheimer's disease (AD). In the brains of AD patients, insulin signalling is desensitised. The incretin hormone Glucagon-like peptide-1 (GLP-1) facilitates insulin signalling, and analogues such as liraglutide are on the market as treatments for type 2 diabetes. We have previously shown that liraglutide showed neuroprotective effects in the APPswe/PS1ΔE9 mouse model of AD. Here, we test the GLP-1 receptor agonist lixisenatide in the same mouse model and compare the effects to liraglutide. After ten weeks of daily i.p. injections with liraglutide (2.5 or 25 nmol/kg) or lixisenatide (1 or 10 nmol/kg) or saline of APP/PS1 mice at an age when amyloid plaques had already formed, performance in an object recognition task was improved in APP/PS1 mice by both drugs at all doses tested. When analysing synaptic plasticity in the hippocampus, LTP was strongly increased in APP/PS1 mice by either drug. Lixisenatide (1 nmol/kg) was most effective. The reduction of synapse numbers seen in APP/PS1 mice was prevented by the drugs. The amyloid plaque load and dense-core Congo red positive plaque load in the cortex was reduced by both drugs at all doses. The chronic inflammation response (microglial activation) was also reduced by all treatments. The results demonstrate that the GLP-1 receptor agonists liraglutide and lixisenatide which are on the market as treatments for type 2 diabetes show promise as potential drug treatments of AD. Lixisenatide was equally effective at a lower dose compared to liraglutide in some of the parameters measured.

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.

Liraglutide can reverse memory impairment, synaptic loss and reduce plaque load in aged APP/PS1 mice, a model of Alzheimer's disease

Type 2 diabetes is a risk factor in the development of Alzheimer's disease (AD). It has been shown that insulin signalling is desensitised in the brains of AD patients. The incretin hormone Glucagon-like peptide-1 (GLP-1) facilitates insulin signalling, and long-lasting analogues such as liraglutide (Victoza(®)) are on the market as type 2 diabetes treatments. We have previously shown that liraglutide improved cognitive function, reduced amyloid plaque deposition, inflammation, overall APP and oligomer levels and enhanced LTP when injected peripherally for two months in 7 month old APPswe/PS1ΔE9 (APP/PS1) mice. This showed that liraglutide has preventive effects at the early stage of AD development. The current study investigated whether Liraglutide would have restorative effects in late-stage Alzheimer's disease in mice. Accordingly, 14-month-old APP/PS1 and littermate control mice were injected with Liraglutide (25 nmol/kg bw) ip. for 2 months. Spatial memory was improved by Liraglutide-treatment in APP/PS1 mice compared with APP/PS1 saline-treated mice. Overall plaque load was reduced by 33%, and inflammation reduced by 30%, while neuronal progenitor cell count in the dentate gyrus was increased by 50%. LTP was significantly enhanced in APP/PS1 liraglutide-treated mice compared with APP/PS1 saline mice, corroborated with increased synapse numbers in hippocampus and cortex. Total brain APP and beta-amyloid oligomer levels were reduced in Liraglutide-treated APP/PS1 mice while IDE levels were increased. These results demonstrate that Liraglutide not only has preventive properties, but also can reverse some of the key pathological hallmarks of AD. Liraglutide is now being tested in clinical trials in AD patients. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Neuroprotective effects of D-Ala(2)GIP on Alzheimer's disease biomarkers in an APP/PS1 mouse model

Introduction

Type 2 diabetes mellitus has been identified as a risk factor for Alzheimer's disease (AD). An impairment of insulin signaling as well as a desensitization of its receptor has been found in AD brains. Glucose-dependent insulinotropic polypeptide (GIP) normalises insulin signaling by facilitating insulin release. GIP directly modulates neurotransmitter release, LTP formation, and protects synapses from the detrimental effects of beta-amyloid fragments on LTP formation, and cell proliferation of progenitor cells in the dentate gyrus. Here we investigate the potential therapeutic property of the new long lasting incretin hormone analogue D-Ala²GIP on key symptoms found in a mouse model of Alzheimer' disease (APPswe/PS1detaE9).

Methods

D-Ala²GIP was injected for 21 days at 25 nmol/kg ip once daily in APP/PS1 male mice and wild type (WT) littermates aged 6 or 12 months of age. Amyloid plaque load, inflammation biomarkers, synaptic plasticity in the brain (LTP), and memory were measured.

Results

D-Ala²GIP improved memory in WT mice and rescued the cognitive decline of 12 months old APP/PS1 mice in two different memory tasks. Furthermore, deterioration of synaptic function in the dentate gyrus and cortex was prevented in 12 months old APP/PS1 mice. D-Ala²GIP facilitated synaptic plasticity in APP/PS1 and WT mice and reduced the number of amyloid plaques in the cortex of D-Ala²GIP injected APP/PS1 mice. The inflammatory response in microglia was also reduced.

Conclusion

The results demonstrate that D-Ala²GIP has neuroprotective properties on key hallmarks found in AD. This finding shows that novel GIP analogues have the potential as a novel therapeutic for AD.

Short-term plasticity constrains spatial organization of a hippocampal presynaptic terminal

Although the CA3-CA1 synapse is critically important for learning and memory, experimental limitations have to date prevented direct determination of the structural features that determine the response plasticity. Specifically, the local calcium influx responsible for vesicular release and short-term synaptic facilitation strongly depends on the distance between the voltage-dependent calcium channels (VDCCs) and the presynaptic active zone. Estimates for this distance range over two orders of magnitude. Here, we use a biophysically detailed computational model of the presynaptic bouton and demonstrate that available experimental data provide sufficient constraints to uniquely reconstruct the presynaptic architecture. We predict that for a typical CA3-CA1 synapse, there are ~70 VDCCs located 300 nm from the active zone. This result is surprising, because structural studies on other synapses in the hippocampus report much tighter spatial coupling. We demonstrate that the unusual structure of this synapse reflects its functional role in short-term plasticity (STP).

Glucose-dependent insulinotropic polypeptide receptor knockout mice are impaired in learning, synaptic plasticity, and neurogenesis

Glucose-dependent insulinotropic polypeptide (GIP) is a key incretin hormone, released from intestine after a meal, producing a glucose-dependent insulin secretion. The GIP receptor (GIPR) is expressed on pyramidal neurons in the cortex and hippocampus, and GIP is synthesized in a subset of neurons in the brain. However, the role of the GIPR in neuronal signaling is not clear. In this study, we used a mouse strain with GIPR gene deletion (GIPR KO) to elucidate the role of the GIPR in neuronal communication and brain function. Compared with C57BL/6 control mice, GIPR KO mice displayed higher locomotor activity in an open-field task. Impairment of recognition and spatial learning and memory of GIPR KO mice were found in the object recognition task and a spatial water maze task, respectively. In an object location task, no impairment was found. GIPR KO mice also showed impaired synaptic plasticity in paired-pulse facilitation and a block of long-term potentiation in area CA1 of the hippocampus. Moreover, a large decrease in the number of neuronal progenitor cells was found in the dentate gyrus of transgenic mice, although the numbers of young neurons was not changed. Together the results suggest that GIP receptors play an important role in cognition, neurotransmission, and cell proliferation.

Val(8)GLP-1 rescues synaptic plasticity and reduces dense core plaques in APP/PS1 mice

Diabetes is a risk factor for Alzheimer's disease. We tested the effects of Val(8)GLP-1, an enzyme-resistant analogue of the incretin hormone glucagon-like peptide 1 originally developed to treat diabetes in a mouse model of Alzheimer's disease that expresses mutated amyloid precursor protein (APP) and presenilin-1. We tested long term potentiation (LTP) of synaptic plasticity, inflammation response, and plaque formation. Val(8)GLP-1 crosses the blood-brain barrier when administered via intraperitoneal injection. Val(8)GLP-1 protected LTP in 9- and 18-month-old Alzheimer's disease mice when given for 3 weeks at 25 nmol/kg intraperitoneally. LTP was also enhanced in 18-month-old wild type mice, indicating that Val(8)GLP-1 also ameliorates age-related synaptic degenerative processes. Paired-pulse facilitation was also enhanced. The number of beta-amyloid plaques and microglia activation in the cortex increased with age but was not reduced by Val(8)GLP-1. In 18-month-old mice, however, the number of Congo red positive dense-core amyloid plaques was reduced. Treatment with Val(8)GLP-1 might prevent or delay neurodegenerative processes.

Synaptic plasticity in the hippocampus of a APP/PS1 mouse model of Alzheimer's disease is impaired in old but not young mice

Background

Alzheimer disease (AD) is a neurodegenerative disorder for which there is no cure. We have investigated synaptic plasticity in area CA1 in a novel AD mouse model (APPPS1-21) which expresses the Swedish mutation of APP and the L166P mutation of human PS-1. This model shows initial plaque formation at 2 months in the neocortex and 4 months in the hippocampus and displays β−amyloid-associated pathologies and learning impairments.

Methodology/Principal Findings

We tested long-term potentiation (LTP) and short term potentiation (paired-pulse facilitation, PPF) of synaptic transmission in vivo in area CA1 of the hippocampus. There was no difference in LTP or PPF at 4–5 months of age in APPPS1-21 mice compared to littermate controls. At 6 months of age there was also no difference in LTP but APPPS1-21 mice showed slightly increased PPF (p<0.03). In 8 months old mice, LTP was greatly impaired in APPPS-21 animals (p<0.0001) while PPF was not changed. At 15 months of age, APPPS1-21 mice showed again impaired LTP compared to littermate controls (p<0.005), and PPF was also significantly reduced at 80 ms (p<0.005) and 160 ms (p<0.01) interstimulus interval. Immunohistological analysis showed only modest amyloid deposition in the hippocampus at 4 and 6 months with a robust increase up to 15 months of age.

Conclusions

Our results suggest that increased formation and aggregation of beta amyloid with aging is responsible for the impaired LTP with aging in this mouse model, while the transient increase of PPF at 6 months of age is caused by some other mechanism.

Determination of the extracellular basal levels of glutamate and GABA at dentate gyrus of streptozotocin-induced diabetic rats

Previous studies have indicated an association between diabetes mellitus and impairments in synaptic plasticity in the hippocampus. However, it is not clear if the impairments of synapses are pre- or post-synaptic or both. The aim of this study was to evaluate the extracellular basal levels of glutamate and GABA at dentate gyrus of anesthetized streptozotocin-induced diabetic rats, after 12 weeks of diabetes induction. Extracellular levels of glutamate and GABA were investigated by using the microdialysis technique coupled to high performance liquid chromatography (HPLC) with fluorescent detection. Experimental groups were the control group and the diabetes group. The results showed that glutamate levels were significantly decreased in diabetes group compared to the control group, while GABA levels showed no changes. The findings support the possibility that alterations in transmission may account, in part, for synaptic plasticity deficits induced in diabetes.

Release dependence to a paired stimulus at a synaptic release site with a small variable pool of immediately releasable vesicles

Monte Carlo simulations were performed on a release model based on experimental data from single glutamatergic synapses containing a single release site in the hippocampal CA1 region of the neonatal rat. These simulations explored what can be learned about the release process by examining how the release probability in response to the second stimulus (P(2)) of a paired stimulus to a synapse depends on the release in response to the first stimulus. Comparisons between experimental data from a number of individual synapses and the simulated data support the notion that the immediately releasable vesicle pool is small (approximately one) and shows substantial intertrial variation. The simulations also show that the release dependence of P(2) is not necessarily an indicator of either intertrial variation in Ca(2+) influx, feedback effects of released transmitter, or activation failure of the axon.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

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.

Calcium- and activity-dependent synaptic plasticity

Calcium ions play crucial signaling roles in many forms of activity-dependent synaptic plasticity. Recent presynaptic [Ca2+]i measurements and manipulation of presynaptic exogenous buffers reveal roles for residual [Ca2+]i following conditioning stimulation in all phases of short-term synaptic enhancement. Pharmacological manipulations implicate mitochondria in post-tetanic potentiation. New evidence supports an influence of Ca2+ in replacing depleted vesicles after synaptic depression. In addition, high-resolution measurements of [Ca2+]i in dendritic spines show how Ca2+ can encode the precise relative timing of presynaptic input and postsynaptic activity and generate long-term synaptic modifications of opposite polarity.

Long-term potentiation involves increases in the probability of neurotransmitter release

There is great interest in understanding the mechanisms of expression underlying long-term potentiation (LTP). They are agreed to involve an increase in synaptic efficacy, which is described by three multiplicative parameters: p, the probability of neurotransmitter release; n, the number of active release sites; and q, the postsynaptic unit response to transmitter release. We report three new lines of evidence suggesting that increases in p contribute to LTP expression. (i) When the contributions to LTP by p, n, and q are maximized, and p alone is decreased, another high-frequency stimulation elicits additional LTP. The additional potentiation is only associated with decreases in paired-pulse facilitation (PPF) suggesting an increase in p. (ii) There is an inverse relationship between baseline p [corrected] and the magnitude of LTP elicited, consistent with p [corrected] having more or less room to increase when p is smaller or greater. (iii) It has been shown that there is an inverse relationship between the magnitude of LTP induced and the associated changes in PPF. Now I find that decreasing p before inducing LTP moves the set-point for measuring those changes in PPF from before to after p is decreased, which would only occur if p contributes to LTP. Three lines of evidence, then, suggest that increases in p contribute to LTP expression, which is consistent with a presynaptic contribution to LTP. These experiments do not address potential postsynaptic contributions.

Excitatory synaptic site heterogeneity during paired pulse plasticity in CA1 pyramidal cells in rat hippocampus in vitro

1. The properties of individual excitatory synaptic sites onto adult CA1 hippocampal neurons were investigated using paired pulse minimal stimulation and low noise whole-cell recordings. Non-NMDA receptor-mediated synaptic responses were isolated using a pharmacological blockade of NMDA and GABAA receptors. Amongst the twenty-five stationary ensembles there were twelve showing paired pulse potentiation, two showing paired pulse depression and eleven with no significant net change. The signal-to-noise ratio averaged 4.5:1. There was no correlation between the amplitude of the first and second responses after separation of failures: the percentage of failures averaged 33.6% for the conditioning pulse and 31.7% for the test pulse. 2. Site-directed Bayesian statistical analysis was developed to predict the likely number of activated synapses, synaptic response amplitudes, probability of release and intrinsic variation at each individual synaptic site. Extensive simulations showed the usefulness of this model and defined appropriate parameters. These simulations demonstrated only small errors in estimating parameters of data sets with a small number of sites (< 10) and similar characteristics to the physiological data sets. 3. Physiological ensembles showed between one and three synaptic sites, which exhibited a wide range of values for release probability (0.03-0.99), synaptic amplitudes (1.46-16.8 pA; approximately 62% coefficient of variation between sites) and intrinsic variation over time (approximately 36%). Paired pulse plasticity occurred primarily from alterations in the release probabilities but a few ensembles also showed small changes in site amplitude. Initial release probability correlated negatively with the degree of paired pulse potentiation. Whilst it was possible to use simple assumptions regarding site homogeneity (such as required for a binomial process) for 48% (12 out of 25) of the data sets, the Bayesian analysis was necessary to reveal the complex changes and heterogeneity that occurred in the other 52% of the data sets. The Bayesian site analysis robustly indicated the presence of considerable site heterogeneity, significant intrinsic site variation over time and changes in parameters at individual synaptic sites with plasticity.