Schaffer Collateral Research Articles

N-(5-Hydroxynicotinoil)-L-Glutamic Acid Calcium Salt Modifies Responses of Rat Hippocampal CA1 Pyramidal Neurons during Orthodromic Stimulation.

The study examined the effect of calcium salt of N-(5-hydroxynicotinoil)-L-glutamic acid (Ampasse preparation) on neuronal activity in hippocampal CA1 area evoked by stimulation of Schaffer collaterals at a rate of 1 Hz (30 impulses during 30 sec) in the surviving hippocampal slices of Wistar rats. The records of 1st and 30th orthodromic population spikes showed that Ampasse in concentrations of 500 μM, 1, 2, and 10 mM facilitated the synaptic transmission in Schaffer collaterals - hippocampal CA1 pyramidal neurons axis; the maximum effect was observed at 2 mM Ampasse. When used in a concentration of 10 mM, Ampasse provoked epileptiform activity, which could be prevented by MK-801, a specific noncompetitive antagonist of the NMDA-receptor complex.

Nano-CuO impairs spatial cognition associated with inhibiting hippocampal long-term potentiation via affecting glutamatergic neurotransmission in rats

Manufactured metal nanoparticles and their applications are continuously expanding because of their unique characteristics while their increasing use may predispose to potential health problems. Several studies have reported the adverse effects of copper oxide nanoparticles (nano-CuO) relative to ecotoxicity and cell toxicity, whereas little is known about the neurotoxicity of nano-CuO. The present study aimed to examine its effects on spatial cognition, hippocampal function, and the possible mechanisms. Male Wistar rats were used to establish an animal model, and nano-CuO was administered at a dose of 0.5 mg/kg/day for 2 weeks. The Morris water maze (MWM) test was employed to evaluate learning and memory. The long-term potentiation (LTP) from Schaffer collaterals to the hippocampal CA1 region, and the effects of nano-CuO on synases were recorded in the hippocampal CA1 neurons of rats. MWM test showed that learning and memory abilities were impaired significantly by nano-CuO ( p < 0.05). The LTP test demonstrated that the field excitatory postsynaptic potential (fEPSP) slopes were significantly lower in nano-CuO-treated groups compared with the control group ( p < 0.01). Furthermore, the data of whole-cell patch-clamp experiments showed that nano-CuO markedly depressed the frequencies of both spontaneous excitatory postsynaptic currents (sEPSCs) and miniature EPSCs (mEPSCs), indicating an effect of nano-CuO on inhibiting the release frequency of glutamate presynapticly ( p < 0.01). Meanwhile, the amplitudes of both sEPSC and mEPSC were significantly reduced in nano-CuO-treated animals, which suggested that the effect of nano-CuO modulates postsynaptic receptor kinetics ( p < 0.01). Paired pulse facilitation (PPF) ( p < 0.05) and the expression of NR2A, but not NR2B, of N-methyl-d-aspartate (NMDA) subunits ( p < 0.05), were decreased significantly. In conclusion, nano-CuO impaired glutamate transmission presynapticly and postsynapticly, which may contribute importantly to diminished LTP and other induced cognitive deficits.

Honokiol and Pioglitazone Ameliorate Alzheimer's Disease Pathologies in vitro and ex vivo

Amyloid beta (AB) plaques are extracellular, neurotoxic protein aggregations. These plaques have historically been considered the hallmark of Alzheimer's disease due to their ability to reduce long term potentiation (LTP) and promote long term depression (LTD). Peroxisome proliferator‐activated receptor gamma (PPARg) agonists have been shown to be a novel and promising class of drugs capable of alleviating AD pathologies. In this study, we observed the neurotoxic effects of amyloid beta on hippocampal field potentials on the Schaffer collateral. In addition, we displayed honokiol, a novel SIRT3 and partial PPARg agonist, and pioglitazone, a full PPARg agonist, is capable of improving mitochondrial function, increasing LTP, and reducing AB aggregation. The hippocampal field potential along the Schaffer collateral was measured by the LTP theta burst protocol, while signaling mechanisms were evaluated using western blot, RT‐qPCR, mitochondrial assays, and imaging techniques. We hypothesize and provide evidence that these compounds regulate mitochondria‐endoplasmic reticulum communication in order to modify AD pathologies providing evidence for the MAM hypothesis. Both compounds performed comparably suggesting honokiol may be a promising compound for the treatment of AD as its blood brain barrier (BBB) permeability is much higher than pioglitazone.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Shaker-related voltage-gated potassium channels Kv1 in human hippocampus.

In this study, we investigated the tissue expression levels, alpha subunit composition and distribution of Shaker-related voltage-dependent potassium Kv1 channels in human hippocampus by combining western blotting experiments, toxin autoradiography, in vivo radioligand binding studies, immunoprecipitation and immunohistochemistry. Tissue expression of Kv1.1 and Kv1.2 α-subunits in human post-mortem brain tissue was confirmed in immunoblot analysis using a panel of specific monoclonal and polyclonal antibodies. Immunoprecipitation experiments using toxin-prelabeled Kv1 channels revealed that all toxin-sensitive Kv1 channels in human hippocampus contained either a Kv1.1 or Kv1.2 α-subunit with the majority being composed of Kv1.1/Kv1.2 heterotetramers. Receptor autoradiography suggested Kv1.1/Kv1.2 channel expression in the molecular layer of dentate gyrus. In accordance, immunohistochemical experiments also observed Kv1.1 and Kv1.2 α-subunits in the molecular layer of the dentate gyrus, in addition to the CA3 stratum lucidum and the CA1 stratum oriens. These findings indicate expression in axons and terminals of hippocampal pathways, namely the perforant path, the mossy fiber pathway and the Schaffer collaterals. Herein we present the first direct demonstration that Kv1.1 and Kv1.2 channel proteins are targeted to distinct compartments of the human hippocampal formation and that this expression pattern largely reflects their distribution profile in murine brain.

Age-dependent changes in vesicular glutamate transporter1and2 expression in the gerbil hippocampus.

Glutamate is a major excitatory neurotransmitter that is stored in vesicles located in the presynaptic terminal. Glutamate is transported into vesicles via the vesicular glutamate transporter (VGLUT). In the present study, the age-associated changes of the major VGLUTs, VGLUT1 and VGLUT2, in the hippocampus were investigated, based on immunohistochemistry and western blot analysis at postnatal month 1 (PM1; adolescent), PM6, PM12 (adult group), PM18 and PM24 (the aged groups). VGLUT1 immunoreactivity was primarily detected in the mossy fibers, Schaffer collaterals and stratum lacunosum-moleculare. By contrast, VGLUT2 immunoreactivity was observed in the granule cell layer and the outer molecular layer of the dentate gyrus, stratum pyramidale, Schaffer collaterals and stratum lacunosum-moleculare in the hippocampal CA1-3 regions. VGLUT1 immunoreactivity and protein levels remained constant across all age groups. However, VGLUT2 immunoreactivity and protein levels decreased in the PM3 group when compared with the PM1 group. VGLUT2 immunoreactivity and protein levels were not altered in the PM12 group; however, they increased in the PM18 group. In addition, in the PM18 group, highly immunoreactive VGLUT2 cells were also identified in the stratum radiatum and oriens of the hippocampal CA1 region. In the PM24 group, VGLUT2 immunoreactivity and protein levels were significantly decreased and were the lowest levels observed amongst the different groups. These results suggested that VGLUT1 may be less susceptible to the aging process; however, the increase of VGLUT2 in the non-pyramidal cells in the PM18 group, and the consequent decrease in VGLUT2, may be closely linked to age-associated memory impairment in the hippocampus.

P.2.015 - The differential effects of short-term exposure to high fat diet on hippocampal function during juvenility and adulthood

Environmental and metabolic challenges during early life can have long-term devastating effects on cognitive and emotional functions. We recently showed in two lines of experiments that the juvenile brain is different from that of the adult brain; we showed that stress has opposite effects on prefrontal cortex plasticity and extinction [1]. Specifically, while stress results in impaired long-term potentiation (LTP) and extinction in the adult animal, it resulted in enhanced extinction and LTP in the juveniles. Similarly, exposure to long-term high fat diet (HFD) from juvenility until adulthood was associated with enhanced activation and plasticity in the amygdala and impaired activation and plasticity in the hippocampus, while similar exposure starting at adulthood was not associated with these effects [2]. Both stress and HFD are associated with activation of the hypothalamic-pituitary-adrenal (HPA) axis. Based on these findings we focused on the effects of short- term exposure to HFD on synaptic plasticity in Schaffer Collateral- CA1 pathway and hippocampal-dependent spatial memory in adults [post-natal day (PND 60)] and juveniles (PND 27). Rats were exposed to HFD for 7-10 days and CA1 LTP was examined or spatial recognition. In the spatial paradigm, two different objects were located in opposite corners; each rat was allowed to explore the objects for 5 min, 24h after the sample phase the rat was presented with the same two objects one of the objects was placed at a new location. A discrimination index calculated for each animal was expressed as TN/(TN+TF) (TN = time spent exploring the object in the novel location; TF = time spent exploring the object in the familiar location). In the last part of this work we examined the ability of the glucocorticoid receptor (GR) antagonist (RU486) to reverse the effects of the HFD on LTP in the CA1 pathway and on spatial memory. The main finding of this experiment was that short-term HFD deferentially modulates plasticity in the adult animals as compared to the juveniles. Specifically, in the Schaffer Collateral- CA1 pathway, in contrast to the adult animals where exposure to short-term HFD enhanced the induction of LTP; in the juvenile animals’ exposure to the same duration of HFD inhibited LTP in the CA1. Furthermore, corresponding to these results, short-term HFD enhanced spatial memory in adult animals while it abolished spatial memory in juvenile rats. In addition, after systemic application of GRs antagonist the effects of HFD were no longer observed; the antagonist abolished the induction of LTP in the HFD-fed adults while enhancing induction in HFD-fed juveniles and reversed the HFD effects on spatial memory.The results were presented as mean ± S.E.M. For statistical analysis, independent t-test and ANOVA for repeated measures tests were used as indicated. These results suggest developmental differences in the way short-term HFD affects CA1 plasticity and the spatial memory. These effects are differently modulated by GRs; inhibition of GRs normalized the potentiation in CA1. These effects of HFD may be relevant to the increase prevalence of emotional and behavioral disorders in children exposed to western diet.

High frequency stimulations change the phase-locking relationship between neuronal firing and the rhythms of field potentials

Deep brain stimulation (DBS) has been successfully used to treat a variety of brain diseases in clinic. Recent investigations have suggested that high frequency stimulation (HFS) of electrical pulses used by DBS might change pathological rhythms in action potential firing of neurons, which may be one of the important mechanisms of DBS therapy. However, experimental data are required to confirm the hypothesis. In the present study, 1 min of 100 Hz HFS was applied to the Schaffer collaterals of hippocampal CA1 region in anaesthetized rats. The changes of the rhythmic firing of action potentials from pyramidal cells and interneurons were investigated in the downstream CA1 region. The results showed that obvious θ rhythms were present in the field potential of CA1 region of the anesthetized rats. The θ rhythms were especially pronounced in the stratum radiatum. In addition, there was a phase-locking relationship between neuronal spikes and the θ rhythms. However, HFS trains significantly decreased the phase-locking values between the spikes of pyramidal cells and the θ rhythms in stratum radiatum from 0.36 ± 0.12 to 0.06 ± 0.04 ( P < 0.001, paired t-test, N = 8). The phase-locking values of interneuron spikes were also decreased significantly from 0.27 ± 0.08 to 0.09 ± 0.05 ( P < 0.01, paired t-test, N = 8). Similar changes were obtained in the phase-locking values between neuronal spikes and the θ rhythms in the pyramidal layer. These results suggested that axonal HFS could eliminate the phase-locking relationship between action potentials of neurons and θ rhythms thereby changing the rhythmic firing of downstream neurons. HFS induced conduction block in the axons might be one of the underlying mechanisms. The finding is important for further understanding the mechanisms of DBS.

Hippocampal Ripple Oscillations and Inhibition-First Network Models: Frequency Dynamics and Response to GABA Modulators

Hippocampal ripples are involved in memory consolidation, but the mechanisms underlying their generation remain unclear. Models relying on interneuron networks in the CA1 region disagree on the predominant source of excitation to interneurons: either "direct," via the Schaffer collaterals that provide feedforward input from CA3 to CA1, or "indirect," via the local pyramidal cells in CA1, which are embedded in a recurrent excitatory-inhibitory network. Here, we used physiologically constrained computational models of basket-cell networks to investigate how they respond to different conditions of transient, noisy excitation. We found that direct excitation of interneurons could evoke ripples (140-220 Hz) that exhibited intraripple frequency accommodation and were frequency-insensitive to GABA modulators, as previously shown in in vitro experiments. In addition, the indirect excitation of the basket-cell network enabled the expression of intraripple frequency accommodation in the fast-gamma range (90-140 Hz), as in vivo In our model, intraripple frequency accommodation results from a hysteresis phenomenon in which the frequency responds differentially to the rising and descending phases of the transient excitation. Such a phenomenon predicts a maximum oscillation frequency occurring several milliseconds before the peak of excitation. We confirmed this prediction for ripples in brain slices from male mice. These results suggest that ripple and fast-gamma episodes are produced by the same interneuron network that is recruited via different excitatory input pathways, which could be supported by the previously reported intralaminar connectivity bias between basket cells and functionally distinct subpopulations of pyramidal cells in CA1. Together, our findings unify competing inhibition-first models of rhythm generation in the hippocampus.SIGNIFICANCE STATEMENT The hippocampus is a part of the brain of humans and other mammals that is critical for the acquisition and consolidation of memories. During deep sleep and resting periods, the hippocampus generates high-frequency (∼200 Hz) oscillations called ripples, which are important for memory consolidation. The mechanisms underlying ripple generation are not well understood. A prominent hypothesis holds that the ripples are generated by local recurrent networks of inhibitory neurons. Using computational models and experiments in brain slices from rodents, we show that the dynamics of interneuron networks clarify several previously unexplained characteristics of ripple oscillations, which advances our understanding of hippocampus-dependent memory consolidation.

Aß Facilitates LTD at Schaffer Collateral Synapses Preferentially in the Left Hippocampus

Promotion of long-term depression (LTD) mechanisms by synaptotoxic soluble oligomers of amyloid-β (Aß) has been proposed to underlie synaptic dysfunction in Alzheimer's disease (AD). Previously, LTD was induced by relatively non-specific electrical stimulation. Exploiting optogenetics, we studied LTD using a more physiologically diffuse spatial pattern of selective pathway activation in the rat hippocampus invivo. This relatively sparse synaptic LTD requires both the ion channel function and GluN2B subunit of the NMDA receptor but, in contrast to electrically induced LTD, is not facilitated by boosting endogenous muscarinic acetylcholine or metabotropic glutamate 5 receptor activation. Although in the absence of Aß, there is no evidence of hippocampal LTD asymmetry, in the presence of Aß, the induction of LTD is preferentially enhanced in the left hippocampus in an mGluR5-dependent manner. This circuit-selective disruption of synaptic plasticity by Aß provides a route to understanding the development of aberrant brain lateralization in AD.

Administration of muscarinic antagonists induce changes in passive avoidance learning and in synaptic transmission in the CA1 area of the hippocampus

Muscarinic acetylcholine receptors (mAChR) are known to be related to learning and memory processes. Inactivation of mAChR by cholinergic antagonists have been shown to produce amnesia in a variety of behavioral tasks. In this study, we investigated the role of M1 and M2 AChR on passive avoidance learning and plasticity of synapses formed by Schaffer collaterals in freely moving rats. Experiments were performed using Wistar male rats. Seven days before testing, a recording electrode was lowered in the CA1 region under chloral hydrate anaesthesia to record the field excitatory postsynaptic potential (fEPSP) in response to Schaffer collateral stimulation. Selective M2 receptor antagonists methoctramine and selective M1 receptors antagonist pirenzepine were intraperitoneally injected immediately after training. The effects on memory retention were examined using passive avoidance training. We measured latency of the first entry into a dark compartment of the chamber. fEPSP amplitude and slope ratio were measured before shock presentation, 90 min after the shock, and 24 hour after the shock. Methoctramine significantly impaired behavior in the passive avoidance test but pirenzepine did not induce any changes compared to control. Our results showed that pirenzepine but not methoctramine supressed the amplitude of fEPSPs. On the other hand, intracerebroventricular methoctramine administration impaired passive avoidance learning and increased the amplitude of fEPSP.

Low Frequency Electrical Stimulation Attenuated The Epileptiform Activity-Induced Changes in Action Potential Features in Hippocampal CA1 Pyramidal Neurons.

Objective Electrical low frequency stimulation (LFS) is a new therapeutic method that moderates hyperexcitability during epileptic states. Seizure occurrence is accompanied by some changes in action potential (AP) features. In this study, we investigated the inhibitory action of LFS on epileptiform activity (EA) induced-changes in AP features in hippocampal CA1 pyramidal neurons. Materials and Methods In this experimental study, we induced EA in hippocampal slices by increasing the extracellular potassium (K+) concentration to 12 mM. LFS (1 Hz) was applied to the Schaffer collaterals at different pulse numbers (600 and 900) at the beginning of the EA. Changes in AP features recorded by whole-cell patch clamp recording were compared using phase plot analysis. Results Induction of EA depolarized membrane potential, decreased peak amplitude, as well as the maximum rise and decay slopes of APs. Administration of 1 Hz LFS at the beginning of EA prevented the above mentioned changes in AP features. This suppressive effect of LFS depended on the LFS pulse number, such that application of 900 pulses of LFS had a stronger recovery effect on AP features that changed during EA compared to 600 pulses of LFS. The constructed phase plots of APs revealed that LFS at 900 pulses significantly decreased the changes in resting membrane potential (RMP), peak amplitude, and maximum rise and decay slopes that appeared during EA. ConclusionIncreasing the numbers of LFS pulses can magnify its inhibitory effects on EA-induced changes in AP features.

Syringaresinol suppresses excitatory synaptic transmission and picrotoxin-induced epileptic activity in the hippocampus through presynaptic mechanisms

Many neuromodulating drugs acting on the nervous system originate from botanical sources. These plant-derived substances modulate the activity of receptors, ion channels, or transporters in neurons. Their properties make the substances useful for medicine and research. Here, we show that the plant lignan (+)-syringaresinol (SYR) suppresses excitatory synaptic transmission via presynaptic modulation. Bath application of SYR rapidly reduced the slopes of the field excitatory postsynaptic potentials (fEPSPs) at the hippocampal Schaffer collateral (SC)-CA1 synapse in a dose-dependent manner. SYR preferentially affected excitatory synapses, while inhibitory synaptic transmission remained unchanged. SYR had no effect on the conductance or the desensitization of AMPARs but increased the paired-pulse ratios of synaptic responses at short (20–200 ms) inter-stimulus intervals. These presynaptic changes were accompanied by a reduction of the readily releasable pool size. Pretreatment of hippocampal slices with the Gi/o protein inhibitor N-ethylmaleimide (NEM) abolished the effect of SYR on excitatory synaptic transmission, while the application of SYR significantly decreased Ca2+ currents and hyperpolarized the resting membrane potentials of hippocampal neurons. In addition, SYR suppressed picrotoxin-induced epileptiform activity in hippocampal slices. Overall, our study identifies SYR as a new neuromodulating agent and suggests that SYR suppresses excitatory synaptic transmission by modulating presynaptic transmitter release.

A gradient of frequency-dependent synaptic properties along the longitudinal hippocampal axis

BackgroundThe hippocampus is a functionally heterogeneous brain structure and specializations of the intrinsic neuronal network may crucially support the functional segregation along the longitudinal axis of the hippocampus. Short-term synaptic plasticity plays fundamental roles in information processing and may be importantly involved in diversifying the properties of local neuronal network along the hippocampus long axis. Therefore, we aimed to examine the properties of the cornu ammonis 1 (CA1) synapses along the entire dorsoventral axis of the rat hippocampus using field excitatory postsynaptic potentials from transverse rat hippocampal slices and a frequency stimulation paradigm.ResultsApplying a ten-pulse stimulus train at frequencies from 0.1 to 100 Hz to the Schaffer collaterals we found a gradually diversified pattern of frequency-dependent synaptic effects along the dorsoventral hippocampus axis. The first conditioned response was facilitated along the whole hippocampus for stimulus frequencies 10–40 Hz. However, steady-state responses or averaged responses generally ranged from maximum synaptic facilitation in the most dorsal segment of the hippocampus to maximum synaptic depression in the most ventral segment of the hippocampus. In particular, dorsal synapses facilitated for stimulus frequency up to 50 Hz while they depressed at higher frequencies (75–100 Hz). Facilitation at dorsal synapses was maximal at stimulus frequency of 20 Hz. On the contrary, the most ventral synapses showed depression regardless of the stimulus frequency, only displaying a transient facilitation at the beginning of 10–50 Hz stimulation. Importantly, the synapses in the medial hippocampus displayed a transitory behavior. Finally, as a whole the hippocampal synapses maximally facilitated at 20 Hz and increasingly depressed at 50–100 Hz.ConclusionThe short-term synaptic dynamics change gradually along the hippocampal long axis in a frequency-dependent fashion conveying distinct properties of information processing to successive segments of the structure, thereby crucially supporting functional segregation along the dorsoventral axis of the hippocampus.

A Comparison of Different Slicing Planes in Preservation of Major Hippocampal Pathway Fibers in the Mouse.

The hippocampus plays a critical role in learning and memory and higher cognitive functions, and its dysfunction has been implicated in various neuropathological disorders. Electrophysiological recording undertaken in live brain slices is one of the most powerful tools for investigating hippocampal cellular and network activities. The plane for cutting the slices determines which afferent and/or efferent connections are best preserved, and there are three commonly used slices: hippocampal-entorhinal cortex (HEC), coronal and transverse. All three slices have been widely used for studying the major afferent hippocampal pathways including the perforant path (PP), the mossy fibers (MFs) and the Schaffer collaterals (SCs). Surprisingly, there has never been a systematic investigation of the anatomical and functional consequences of slicing at a particular angle. In the present study, we focused on how well fiber pathways are preserved from the entorhinal cortex (EC) to the hippocampus, and within the hippocampus, in slices generated by sectioning at different angles. The postmortem neural tract tracer 1,1′-dioctadecyl-3,3,3′3′-tetramethylindocarbocyanine perchlorate (DiI) was used to label afferent fibers to hippocampal principal neurons in fixed slices or whole brains. Laser scanning confocal microscopy was adopted for imaging DiI-labeled axons and terminals. We demonstrated that PP fibers were well preserved in HEC slices, MFs in both HEC and transverse slices and SCs in all three types of slices. Correspondingly, field excitatory postsynaptic potentials (fEPSPs) could be consistently evoked in HEC slices when stimulating PP fibers and recorded in stratum lacunosum-moleculare (sl-m) of area CA1, and when stimulating the dentate granule cell layer (gcl) and recording in stratum lucidum (sl) of area CA3. The MF evoked fEPSPs could not be recorded in CA3 from coronal slices. In contrast to our DiI-tracing data demonstrating severely truncated PP fibers in coronal slices, fEPSPs could still be recorded in CA1 sl-m in this plane, suggesting that an additional afferent fiber pathway other than PP might be involved. The present study increases our understanding of which hippocampal pathways are best preserved in the three most common brain slice preparations, and will help investigators determine the appropriate slices to use for physiological studies depending on the subregion of interest.

Seizure liability assessments using the hippocampal tissue slice: Comparison of non-clinical species

IntroductionTraditionally, rat hippocampal tissue slice models are used as an in vitro electrophysiology assay to assess seizurogenic potential in early drug development despite non-clinical species-specific differences noted during in vivo seizure studies. MethodsHippocampal tissue slices were acutely isolated from rats, minipigs, dogs and nonhuman primates (NHP). Population spikes (PS) were evoked through stimulation of the CA3 Schaffer collateral pathway and recorded using in vitro electrophysiological techniques via an extracellular electrode placed within the CA1 stratum pyramidale cell body layer. ResultsHippocampal slices, across all species, displayed a concentration-dependent increase in PS area and number with the pro-convulsant pentylenetetrazol (PTZ; 0.1–10mM). Beagle dogs exhibited higher sensitivities to PTZ-induced changes in PS area and number compared to both rats and NHPs which presented nuanced differences in their responsiveness to PTZ modulation. Minipigs were comparatively resistant to PTZ-induced changes in both PS area and number. Rat and NHP hippocampal tissues were further characterized with the pro-convulsant agents 4-aminopyradine (4-AP; 1–100μM) and cefazolin (0.001–10mM). Rats possessed higher sensitivities to 4-AP- and cefazolin-induced changes to both PS area and number whereas NHP displayed greater modulation in PS duration. The anti-convulsant agents, diazepam (10–500μM) and lidocaine (1–500μM), were also tested on either rat and/or NHP tissue with both drugs repressing PS activation at high concentrations. DiscussionHippocampal tissue slices, across all species, possessed distinct sensitivities to pro- and anti-convulsant agents which may benefit the design of non-clinical seizure liability studies and their associated data interpretation.

Effect of Low-Frequency Electrical Stimulation on the High-K+-Induced Neuronal Hyperexcitability in Rat Hippocampal Slices

Low-frequency electrical stimulation (LFS) is a potential therapeutic method for epilepsy treatment. However, the effect of different LFS characteristics including the number of pulses, intensity and the time of application on its antiepileptic action has not been completely determined. In the present study, epileptiform activity (EA) was induced in hippocampal slices by high-K+ solution which was washed out after 20 min. The changes in the electrophysiological properties of CA1 pyramidal neurons were measured during and 30 min after EA using whole-cell patch-clamp recording. EA occurrence resulted in neuronal hyperexcitability. Application of 1-Hz LFS to the Schaffer collaterals at 600 and 900 pulses and two intensities (equal and 1.5 times more than an intensity sufficient to elicits a 5-mV EPSP) at the beginning of EA showed that 900-pulse LFS at high intensity had stronger preventing effect on high-K+-induced neuronal hyperexcitability by increasing the rheobase current, utilization time, first-spike latency, delay to first-rebound action potential and decreasing the number of rebound action potential. In addition, application of high-intensity 900-pulse LFS had better inhibitory effect on the neuronal hyperexcitability when applied at the beginning of EA compared to its administration before or at 8–10 min after EA. Therefore, it may suggest the inhibitory action of LFS on the neuronal hyperexcitability is augmented by increasing its number of pulses and intensity. In addition, there is a time window for LFS application so that its application at the beginning of EA has better inhibitory effect.

Status epilepticus induced by pentylenetetrazole increases short-term synaptic facilitation in the hippocampus of juvenile rats.

We studied the effect of status epilepticus (SE) on short-term synaptic plasticity. The amplitudes of field potentials in response to extracellular stimulation of the Schaffer collaterals were recorded in hippocampal slices. Subtle modifications were revealed on day 1 after SE, whereas on days 3 and 7 we did not find any differences from the control. These data show that, one day after SE, the probability of a transmitter release in hippocampal synapses decreases that serves as a compensatory mechanism, which prevents seizure activity.

Plasticity of intrinsic excitability during LTD is mediated by bidirectional changes in h-channel activity

The polarity of excitability changes associated with induction of Long-Term synaptic Depression (LTD) in CA1 pyramidal neurons is a contentious issue. Postsynaptic neuronal excitability after LTD induction is found to be reduced in certain cases (i.e. synergistic changes) but enhanced in others (i.e. compensatory or homeostatic). We examined here whether these divergent findings could result from the activation of two separate mechanisms converging onto a single learning rule linking synergistic and homeostatic plasticity. We show that the magnitude of LTD induced with low frequency stimulation (LFS) of the Schaffer collaterals determines the polarity of intrinsic changes in CA1 pyramidal neurons. Apparent input resistance (Rin) is reduced following induction of moderate LTD (<20–30%). In contrast, Rin is increased after induction of large LTD (>40%) induced by repetitive episodes of LFS. The up-regulation of Ih observed after moderate LTD results from the activation of NMDA receptors whereas the down-regulation of Ih is due to activation of mGluR1 receptors. These changes in Rin were associated with changes in intrinsic excitability. In conclusion, our study indicates that changes in excitability after LTD induction follow a learning rule describing a continuum linking synergistic and compensatory changes in excitability.

Tight coupling of astrocyte energy metabolism to synaptic activity revealed by genetically encoded FRET nanosensors in hippocampal tissue

The potassium ion, K+, a neuronal signal that is released during excitatory synaptic activity, produces acute activation of glucose consumption in cultured astrocytes, a phenomenon mediated by the sodium bicarbonate cotransporter NBCe1 (SLC4A4). We have explored here the relevance of this mechanism in brain tissue by imaging the effect of neuronal activity on pH, glucose, pyruvate and lactate dynamics in hippocampal astrocytes using BCECF and FRET nanosensors. Electrical stimulation of Schaffer collaterals produced fast activation of glucose consumption in astrocytes with a parallel increase in intracellular pyruvate and biphasic changes in lactate. These responses were blocked by TTX and were absent in tissue slices prepared from NBCe1-KO mice. Direct depolarization of astrocytes with elevated extracellular K+ or Ba2+ mimicked the metabolic effects of electrical stimulation. We conclude that the glycolytic pathway of astrocytes in situ is acutely sensitive to neuronal activity, and that extracellular K+ and the NBCe1 cotransporter are involved in metabolic crosstalk between neurons and astrocytes. Glycolytic activation of astrocytes in response to neuronal K+ helps to provide an adequate supply of lactate, a metabolite that is released by astrocytes and which acts as neuronal fuel and an intercellular signal.

Substance P induces plasticity and synaptic tagging/capture in rat hippocampal area CA2

The hippocampal area Cornu Ammonis (CA) CA2 is important for social interaction and is innervated by Substance P (SP)-expressing supramammillary (SuM) nucleus neurons. SP exerts neuromodulatory effects on pain processing and central synaptic transmission. Here we provide evidence that SP can induce a slowly developing NMDA receptor- and protein synthesis-dependent potentiation of synaptic transmission that can be induced not only at entorhinal cortical (EC)-CA2 synapses but also at long-term potentiation (LTP)-resistant Schaffer collateral (SC)-CA2 synapses. In addition, SP-induced potentiation of SC-CA2 synapses transforms a short-term potentiation of EC-CA2 synaptic transmission into LTP, consistent with the synaptic tagging and capture hypothesis. Interestingly, this SP-induced potentiation and associative interaction between the EC and SC inputs of CA2 neurons is independent of the GABAergic system. In addition, CaMKIV and PKMζ play a critical role in the SP-induced effects on SC-CA2 and EC-CA2 synapses. Thus, afferents from SuM neurons are ideally situated to prime CA2 synapses for the formation of long-lasting plasticity and associativity.