On the sites of presynaptic inhibition by neuropeptide y in rat hippocampus in vitro

Neuropeptide Y (NPY) reduces excitatory synaptic transmission between stratum radiatum and CA1 pyramidal cells in rat hippocampal slice in vitro by a presynaptic action. To understand NPY's role in the control of excitability in hippocampus, its actions on excitatory and inhibitory synaptic transmission were examined, using intracellular, sharp microelectrode, and tight-seal, whole cell recordings from principal neurons in areas CA1, CA3, and dentate. Bath application of 1 μM NPY reversibly inhibited excitatory postsynaptic potentials (EPSPs) evoked in CA1 pyramidal cells from either stratum radiatum or stratum oriens by about 50%. Neuropeptide Y also inhibited EPSPs at mossy fiber-CA3, stratum oriens-CA3, and CA3-CA3 synapses by between 45% and 55%. As in CA1, the action of NPY was presynaptic. By contrast, NPY did not inhibit EPSPs evoked in dentate granule cells from either perforant path or commissural inputs. Neuropeptide Y did not alter postsynaptic membrane properties in any cell type. Although NPY attenuated the orthodromically evoked (stratum radiatum) inhibitory postsynaptic potentials in CA1 pyramidal cells by about the same amount as it inhibited the EPSPs, it did not affect the IPSPs evoked in the same cells by antidromic stimulation from alveus. Inhibitory postsynaptic potentials evoked in pharmacological isolation in CA1, CA3, or dentate were also not significantly affected by NPY. The evidence supports the hypothesis that NPY acts at feedforward excitatory synapses to presynaptically reduce the amplitude of excitation as it travels through hippocampal circuits. By contrast, synaptically mediated inhibition is not directly affected by NPY. Neuropeptide Y is the only known endogenous substance that selectively reduces feedforward excitatory transmission without causing changes in other properties of the hippocampal circuitry.     

Elevation of extracellular potassium facilitates the induction of hippocampal long-term potentiation.

Hippocampal long-term potentiation (LTP) is widely believed to be a cellular substrate for learning and memory. A likely physiological stimulus for initiating LTP is repetitive neuronal activity, which also results in K+ accumulation extracellularly. Therefore, the involvement of elevated extracellular K+ concentrations in the induction of LTP of the stratum radiatum-CA1 neuronal synapse was investigated in the hippocampal slice preparation. Increasing the K+ content in extracellular perfusing medium from 3.1 to 15 mM resulted in facilitation of LTP induction in weak excitatory postsynaptic potentials (EPSPs). Since changes that occur to generate LTP are thought to be localized to synaptic regions, it would be relevant to selectively increase synaptic K+ levels. To this end, the following experiments were conducted: i) baclofen, a GABAB receptor agonist which, in addition to having a disinhibitory presynaptic action, activates a K+ conductance in CA1 neuronal dendrites, was applied to the slice; ii) K+ was directly applied by iontophoresis. At a concentration of 5 microM baclofen, as well as with K+ iontophoresis (200-300 nA), LTP of weak EPSPs was facilitated. The present data suggest that an increase in synaptic K+ levels can fulfill the condition of cooperativity for LTP induction, raising the possibility that an elevation of this monovalent ion plays a physiological role in triggering LTP.     

Induction of hippocampal long-term potentiation in the absence of extracellular Ca2+

Extracellular Ca2+, synaptic transmission, and the activation of subsynaptic receptors are not required for the induction of long-term potentiation of excitatory synaptic transmission at stratum radiatum-CA1 neuron junctions as long as sufficient depolarizations of the presynaptic terminals and the postsynaptic neurons co-occur.     

Topiramate alters excitatory synaptic transmission in mouse hippocampus

Antiepileptic drugs may exert neuroprotective effects by decreasing excessive membrane excitability, neurotransmitter release, or postsynaptic Ca2+ entry. To assess these sites of action, we combined fluorescence Ca2+ imaging with extracellular field recording to analyze axonal excitability, evoked presynaptic Ca2+ entry through presynaptic Ca2+ channels, postsynaptic excitatory field potentials (fEPSP), and postsynaptic Ca2+ buildup ([Capost]) at the mouse hippocampal CA3-CA1 synapse exposed to topiramate (TPM). Topiramate had no effect on presynaptic Ca2+ entry, and produced only a minor inhibition of axonal excitability. Topiramate at concentrations up to 100 microM only slightly reduced the amplitude of the evoked fEPSP, but strongly inhibited the [Capost] evoked by repetitive synaptic activation. Postsynaptically, the action of TPM on the fEPSP and [Capost] was not mediated by an inhibition of the NMDA receptor, or by direct modulation of voltage-dependent Ca2+ channels, but reflected reduced somatic or dendritic membrane depolarization by AMPA and kainate receptors. These results are consistent with the known anticonvulsant properties of TPM. In addition, the ability of TPM to reduce postsynaptic Ca2+ buildup may provide a potential mechanism for neuronal protection during paroxysmal firing associated with epileptic seizures.     

Spontaneous epileptiform discharges in hippocampal slices induced by 4-aminopyridine

4-Aminopyridine (4-AP) induced 2 types of spontaneous field potentials (SFPs) in the hippocampal slice. Type I resembled spontaneous activity induced by other convulsants. They occurred at a rate of approximately 1 Hz, started in the CA2/CA3 region and spread at a velocity of 0.3 m/s to area CA1. Transsection experiments and laminar profiles indicated that they spread synaptically along the Schaffer collateral pathway. Synaptic blockade by low Ca2+/high Mg2+ or kynurenic acid reversibly abolished type I SFPs. Increasing [Ca2+]o lowered the rate and slightly increased the amplitude. Possibly, increased spontaneous transmitter release, and not disinhibition, is responsible for the generation of type I SFPs. Type II occurred at a rate of about 0.15 Hz and travelled in the same direction, but a factor 10 slower. They could not be blocked by separation of the CA1 and CA3 region; coupling remained until stratum moleculare was severed. Type II could not be suppressed by blockade of synaptic transmission. The laminar profile is similar in shape to that of type I but not identical. Increasing [Ca2+]o had the same but stronger effect as on type I. Type II SFPs depressed evoked population spikes up to a second and delayed the next type I SFP. The mechanisms involved remain largely speculative; further analysis is needed to help understand the epileptogenic action of 4-AP.     

Valproate reduced synaptic activity increase induced by 4-aminopyridine at the hippocampal CA3-CA1 synapse

PURPOSE: We investigated the effects of valproate (VPA) on excitatory synaptic transmission changes induced by 4-aminopyridine (4-AP) to determine whether the antiepileptic effects shown by VPA can be ascribed to a modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in the CA3-CA1 synapse. METHODS: Rat hippocampal slices were prepared and maintained in vitro with standard methods. Whole-cell current and voltage-clamp recordings were obtained from CA1 pyramidal neurons by using the "blind" patch-clamp technique in an immersion recording chamber. Increase in the spontaneous excitatory synaptic activity was induced by addition of 4-AP to the medium. RESULTS: Perfusion with VPA significantly counteracted the increase of frequency and amplitude of the sEPSCs induced by application of 4-AP and suppressed the epileptiform activity. CONCLUSIONS: We conclude that VPA decreases the 4-AP-induced enhancement of excitatory synaptic activity at the CA3-CA1 synapse, and that this reduction of excitation input to CA1 contributes to the anticonvulsant effects of VPA.     

Phorbol ester uncouples adenosine inhibition of presynaptic Ca2+ transients at hippocampal synapses

Synaptic transmission involves Ca2+ influx at presynaptic terminals. Adenosine receptors inhibit transmission, and this effect can be abolished by activation of PKC with phorbol esters. Whether protein kinase C (PKC) acts via alterations in Ca2+ entry at the presynaptic terminal is unknown. In the present study, we recorded the presynaptic Ca2+ transients (preCa(delta)) in hippocampal stratum radiatum, using fluorescence photometry. The calcium dye Fura-2 AM was used to load the Schaffer collateral/commissural tract and its terminals. Tetrodotoxin (TTX)-sensitive Na+ channels and Cd2+-sensitive, high-voltage activated Ca2+ channels (HVACCs) were required to elicit the preCa(delta). Application of the phorbol ester phorbol-12,13-dibutyrate (PDBu) abolished the adenosine inhibition of both preCa(delta) and the field excitatory postsynaptic potentials (fEPSPs). PDBu consistently potentiated fEPSPs, and also increased preCa(delta) in a large majority of the slices examined. Regardless of whether potentiation was observed, PDBu always prevented adenosine inhibition of preCa(delta). In contrast, the inactive phorbol ester, 4alpha-phorbol, did not alter adenosine inhibition of preCa(delta), indicating that PKC activation is necessary for the occurrence of the observed effects. Our findings suggest that PKC activation abolishes adenosine's inhibitory effect on synaptic activity involving presynaptic Ca2+ entry.     

Action and location of neuropeptide tyrosine (Y) on hippocampal neurons of the rat in slice preparations

The action of bath applied NPY (1-1,000 nM) was investigated on hippocampal slices of the rat with extra- and intracellular recording. Neuropeptide Y (NPY) at 10-1,000 nM caused a concentration-dependent, long-lasting reduction of excitatory postsynaptic potentials (EPSPs) in the hippocampal subfield CA1 and the area dentata, and an even stronger reduction of population spikes. Paired pulse experiments with low intensity, stimulation-evoked PSPs showed a marked increase in facilitation in the presence of NPY, indicating a presynaptic action. Spontaneous burst firing of CA1 pyramidal cells in low calcium, high magnesium medium was reduced, indicating a partially postsynaptic inhibitory action of NPY on their dendrites. Intracellular recording from CA1 somata during NPY administration revealed a reduction of the amplitudes of excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequences in the absence of changes in membrane potential and conductance. Accommodation of firing during long depolarizing pulses and afterhyperpolarizations were unchanged. The innervation pattern of NPY immunoreactive fibers in the same regions was studied in slices adjacent to the ones used for electrophysiology by using antisera against NPY and light and electron microscopy. There is a dense innervation of CA1 by NPY-immunoreactive axons and terminals, particularly in the stratum moleculare. NPY-immunoreactive neurons are present in the stratum oriens and pyramidale. The NPY labeled axons of the stratum moleculare participate in numerous synaptic contacts with the smaller dendritic elements in this layer, many of which belong to pyramidal neurons. These observations provide evidence for a dendritic NPY-immunoreactive innervation of CA1 neurons, which is in keeping with the electrophysiological effects of NPY on pyramidal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ion channel involvement in hypoxia-induced spreading depression in hippocampal slices.

Rat hippocampal tissue slices were made hypoxic in control medium and in medium containing the ion channel blockers tetraethylammonium (TEA), 4-aminopyridine (4-AP), or tetrodotoxin (TTX). Postsynaptic evoked potentials, extracellular DC potential Vec, and in some experiments extracellular potassium concentration [K+]o were monitored in stratum pyramidale of the CA1 region. TEA (10 mM) decreased the latency of hypoxia-induced spreading depression (SD), and reduced the amplitudes of the changes in Vec and [K+]o. 4-AP (50 microM) also decreased the latency of SD but had no effect on the Vec shift. In most slices, TTX (1 microM) increased SD latency but had no effect on the Vec shift. In some slices, TTX blocked the occurrence of SD.     

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.     

Characterization of l-2-Amino-4-Phosphonobutanoate Action Following Sensitization by Quisqualate in Rat Hippocampal Slice Cultures

An excitatory action of l-2-amino-4-phosphonobutanoate (l-AP4), a glutamate analogue, is observed following pre-exposure of tissue to quisqualate. We have studied the mechanism of sensitization of l-AP4 responses by quisqualate in voltage-clamped CA3 pyramidal cells in rat hippocampal slice cultures in the presence of tetrodotoxin. Prior to quisqualate addition, CA3 cells did not respond to l-AP4 (50 - 1000 microM). Following brief application of quisqualate (500 nM for 30 s), l-AP4 (50 - 200 microM) induced a complex excitatory response which could be obtained for >1 h. l-AP4 caused an ionotropic inward current associated with a conductance increase. This response was in part sensitive to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and in part sensitive to d-2-amino-5-phosphonovalerate (d-AP5) and Mg2+ ions. At depolarizing potentials, in the presence of CNQX and d-AP5, l-AP4 caused excitation by depressing K+ currents, mimicking the metabotropic action of glutamate. This indicates that the action of l-AP4 is mediated by three different receptor types: N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors, and glutamatergic metabotropic receptors. The l-AP4 response persisted in solutions containing low Ca2+ and high Mg2+ concentrations or 100 - 200 microM Cd2+, suggesting that it is independent of extracellular Ca2+. We were unable to identify any substance other than quisqualate capable of sensitizing the l-AP4 action. This effect also occurred when quisqualate was applied in Ca2+-free solution or in solutions containing low concentrations of Na+ or Cl-. Sensitization of l-AP4 responses by quisqualate was not observed in acutely dissociated pyramidal cells recorded by means of the whole-cell recording mode, although ionotropic quisqualate responses were present. Sensitization was readily reversed by short applications of the endogenous excitatory amino acids glutamate, aspartate and homocysteate at concentrations of 10 - 100 microM. Our data are consistent with the hypothesis that the excitatory action of l-AP4 results from a Ca2+-independent release of endogenous excitatory amino acids from some presynaptic neuronal or glial site.     

Modulation by substance P of synaptic transmission in the mouse hippocampal slice

The modulatory action of substance P on synaptic transmission of CA1 neurons was studied using intra‐ or extracellular recording from the mouse hippocampal slice preparation. Bath‐applied substance P (2–4 μ m ) or the selective NK₁ receptor agonist substance P methylester (SPME, 10 n m –5 μ m ) depressed field potentials (recorded from stratum pyramidale) evoked by focal stimulation of Schaffer collaterals. This effect was apparently mediated via NK₁ receptors since it was completely blocked by the selective NK₁ antagonist SR 140333. The field potential depression by SPME was significantly reduced in the presence of bicuculline. Intracellular recording from CA1 pyramidal neurons showed that evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs) were similarly depressed by SPME, which at the same time increased the frequency of spontaneous GABAergic events and reduced that of spontaneous glutamatergic events. The effects of SPME on spontaneous and evoked IPSPs were prevented by the ionotropic glutamate receptor blocker kynurenic acid. In tetrodotoxin (TTX) solution, no change in either the frequency of spontaneous GABAergic and glutamatergic events or in the amplitude of responses of pyramidal neurons to 4 μ m α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) or 10 μ m N ‐methyl‐ d ‐aspartate (NMDA) was observed. On the same cells, SPME produced minimal changes in passive membrane properties unable to account for the main effects on synaptic transmission. The present data indicate that SPME exerted its action on CA1 pyramidal neurons via a complex network mechanism, which is hypothesized to involve facilitation of a subset of GABAergic neurons with widely distributed connections to excitatory and inhibitory cells in the CA1 area.     

GluK1 inhibits calcium dependent and independent transmitter release at associational/commissural synapses in area CA3 of the hippocampus

CA3 pyramidal cells receive three main excitatory inputs: the first one is the mossy fiber input, synapsing mainly on the proximal apical dendrites. Second, entorhinal cortex cells form excitatory connections with CA3 pyramidal cells via the perforant path in the stratum lacunosum moleculare. The third input involves the ipsi‐and contralateral connections, termed the associational/commissural (A/C) pathway terminating in the stratum radiatum of CA3, thus forming a feedback loop within this region. Since this excitatory recurrent synapse makes the CA3 region extremely prone to seizure development, understanding the regulation of synaptic strength of this connection is of crucial interest. Several studies suggest that kainate receptors (KAR) play a role in the regulation of synaptic strength. Our aim was to characterize the influence of KAR on A/C synaptic transmission: application of ATPA, a selective agonist of the GluK1 KAR, depressed the amplitude fEPSP without affecting the size of the fiber volley. Blockade of GABA receptors had no influence on this effect, arguing against the influence of interneuronal KARs. Pharmacological and genetic deletion studies could show that this effect was selectively due to GluK1 receptor activation. Several lines of evidence, such as PPF changes, coefficient of variance–analysis and glutamate uncaging experiments strongly argue for a presynaptic locus of suppression. This is accompanied by an ATPA‐mediated reduction in Ca²⁺ influx at excitatory synaptic terminals, which is most likely mediated by a G‐Protein dependent mechanism, as suggested by application of pertussis toxin. Finally, analysis of miniature EPSCs in the presence and absence of extracellular Ca²⁺ suggest that presynaptic KAR can also reduce transmitter release downstream and therefore independent of Ca²⁺ influx. © 2010 Wiley Periodicals, Inc., Inc.     

Effect of 4-aminopyridine on synaptic transmission in rat hippocampal slices

Extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded in area CA1 of rat hippocampal slices in vitro. The responses evoked by spontaneously released glutamate and GABA were recorded from area CA1 pyramidal neurons in rat hippocampal slices in whole-cell mode. The glutamate and GABA receptor-associated ligand-gated currents were obtained from dissociated single hippocampal pyramidal cells. The results showed that 4-aminopyridine (4-AP) had obvious effects on both presynaptic and postsynaptic events. Applications of 4-AP in micromolar concentration resulted in persistent enhancement of the initial slope of fEPSPs with the half-maximal enhancement concentration (EC(50)) of 46.7+/-2.68 microM. At the concentration of 200 microM, 4-AP increased the initial slopes of the total fEPSPs, NMDA- and AMPA-mediated fEPSPs components to 225.6+/-23.8%, 177.4+/-20.1% and 142.3+/-18.9%, respectively, but had no effect on the fiber volley. The half-maximal stimulus intensity to induce responses was reduced from 5.14+/-0.27 to 3.58+/-0.23 V. The frequencies of mEPSCs and mIPSCs were increased to 324.2+/-25.4% and 287.3+/-36.3% by 200 microM 4-AP. The amplitude histograms of mEPSCs and mIPSCs were fitted with Gaussian distributions. After 200 microM 4-AP application, the first and second peaks in Gaussian distributions of mEPSCs were shifted from 8.73+/-0.94 and 17.78+/-2.13pA to 10.48+/-0.82 and 21.14+/-2.45 pA, while those of mIPSCs were shifted from 13.65+/-0.96 and 25.51+/-2.95 pA to 11.21+/-1.04 and 23.08+/-2.37 pA. At 200 microM, 4-AP reduced paired-pulse facilitation and accelerated synaptic fatigue induced by stimulation at 10 Hz (for 1 s) and the ratio of fEPSPs(10)/fEPSPs(1) was decreased from 1.62+/-0.16 to 0.61+/-0.15. At 200 microM, 4-AP inhibited postsynaptic GABA currents induced by 5 microM GABA to 68.2+/-15.5%: by countering the effect of enhanced release of GABA from presynaptic terminals, this could depress the inhibitory pathway. Also at 200 microM, 4-AP increased NMDA currents to 155.3+/-17.8%, but had no significant effect on AMPA currents (94.2+/-15.6%). Our experimental results thus show that 4-AP-induced changes of synaptic transmission in area CA1 of rat hippocampus may be attributed to 4-AP's effects on both presynaptic terminals and postsynaptic receptors.     

The Nitric Oxide - Cyclic GMP Pathway and Synaptic Depression in Rat Hippocampal Slices

The ability of exogenous nitric oxide (NO) to modify synaptic transmission was investigated in area CA1 of the rat hippocampal slice. The NO donors S -nitroso- N -acetylpenicillamine (SNAP) and S -nitrosoglutathione (SNOG) depressed field excitatory postsynaptic potentials evoked by low frequency stimulation of the Schaffer collateral - commissural pathway. Upon washout of the NO donors, synaptic transmission rapidly returned to control levels. A similar reversible synaptic depression was produced by SNAP when tetanic stimulation (100 Hz; 1 s) was delivered in its presence. The effect of SNAP was not mimicked by its precursor or breakdown product and was blocked by haemoglobin, indicating that the effect involved NO. Roussin's black salt, a photolabile NO donor, also depressed transiently field excitatory postsynaptic potentials following photolysis. The depression was induced rapidly following a flash of UV light (20 s duration) focused onto the slice using a confocal microscope. The depressant effect of the NO donors on synaptic transmission was mimicked by zaprinast, a specific cGMP - phosphodiesterase inhibitor. Zaprinast depressed to a similar extent both the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate and N -methyl- d -aspartate receptor-mediated components of excitatory postsynaptic currents without affecting passive membrane properties, indicating a presynaptic locus of action. SNAP, SNOG and zaprinast all elevated cGMP levels in rat hippocampal slices. Immunocytochemical staining revealed that the cGMP accumulation was mainly in a network of varicose fibres running throughout the CA1 region, consistent with a presynaptic site of action of NO. We conclude that NO, possibly through activation of guanylate cyclase, may be involved in transient presynaptic depression in the CA1 region of the hippocampus.     

Stimulation-dependent calcium binding sites in the guinea pig hippocampal slice: and electrophysiological and electron microscopic study

Transverse slices of the hippocampus of guinea pigs were prepared in order to investigate Ca2+ binding sites in CA1. Electrical stimulation (Schaffer collaterals and stratum oriens) combined with different aminopyridine compounds (AP) were used for neuronal activation. With histochemical methods Ca2+ binding sites were identified and localized at the electron microscopic level as electron dense deposits of granular or elongated shape. After electrical stimulation, electron dense deposits of 30-50 nm diameter were spread at low density over all layers of CA1. Electrical stimulation combined with application of aminopyridine compounds led to electron dense deposits of 60-400 nm diameter, mainly restricted to the activated input layers. Deposits were predominantly found at presynaptic sides, with few at dendrites and glial cells. Application of aminopyridine alone led to very few deposits, spread over the total CA1 area. The results indicate that aminopyridines, if combined with electrical stimulation, display a strong presynaptic action, which results in a remarkable Ca2+-translocation at the preterminal and terminal level. On the dendritic side aminopyridines in the concentrations used for the study weakly activate Ca2+ movements.     

4-aminopyridine-induced epileptiform activity and a GABA-mediated long-lasting depolarization in the rat hippocampus.

Two types of spontaneous filed potentials were recorded in rat hippocampal slices after addition of 4-aminopyridine (4-AP; 50 microM). One consisted of brief, epileptiform discharges that occurred at 0.6 +/- 0.2 sec-1 in the CA3 and CA1 areas. The other type occurred less frequently (0.036 +/- 0.013 sec-1) and was recorded in CA1, CA3, and dentate areas. It corresponded in all regions to an intracellular long-lasting depolarization (LLD; duration, 300-1200 msec; peak amplitude, 2-15 mV) that was abolished by bicuculline methiodide; therefore, it was mediated by GABAA receptors. Sectioning experiments and the occurrence of propagation failures indicated that LLDs could be initiated by any area of the slice. Furthermore, the propagation of LLDs did not follow any consistent or predictable pattern along known anatomical hippocampal pathways. Finally, neither the occurrence nor the propagation of LLDs was affected when excitatory synaptic transmission was blocked by NMDA and non-NMDA receptor antagonists. In the presence of antagonists of glutamatergic receptors, LLDs disappeared after the omission of Ca2+ or the addition of Cd2+ to the perfusing solution, suggesting that synaptic transmission was required for their generation. These data indicate that 4-AP discloses both interictal epileptiform discharges and LLDs in the rat hippocampus. The first type of activity is presumably related to certain properties of CA3 pyramidal neurons and the neuronal circuit, whereas LLDs originate from the spontaneous, periodic activity of GABAergic interneurons located in any area of the hippocampus, and can propagate to the other areas by the use of nonsynaptic mechanisms. We propose that 4-AP reveals a novel type of interaction among GABAergic interneurons that is based on the accumulation and the dispersion of K+.     

Epileptogenic properties of the mast cell degranulating peptide in CA3 hippocampal neurones

The epileptogenic properties of the mast cell degranulating peptide (MCD) have been investigated in the CA3 region of the hippocampal slice preparation. Brief (3-5 min) bath application of MCD (0.5-2 microM) to CA3 hippocampal neurones produced an enhancement of the spontaneous synaptic activity and the appearance of spontaneous bursts that persisted for several hours. These bursts were network driven and the underlying paroxysmal depolarizing shift met the criteria for a giant excitatory postsynaptic potential (EPSP), with a reversal potential close to 0 mV. Furthermore following the application of MCD, stimulation of the mossy fibres, commissural or temporo-ammonic pathway evoked an EPSP followed by an evoked network burst. The bursts which could be elicited for several hours were reversibly blocked by a brief application of tetrodotoxin (TTX; 1 microM) or cobalt (2 mM). In contrast, prior and concomitant treatment with TTX or cobalt prevented the occurrence of the bursts induced by MCD. The effects of MCD were not due to a blockade of GABAergic inhibition since the toxin did not reduce the fast and slow IPSP. Furthermore, the N-methyl-D-aspartate (NMDA) antagonists D-2-amino-phosphonovalerate (D-APV; 30 microM) or DL-amino-phosphoheptanoic acid (AP-7, 30 microM) did not block the action of MCD, suggesting that the activation of NMDA receptors are neither necessary nor sufficient for MCD-induced bursts. It is concluded that MCD induces in the CA3 region long-lasting changes in the synaptic responses which may be mediated through a presynaptic mechanism.     

Interplay between facilitation, depression, and residual calcium at three presynaptic terminals.

Synapses display remarkable alterations in strength during repetitive use. Different types of synapses exhibit distinctive synaptic plasticity, but the factors giving rise to such diversity are not fully understood. To provide the experimental basis for a general model of short-term plasticity, we studied three synapses in rat brain slices at 34 degrees C: the climbing fiber to Purkinje cell synapse, the parallel fiber to Purkinje cell synapse, and the Schaffer collateral to CA1 pyramidal cell synapse. These synapses exhibited a broad range of responses to regular and Poisson stimulus trains. Depression dominated at the climbing fiber synapse, facilitation was prominent at the parallel fiber synapse, and both depression and facilitation were apparent in the Schaffer collateral synapse. These synapses were modeled by incorporating mechanisms of short-term plasticity that are known to be driven by residual presynaptic calcium (Ca(res)). In our model, release is the product of two factors: facilitation and refractory depression. Facilitation is caused by a calcium-dependent increase in the probability of release. Refractory depression is a consequence of release sites becoming transiently ineffective after release. These sites recover with a time course that is accelerated by elevations of Ca(res). Facilitation and refractory depression are coupled by their common dependence on Ca(res) and because increased transmitter release leads to greater synaptic depression. This model captures the behavior of three different synapses for various stimulus conditions. The interplay of facilitation and depression dictates synaptic strength and variability during repetitive activation. The resulting synaptic plasticity transforms the timing of presynaptic spikes into varying postsynaptic response amplitudes.     

l-Carnitine: therapeutic strategy for metabolic encephalopathy

The effects of 4-aminopyridine (4-AP) on membrane electrical properties and synaptic transmission in neurons of the isolated rabbit superior cervical ganglion were investigated. 4-AP (0.03-0.1 mM) increased the amplitude of the fast excitatory postsynaptic potential (f-EPSP) without affecting appreciably either the acetylcholine (ACh) depolarization induced by iontophoresis of ACh or the passive and active membrane properties of the neurons. At concentrations of 1-5 mM, 4-AP reversibly depressed the amplitude of the f-EPSP as well as the ACh depolarization; a slight to moderate prolongation of the action potential duration was observed. In addition to the effects on evoked synaptic potentials, 4-AP induced spontaneous discharges which were abolished reversibly by curare, low Ca solution or Co. The results indicate that 4-AP at low concentrations facilitated evoked as well as spontaneous release of ACh by a presynaptic mechanism, whereas at higher concentrations it exerted a curare-like effect on the postsynaptic membrane.