Stimulation Of Stratum Radiatum Research Articles

Selective inhibition of local excitatory synaptic transmission by serotonin through an unconventional receptor in the CA1 region of rat hippocampus.

1. The modulation of synaptic transmission by serotonin (5-HT) was studied using whole-cell voltage-clamp and sharp-electrode current-clamp recordings from CA1 pyramidal neurones in transverse rat hippocampal slices in vitro. 2. With GABA(A) receptors blocked, polysynaptic transmission evoked by stratum radiatum stimulation was inhibited by submicromolar concentrations of 5-HT, while monosynaptic excitatory transmission and CA1 pyramidal neurone excitability were unaffected. The effect persisted following pharmacological blockade of 5-HT(1A) and 5-HT(4) receptors, which directly affect CA1 pyramidal neurone excitability. 3. Concentration-response relationships for 5-HT were determined in individual neurones; the EC(50) values for block of polysynaptic excitation and inhibition by 5-HT were approximately 230 and approximately 160 nM, respectively. The 5-HT receptor type responsible for the observed effect does not fall easily into the present classification of 5-HT receptors. 4. 5-HT inhibition of polysynaptic EPSCs persisted following complete block of GABAergic transmission and in CA1 minislices, ruling out indirect effects through interneurones and non-CA1 pyramidal neurones, respectively. 5. Monosynaptic EPSCs evoked by stimulation of CA1 afferent pathways appeared to be unaffected by 5-HT. Monosynaptic EPSCs evoked by stimulation of the alveus, which contains CA1 pyramidal neurone axons, were partially inhibited by 5-HT. 6. We conclude that 5-HT inhibited synaptic transmission by acting at local recurrent collaterals of CA1 pyramidal neurones. This may represent an important physiological action of 5-HT in the hippocampus, since it occurs over a lower concentration range than the 5-HT effects reported so far.

Neuropeptide Y(5) receptors reduce synaptic excitation in proximal subiculum, but not epileptiform activity in rat hippocampal slices.

Neuropeptide Y (NPY) potently inhibits excitatory synaptic transmission in the hippocampus, acting predominantly via a presynaptic Y(2) receptor. Recent reports that the Y(5) receptor may mediate the anticonvulsant actions of NPY in vivo prompted us to test the hypothesis that Y(5) receptors inhibit synaptic excitation in the hippocampal slice and, furthermore, that they are effective in an in vitro model of anticonvulsant action. Two putative Y(5) receptor-preferring agonists inhibited excitatory postsynaptic currents (EPSCs) evoked by stimulation of stratum radiatum in pyramidal cells. We recorded initially from area CA1 pyramidal cells, but subsequently switched to cells from the subiculum, where a much greater frequency of response was observed to Y(5) agonist application. Both D-Trp(32)NPY (1 microM) and [ahx(8-20)]Pro(34)NPY (3 microM), a centrally truncated, Y(1)/Y(5) agonist we synthesized, inhibited stimulus-evoked EPSCs in subicular pyramidal cells by 44.0 +/- 5.7% and 51.3 +/- 3.5% (mean +/- SE), in 37 and 58% of cells, respectively. By contrast, the less selective centrally truncated agonist, [ahx(8-20)] NPY (1 microM), was more potent (66.4 +/- 4.1% inhibition) and more widely effective, suppressing the EPSC in 86% of subicular neurons. The site of action of all NPY agonists tested was most probably presynaptic, because agonist application caused no changes in postsynaptic membrane properties. The selective Y(1) antagonist, BIBP3226 (1 microM), did not reduce the effect of either more selective agonist, indicating that they activated presynaptic Y(5) receptors. Y(5) receptor-mediated synaptic inhibition was more frequently observed in slices from younger animals, whereas the nonselective agonist appeared equally effective at all ages tested. Because of the similarity with the previously reported actions of Y(2) receptors, we tested the ability of Y(5) receptor agonists to suppress stimulus train-induced bursting (STIB), an in vitro model of ictaform activity, in both area CA3 and the subiculum. Neither [ahx(8-20)]Pro(34)NPY nor D-Trp(32)NPY were significantly effective in suppressing or shortening STIB-induced afterdischarge, with <20% of slices responding to these agonists in recordings from CA3 and none in subiculum. By contrast, 1 microM each of [ahx(8-20)]NPY, the Y(2) agonist, [ahx(5-24)]NPY, and particularly NPY itself suppressed the afterdischarge in area CA3 and the subiculum, as reported earlier. We conclude that Y(5) receptors appear to regulate excitability to some degree in the subiculum of young rats, but their contribution is relatively small compared with those of Y(2) receptors, declines with age, and is insufficient to block or significantly attenuate STIB-induced afterdischarges.

2-Deoxyglucose-induced long-term potentiation in CA1 is not prevented by intraneuronal chelator.

In hippocampal slices, temporary (10-20 min) replacement of glucose with 10 mM 2-deoxyglucose is followed by marked and very sustained potentiation of EPSPs (2-DG LTP). To investigate its mechanism, we examined 2-DG's effect in CA1 neurons recorded with sharp 3 M KCl electrodes containing a strong chelator, 50 or 100 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). In most cases, field EPSPs were simultaneously recorded and conventional LTP was also elicited in some cells by tetanic stimulation of stratum radiatum. 2-DG potentiated intracellular EPSP slopes by 48 +/- 5.1% (SE) in nine cells recorded with plain KCl electrodes and by 52 +/- 6.2% in seven cells recorded with EGTA-containing electrodes. In four of the latter cells, tetanic stimulation (twice 100 Hz for 1 s) failed to evoke LTP (2 +/- 1.1%), although field EPSPs were clearly potentiated (by 28 +/- 6.9%). Thus unlike tetanic LTP, 2-DG LTP is not readily prevented by postsynaptic intraneuronal injection of EGTA. These findings agree with other evidence that the rise in postsynaptic (somatic) [Ca(2+)](i) caused by 2-DG is not the principal trigger for the subsequent 2-DG LTP and that it may be a purely presynaptic phenomenon.

Laminar difference in GABA uptake and GAT-1 expression in rat CA1.

1. The axonal plexus of most hippocampal interneurons is restricted to certain strata within the target region. This lamination suggests a possible functional heterogeneity of inhibitory synapses between different interneurons and CA1 pyramidal cells. 2. We therefore compared inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in CA1 pyramidal cells, which were evoked from two stimulation sites (stratum oriens and stratum radiatum). Stimulation in stratum oriens yielded faster decaying IPSPs and IPSCs than stimulation in stratum radiatum. 3. IPSP and IPSC kinetics were regulated by GABA uptake in both layers as indicated by the prolongation of the signals under tiagabine, a GAT-1 (neuronal GABA plasma membrane transporter)-specific GABA-uptake blocker. However, the effect of tiagabine was significantly more pronounced following stimulation in stratum radiatum than in stratum oriens (prolongation of IPSC half-decay time by 167 vs. 115 %, respectively). 4. In situ hybridization with antisense mRNA for the GABA-synthesizing enzyme glutamate decarboxylase (GAD65/67) and the GABA transporter GAT-1 showed that the proportion of interneurons expressing GAT-1 was lower in stratum oriens than in stratum radiatum/lacunosum-moleculare. 5. From these functional and molecular data we conclude that the regulation of IPSP and IPSC kinetics in CA1 pyramidal cells by neuronal GABA uptake differs between layers. Our findings suggest that this laminar difference is caused by a lower expression of GAT-1 in interneurons in stratum oriens than in stratum radiatum/lacunosum-moleculare.

Long-term plasticity at excitatory synapses on aspinous interneurons in area CA1 lacks synaptic specificity.

The synaptic specificity of long-term potentiation (LTP) was examined at synapses formed on aspinous dendrites of interneurons whose somata were located in the pyramidal cell layer of hippocampal area CA1. Intracellular recordings from slices prepared from rats were used to monitor excitatory postsynaptic potentials (EPSPs) elicited by extracellular stimulation in stratum radiatum. Two synaptic inputs were evoked at 0.5 Hz by stimulating axons adjacent to stratum pyramidale and s. lacunosum-moleculare. After obtaining baseline recordings (>/=10 min), one of the EPSPs was conditioned. The protocol involved tetanic stimulation, sometimes combined with somatic depolarization. Low-frequency stimulation of the two pathways was then resumed and EPSPs were recorded for <30 min. We observed both homosynaptic and heterosynaptic changes in synaptic strength. LTP and long-term depression (LTD) were seen in both pathways and all possible combinations of changes in the two EPSPs were observed, including heterosynaptic LTP associated with either homosynaptic LTP or LTD. Intracellular 1,2-bis (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) abolished alterations in synaptic strength. When axons in s. radiatum synapse onto a spiny pyramidal cell, synaptic specificity of LTP is preserved. However the results obtained from aspinous interneurons show that synaptic specificity of LTP is lost. These results are consistent with the hypothesis that spines provide postsynaptic mechanism(s) for conferring specificity to LTP.

Simultaneous release of glutamate and GABA might be induced in mossy fibers after kindling

The functional relevance of presynaptic glutamate receptors in controlling presynaptic Ca2+ influx and thereby transmitter release is unknown. To test if presynaptic Ca2+ entry in the hippocampus is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presynaptic Ca2+ signals with Ca2+-sensitive microelectrodes after lesioning of neurons by glucose deprivation or kainate. Stratum radiatum and alveus stimulation-induced postsynaptic field potential components were irreversibly abolished in areas CA1 and CA3 of lesioned slices, whereas stratum radiatum stimulation still evoked afferent volleys. Repetitive stimulation of the stratum radiatum still induced decreases in extracellular Ca2+ concentration. Repetitive stimulation of the alveus no longer induced decreases in extracellular Ca2+ concentration, suggesting complete damage of pyramidal cells. The stratum radiatum stimulation-induced decreases in extracellular Ca2+ concentration in lesioned slices were comparable to those elicited during application of the glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate.In lesioned slices the stimulus-induced presynaptic Ca2+ influx was reversibly reduced by kainate, RS-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), N-methyl-d-aspartate and glutamate without effects on afferent volleys. The kainate and N-methyl-d-aspartate effects on presynaptic Ca2+ signals were partly sensitive to 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate, respectively, while the AMPA effects were not significantly affected by 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline, suggesting involvement of a novel glutamate receptor subtype. The involvement of a novel glutamate receptor subtype was supported by our findings that ionotropic glutamate receptor agonists also reduce presynaptic Ca2+ influx under conditions of blocked synaptic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate. 1-Aminocyclopentane-trans-1,3-dicarboxylic acid had no significant effect on presynaptic Ca2+ entry. Also, the presynaptic Ca2+ influx was not influenced by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate when applied alone.Low kainate concentrations (5 μM) reduced presynaptic Ca2+ signals in area CA3 but not in area CA1, demonstrating the higher affinity of presynaptic kainate receptors on mossy fibre terminals.

Structure-dependent differences in the effects of the Aconitum alkaloids lappaconitine, N-desacetyllappaconitine and lappaconidine in rat hippocampal slices

Lappaconitine, a C 19 diterpenoid alkaloid from Aconitum sinomontanum has been reported to possess analgesic and antiinflammatory properties in vivo and to inhibit neuronal activity in brain slices. In the present study the effect of lappaconitine has been compared with the effects of its main metabolite N-desacetyllappaconitine and the structurally related alkaloid lappaconidine. For comparison of drug effects population spikes and field excitatory postsynaptic potentials (EPSPs) evoked by stimulation of stratum radiatum or the alveus were studied in normal rat hippocampal slices and in slices treated with low Mg 2+-medium. At concentrations of 3–100 μM, both lappaconitine and N-desacetyllappaconitine inhibited population spikes elicited by stratum radiatum and alvear stimulation as well as the field EPSP recorded in CA1 stratum radiatum. The drug-induced depression of field potential responses was increased with rising stimulus frequency, indicating an activity-dependent mode of action. The effect of N-desacetyllappaconitine on each parameter investigated was significantly stronger than the effect of lappaconitine. Despite the structural relationship, lappaconidine failed to affect neuronal excitability in concentration below 100 μM, and an increase in stimulus frequency did not potentiate its effect. Moreover, lappaconitine and N-desacetyllappaconitine suppressed epileptiform activity induced by bicuculline or by omission of Mg 2+ from the bathing medium.

Potentiation of glutamatergic EPSPs in rat CA1 hippocampal neurons after selective cholinergic denervation by 192 IgG-saporin.

A complete and selective destruction of the basal forebrain cholinergic neurons projecting to the cerebral cortex and the hippocampus was induced in the rat by the toxin 192 IgG-saporin. Using electrophysiologic techniques, we have investigated the consequences of this cholinergic denervation on inhibitory and excitatory synaptic responses of CA1 pyramidal cells in rat hippocampal slices ex vivo. Histochemical experiments were performed in slices from control and 192 IgG-saporin-treated rats to check the efficacy of the intracerebroventricular injection of the immunotoxin. Stimulation of stratum radiatum elicits a glutamatergic excitatory postsynaptic potentials followed by a biphasic GABAergic inhibitory postsynaptic potential (IPSP). No significant change in IPSP was observed in 192 IgG-saporin-treated rats. By contrast, the N-methyl-D-aspartate (NMDA) and to a lesser extent the non-NMDA components of the glutamatergic response were potentiated in these animals. The possible pre- and postsynaptic mechanisms of this potentiation were discussed.

Ca2+ oscillations mediated by the synergistic excitatory actions of GABA(A) and NMDA receptors in the neonatal hippocampus.

We asked whether GABA(A) and NMDA receptors may act in synergy in neonatal hippocampal slices, at a time when GABA exerts a depolarizing action. The GABA(A) receptor agonist isoguvacine reduced the voltage-dependent Mg2+ block of single NMDA channels recorded in cell-attached configuration from P(2-5) CA3 pyramidal neurons and potentiated the Ca2+ influx through NMDA channels. The synaptic response evoked by electrical stimulation of stratum radiatum was mediated by a synergistic interaction between GABA(A) and NMDA receptors. Network-driven Giant Depolarizing Potentials, which are a typical feature of the neonatal hippocampal network, provided coactivation of GABA(A) and NMDA receptors and were associated with spontaneous and synchronous Ca2+ increases in CA3 pyramidal neurons. Thus, at the early stages of development, GABA is a major excitatory transmitter that acts in synergy with NMDA receptors. This provides in neonatal neurons a hebbian stimulation that may be involved in neuronal plasticity and network formation in the developing hippocampus.

Effects of glutamate receptor agonists and antagonists on Ca 2+ uptake in rat hippocampal slices lesioned by glucose deprivation or by kainate

The functional relevance of presynaptic glutamate receptors in controlling presynaptic Ca 2+ influx and thereby transmitter release is unknown. To test if presynaptic Ca 2+ entry in the hippocampus is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presynaptic Ca 2+ signals with Ca 2+-sensitive microelectrodes after lesioning of neurons by glucose deprivation or kainate. Stratum radiatum and alveus stimulation-induced postsynaptic field potential components were irreversibly abolished in areas CA1 and CA3 of lesioned slices, whereas stratum radiatum stimulation still evoked afferent volleys. Repetitive stimulation of the stratum radiatum still induced decreases in extracellular Ca 2+ concentration. Repetitive stimulation of the alveus no longer induced decreases in extracellular Ca 2+ concentration, suggesting complete damage of pyramidal cells. The stratum radiatum stimulation-induced decreases in extracellular Ca 2+ concentration in lesioned slices were comparable to those elicited during application of the glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate. In lesioned slices the stimulus-induced presynaptic Ca 2+ influx was reversibly reduced by kainate, RS-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), N-methyl- d-aspartate and glutamate without effects on afferent volleys. The kainate and N-methyl- d-aspartate effects on presynaptic Ca 2+ signals were partly sensitive to 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate, respectively, while the AMPA effects were not significantly affected by 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline, suggesting involvement of a novel glutamate receptor subtype. The involvement of a novel glutamate receptor subtype was supported by our findings that ionotropic glutamate receptor agonists also reduce presynaptic Ca 2+ influx under conditions of blocked synaptic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate. 1-Aminocyclopentane- trans-1,3-dicarboxylic acid had no significant effect on presynaptic Ca 2+ entry. Also, the presynaptic Ca 2+ influx was not influenced by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate when applied alone. Low kainate concentrations (5 μM) reduced presynaptic Ca 2+ signals in area CA3 but not in area CA1, demonstrating the higher affinity of presynaptic kainate receptors on mossy fibre terminals.

Model of spatio-temporal propagation of action potentials in the Schaffer collateral pathway of the CA1 area of the rat hippocampus.

There is a sharp contrast between the profuse in vivo axonal arborization of CA3 pyramidal cells in the CA1 area and the low probability of finding pairs of connected CA3-CA1 pyramidal cells in vitro. These anatomical differences contribute to a connectivity argument for discrepancies between electrophysiological data recorded in vitro and in vivo. In order to investigate this issue, we have developed a realistic computer model of the Schaffer collateral pathway of the hippocampus and analyzed the spatio-temporal distribution of action potentials along this pathway following three different types of electrical test stimulus. Direct activation of mossy fibers, CA3 pyramidal cells and focal stimulation of CA1 stratum radiatum were investigated. The parameters of the model were selected from available biological data. Spikes in Schaffer collaterals were followed from their onset in the CA3 pyramidal cell initial segment to the last order branches of their axonal tree in two types of configuration: the whole hippocampus and the slice configuration. The anatomical and electropysiological characteristics of the mossy fibre and Schaffer collateral pathways were found to impose strong constraints on the spatio-temporal distribution of action potentials in the CA1 area. Specific projection zones are determined by the spatial localization of the emitting CA3 pyramidal cells. Their position also defines precise time windows during which some CA1 projection zones receive a large number of correlated signals. Moreover, the variability of the delay at the mossy fibre/CA3 pyramidal cell synapse seems to provide the CA1 projection zones with a background level of excitation. Finally, we show how the patterns of activation obtained in the whole hippocampus are different from those obtained in the slice.

Model of spatio‐temporal propagation of action potentials in the Schaffer collateral pathway of the CA1 area of the rat hippocampus

There is a sharp contrast between the profuse in vivo axonal arborization of CA3 pyramidal cells in the CA1 area and the low probability of finding pairs of connected CA3-CA1 pyramidal cells in vitro. These anatomical differences contribute to a connectivity argument for discrepancies between electrophysiological data recorded in vitro and in vivo. In order to investigate this issue, we have developed a realistic computer model of the Schaffer collateral pathway of the hippocampus and analyzed the spatio-temporal distribution of action potentials along this pathway following three different types of electrical test stimulus. Direct activation of mossy fibers, CA3 pyramidal cells and focal stimulation of CA1 stratum radiatum were investigated. The parameters of the model were selected from available biological data. Spikes in Schaffer collaterals were followed from their onset in the CA3 pyramidal cell initial segment to the last order branches of their axonal tree in two types of configuration: the whole hippocampus and the slice configuration. The anatomical and electropysiological characteristics of the mossy fibre and Schaffer collateral pathways were found to impose strong constraints on the spatio-temporal distribution of action potentials in the CA1 area. Specific projection zones are determined by the spatial localization of the emitting CA3 pyramidal cells. Their position also defines precise time windows during which some CA1 projection zones receive a large number of correlated signals. Moreover, the variability of the delay at the mossy fibre/CA3 pyramidal cell synapse seems to provide the CA1 projection zones with a background level of excitation. Finally, we show how the patterns of activation obtained in the whole hippocampus are different from those obtained in the slice. Hippocampus 7:58–72, 1997. © 1997 Wiley-Liss, Inc.

Long-lasting changes in synaptic excitability induced bt anoxia in the rat hippocampus

1. 1. Field-potential and intracellular recordings in the CA1 subfield of rat hippocampal slices were employed to study the long-lasting changes in synaptic excitability that follow brief (< 7min) episodes of anoxia. 2. 2. Disappearance of the stratum radiatum-induced population spike and/or substantial reduction of the corresponding field excitatory postsynaptic potential (EPSP) occurred with anoxia. During reoxygenation the population spike amplitude increased in 67% of trials by 20–360% (87 ± 28%, mean ± SEM, n=35) as compared to control; an enhancement of the postanoxic field EPSP was also observed. Both types of increase in synaptic excitability were long-lasting (up to 160 min after reoxygenation). 3. 3. Further anoxic episodes made epileptiform bursts appear in CA1 in response to stratum radiatum stimulation. These postanoxic, epileptiform responses were associated with depolarization of CA1 pyramidal cells (mean reversal potential = 16 ± 7 mV, n=4), and were also seen after surgical isolation from the CA3 subfield. 4. 4. N-methyl-D-aspartate (NMDA) receptor antagonists did not influence the postanoxic increase in population spike or field EPSP but reduced the duration of stratum radiatum-induced epileptiform bursts. Application of a non-NMDA receptor antagonist could abolish both postanoxic synaptic responses and epileptiform bursts. Paired-pulse stimulation protocols revealed a persistent decrease of this type of inhibition (up to 45%) following a single episode of anoxia. 5. 5. The present findings indicate that anoxia can induce a long-lasting enhancement of synaptic excitability as well as a reduction of polysynaptic inhibitory mechanisms in the CA1 subfield. Moreover, repeated anoxic episodes reveal an NMDA-mediated component of excitatory synaptic transmission that contributes to the appearance of epileptiform discharges.

Frequency facilitation in the hippocampal slices of conditioned rats.

Electrophysiological parameters in the hippocampal slices of two experimental (prepared immediately (n = 27) and one week (n = 9) after conditioning) and two control (passive (n = 10) and active (n = 8)) groups of rats have been compared. The experimental rats were trained in a two-way avoidance chamber. The active control rats received the same number of conditioned and unconditioned stimuli without pairing. The threshold and amplitude of population spikes recorded from the CA1 pyramidal cell body layer to the stimulation of stratum radiatum and its modification in a train of 10 pulses (0.2 Hz) were measured. The mean values of the threshold intensity were not significantly different between any pair of these groups. The direction of the changes in the population spike amplitudes following pseudo-conditioning or conditioning was the same. However, the population spike amplitudes decreased more significantly in the slices from the conditioned rats. The increase in the frequency facilitation was specific for the slices of conditioned rats. The modifications in the mechanism of frequency facilitation in the reinforced pathways may represent an important mechanism for behavioural learning.

Spontaneous and stimulation-induced synchronized burst afterdischarges in the isolated CA1 of kainate-treated rats.

1. Kainate treatment preferentially kills dentate hilar neurons and CA3 pyramidal cells and ultimately leads to a chronic epileptic state. Bicuculline-induced epileptiform bursts were studied to test the hypothesis that multiple kainate injections and consequent status epilepticus would lead-after weeks to months of recovery-to prolonged synchronous afterdischarges in the isolated CA1 area of rat hippocampal slices, as would be expected if new recurrent excitatory circuits had formed. 2. Synaptic responses evoked in CA1 pyramidal cells of rats injected subcutaneously with kainate (10 hourly injections, 5 mg/kg each) 24-316 days before the slice experiment were compared with responses in slices from untreated and saline-injected controls. The maximal response to stratum radiatum stimulation in normal solution consisted of two to eight population spikes. 3. When gamma-aminobutyric acid-A receptor-mediated inhibition was reduced with bicuculline, synchronized burst afterdischarges after the initial stimulation-evoked burst, similar to the type of activity described in area CA3 under conditions where inhibition is impaired, occurred in 23% of slices. 4. The prolonged synchronized burst afterdischarges in the isolated CA1 area of kainate-treated rats were associated with large excitatory postsynaptic potentials (EPSPs). These prolonged bursts were not graded with the stimulus intensity; rather, they were triggered in an all-or-none manner, even though there was some variability across bursts. The bursts of population spikes also were correlated with subthreshold EPSPs. 5. Slices that had synchronized burst afterdischarges had significantly more damage in area CA3 than slices without afterdischarges. 6. The data indicate that kainate-induced damage in CA3 can lead to prolonged synchronous afterdischarges, even after CA1 is surgically isolated from the CA3 area. Because the repetitive bursts during the prolonged and synchronous afterdischarges were associated with large EPSPs, these data suggest that kainate-induced damage to CA3 and subsequent degeneration of synaptic terminals in the CA1 area causes the formation of new recurrent excitatory circuits that could be involved in the development of chronic epilepsy.

Effects of glutamate receptor agonists on presumed presynaptic Ca2+-signals in juvenile rat hippocampal area CA1

The physiological role of presynaptic glutamate receptors in controlling presynaptic Ca2+-influx and thereby transmitter release is unknown. To test if presynaptic Ca2+-uptake in the hippocampas is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presumed presynaptic Ca2+-signals with ion-sensitive microelectrodes after lesioning of postsynaptic neurons by glucose deprivation. After prolonged glucose deprivation in slices from juvenile animals of postnatal days 13–15 and 20–22, stratum radiatum (SR) and alveus stimulation-induced postsynaptic field potential (fp) components were irreversibly abolished in area CA1, whereas SR stimulation still evoked afferent volleys. Repetitive stimulation of the SR still induced small decreases in the extracellular Ca2+ concentration ([Ca2+]0), but repetitive alveus stimulation no longer induced decreases in [Ca+]0, suggesting a complete damage of pyramidal cells. In lesioned slices the remaining SR stimulation-induced small decreases in [Ca2+]0 presumably reflect presynaptic Ca 2+-influx. These small decreases in [Ca2+]0 were reversibly reduced by kainate, RS-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-d-aspartate (NMDA) and l-glutamic acid (glutamate), without effects on afferent volleys.

Effects of glutamate receptor agonists on presumed presynaptic Ca 2+-signals in juvenile rat hippocampal area CA1

The physiological role of presynaptic glutamate receptors in controlling presynaptic Ca 2+-influx and thereby transmitter release is unknown. To test if presynaptic Ca 2+-uptake in the hippocampas is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presumed presynaptic Ca 2+-signals with ion-sensitive microelectrodes after lesioning of postsynaptic neurons by glucose deprivation. After prolonged glucose deprivation in slices from juvenile animals of postnatal days 13–15 and 20–22, stratum radiatum (SR) and alveus stimulation-induced postsynaptic field potential (fp) components were irreversibly abolished in area CA1, whereas SR stimulation still evoked afferent volleys. Repetitive stimulation of the SR still induced small decreases in the extracellular Ca 2+ concentration ([Ca 2+] 0), but repetitive alveus stimulation no longer induced decreases in [Ca +] 0, suggesting a complete damage of pyramidal cells. In lesioned slices the remaining SR stimulation-induced small decreases in [Ca 2+] 0 presumably reflect presynaptic Ca 2+-influx. These small decreases in [Ca 2+] 0 were reversibly reduced by kainate, RS-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl- d-aspartate (NMDA) and l-glutamic acid (glutamate), without effects on afferent volleys.

Effects of T-type, L-type, N-type, P-type, and Q-type calcium channel blockers on stimulus-induced pre- and postsynaptic calcium fluxes in rat hippocampal slices.

The contribution of T-, L-, N-, P-, and Q-type Ca2+ channels to pre- and postsynaptic Ca2+ entry during stimulus-induced high neuronal activity in area CA1 of rat hippocampal slices was investigated by measuring the effect of specific blockers on stimulus-induced decreases in extracellular Ca2+ concentration ([Ca2+]o). [Ca2+]o was measured with ion-selective electrodes in stratum radiatum (SR) and stratum pyramidale (SP), while Ca2+ entry into neurons was induced with stimulus trains (20 Hz for 10 s) alternately delivered to SR and the alveus, respectively. The [Ca2+]o decreases recorded in SR in response to SR stimulation represented mainly presynaptic Ca2+ entry (Capre), while [Ca2+]o decreases recorded in SP in response to alvear stimulation were predominantly based on postsynaptic Ca2+ entry (Capost). Ethosuximide and trimethadione were ineffective in concentrations up to 1 mM. At 10 mM, they reduced Capost and, much less, also Capre. Nimodipine (25 microM) reduced Capost and, to a minor extent, Capre. omega-Agatoxin IVA (0.4-1 microM) and omega-contoxin MVIIC (1 microM) also reduced both Capre and Capost, but with a stronger action on Capre. omega-Conotoxin GVIA (3-8 microM) reduced Capost without effect on Capre. We conclude that during stimulus-induced, high-frequency neuronal activity Capost is carried by P/Q-, N-, and L- type channels and probably a further channel type different from these channels. Capre includes at least P/Q- and possibly L-type channels. N-type channels did not contribute to Capre in our experiments. Since ethosuximide and trimethadione were only effective in high concentrations, their action may be unspecific. Thus, T-type channels do not seem to play a major part in Ca2+ entry in this situation.

Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus.

We recorded in the CA1 region from hippocampal slices of prion protein (PrP) gene knockout mice to investigate whether the loss of the normal form of prion protein (PrPC) affects neuronal excitability as well as synaptic transmission in the central nervous system. No deficit in synaptic inhibition was found using field potential recordings because (i) responses induced by stimulation in stratum radiatum consisted of a single population spike in PrP gene knockout mice similar to that recorded from control mice and (ii) the plot of field excitatory postsynaptic potential slope versus the population spike amplitude showed no difference between the two groups of mice. Intracellular recordings also failed to detect any difference in cell excitability and the reversal potential for inhibitory postsynaptic potentials. Analysis of the kinetics of inhibitory postsynaptic current revealed no modification. Finally, we examined whether synaptic plasticity was altered and found no difference in long-term potentiation between control and PrP gene knockout mice. On the basis of our findings, we propose that the loss of the normal form of prion protein does not alter the physiology of the CA1 region of the hippocampus.

Changes in quantal parameters of EPSCs in rat CA1 neurones in vitro after the induction of long-term potentiation.

1. Long-term potentiation (LTP) was induced in EPSCs evoked in CA1 pyramidal neurones of young rats in vitro by extracellular stimulation of stratum radiatum. Low frequency stimulation was paired with postsynaptic depolarization to induce LTP, using whole-cell recording techniques. 2. Sufficient control and potentiated records were obtained under stable recording conditions to allow a quantal analysis of eleven EPSCs. The fluctuations in amplitude of all eleven EPSCs were quantized before conditioning stimulation, and they remained quantized after LTP induction, usually with an increased quantal variance. 3. Quantal current was increased by conditioning for nine out of eleven EPSCs. The increase in quantal current was correlated with the percentage increase in the EPSC. For only two EPSCs could the entire potentiation be attributed to an increase in quantal current. 4. The amplitude fluctuations of five control EPSCs could be described by binomial statistics, but after conditioning the binomial description held for only one of these EPSCs. For this EPSC, conditioning caused the release probability to increase from 0.39 +/- 0.05 to 0.47 +/- 0.02. 5. Quantal content was increased by conditioning stimulation for ten out of eleven EPSCs. The increase in quantal content was correlated with the percentage increase in the EPSC. However, for only four EPSCs could the entire potentiation be attributed to an increase in quantal content. 6. Most EPSCs were evoked with a high proportion of response failures. The probability of response failures decreased in eight out of eleven EPSCs following the induction of LTP. There was a negative correlation between the change in the probability of response failures and the amount of LTP. 7. The minimal number of sites at which transmission occurred increased for ten out of eleven EPSCs following LTP induction. Increases in the minimal number of active sites following conditioning were associated with decreases in the probability of response failures for seven out of eleven EPSCs. 8. The induction of LTP usually resulted in changes in the time course of the EPSCs. Cable analysis using a passive compartmental model of a CA1 pyramidal cell suggested that these time course changes were associated with shifts in the average electrotonic location of the active sites following LTP induction, rather than being caused by an increased duration of synaptic current. 9. LTP expression involves postsynaptic modifications to enhance the synaptic current at active sites. New sites are recruited, and our data cannot be used to determine if this is a result of a pre- or a postsynaptic change. Evidence for an increase in release probability was found for one EPSC.