Dendritic excitability during increased synaptic activity in rat neocortical L5 pyramidal neurons

Recent years have seen increased study of dendritic integration, mostly in acute brain slices. However, due to the low background activity in brain slices the integration of synaptic input in slice preparations may not truly reflect conditions in vivo. To investigate dendritic integration, back‐propagation of the action potential (AP) and initiation of the dendritic Ca²⁺ spike we simultaneously recorded membrane potential at the soma and apical dendrite of layer 5 (L5) pyramidal neurons in quiescent and excited acute brain slices. After excitation of the brain slice the somatic input resistance decreased and the apparent passive space constant shortened. However, the back‐propagating AP and dendritic Ca²⁺ spike were robust during increased synaptic activity. The dendritic Ca²⁺ spike was suppressed by the ionic composition of the bath solution required for slice excitation, suggesting that Ca²⁺ spikes may be smaller in vivo than in the acute slice preparation. The results presented here suggest that, under the conditions of slice excitation examined in this study, the increased membrane conductance induced by activation of voltage‐gated channels during back‐propagation of the AP and dendritic Ca²⁺ spike initiation is sufficiently larger than the membrane conductance at subthreshold potentials to allow these two regenerative dendritic events to remain robust over several levels of synaptic activity in the apical dendrite of L5 pyramidal neurons.     

The effects of moderate changes of extracellular K and Ca on synaptic and neural function in the CA1 region of the hippocampal slice

The effects of moderate changes of the concentration of ions on the function of mammalian central nervous tissue have not exactly been determined. We placed tissue slices from rat hippocampal formation in an interface chamber for study in vitro. Extracellular potentials were recorded in stratum radiatum and stratum pyramidale in response to stimuli of varying intensity applied to the Schaffer collateral bundle. The overall input-output relationship of excitatory synaptic transmission was gauged by expressing postsynaptic population spike amplitude as a function of presynaptic volley amplitude. The components of the transmission process were also examined by plotting (1) the maximal rate of rise (slope) of the focally recorded synaptic potential (fEPSP) as a function of presynaptic volley amplitude, and (2) the population spike amplitude as a function of the fEPSP slope. Raising the concentration of K⁺ from the normal level of 3.5 mM to 5 mM caused an average increase of 48% in the population spike evoked by a given presynaptic volley. This was due to an increased electrical excitability of pyramidal cells, as indicated by an increase of the population spike evoked by a given magnitude of fEPSP. Conversely, lowering [K⁺]_o from 3.5 to 2 mM caused a decrease of the population spike relative to a given magnitude of either the presynaptic volley or the fEPSP. Changing [K⁺]_o within these limits caused no significant change of the fEPSP evoked by a given presynaptic volley. Raising [Ca²⁺]_o from 1.2 to 1.8 mM caused a 35% increase in both the fEPSP and the population spike evoked by a given presynaptic volley, and lowering [Ca²⁺]_o to 0.8 mM caused a decrease of both these functions. The amplitude of the population spikes evoked by given fEPSPs changed surprisingly little (but consistently) when [Ca²⁺]_o was varied within these limits. We conclude that moderate changes of [K⁺]_o influence mainly the electric excitability of hippocampal pyramidal cells, with little effect on transmitter release or on the response of the postsynaptic membrane to transmitter, while moderate changes of [Ca²⁺]_o affect the release of excitatory synaptic transmitter more than they affect postsynaptic membrane function.     

Action potential broadening induced by lithium may cause a presynaptic enhancement of excitatory synaptic transmission in neonatal rat hippocampus

Lithium enhances excitatory synaptic transmission in CA1 pyramidal cells, but the mechanisms remain unclear. The present study demonstrates that lithium enhances the N-methyl-d -aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) receptor-mediated components of the excitatory postsynaptic current (EPSC). Lithium decreased the magnitude of paired-pulse facilitation and presented an inverse correlation between the lithium-induced enhancement of synaptic transmission and initial paired-pulse facilitation, which is consistent with a presynaptic mode of action. The enhancement of synaptic strength is likely to act, at least in part, by increasing the amplitude of the presynaptic Ca²⁺ transient. One mechanism which could account for this change of the presynaptic Ca²⁺ transient is an increase in the duration of the action potential. We investigated action potential in hippocampal pyramidal neurons and found that lithium (0.5–6 mm ) increased the half-amplitude duration and reduced the rate of repolarization, whereas the rate of depolarization remained similar. To find out whether the lithium synaptic effects might be explained by spike broadening, we investigated the field recording of the excitatory postsynaptic potential (EPSP) in hippocampal slices and found three lines of evidence. First, the prolongation of the presynaptic action potential with 4-aminopyridine and tetraethylammonium blocked or reduced the synaptic effects of lithium. Second, the lithium-induced synaptic enhancement was modulated when presynaptic Ca²⁺ influx was varied by changing the external Ca²⁺ concentration. Finally, both effects, the synaptic transmission increment and the action potential broadening, were independent of inositol depletion. These results suggest that lithium enhances synaptic transmission in the hippocampus via a presynaptic site of action: the mechanism underlying the potentiating effect may be attributable to an increased Ca²⁺ influx consequent to the broadening effect of lithium on the action potential.     

A new type of Ca channel blocker, NC-1100, inhibits the low- and high-threshold Ca currents in the rat CNS neurons

The effect of a new type of organic Ca²⁺ channel blocker, NC-1100 [(±)-1-(3,4-dimethoxyphenyl)-2-(4-diphenylmethylpiperazinyl)ethanol dihydrochloride], on both low- and high-threshold Ca²⁺ currents was studied in the whole-cell mode of the pyramidal neurons freshly dissociated from rat hippocampal CA1 region under voltage-clamp condition. The NC-1100 reversibly reduced the high-threshold Ca²⁺ current (HVAI_Ca) in a concentration-dependent manner without affecting the current-voltage relationship. The values of half-inhibition (IC₅₀) were 1.3 × 10⁻⁵ and 9.1 × 10⁻⁶M in external solution containing 10 and 2.5 mM Ca²⁺, respectively. The NC-1100 also decreased the low-threshold Ca²⁺ current (LVAI_Ca) in a concentration-dependent manner. The inhibitory potency was augmented by increasing the stimulation frequency and / or decreasing the extracellular Ca²⁺ concentration to a physiological range (2.5 mM). The IC₅₀ value decreased to 7.7 × 10⁻⁷M in external solution containing 2.5 mM Ca²⁺ at a stimulation frequency of 1 Hz. The NC-1100 delayed the reactivation of LVA Ca²⁺ channel and enhanced voltage-dependently the steady-state inactivation, suggesting that this drug bound not only the resting LVA Ca²⁺ channel but also the inactivated one.     

Preferential inhibition of L- and N-type calcium channels in the rat hippocampal neurons by cilnidipine

The effect of a dihydropyridine Ca²⁺ antagonist, cilnidipine, on voltage-dependent Ca²⁺ channels was studied in acutely dissociated rat CA1 pyramidal neurons using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Cilnidipine had no effect on low-voltage-activated (LVA) Ca²⁺ channels at the low concentrations under 10⁻⁶ M. On the other hand, cilnidipine inhibited the high-voltage-activated (HVA) Ca²⁺ current (I_Ca) in a concentration-dependent manner and the inhibition curve showed a step-wise pattern; cilnidipine selectively reduced only L-type HVA I_Ca at the low concentrations under 10⁻⁷ and 10⁻⁶ M cilnidipine blocked not only L- but also N-type HVA I_Ca. At the high concentration over 10⁻⁶ M cilnidipine non-selectively blocked the T-type LVA and P/Q- and R-type HVA Ca²⁺ channels. This is the first report that cilnidipine at lower concentration of 10⁻⁶ M blocks both L- and N-type HVA I_Ca in the hippocampal neurons.     

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.     

Calcium Channel-Dependent Induction of Long-Term Synaptic Plasticity at Excitatory Golgi Cell Synapses of Cerebellum

Golgi cells, together with granule cells and mossy fibers, form a neuronal microcircuit regulating information transfer at the cerebellum input stage. Despite theoretical predictions, little was known about long-term synaptic plasticity at Golgi cell synapses. Here, we have used whole-cell patch-clamp recordings and calcium imaging to investigate long-term synaptic plasticity at excitatory synapses impinging on Golgi cells. In acute mouse cerebellar slices, mossy fiber theta-burst stimulation (TBS) could induce either long-term potentiation (LTP) or long-term depression (LTD) at mossy fiber-Golgi cell and granule cell-Golgi cell synapses. This synaptic plasticity showed a peculiar voltage dependence, with LTD or LTP being favored when TBS induction occurred at depolarized or hyperpolarized potentials, respectively. LTP required, in addition to NMDA channels, activation of T-type Ca²⁺ channels, while LTD required uniquely activation of L-type Ca²⁺ channels. Notably, the voltage dependence of plasticity at the mossy fiber-Golgi cell synapses was inverted with respect to pure NMDA receptor-dependent plasticity at the neighboring mossy fiber-granule cell synapse, implying that the mossy fiber presynaptic terminal can activate different induction mechanisms depending on the target cell. In aggregate, this result shows that Golgi cells show cell-specific forms of long-term plasticity at their excitatory synapses, that could play a crucial role in sculpting the response patterns of the cerebellar granular layer.SIGNIFICANCE STATEMENT This article shows for the first time a novel form of Ca²⁺ channel-dependent synaptic plasticity at the excitatory synapses impinging on cerebellar Golgi cells. This plasticity is bidirectional and inverted with respect to NMDA receptor-dependent paradigms, with long-term depression (LTD) and long-term potentiation (LTP) being favored at depolarized and hyperpolarized potentials, respectively. Furthermore, LTP and LTD induction requires differential involvement of T-type and L-type voltage-gated Ca²⁺ channels rather than the NMDA receptors alone. These results, along with recent computational predictions, support the idea that Golgi cell plasticity could play a crucial role in controlling information flow through the granular layer along with cerebellar learning and memory.     

Action of vasopressin on CA1 pyramidal neurons in rat hippocampal slices

The effects of arginine⁸-vasopressin (AVP) on the excitability of 47 pyramidal cells of the CA1 region of the hippocampus were determined by using intracellular recording techniques in a submerged slice preparation. Addition of 10⁻⁶ M AVP to the bathing medium evoked an increase in spike discharge which was slow in onset and only gradually reversible. The discharge was accompanied by an increase in excitatory postsynaptic potentials without significant change of the resting input resistance. AVP-induced excitation was found in 81% of ventral and 29% of dorsal hippocampal CA1 pyramidal cells. In low Ca²⁺, high Mg²⁺ solution this excitatory action by AVP was blocked. Microiontophoretic application of AVP onto apical or basal dendrites or the cell body did not result in excitation. These observations suggest that the action of AVP on CA1 pyramidal cells is transsynaptic and is more pronounced in ventral than dorsal CA1.     

GABAergic interneurons targeting dendrites of pyramidal cells in the CA1 area of the hippocampus

The dendrites of pyramidal cells are active compartments capable of independent computations, input/output transformation and synaptic plasticity. Pyramidal cells in the CA1 area of the hippocampus receive 92% of their GABAergic input onto dendrites. How does this GABAergic input participate in dendritic computations of pyramidal cells? One key to understanding their contribution to dendritic computation lies in the timing of GABAergic input in relation to excitatory transmission, back‐propagating action potentials, Ca²⁺ spikes and subthreshold membrane dynamics. The issue is further complicated by the fact that dendritic GABAergic inputs originate from numerous distinct sources operating with different molecular machineries and innervating different subcellular domains of pyramidal cell dendrites. The GABAergic input from distinct sources is likely to contribute differentially to dendritic computations. In this review, I describe four groups of GABAergic interneuron according to their expression of parvalbumin, cholecystokinin, axonal arborization density and long‐range projections. These four interneuron groups contain at least 12 distinct cell types, which innervate mainly or exclusively the dendrites of CA1 pyramidal cells. Furthermore, I summarize the different spike timing of distinct interneuron types during gamma, theta and ripple oscillations in vivo, and I discuss some of the open questions on how GABAergic input modulates dendritic operations in CA1 pyramidal cells.     

Serotonin, via 5-HT2A receptors, increases EPSCs in layer V pyramidal cells of prefrontal cortex by an asynchronous mode of glutamate release

Previously, serotonin (5-HT) was found to induce a marked increase in glutamatergic spontaneous excitatory postsynaptic currents (EPSCs) in apical dendrites of layer V pyramidal cells of prefrontal cortex; this effect was mediated by 5-HT_2A receptors, a proposed site of action of hallucinogenic and atypical antipsychotic drugs. Unexpectedly, although the effect of 5-HT was Ca²⁺-dependent and tetrodotoxin-sensitive, it did not appear to involve the activation of excitatory afferent impulse flow. This paradox prompted us to investigate (in rat brain slices) whether 5-HT was acting through an atypical mode of excitatory transmitter release. We found that the frequency of 5-HT-induced spontaneous EPSCs was fully supported by Sr²⁺ in the absence of added Ca²⁺, implicating the mechanism of asynchronous transmitter release which has been linked to the high-affinity Ca²⁺-sensor synaptotagmin III. Although the early, synchronous component of electrically evoked EPSCs was reduced while 5-HT was being applied, late, nonsynchronous components were enhanced during 5-HT washout and also by the 5-HT₂ partial agonist 1-(2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI); the effect of DOI was blocked by a selective 5-HT_2A antagonist (MDL 100,907). This late, nonsynchronous component was distinct from conventional polysynaptic EPSCs evoked in the presence of the GABA_A antagonist bicuculline, but resembled asynchronous glutamatergic excitatory postsynaptic potentials (EPSPs) evoked in the presence of Sr²⁺. An enhancement of asynchronous EPSCs by a specific neurotransmitter receptor has not been reported previously. The possible role of excessive asynchronous transmission in the cerebral cortex in mediating the hallucinogenic effects of 5-HT_2A agonists such as DOI is discussed.     

Intracellular calcium elevation during plateau potentials mediated by extrasynaptic NMDA receptor activation in rat hippocampal CA1 pyramidal neurons is primarily due to calcium entry through voltage‐gated calcium channels

We reported previously that plateau potentials mediated by extrasynaptic N‐methyl‐d ‐aspartate receptors (NMDARs) can be induced either by synaptic stimulation in the presence of glutamate transporter antagonist or by iontophoresis of NMDA in rat hippocampal CA1 pyramidal neurons. To examine whether the plateau potentials are accompanied by an elevation of intracellular Ca²⁺ and to determine the source of Ca²⁺ elevation, we performed Ca²⁺ imaging during the plateau potential. Neurons were loaded with Ca²⁺ indicator fluo‐4, and the plateau potentials were generated either synaptically in the presence of glutamate transporter antagonist or by iontophoretically applying NMDA. We have found that a transient elevation in intracellular Ca²⁺ accompanies the plateau potential. The synaptically induced plateau potential and the Ca²⁺ elevation were blocked by 5,7‐dichlorokynurenic acid (5,7‐dCK), an antagonist for the glycine‐binding sites of NMDAR. A mixture of Cd²⁺ and tetrodotoxin did not block NMDA‐induced plateau potentials, but completely abolished the accompanying Ca²⁺ elevation in both the presence and absence of Mg²⁺ ions in the bathing solution. The NMDA‐induced plateau potential was blocked by further adding 5,7‐dCK. Our results show that the NMDAR‐mediated plateau potential is accompanied by elevation of intracellular Ca²⁺ that is primarily caused by the influx of Ca²⁺ through voltage‐gated Ca²⁺ channels.     

The action of valproate on spontaneous epileptiform activity in the absence of synaptic transmission and on evoked changes in [Ca]0 and [K]0 in the hippocampal slice

The effects of valproate (VPA) on neuronal excitability and on changes in extracellular potassium ([K⁺]₀) and calcium ([Ca²⁺]₀) were investigated with ion selective-reference electrode pairs in area CA₁ of rat hippocampal slices. Field potential responses to single ortho- and antidromic stimuli were unaltered by VPA (1–5 mM). The afferent volley evoked in the Schaffer-commissural fibers was also unaffected. In contrast, VPA (1 mM) depressed frequency potentiation and paired pulse facilitation markedly. Decreases in [Ca²⁺]₀ induced either by repetitive stimulation or by application of the excitatory amino acids N-methyl-d-aspartate and quisqualate were reduced, and the latter results suggest that VPA interferes with postsynaptic Ca²⁺ entry. When synaptic transmission was blocked by lowering [Ca²⁺]₀ (0.2 mM) and elevating [Mg²⁺]₀ (7 mM), prolonged afterdischarges elicited by antidromic stimulation were blocked by VPA. VPA also suppressed the spontaneous epileptiform activity seen when [Ca²⁺]₀ was lowered to 0.2 mM, without elevating [Mg²⁺]₀. The amplitudes of the rises in [K⁺]₀ induced by repetitive orthodromic stimulation were only slightly depressed and those elicited by antidromic stimulation were generally unaltered by VPA, as were laminar profiles of stimulus-evoked [K⁺]₀ signals. These results indicate that VPA has membrane actions in addition to known effects on excitatory and inhibitory transmitter pools.     

Chronic lead exposure alters presynaptic calcium regulation and synaptic facilitation in Drosophila larvae

Prolonged exposure to inorganic lead (Pb²⁺) during development has been shown to influence activity-dependent synaptic plasticity in the mammalian brain, possibly by altering the regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i). To explore this possibility, we studied the effect of Pb²⁺ exposure on [Ca²⁺]_i regulation and synaptic facilitation at the neuromuscular junction of larval Drosophila. Wild-type Drosophila (CS) were raised from egg stages through the third larval instar in media containing either 0 μM, 100 μM or 250 μM Pb²⁺ and identified motor terminals were examined in late third-instar larvae. To compare resting [Ca²⁺]_i and the changes in [Ca²⁺]_i produced by impulse activity, the motor terminals were loaded with a Ca²⁺ indicator, either Oregon Green 488 BAPTA-1 (OGB-1) or fura-2 conjugated to a dextran. We found that rearing in Pb²⁺ did not significantly change the resting [Ca²⁺]_i nor the Ca²⁺ transient produced in synaptic boutons by single action potentials (APs); however, the Ca²⁺ transients produced by 10 Hz and 20 Hz AP trains were larger in Pb²⁺-exposed boutons and decayed more slowly. For larvae raised in 250 μM Pb²⁺, the increase in [Ca²⁺]_i during an AP train (20 Hz) was 29% greater than in control larvae and the [Ca²⁺]_i decay τ was 69% greater. These differences appear to result from reduced activity of the plasma membrane Ca²⁺ ATPase (PMCA), which extrudes Ca²⁺ from these synaptic terminals. These findings are consistent with studies in mammals showing a Pb²⁺-dependent reduction in PMCA activity. We also observed a Pb²⁺-dependent enhancement of synaptic facilitation at these larval neuromuscular synapses. Facilitation of EPSP amplitude during AP trains (20 Hz) was 55% greater in Pb²⁺-reared larvae than in controls. These results showed that Pb²⁺ exposure produced changes in the regulation of [Ca²⁺]_i during impulse activity, which could affect various aspects of nervous system development. At the mature synapse, this altered [Ca²⁺]_i regulation produced changes in synaptic facilitation that are likely to influence the function of neural networks.     

IP3 mobilization and diffusion determine the timing window of Ca2+ release by synaptic stimulation and a spike in rat CA1 pyramidal cells

Synaptically activated calcium release from internal stores in CA1 pyramidal neurons is generated via metabotropic glutamate receptors by mobilizing IP₃. Ca²⁺ release spreads as a large amplitude wave in a restricted region of the apical dendrites of these cells. These Ca²⁺ waves have been shown to induce certain forms of synaptic potentiation and have been hypothesized to affect other forms of plasticity. Pairing a single backpropagating action potential (bAP) with repetitive synaptic stimulation evokes Ca²⁺ release when synaptic stimulation alone is subthreshold for generating release. We examined the timing window for this synergistic effect under conditions favoring Ca²⁺ release. The window, measured from the end of the train, lasted 250–500 ms depending on the duration of stimulation tetanus. The window appears to correspond to the time when both IP₃ concentration and [Ca²⁺]_i are elevated at the site of the IP₃ receptor. Detailed analysis of the mechanisms determining the duration of the window, including experiments using different forms of caged IP₃ instead of synaptic stimulation, suggest that the most significant processes are the time for IP₃ to diffuse away from the site of generation and the time course of IP₃ production initiated by activation of mGluRs. IP₃ breakdown, desensitization of the IP₃ receptor, and the kinetics of IP₃ unbinding from the receptor may affect the duration of the window but are less significant. The timing window is short but does not appear to be short enough to suggest that this form of coincidence detection contributes to conventional spike timing‐dependent synaptic plasticity in these cells. © 2009 Wiley‐Liss, Inc.     

Enhanced Synaptic Inhibition Disrupts the Efferent Code of Cerebellar Purkinje Neurons in Leaner Cav2.1 Ca2+ Channel Mutant Mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca²⁺ current carried by Ca_v2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca_v2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca²⁺ release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca²⁺ pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca_V2.1 channel gene.     

Erratum to: Differential alterations in the distribution of voltage-gated calcium channels in aged rat cerebellum: [Brain Research 903 (2001) 247–252]

In the present study, we have investigated the spatial and temporal distribution of voltage-gated calcium channels in the gerbil model of global cerebral ischemia using immunohistochemistry. Distinct localizations of P-type (α_1A), N-type (α_1B), and L-type (α_1C and α_1D) Ca²⁺ channels were observed in the hippocampus at days 1–5 after ischemic injury. However, increased expression of N-type Ca²⁺ channels was detectable in brain regions vulnerable to ischemia only at days 2 and 3 after ischemic injury. The pyramidal cell bodies of CA1-3 areas and the granule cell bodies of the dentate gyrus were intensely stained at days 2 and 3 following ischemic injury. Transient changes in N-type Ca²⁺ channel expression were also observed in the affected cerebral cortex and striatum at days 2 and 3 after ischemic injury. Although the present study has not addressed the multiple mechanisms contributing to the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) increase in the ischemic brain, the first demonstration of the transient increase in N-type Ca²⁺ channels may prove useful for future investigations.     

Action of GP 47779, the Active Metabolite of Oxcarbazepine,on the Corticostriatal System. II. Modulation of High-Voltage-Activated Calcium Currents

Summary: GP 47779, the active metabolite of oxcarbazepine (OCBZ) inhibits glutamatergic excitatory postsynaptic potentials (EPSPs) in rat striatum (described in the accompanying article). This effect was presumed to involve the modulation of the calcium (Ca²⁺) signals at either pre- or postsynaptic level. Therefore, we directly tested whether GP 47779 could modulate Ca²⁺ conductances in cortical as well as in striatal neurons. GP 47779 produced a reversible dose-dependent decrease in high-voltage-activated (HVA) Ca²⁺ currents evoked by membrane depolarization in isolated cortical pyramidal cells. GP 47779-mediated reduction in HVA Ca2+ currents, if occurring also at corticostriatal axon terminals, might explain the reduction of glutamate release in the striatum. An inhibitory action of GP 47779 on HVA Ca²⁺ currents was also observed in isolated striatal neurons. The effect on HVA Ca²⁺ currents in cortical and striatal neurons persisted in the presence of nifedipine, suggesting that dihydropyridine-sensitive channels were not involved in the GP 47779-mediated responses. We propose that the modulation of HVA Ca2+ channels by this carbamazepine (CBZ) analogue may account for its inhibitory action on transmitter release.     

Effects of trifluoperazine on synaptically evoked potentials and membrane properties of CA1 pyramidal neurons of rat hippocampus in situ and in vitro

The effects of trifluoperazine (TFP), a phenothiazine antipsychotic, on hippocampal activity were studied in the CA1 subfield, both in situ and in slices. In the extracellular studies in situ and in vitro, both somatic population spikes and dendritic excitatory postsynaptic potentials (EPSP) fields were depressed reversibly by TFP, applied by microiontophoresis or in the bath (50-100 μM). Similar effects were also seen during iontophoretic applications of sphingosine in situ. Like TFP (at micromolar concentrations) sphingosine is a dual Ca²⁺/calmodulin-dependent kinase and protein kinase C (PKC) inhibitor. In intracellular recordings from slices, 50-100 μM TFP induced a slow depolarization and a decrease in input resistance (R_N), probably through a β-aminobutyric acid (GABA)-mediated increase in Cl^? conductance (G_Cl). TFP also reduced the slow afterhyperpolarization (AHP) as well as electrically evoked inhibitory postsynaptic potentials (IPSPs), but EPSPs were augmented in both amplitude and duration. When CA1 neurons were voltage clamped, TFP elicited a corresponding inward current (consistent with depolarization), increased the leak conductance, and enhanced excitatory synaptic currents; whereas inhibitory synaptic currents and high-threshold Ca²⁺ currents were reduced. In conclusion, these effects of TFP–which cannot be readily explained by its potent antidopamine action–are in keeping with other evidence that both Ca²⁺/calmodulin-dependent kinase and PKC can modulate G_Cl-conductance and high-threshold Ca²⁺ -conductance, as well as inhibitory and excitatory postsynaptic currents. © 1993 Wiley-Liss, Inc.     

Integration of synchronous synaptic input in CA1 pyramidal neuron depends on spatial and temporal distributions of the input

Highly synchronized neural firing has been discussed in relation to learning and memory, for instance sharp‐wave activity in hippocampus. We were interested to study how a postsynaptic CA1 pyramidal neuron would integrate input of different levels of synchronicity. In previous work using computational modeling we studied how the integration depends on dendritic conductances. We found that the transient A‐type potassium channel K_A was able to selectively suppress input of high synchronicity. In recent years, compartmentalization of dendritic integration has been shown. We were therefore interested to study the influence of localization and pattern of synaptic input over the dendritic tree of the CA1 pyramidal neuron. We find that the selective suppression increases when synaptic inputs are placed on oblique dendrites further out from the soma. The suppression also increases along the radial axis from the apical trunk out to the end of oblique dendrites. We also find that the K_A channel suppresses the occurrence of dendritic spikes. Moreover, recent studies have shown interaction between synaptic inputs. We therefore studied the influence of apical tuft input on the integration studied above. We find that excitatory input provides a modulatory influence reducing the capacity of K_A to suppress synchronized activity, thus facilitating the excitatory drive of oblique dendritic input. Conversely, inhibitory tuft input increases the suppression by K_A providing a larger control of oblique depolarizing factors on the CA1 pyramidal neuron in terms of what constitutes the most effective level of synchronicity. Furthermore, we show that the selective suppression studied above depends on the conductance of the K_A channel. K_A, as several other potassium channels, is modulated by several neuromodulators, for instance acetylcholine and dopamine, both of which have been discussed in relation to learning and memory. We suggest that dendritic conductances and their modulatory systems may be part of the regulation of processing of information, in particular for how network synchronicity affects learning and memory. © 2012 Wiley Periodicals, Inc.     

Glutamate released spontaneously from astrocytes sets the threshold for synaptic plasticity

Astrocytes exhibit spontaneous calcium oscillations that could induce the release of glutamate as gliotransmitter in rat hippocampal slices. However, it is unknown whether this spontaneous release of astrocytic glutamate may contribute to determining the basal neurotransmitter release probability in central synapses. Using whole‐cell recordings and Ca²⁺ imaging, we investigated the effects of the spontaneous astrocytic activity on neurotransmission and synaptic plasticity at CA3–CA1 hippocampal synapses. We show here that the metabolic gliotoxin fluorocitrate (FC) reduces the amplitude of evoked excitatory postsynaptic currents and increases the paired‐pulse facilitation, mainly due to the reduction of the neurotransmitter release probability and the synaptic potency. FC also decreased intracellular Ca²⁺ signalling and Ca²⁺‐dependent glutamate release from astrocytes. The addition of glutamine rescued the effects of FC over the synaptic potency; however, the probability of neurotransmitter release remained diminished. The blockage of group I metabotropic glutamate receptors mimicked the effects of FC on the frequency of miniature synaptic responses. In the presence of FC, the Ca²⁺ chelator 1,2‐bis(2‐aminophenoxy)ethane‐N,N,N ′,N ′‐tetra‐acetate or group I metabotropic glutamate receptor antagonists, the excitatory postsynaptic current potentiation induced by the spike‐timing‐dependent plasticity protocol was blocked, and it was rescued by delivering a stronger spike‐timing‐dependent plasticity protocol. Taken together, these results suggest that spontaneous glutamate release from astrocytes contributes to setting the basal probability of neurotransmitter release via metabotropic glutamate receptor activation, which could be operating as a gain control mechanism that regulates the threshold of long‐term potentiation. Therefore, endogenous astrocyte activity provides a novel non‐neuronal mechanism that could be critical for transferring information in the central nervous system.