Tetanus-induced potentiation of inhibitory postsynaptic potentials in hippocampal CA1 neurons

Sleep-deprivation induces changes in GABA(B) and mGlu receptor expression and has consequences for synaptic long-term depression

Long term depression (LTD) in the CA1 region of the hippocampus, induced with a 20-Hz, 30 s tetanus to Schaffer collaterals, is enhanced in sleep-deprived (SD) rats. In the present study, we investigated the role of metabotropic glutamate receptors (mGluRs), γ-aminobutyric acid (GABA) B receptors (GABA(B)-Rs) and N-methyl-D-aspartic acid receptors (NMDARs) in the LTD of the population excitatory postsynaptic potential (pEPSP). The requirement of Ca(2+) from L- and T-type voltage-gated calcium channels (VGCCs) and intracellular stores was also studied. Results indicate that mGluRs, a release of Ca(2+) from intracellular stores and GABA(B)-Rs are required for LTD. Interestingly, while mGlu1Rs seem to be involved in both short-term depression and LTD, mGlu5Rs appear to participate mostly in LTD. CGP 55845, a GABA(B)-R antagonist, partially suppressed LTD in normally sleeping (NS) rats, while completely blocking LTD in SD rats. Moreover, GS-39783, a positive allosteric modulator for GABA(B)-R, suppressed the pEPSP in SD, but not NS rats. Since both mGluRs and GABA(B)-Rs seem to be involved in the LTD, especially in SD rats, we examined if the receptor expression pattern and/or dimerization changed, using immunohistochemical, co-localization and co-immunoprecipitation techniques. Sleep-deprivation induced an increase in the expression of GABA(B)-R1 and mGlu1αR in the CA1 region of the hippocampus. In addition, co-localization and heterodimerization between mGlu1αR/GABA(B)-R1 and mGlu1αR/GABA(B)-R2 is enhanced in SD rats. Taken together, our findings present a novel form of LTD sensitive to the activation of mGluRs and GABA(B)-Rs, and reveal, for the first time, that sleep-deprivation induces alterations in the expression and dimerization of these receptors.

The effect of a new water-soluble sedative-hypnotic drug, JM-1232(-), on long-term potentiation in the CA1 region of the mouse hippocampus

BACKGROUND

JM-1232(-) {(-)-3-[2-(4-methyl-1-piperazinyl)-2-oxoethyl]-2-phenyl-3,5,6,7-tetrahydrocyclopenta[f]isoindol-1(2H)-one} is a new water-soluble sedative-hypnotic drug with affinity for the benzodiazepine binding site on γ-aminobutyric acid A receptors. The effects of JM-1232(-) on synaptic transmission in the brain are not known. In the present study, we investigated the effects of JM-1232(-) on synaptic transmission, synaptic plasticity (i.e., long-term potentiation [LTP] and paired-pulse facilitation), and excitatory/inhibitory postsynaptic currents (EPSCs/IPSCs) of pyramidal neurons in the CA1 region of mouse hippocampal slices.

METHODS

We recorded Schaffer collateral-evoked field excitatory postsynaptic potentials and EPSCs and IPSCs of pyramidal neurons using whole-cell patch-clamp techniques in the CA1 region of mouse hippocampal slices.

RESULTS

JM-1232(-) had no significant effect on the field excitatory postsynaptic potentials. Application of JM-1232(-) for 20 minutes before theta-burst stimulation dose dependently impaired LTP. JM-1232(-) impaired paired-pulse facilitation. The benzodiazepine antagonist flumazenil abolished the inhibitory effect of JM-1232(-) on LTP and paired-pulse facilitation. JM-1232(-) had no effect on Schaffer collateral stimulation-evoked EPSCs, whereas it potentiated the amplitude and prolonged the decay of evoked IPSCs in CA1 pyramidal neurons. Flumazenil blocked the effect of JM-1232(-) on the amplitude and decay of evoked IPSCs. JM-1232(-) suppressed the action potential discharge in the CA1 pyramidal neurons during theta-burst stimulation, which was reversed by flumazenil.

CONCLUSION

JM-1232(-) enhances synaptic inhibition and impairs LTP and paired-pulse facilitation in area CA1 of the mouse hippocampus. These effects were mediated by benzodiazepine binding sites on γ-aminobutyric acid A receptors.

Differential effects of excitatory and inhibitory plasticity on synaptically driven neuronal input-output functions

Ultimately, whether or not a neuron produces a spike determines its contribution to local computations. In response to brief stimuli the probability a neuron will fire can be described by its input-output function, which depends on the net balance and timing of excitatory and inhibitory currents. While excitatory and inhibitory synapses are plastic, most studies examine plasticity of subthreshold events. Thus, the effects of concerted regulation of excitatory and inhibitory synaptic strength on neuronal input-output functions are not well understood. Here, theoretical analyses reveal that excitatory synaptic strength controls the threshold of the neuronal input-output function, while inhibitory plasticity alters the threshold and gain. Experimentally, changes in the balance of excitation and inhibition in CA1 pyramidal neurons also altered their input-output function as predicted by the model. These results support the existence of two functional modes of plasticity that can be used to optimize information processing: threshold and gain plasticity.

A method for recording miniature inhibitory postsynaptic currents in the central nervous system suitable for quantal analysis

Quantal analysis of transmitter release is useful in examining presynaptic mechanisms involved in synaptic transmission. However, in central neurons, the presence of multiple synapses makes it difficult to use the traditional quantal analysis, developed for the neuromuscular transmission. We developed a method to minimize these difficulties. Experiments were performed, using the whole-cell patch-clamp recording technique, on rat CA1 pyramidal neurons in a hippocampal slice preparation. When the stratum radiatum was stimulated, mixed current signals including, miniature inhibitory postsynaptic currents (mIPSCs), miniature excitatory postsynaptic currents (mEPSCs), evoked inhibitory postsynaptic currents (eIPSCs) and evoked excitatory postsynaptic currents (eEPSCs), could be observed in CA1 pyramidal cells while slices were superfused with the normal, Na(+)-containing, medium. The mIPSCs could be blocked by bicuculline (10 microm). mEPSCs, eEPSCs and eIPSCs could not be observed when the Na(+)-containing perfusion medium was replaced by a Na(+)-free medium but reappeared when the Na(+)-containing medium was re-introduced. When a polarizing electrode was placed near the recorded neuron, while slices were superfused with the Na(+)-free medium, and depolarizing rectangular current pulses of different magnitudes were applied, the number of mIPSCs increased with increasing amount of the current. Amplitudes of the mIPSCs showed a Gaussian distribution and the coefficient of variation was small. These observations indicate that a combination of the Na(+)-free superfusing medium and local depolarizations with a polarizing electrode is useful for recording mIPSCs from a localized area of the recorded neuron and for quantal analysis.

Differential induction of long term synaptic plasticity in inhibitory synapses of the hippocampus

Long term synaptic plasticity has been more extensively studied in excitatory synapses, but it is also a property of inhibitory synapses. Many inhibitory synapses target hippocampal pyramidal neurons of the CA1 region. They originate from several interneuron classes that subdivide the surface area that they target on the pyramidal cell. Thus, many interneurons preferentially innervate the perisomatic area and axon hillock of the pyramidal cells while others preferentially target dendritic branches and spines. Methods to preferentially activate dendritic or somatic inhibitory synapses onto pyramidal neurons have been devised. By using these methods, the present work demonstrates that a stimulation pattern that induces long term potentiation (LTP) in excitatory synapses of the Schaffer collaterals is also capable of inducing distinct types of long term plastic changes in different classes of inhibitory synapses: Induction of long term depression (LTD) was seen in dendritic inhibitory synapses whereas LTP was observed in somatic inhibitory synapses. These findings suggest that inhibitory synapses arising from different interneuron classes may respond to the same stimulus according to their specific plastic potential enabling a spatial combinatorial pattern of inhibitory effects onto the pyramidal cell.

A possible mechanism for the effect of modifiable lateral inhibition in the striatum on the selection of conditioned reflex motor responses

A mechanism is proposed for the effects of striatal dopamine-modifiable lateral inhibition on the selection of conditioned reflex motor responses. According to this mechanism, activation of dopamine D1 (D2) receptors on strionigral (striopallidal) neurons facilitates long-term depression (potentiation) of the inhibitory inputs simultaneously with potentiation (depression) of the excitatory inputs, of sufficient strength to open NMDA channels. For " weak" excitation, insufficient to open NMDA channels, the modification rules were of the opposite sign. Activation of presynaptic D2 (D1) receptors leads to decreases (increases) in GABA release from strionigral (striopallidal) axon terminals innervating strionigral (striopallidal) cells. As a result, dopamine-modifiable lateral inhibition simultaneously increases both the potentiation (depression) of the excitatory inputs to "strongly" activated strionigral (striopallidal) neurons, increasing (decreasing) their activity, and increases the depression (potentiation) of the excitatory inputs to the "weakly" activated strionigral (striopallidal) neurons, decreasing (increasing) their activity. Subsequent reorganization of neuron activity in the cortex-basal ganglia-thalamus-cortex circuit facilitates selection of conditioned reflex motor responses by further increasing (decreasing) the activity of those motor cortex neurons which were "strongly" ("weakly") excited by the striatum in conditions of dopamine release in response to the conditioned stimulus.

Different forms of homeostatic plasticity are engaged with distinct temporal profiles

Global changes in network activity have been reported to induce homeostatic plasticity at multiple synaptic and cellular loci. Though individual types of plasticity are normally examined in isolation, it is their interactions and net effect that will ultimately determine their functional consequences. Here we examine homeostatic plasticity of both inhibition and intrinsic excitability in parallel in rat organotypic hippocampal slices. As previous studies have not examined inhibitory plasticity using a functional measure, inhibition was measured by the ability of evoked inhibitory postsynaptic potentials (IPSPs) to suppress action potentials, as well as IPSP amplitude. We show that manipulations of network activity can both up- and downregulate functional inhibition, as well as intrinsic excitability. However, these forms of plasticity are dissociable. Specifically, robust changes in intrinsic excitability were observed in the absence of inhibitory plasticity, and shifts in inhibition, but not excitability, appear to be sensitive to developmental stage. Our data establish that while the two forms of homeostatic plasticity can be engaged in parallel, there is a specific order in which they are expressed, with changes in excitability preceding those in inhibition. We propose that changes in intrinsic excitability occur first in order to stabilize network activity while optimizing the preservation of information stored in synaptic strengths by restricting changes that will disrupt the balance of synaptic excitation and inhibition.

Benzodiazepine involvement in LTP of the GABA-ergic IPSC in rat hippocampal CA1 neurons

Benzodiazepine binding sites are present on gamma-aminobutyric acid (GABA) receptors in hippocampal neurons. Diazepam is known to potentiate the amplitude and prolong the decay of GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs). In this study, benzodiazepine involvement in long-term potentiation (LTP) of the IPSC was examined. Whole-cell recordings of IPSCs were made from rat hippocampal CA1 neurons in a slice preparation. LTP was induced by a tetanic stimulation in the stratum radiatum (2 trains of 100 Hz for 1 s, 20 s inter-train interval) while pharmacologically blocking ionotropic glutamate receptors. During LTP, the amplitude of the IPSCs was potentiated in the majority of neurons with the IPSC decay and shape unaffected. Diazepam (5 microM) potentiated the IPSC amplitude and prolonged the decay when applied before, but not during, LTP. In neurons in which LTP could not be induced by a tetanic stimulation, diazepam did not increase the amplitude of the pre-tetanic IPSC. Flumazenil, at a concentration (10 microM) that blocked the enhancement of the IPSC by applied diazepam, had no effect on the IPSC amplitude when applied before LTP induction but significantly decreased the IPSC when applied during LTP maintenance. The antagonist, when applied during the tetanic stimulation, did not block LTP, suggesting that benzodiazepine receptors do not participate in LTP induction. These results indicate that the maintenance of LTP of the IPSC involves (a) the release of endogenous benzodiazepine agonist(s) and/or (b) the participation of benzodiazepine binding sites on subsynaptic GABA(A) receptors.

Timing and balance of inhibition enhance the effect of long-term potentiation on cell firing

The role of a neuron in neural processing is ultimately determined by whether or not it fires an action potential in a given context. Studies on synaptic plasticity have focused primarily on changes in EPSPs, and not on whether plasticity translates into changes in firing. However, this issue has been addressed by examining EPSP-spike (E-S) potentiation, which enhances the ability of an EPSP of a fixed slope to elicit spikes after long-term potentiation (LTP). Although LTP is thought to underlie learning and memory, E-S potentiation could play an equally important role by potentiating the neuronal input-output function. Here, we used a combined experimental and theoretical approach to examine both the mechanisms underlying E-S potentiation as well as the role of inhibition in shaping the input-output function of neurons. Whereas previous studies examined tetanus-LTP, in which inhibitory synapses may have undergone plasticity, here we examined pairing-induced associative LTP. We determined that although intact inhibition was necessary for pairing-induced E-S potentiation, inhibitory plasticity was not. We further established using computer simulations that a primary mechanism of E-S potentiation was a change in the relative recruitment and latency of inhibitory neurons. Although these studies do not exclude the presence of additional mechanisms of E-S potentiation that may be engaged depending on the induction protocol, they do establish that under intact pharmacology, LTP of the Schaffer collateral to CA1 pyramidal neuron synapses will produce E-S potentiation as a result of changes in the balance and timing of excitation and inhibition.

RS-100642-198, a novel sodium channel blocker, provides differential neuroprotection against hypoxia/hypoglycemia, veratridine or glutamate-mediated neurotoxicity in primary cultures of rat cerebellar neurons

The present study investigated the effects of RS-100642-198 (a novel sodium channel blocker), and two related compounds (mexiletine and QX-314), in in vitro models of neurotoxicity. Neurotoxicity was produced in primary cerebellar cultures using hypoxia/hypoglycemia (H/H), veratridine or glutamate where, in vehicle-treated neurons, 65%, 60% and 75% neuronal injury was measured, respectively. Dose-response neuroprotection experiments were carried out using concentrations ranging from 0.1-500 micro M. All the sodium channel blockers were neuroprotective against H/H-induced injury, with each exhibiting similar potency and efficacy. However, against veratridine-induced neuronal injury only RS-100642-198 and mexiletine were 100% protective, whereas QX-314 neuroprotection was limited (i.e. only 54%). In contrast, RS-100642-198 and mexiletine had no effect against glutamate-induced injury, whereas QX-314 produced a consistent, but very limited (i.e. 25%), neuroprotection. Measurements of intraneuronal calcium [Ca(2+)]i) mobilization revealed that glutamate caused immediate and sustained increases in [Ca(2+)]i which were not affected by RS-100642-198 or mexiletine. However, both drugs decreased the initial amplitude and attenuated the sustained rise in [Ca(2+)]i mobilization produced by veratridine or KCl depolarization. QX-314 produced similar effects on glutamate-, veratridine- or KCl-induced [Ca(2+)]i dynamics, effectively decreasing the amplitude and delaying the initial spike in [Ca(2+)]i, and attenuating the sustained increase in [Ca(2+)]i mobilization. By using different in vitro models of excitotoxicity, a heterogeneous profile of neuroprotective effects resulting from sodium channel blockade has been described for RS-100642-198 and related drugs, suggesting that selective blockade of neuronal sodium channels in pathological conditions may provide therapeutic neuroprotection against depolarization/excitotoxicity via inhibition of voltage-dependent Na(+) channels.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Differential effects of short- and long-term potentiation on cell firing in the CA1 region of the hippocampus

Long-term potentiation (LTP) in the hippocampus enhances the ability of a stimulus to produce cell firing, not only by increasing the strength of the EPSPs, but also by increasing the efficiency of the input/output (I/O) function of pyramidal neurons. This means that EPSPs of a given size more easily elicit spikes after LTP, a process known as EPSP-spike (E-S) potentiation. In contrast to LTP, it is not known whether the synaptic strengthening produced by paired-pulse facilitation (PPF) also results in changes in the I/O function. We have addressed this question by examining E-S curves from rat hippocampal area CA1 in response to both PPF and LTP. We describe a novel form of I/O modulation in which PPF produces E-S depression; that is, the E-S curve is shifted to the right, indicating a decreased ability of EPSPs to elicit action potentials. Consistent with the notion that E-S potentiation observed with LTP is caused by long-term increases in the excitatory-inhibitory ratio, we show that PPF-induced E-S depression relies on short-term decreases in this ratio. These results indicate that different forms of synaptic plasticity that produce the same degree of EPSP potentiation can result in dramatically different effects on cell firing, because of the dynamic changes in the excitatory-inhibitory balance within local circuits.

Down regulation of sodium channel Na(v)1.1 expression by veratridine and its reversal by a novel sodium channel blocker, RS100642, in primary neuronal cultures

This study investigated the effects of veratridine-induced neuronal toxicity on sodium channel gene (NaCh) expression in primary forebrain cultures enriched in neurons, and its reversal by a novel sodium channel blocker, RS100642. Using quantitative RT-PCR, our findings demonstrated the expression ratio of NaCh genes in normal fetal rat forebrain neurons to be Na(v)1.2 > Na(v)1.3 > Na(v)1.8 > Na(v)1.1 > Na(v)1.7 (rBII > rBIII > PN3 > rBI > PN1). Veratridine treatment of neuronal cells produced neurotoxicity in a dose-dependent manner (0.25-20 micro M). Neuronal injury caused by a dose of veratridine producing 80% cell death (2.5 micro M) significantly, and exclusively down-regulated the Na(v)1.1 gene. However, treatment of neurons with RS100642 (200 micro M) reversed the down-regulation of the Na(v)1.1 gene expression caused by veratridine. Our findings document for the first time quantitative and relative changes in the expression of various NaCh genes in neurons following injury produced by selective activation of voltage-gated sodium channels, and suggest that the Na(v)1.1 sodium channel gene may play a key role in the neuronal injury/recovery process.

Post-tetanic depression of GABAergic synaptic transmission in rat hippocampal cell cultures

The effect of tetanic stimulation (30 Hz, 4 s) on evoked GABAergic inhibitory postsynaptic currents (IPSCs) was studied in cell cultures of dissociated hippocampal neurons with established synaptic connections. It was found that tetanic stimulation elicited post-tetanic depression (PTD) of the evoked IPSCs with a duration of more than 50 s in about 60% of the connections tested; post-tetanic potentiation was induced in 25% of the connections. We propose that the opposite effects of tetanization on IPSC amplitude are due to differences in the type of the interneuron that was tetanized. Since PTD in our experiments was usually accompanied by changes in the IPSC coefficient of variation and changes of a paired pulse depression, which are thought to reflect presynaptic mechanisms of modulation, we suggest that part of the PTD is due to a presynaptic mechanism(s).

Differential pattern of expression of voltage-gated sodium channel genes following ischemic brain injury in rats

This study investigated the effects of brain ischemia on sodium channel gene (NaCh) expression in rats. Using quantitative RT-PCR, our findings demonstrated the expression ratio of NaCh genes in normal rat brain to be Na(v)1.1 > Na(v)1.8 > Na(v)1.3 > Na(v)1.7 (rBI > PN3 > rBIII > PN1). In contrast, brain injury caused by middle cerebral artery occlusion (MCAo) for 2 h followed by reperfusion significantly down-regulated Na(v)1.3 and Na(v)1.7 genes in both injured and contralateral hemispheres; whereas the Na(v)1.8 gene was down regulated in only the injured hemisphere (though only acutely at 2 or 2-6 h post-MCAo). However, the time-course of NaCh gene expression revealed a significant down-regulation of Na(v)1.1 only in the ischemic hemisphere beginning 6 h post-MCAo and measured out to 48 h post-MCAo. In a separate preliminary study Na(v)1.2 (rBII) gene was found to be expressed at levels greater than that of Na(v)1.1 in normal rats and was significantly down regulated at 24 h post-MCAo). Our findings document, for the first time, quantitative and relative changes in the expression of various NaCh genes following ischemic brain injury and suggest that the Na(v)1.1 sodium channel gene may play a key role in ischemic injury/recovery.

Intracellular correlates of spatial memory acquisition in hippocampal slices: long-term disinhibition of CA1 pyramidal cells

Despite many advances in our understanding of synaptic models of memory such as long-term potentiation and depression, cellular mechanisms that correlate with and may underlie behavioral learning and memory have not yet been conclusively determined. We used multiple intracellular recordings to study learning-specific modifications of intrinsic membrane and synaptic responses of the CA1 pyramidal cells (PCs) in slices of the rat dorsal hippocampus prepared at different stages of the Morris water maze (WM) task acquisition. Schaffer collateral stimulation evoked complex postsynaptic potentials (PSP) consisting of the excitatory and inhibitory postsynaptic potentials (EPSP and IPSP, respectively). After rats had learned the WM task, our major learning-specific findings included reduction of the mean peak amplitude of the IPSPs, delays in the mean peak latencies of the EPSPs and IPSPs, and correlation of the depolarizing-shifted IPSP reversal potentials and reduced IPSP-evoked membrane conductance. In addition, detailed isochronal analyses revealed that amplitudes of both early and late IPSP phases were reduced in a subset of the CA1 PCs after WM training was completed. These reduced IPSPs were significantly correlated with decreased IPSP conductance and with depolarizing-shifted IPSP reversal potentials. Input-output relations and initial rising slopes of the EPSP phase did not indicate learning-related facilitation as compared with the swim and naïve controls. Another subset of WM-trained CA1 PCs had enhanced amplitudes of action potentials but no learning-specific synaptic changes. There were no WM training-specific modifications of other intrinsic membrane properties. These data suggest that long-term disinhibition in a subset of CA1 PCs may facilitate cell discharges that represent and record the spatial location of a hidden platform in a Morris WM.

Mechanisms involved in tetanus-induced potentiation of fast IPSCs in rat hippocampal CA1 neurons

In the present study, possible mechanisms involved in the tetanus-induced potentiation of gamma-aminobutyric acid-A (GABA-A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were investigated using the whole cell voltage-clamp technique on CA1 neurons in rat hippocampal slices. Stimulations (100 Hz) of the stratum radiatum, while voltage-clamping the membrane potential of neurons, induces a long-term potentiation (LTP) of evoked fast IPSCs while increasing the number but not the amplitude of spontaneous IPSCs (sIPSCs). The potentiation of fast IPSCs was input specific. During the period of IPSC potentiation, postsynaptic responses produced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride and baclofen, GABA-A and GABA-B agonists respectively, were not significantly different from control. CGP 36742, a GABA-B antagonist, blocked the induction of tetanus-induced potentiation of evoked and spontaneous IPSCs, while GTPgammaS, an activator of G proteins, substitution for GTP in the postsynaptic recording electrode did not occlude potentiation. Since GABA-B receptors work through G proteins, our results suggest that pre- but not postsynaptic GABA-B receptors are involved in the potentiation of fast IPSCs. A tetanus delivered when GABA-A responses were completely blocked by bicuculline suggests that GABA-A receptor activation during tetanus is not essential for the induction of potentiation. Rp-cAMPs, an antagonist of protein kinase A (PKA) activation, blocks the induction of potentiation of fast IPSCs. Forskolin, an activator of PKA, increases baseline evoked IPSCs as well as the number of sIPSCs, and a tetanic stimulation during this enhancement uncovers a long-term depression of the evoked IPSC. Sulfhydryl alkylating agents, N-ethylmaleimide and p-chloromercuribenzoic acid, which have been found to presynaptically increase GABA release and have been suggested to have effects on proteins involved in transmitter release processes occurring in nerve terminals, occlude tetanus-induced potentiation of evoked and spontaneous IPSCs. Taken together our results suggest that LTP of IPSCs originates from a presynaptic site and that GABA-B receptor activation, cyclic AMP/PKA activation and sulfhydryl-alkylation are involved. Plasticity of IPSCs as observed in this study would have significant implications for network behavior in the hippocampus.

2-Deoxyglucose-induced long-term potentiation of monosynaptic IPSPs in CA1 hippocampal neurons

In previous experiments on excitatory synaptic transmission in CA1, temporary (10-20 min) replacement of glucose with 10 mM 2-deoxyglucose (2-DG) consistently caused a marked and very sustained potentiation (2-DG LTP). To find out whether 2-DG has a similar effect on inhibitory synapses, we recorded pharmacologically isolated mononosynaptic inhibitory postsynaptic potentials (IPSPs; under current clamp) and inhibitory postsynaptic currents (IPSCs; under voltage clamp); 2-DG was applied both in the presence and the absence of antagonists of N-methyl-D-aspartate (NMDA). In spite of sharply varied results (some neurons showing large potentiation, lasting for >1 h, and many little or none), overall there was a significant and similar potentiation of IPSP conductance, both for the early (at approximately 30 ms) and later (at approximately 140 ms) components of IPSPs or IPSCs: by 35.1 +/- 10.25% (mean +/- SE; for n = 24, P = 0.0023) and 36.5 +/- 16.3% (for n = 19, P = 0.038), respectively. The similar potentiation of the early and late IPSP points to a presynaptic mechanism of LTP. Overall, the LTP was statistically significant only when 2-DG was applied in the absence of glutamate antagonists. Tetanic stimulations (in presence or absence of glutamate antagonists) only depressed IPSPs (by half). In conclusion, although smaller and more variable, 2-DG-induced LTP of inhibitory synapses appears to be broadly similar to the 2-DG-induced LTP of excitatory postsynaptic potentials previously observed in CA1.

Post-tetanic potentiation of GABAergic IPSCs in cultured rat hippocampal neurones

1. Dual whole-cell patch-clamp recording was used to investigate post-tetanic potentiation (PTP) of GABAergic IPSCs evoked between pairs of cultured rat hippocampal neurones. Tetanization of the presynaptic neurone at frequencies (f) ranging from 5 to 100 Hz resulted in PTP of the IPSCs. Maximum PTP had a magnitude of 51.6 % just after the stimulus train, and lasted up to 1 min. PTP was shown to be dependent on the number of stimuli in the train, but independent of f at frequencies > or =5 Hz. 2. Blocking postsynaptic GABAA receptors with bicuculline during the tetanus did not affect the expression of PTP, showing that it is a presynaptic phenomenon. PTP was strongly affected by changing [Ca2+]o during the tetanus: PTP was reduced by lowering [Ca2+]o, and increased by high [Ca2+]o. 3. PTP was still present after presynaptic injection of BAPTA or EGTA, or following perfusion of the membrane-permeable ester EGTA-tetraacetoxymethyl ester (EGTA AM, 50 microM). On the other hand, EGTA AM blocked spontaneous, asynchronous IPSCs (asIPSCs), which were often associated with tetanic stimulation. 4. Tetanic stimulation in the presence of 4-aminopyridine (4-AP), which promotes presynaptic Ca2+ influx, evoked sustained PTP of IPSCs in half of the neurones tested. 5. The results indicate that PTP at inhibitory GABAergic synapses is related to the magnitude of presynaptic Ca2+ influx during the tetanic stimulation, leading to an enhanced probability of vesicle release in the post-tetanic period. The increase in [Ca2+]i occurs despite the presence of high-affinity exogenous and endogenous intracellular Ca2+ buffers. That PTP of IPSCs depends on the number, and not the frequency, of spikes in the GABAergic neurone is in accordance with a slow clearing of intracellular Ca2+ from the presynaptic terminals.

Febrile seizures in the developing brain result in persistent modification of neuronal excitability in limbic circuits

Febrile (fever-induced) seizures affect 3-5% of infants and young children. Despite the high incidence of febrile seizures, their contribution to the development of epilepsy later in life has remained controversial. Combining a new rat model of complex febrile seizures and patch clamp techniques, we determined that hyperthermia-induced seizures in the immature rat cause a selective presynaptic increase in inhibitory synaptic transmission in the hippocampus that lasts into adulthood. The long-lasting nature of these potent alterations in synaptic communication after febrile seizures does not support the prevalent view of the 'benign' nature of early-life febrile convulsions.

Differential induction of long-lasting potentiation of inhibitory postsynaptic potentials by theta patterned stimulation versus 100-Hz tetanization in hippocampal pyramidal cells in vitro

Tetanization of Schaffer collaterals, which induces long-term potentiation of excitatory transmission in the hippocampus of the rat, also affects local inhibitory circuits. Mechanisms controlling plasticity of early and late components of inhibitory postsynaptic potentials in CA1 pyramidal cells were studied using intracellular recordings and Ca2+ imaging in rat hippocampal slices. High-frequency stimulation (100 Hz/s) of Schaffer collaterals resulted in no change in the mean amplitude of early or late inhibitory postsynaptic potentials 30 min post-tetanus. However, intracellular injection of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate unmasked a significant increase in mean amplitude of both inhibitory postsynaptic potentials 30 min post-tetanus and the induction of this potentiation was blocked by the N-methyl-D-aspartate receptor antagonist(+/-)-2-amino-5-phosphopentanoic acid. In contrast to high-frequency tetanization, "theta-burst" stimulation in normal medium resulted in a significant potentiation of the mean amplitude of both early and late inhibitory postsynaptic potentials 30 min post-tetanus. This potentiation was blocked by the N-methyl-D-aspartate receptor antagonist. The more physiological tetanization pattern, which mimics the endogenous theta rhythm, therefore resulted in an N-methyl-D-aspartate-dependent increase in inhibition 30 min post-tetanus. Calcium imaging during whole-cell recordings from pyramidal cells revealed differences in the Ca2+ signal associated with high-frequency and theta-burst stimulations. During theta-burst stimulation of Schaffer collaterals, the mean time to peak of Ca2+ signals was significantly longer, and the mean peak amplitude and area under the Ca2+ response were larger than during high-frequency stimulation. These results indicate that tetanization induces long-lasting synaptic plasticity in hippocampal inhibitory circuits. This plasticity involves an interaction between a Ca2(+)-mediated postsynaptic depression and an N-methyl-D-aspartate-mediated potentiation of GABAA and GABAB inhibition, and these processes are differentially sensitive to tetanization parameters.

Astrocyte-mediated potentiation of inhibitory synaptic transmission

We investigated the role of astrocytes in activity-dependent modulation of inhibitory synaptic transmission in hippocampal slices. Repetitive firing of an interneuron decreased the probability of synaptic failures in spike-evoked inhibitory postsynaptic currents (unitary IPSCs) in CA1 pyramidal neurons. The GABAB-receptor antagonist CGP55845A abolished this effect. Direct stimulation of astrocytes, or application of the GABAB-receptor agonist baclofen, potentiated miniature inhibitory postsynaptic currents (mIPSCs) in pyramidal neurons. These effects were blocked by inhibition of astrocytic calcium signaling with the calcium chelator BAPTA or by antagonists of the ionotropic glutamate receptors. These observations suggest that interneuronal firing elicits a GABAB-receptor-mediated elevation of calcium in surrounding astrocytes, which in turn potentiates inhibitory transmission. Astrocytes may therefore be a necessary intermediary in activity-dependent modulation of inhibitory synapses in the hippocampus.

Effects of GABA(A) inhibition on the expression of long-term potentiation in CA1 pyramidal cells are dependent on tetanization parameters

Long-term potentiation (LTP) of excitatory synaptic responses of principal neurons in the hippocampus is accompanied by changes in GABAergic inhibition mediated by interneurons. The impact of inhibition on LTP of excitatory postsynaptic responses in CA1 pyramidal cells was assessed by monitoring changes in field potentials evoked by Schaffer collateral stimulation in hippocampal slices in vitro. First, to determine the effect of inhibition on population EPSPs, slices were exposed to the GABA(A) receptor antagonist bicuculline (10 microM). Both the slope and amplitude of field EPSPs (fEPSPs) were significantly enhanced by bicuculline indicating that inhibition modulates excitatory postsynaptic responses of pyramidal cells. To assess if stimulation-dependent changes in inhibition influence LTP of excitatory responses of pyramidal cells, LTP was examined in the presence and absence of bicuculline (20 microM) following either 100 Hz tetanization, or theta-patterned stimulation (short bursts delivered at 5 Hz). In normal medium, 100 Hz stimulation produced marked short-term potentiation that decayed 5-10 min post-tetanus and both stimulation paradigms produced similar LTP at 30 min post-tetanus. In comparison, LTP of the fEPSP slope and amplitude was significantly enhanced after theta-patterned stimulation, but not after 100 Hz stimulation, in bicuculline. The greater potentiation of field responses following theta-patterned stimulation in the presence of bicuculline indicates that a larger potentiation of excitatory responses was unmasked during suppression of inhibitory inputs. These results suggest that a long-lasting enhancement of inhibition in pyramidal cells was also induced following theta-patterned stimulation in normal ACSF. Since suppression of inhibition did not uncover a significantly larger potentiation following 100 Hz tetanization, the influence of inhibition on LTP of excitatory responses appears to be stimulation-dependent. In conclusion, theta-patterned stimulation appears to be more effective at inducing plasticity within inhibitory circuits, and this plasticity may partially offset concurrent increases in the excitability of the CA1 network.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.