Restrictions on inhibitory circuits contribute to limited recruitment of fast inhibition in rat neocortical pyramidal cells

Estimation of cumulative amplitude distributions of miniature postsynaptic currents allows characterising their multimodality, quantal size and variability

A miniature postsynaptic current (mPSC) is a small, rare, and highly variable spontaneous synaptic event that is generally caused by the spontaneous release of single vesicles. The amplitude and variability of mPSCs are key measures of the postsynaptic processes and are taken as the main characteristics of an elementary unit (quantal size) in traditional quantal analysis of synaptic transmission. Due to different sources of biological and measurement noise, recordings of mPSCs exhibit high trial-to-trial heterogeneity, and experimental measurements of mPSCs are usually noisy and scarce, making their analysis demanding. Here, we present a sequential procedure for precise analysis of mPSC amplitude distributions for the range of small currents. To illustrate the developed approach, we chose previously obtained experimental data on the effect of the extracellular matrix on synaptic plasticity. The proposed statistical technique allowed us to identify previously unnoticed additional modality in the mPSC amplitude distributions, indicating the formation of new immature synapses upon ECM attenuation. We show that our approach can reliably detect multimodality in the distributions of mPSC amplitude, allowing for accurate determination of the size and variability of the quantal synaptic response. Thus, the proposed method can significantly expand the informativeness of both existing and newly obtained experimental data. We also demonstrated that mPSC amplitudes around the threshold of microcurrent excitation follow the Gumbel distribution rather than the binomial statistics traditionally used for a wide range of currents, either for a single synapse or when taking into consideration small influences of the adjacent synapses. Such behaviour is argued to originate from the theory of extreme processes. Specifically, recorded mPSCs represent instant random current fluctuations, among which there are relatively larger spikes (extreme events). They required more level of coherence that can be provided by different mechanisms of network or system level activation including neuron circuit signalling and extrasynaptic processes.

Topology of synaptic connectivity constrains neuronal stimulus representation, predicting two complementary coding strategies

In motor-related brain regions, movement intention has been successfully decoded from in-vivo spike train by isolating a lower-dimension manifold that the high-dimensional spiking activity is constrained to. The mechanism enforcing this constraint remains unclear, although it has been hypothesized to be implemented by the connectivity of the sampled neurons. We test this idea and explore the interactions between local synaptic connectivity and its ability to encode information in a lower dimensional manifold through simulations of a detailed microcircuit model with realistic sources of noise. We confirm that even in isolation such a model can encode the identity of different stimuli in a lower-dimensional space. We then demonstrate that the reliability of the encoding depends on the connectivity between the sampled neurons by specifically sampling populations whose connectivity maximizes certain topological metrics. Finally, we developed an alternative method for determining stimulus identity from the activity of neurons by combining their spike trains with their recurrent connectivity. We found that this method performs better for sampled groups of neurons that perform worse under the classical approach, predicting the possibility of two separate encoding strategies in a single microcircuit.

Data-Driven Modeling of Cholinergic Modulation of Neural Microcircuits: Bridging Neurons, Synapses and Network Activity

Neuromodulators, such as acetylcholine (ACh), control information processing in neural microcircuits by regulating neuronal and synaptic physiology. Computational models and simulations enable predictions on the potential role of ACh in reconfiguring network activity. As a prelude into investigating how the cellular and synaptic effects of ACh collectively influence emergent network dynamics, we developed a data-driven framework incorporating phenomenological models of the physiology of cholinergic modulation of neocortical cells and synapses. The first-draft models were integrated into a biologically detailed tissue model of neocortical microcircuitry to investigate the effects of levels of ACh on diverse neuron types and synapses, and consequently on emergent network activity. Preliminary simulations from the framework, which was not tuned to reproduce any specific ACh-induced network effects, not only corroborate the long-standing notion that ACh desynchronizes spontaneous network activity, but also predict that a dose-dependent activation of ACh gives rise to a spectrum of neocortical network activity. We show that low levels of ACh, such as during non-rapid eye movement (nREM) sleep, drive microcircuit activity into slow oscillations and network synchrony, whereas high ACh concentrations, such as during wakefulness and REM sleep, govern fast oscillations and network asynchrony. In addition, spontaneous network activity modulated by ACh levels shape spike-time cross-correlations across distinct neuronal populations in strikingly different ways. These effects are likely due to the regulation of neurons and synapses caused by increasing levels of ACh, which enhances cellular excitability and decreases the efficacy of local synaptic transmission. We conclude by discussing future directions to refine the biological accuracy of the framework, which will extend its utility and foster the development of hypotheses to investigate the role of neuromodulators in neural information processing.

Activity-induced spontaneous spikes in GABAergic neurons suppress seizure discharges: an implication of computational modeling

Background

Epilepsy, a prevalent neurological disorder, appears self-termination. The endogenous mechanism for seizure self-termination remains to be addressed in order to develop new strategies for epilepsy treatment. We aim to examine the role of activity-induced spontaneous spikes at GABAergic neurons as an endogenous mechanism in the seizure self-termination.

Methods and Results

Neuronal spikes were induced by depolarization pulses at cortical GABAergic neurons from temporal lobe epilepsy patients and mice, in which some of these neurons fired activity-induced spontaneous spikes. Neural networks including excitatory and inhibitory neurons were computationally constructed, and their functional properties were based on our studies from whole-cell recordings. With the changes in the portion and excitability of inhibitory neurons that generated activity-induced spontaneous spike, the efficacies to suppress synchronous seizure activity were analyzed, such as its onset time, decay slope and spike frequency. The increases in the proportion and excitability of inhibitory neurons that generated activity-induced spontaneous spikes effectively suppressed seizure activity in neural networks. These factors synergistically strengthened the efficacy of seizure activity suppression.

Conclusion

Our study supports a notion that activity-induced spontaneous spikes in GABAergic neurons may be an endogenous mechanism for seizure self-termination. A potential therapeutic strategy for epilepsy is to upregulate the cortical inhibitory neurons that generate activity-induced spontaneous spikes.

Glutamatergic input is selectively increased in dorsal raphe subfield 5-HT neurons: role of morphology, topography and selective innervation

Characterization of glutamatergic input to dorsal raphe (DR) serotonin (5-HT) neurons is crucial for understanding how the glutamate and 5-HT systems interact in psychiatric disorders. Markers of glutamatergic terminals, vGlut1, 2 and 3, reflect inputs from specific forebrain and midbrain regions. Punctate staining of vGlut2 was homogeneous throughout the mouse DR whereas vGlut1 and vGlut3 puncta were less dense in the lateral wing (lwDR) compared with the ventromedial (vmDR) subregion. The distribution of glutamate terminals was consistent with the lower miniature excitatory postsynaptic current frequency found in the lwDR; however, it was not predictive of glutamatergic synaptic input with local activity intact, as spontaneous excitatory postsynaptic current (sEPSC) frequency was higher in the lwDR. We examined the morphology of recorded cells to determine if variations in dendrite structure contributed to differences in synaptic input. Although lwDR neurons had longer, more complex dendrites than vmDR neurons, glutamatergic input was not correlated with dendrite length in the lwDR, suggesting that dendrite length did not contribute to subregional differences in sEPSC frequency. Overall, glutamatergic input in the DR was the result of selective innervation of subpopulations of 5-HT neurons and was rooted in the topography of DR neurons and the activity of glutamate neurons located within the midbrain slice. Increased glutamatergic input to lwDR cells potentially synergizes with previously reported increased intrinsic excitability of lwDR cells to increase 5-HT output in lwDR target regions. Because the vmDR and lwDR are involved in unique circuits, subregional differences in glutamate modulation may result in diverse effects on 5-HT output in stress-related psychopathology.

Laminar-specific maturation of GABAergic transmission and susceptibility to visual deprivation are related to endocannabinoid sensitivity in mouse visual cortex

The developmental period when neuronal responses are modified by visual experience is reported to start and end earlier in layer 4 than in layer 2/3 of the visual cortex, and the maturation of GABAergic inhibitory circuits is suggested to determine the timing of this period. Here, we examine whether the laminar difference in such timing corresponds to a difference in the time course of the functional maturation of GABAergic synaptic transmission to star pyramidal and pyramidal cells in layers 4 and 2/3, respectively, of the mouse visual cortex and whether the development of the strength of GABAergic transmission is affected by visual deprivation in a laminar-specific manner. Our analysis of developmental changes in inhibitory postsynaptic currents of star pyramidal and pyramidal cells evoked by electrical stimulation of afferents or action potentials of fast-spiking GABAergic neurons revealed that there was a sequential maturation of GABAergic function from layers 4 to 2/3. The maturation of inhibition in layer 4 occurred at postnatal week 3, which preceded by 1 week that of layer 2/3. Visual deprivation by dark rearing arrested the functional development of GABAergic transmission in layer 2/3, whereas dark rearing was not so effective in layer 4. GABAergic synapses in layer 2/3 were sensitive to an agonist for cannabinoid type 1 receptors and not normally matured in receptor knock-out mice, whereas those in layer 4 were not so. These results suggest laminar-specific maturation of inhibition and susceptibility to visual deprivation, which may be related to the laminar difference in sensitivity to endocannabinoids.

Swim stress differentially blocks CRF receptor mediated responses in dorsal raphe nucleus

Modulation of the serotonergic (5-HT) neurotransmitter system arising from the dorsal raphe nucleus (DR) is thought to support the behavioral effects of swim stress, i.e., immobility. In vivo pharmacological and anatomical studies suggest that corticotropin-releasing factor (CRF) and γ-aminobutyric acid (GABA) synaptic transmission closely interact to set the response of the DR to swim stress. To investigate the cellular basis of these physiological mechanisms the effects of ovine CRF (oCRF) on GABA(A)-dependent miniature inhibitory postsynaptic currents (mIPSCs) in 5-HT and non-5-HT DR neurons in acute mesencephalic slices obtained from rats either naïve or 24h after a 15 min swim stress session were tested. In this study, the effect of swim stress alone was to decrease the holding current, i.e., hyperpolarize the neuron, and to increase the amplitude and charge of mIPSCs recorded from non-5-HT neurons. Ovine CRF (10 nM) induced an increase in mIPSC frequency in 5-HT neurons recorded from naïve rats, an effect that was suppressed by swim stress. The inward current elicited by oCRF in both 5-HT and non-5-HT neurons was also blocked by swim stress. Ovine CRF increased mIPSCs amplitude and charge in both 5-HT and non-5-HT neurons, but this effect was not modified by swim stress. In concert with our previous findings that swim stress decreased input resistance, action potential threshold and action potential duration and increased glutamatergic synaptic activity the overall primary effect of swim stress is to increase the excitability of 5-HT neurons. These data provide a mechanism at the cellular level for the immobility induced by swim stress and identifies critical components of the raphe circuitry responsible for the altered output of 5-HT neurons induced by swim stress.

Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function

This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at 'very low' intensities which elicit only short-latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non-linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity-dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.

State-dependent bidirectional modification of somatic inhibition in neocortical pyramidal cells

Cortical pyramidal neurons alter their responses to input signals depending on behavioral state. We investigated whether changes in somatic inhibition contribute to these alterations. In layer 5 pyramidal neurons of rat visual cortex, repetitive firing from a depolarized membrane potential, which typically occurs during arousal, produced long-lasting depression of somatic inhibition. In contrast, slow membrane oscillations with firing in the depolarized phase, which typically occurs during slow-wave sleep, produced long-lasting potentiation. The depression is mediated by L-type Ca2+ channels and GABA(A) receptor endocytosis, whereas potentiation is mediated by R-type Ca2+ channels and receptor exocytosis. It is likely that the direction of modification is mainly dependent on the ratio of R- and L-type Ca2+ channel activation. Furthermore, somatic inhibition was stronger in slices prepared from rats during slow-wave sleep than arousal. This bidirectional modification of somatic inhibition may alter pyramidal neuron responsiveness in accordance with behavioral state.

Slow GABA(A) mediated synaptic transmission in rat visual cortex

BACKGROUND

Previous reports of inhibition in the neocortex suggest that inhibition is mediated predominantly through GABA(A) receptors exhibiting fast kinetics. Within the hippocampus, it has been shown that GABA(A) responses can take the form of either fast or slow response kinetics. Our findings indicate, for the first time, that the neocortex displays synaptic responses with slow GABA(A) receptor mediated inhibitory postsynaptic currents (IPSCs). These IPSCs are kinetically and pharmacologically similar to responses found in the hippocampus, although the anatomical specificity of evoked responses is unique from hippocampus. Spontaneous slow GABA(A) IPSCs were recorded from both pyramidal and inhibitory neurons in rat visual cortex.

RESULTS

GABA(A) slow IPSCs were significantly different from fast responses with respect to rise times and decay time constants, but not amplitudes. Spontaneously occurring GABA(A) slow IPSCs were nearly 100 times less frequent than fast sIPSCs and both were completely abolished by the chloride channel blocker, picrotoxin. The GABA(A) subunit-specific antagonist, furosemide, depressed spontaneous and evoked GABA(A) fast IPSCs, but not slow GABA(A)-mediated IPSCs. Anatomical specificity was evident using minimal stimulation: IPSCs with slow kinetics were evoked predominantly through stimulation of layer 1/2 apical dendritic zones of layer 4 pyramidal neurons and across their basal dendrites, while GABA(A) fast IPSCs were evoked through stimulation throughout the dendritic arborization. Many evoked IPSCs were also composed of a combination of fast and slow IPSC components.

CONCLUSION

GABA(A) slow IPSCs displayed durations that were approximately 4 fold longer than typical GABA(A)fast IPSCs, but shorter than GABA(B)-mediated inhibition. The anatomical and pharmacological specificity of evoked slow IPSCs suggests a unique origin of synaptic input. Incorporating GABA(A) slow IPSCs into computational models of cortical function will help improve our understanding of cortical information processing.

Rate coding and spike-time variability in cortical neurons with two types of threshold dynamics

Neurons and dynamical models of spike generation display two different classes of threshold behavior: type 1 [firing frequency vs. current (f-I) relationship is continuous at threshold] and type 2 (discontinuous f-I). With steady current or conductance stimulation, regular-spiking (RS) pyramidal neurons and fast-spiking (FS) inhibitory interneurons in layer 2/3 of somatosensory cortex exhibit type 1 and type 2 threshold behaviors, respectively. We compared the postsynaptic firing variability of type 1 RS and type 2 FS cells, during naturalistic, fluctuating conductance input. In RS neurons, increasing the level of independently random, shunting inhibition caused a monotonic increase in spike reliability, whereas in FS interneurons, there was an optimum level of shunting inhibition for achieving the most reliable spike generation and the most precise spike-time encoding. This was observed over a range of different degrees of synchrony, or correlation, in the input. RS cells displayed a progressive rise in spike jitter during natural-like transient burst inputs, whereas for FS cells, jitter was mostly kept low. Furthermore, RS cells showed encoding of the input level in the spike shape, whereas FS cells did not. These differences between the two cell types are consistent with a role of RS neurons as rate-coding integrators, and a role of FS neurons as resonators controlling the coherence of synchronous firing.

Acute injury to superficial cortex leads to a decrease in synaptic inhibition and increase in excitation in neocortical layer V pyramidal cells

Injury to the superficial layers of cerebral cortex produces alterations in the synaptic responses of local circuits that promote the development of seizures. To further delineate the specific changes in synaptic strength that are induced by this type of cortical injury, whole cell voltage-clamp recordings were used to examine evoked and spontaneous synaptic events from layer V pyramidal cells in coronal slices prepared from surgically traumatized rat neocortices in which the superficial third of the cortex (layers I, II, and part of III) was removed. Slices from intact neocortices were used as controls. Examinations of fast inhibitory postsynaptic currents (IPSCs) indicated that traumatized slices were disinhibited, exhibiting evoked IPSCs (eIPSCs) with lower peak amplitudes. Measurements of spontaneous IPSCs (sIPSCs) revealed no difference in the mean amplitudes of sIPSCs recorded in traumatized versus control slices. However, the mean sIPSC frequency was lower in traumatized slices, indicative of a decrease in GABA release at these inhibitory synapses. Traumatized slices also displayed an increase in synaptic excitation, exhibiting spontaneous EPSCs (sESPCs) with larger peak amplitudes and higher frequencies. Peak-scaled nonstationary fluctuation analysis of sEPSCs and sIPSCs was used to obtain estimates of the unit conductance and number of functional receptor channels. EPSC and IPSC channel numbers and IPSC unit conductance did not differ between traumatized and intact slices. However, the mean unit conductance of EPSCs was higher (+25%) in traumatized slices. These findings suggest that acute injury to the superficial neocortical layers results in a disinhibition of cortical circuits that stems from a decline in GABA release likely due to the loss of superficial inhibitory interneurons and an enhancement of synaptic excitation consequent to an increase in the AMPA receptor unit conductance.

Protein kinase Mzeta enhances excitatory synaptic transmission by increasing the number of active postsynaptic AMPA receptors

Protein kinase Mzeta (PKMzeta), a constitutively active, atypical PKC isoform, enhances synaptic strength during the maintenance of long-term potentiation (LTP). Here we examine the mechanism by which PKMzeta increases synaptic transmission. Postsynaptic perfusion of PKMzeta during whole-cell recordings of CA1 pyramidal cells strongly potentiated the amplitude of AMPA receptor (AMPAR)-mediated miniature EPSCs (mEPSCs). Nonstationary fluctuation analysis of events recorded before and after PKMzeta enhancement showed that the kinase doubled the number of functional postsynaptic AMPAR channels. After sustained potentiation, application of a PKMzeta inhibitor reversed the increase in functional channel number to basal levels, suggesting that persistent increase of PKMzeta is required to maintain the postsynaptic localization of a mobile subpopulation of receptors. The kinase did not affect other sites of LTP expression, including presynaptic transmitter release, silent synapse conversion, or AMPAR unit conductance. Thus PKMzeta functions specifically to establish and maintain long-term increases in active postsynaptic AMPAR number.

A Reconfiguration of CaV2 Ca2+ Channel Current and Its Dopaminergic D2 Modulation in Developing Neostriatal Neurons

The modulatory effect of D2 dopamine receptor activation on calcium currents was studied in neostriatal projection neurons at two stages of rat development: postnatal day (PD)14 and PD40. D2-class receptor agonists reduced whole cell calcium currents by about 35% at both stages, and this effect was blocked by the D2 receptor antagonist sulpiride. Nitrendipine partially occluded this modulation at both stages, indicating that modulation of CaV1 channels was present throughout this developmental interval. Nevertheless, modulation of CaV1 channels was significantly larger in PD40 neurons. ω-Conotoxin GVIA occluded most of the Ca2+ current modulation in PD14 neurons. However, this occlusion was greatly decreased in PD40 neurons. ω-Agatoxin TK occluded a great part of the modulation in PD40 neurons but had a negligible effect in PD14 neurons. The data indicate that dopaminergic D2-mediated modulation undergoes a change in target during development: from CaV2.2 to CaV2.1 Ca2+ channels. This change occurred while CaV2.2 channels were being down-regulated and CaV2.1 channels were being up-regulated. Presynaptic modulation mediated by D2 receptors reflected these changes; CaV2.2 type channels were used for release in young animals but very little in mature animals, suggesting that changes took place simultaneously at the somatodendritic and the synaptic membranes.

Nootropic Agents Enhance the Recruitment of Fast GABAA Inhibition in Rat Neocortex

It is widely believed that nootropic (cognition-enhancing) agents produce their therapeutic effects by augmenting excitatory synaptic transmission in cortical circuits, primarily through positive modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors (AMPARs). However, GABA-mediated inhibition is also critical for cognition, and enhanced GABA function may be likewise therapeutic for cognitive disorders. Could nootropics act through such a mechanism as well? To address this question, we examined the effects of nootropic agents on excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) recorded from layer V pyramidal cells in acute slices of somatosensory cortex. Aniracetam, a positive modulator of AMPA/kainate receptors, increased the peak amplitude of evoked EPSCs and the amplitude and duration of polysynaptic fast IPSCs, manifested as a greater total charge carried by IPSCs. As a result, the EPSC/IPSC ratio of total charge was decreased, representing a shift in the excitation-inhibition balance that favors inhibition. Aniracetam did not affect the magnitude of either monosynaptic IPSCs (mono-IPSCs) recorded in the presence of excitatory amino acid receptor antagonists, or miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin. However, the duration of both mono-IPSCs and mIPSCs was prolonged, suggesting that aniracetam also directly modulates GABAergic transmission. Cyclothiazide, a preferential modulator of AMPAR function, enhanced the magnitude and duration of polysynaptic IPSCs, similar to aniracetam, but did not affect mono-IPSCs. Concanavalin A, a kainate receptor modulator, had little effect on EPSCs or IPSCs, suggesting there was no contribution from kainate receptor activity. These findings indicate that AMPAR modulators strengthen inhibition in neocortical pyramidal cells, most likely by altering the kinetics of AMPARs on synaptically connected interneurons and possibly by modulating GABA(A) receptor responses in pyramidal cells. This suggests that the therapeutic actions of nootropic agents may be partly mediated through enhanced cortical GABAergic inhibition, and not solely through the direct modification of excitation, as previously thought.

Dopamine-glutamate interactions controlling prefrontal cortical pyramidal cell excitability involve multiple signaling mechanisms

Although the importance of dopamine (DA) for prefrontal cortical (PFC) cognitive functions is widely recognized, the nature of DA actions in the PFC remains controversial. A critical component in DA actions is its modulation of glutamate transmission, which can be different when specific receptors are activated. To obtain a clear picture of cellular mechanisms involved in these interactions, we studied the effects of DA-glutamate coactivation on pyramidal cell excitability in brain slices obtained from developmentally mature rats using whole-cell patch-clamp recordings. Bath application of NMDA, AMPA, and the D1 agonist SKF38393 induced concentration-dependent excitability increases, whereas bath application of the D2 receptor agonist quinpirole induced a concentration-dependent excitability decrease. The NMDA-mediated response was potentiated by SKF38393. This NMDA-D1 synergism required postsynaptic intracellular Ca2+ and protein kinase A (PKA) and was independent of membrane depolarization. On the other hand, the excitatory effects of both NMDA and AMPA were attenuated by a D2 agonist. Surprisingly, the D2-NMDA interaction was also blocked by the GABA(A) antagonists bicuculline and picrotoxin, suggesting that the inhibitory action of D2 receptors on NMDA-induced responses in the PFC may be mediated by GABAergic interneurons. In contrast, the D2-AMPA interaction involves inhibition of PKA and activation of phospholipase lipase C-IP3 and intracellular Ca2+ at a postsynaptic level. Thus, the modulatory actions of D1 and D2 receptors on PFC pyramidal cell excitability are mediated by multiple intracellular mechanisms and by activation of GABA(A) receptors, depending on the glutamate receptor subtypes involved.

Spontaneous synaptic activity is required for the formation of functional GABAergic synapses in the developing rat hippocampus

Here we examine the role of the spontaneous synaptic activity generated by the developing rat hippocampus in the formation of functional gamma-aminobutyric acid (GABA) synapses. Intact hippocampal formations (IHFs) were dissected at birth and incubated for 1 day in control or tetrodotoxin (TTX)-supplemented medium at 25 degrees C. After the incubation, miniature GABA(A)-mediated postsynaptic currents (mGABA(A)-PSCs) were recorded in whole-cell voltage-clamped CA3 pyramidal neurones from IHF-derived slices. After 1 day in vitro in control medium, the frequency of mGABA(A)-PSCs was similar to that recorded in acute slices obtained 1 day after birth, but significantly higher than the frequency recorded from acute slices just after birth. These results suggest that the factors required in vivo for the formation of functional GABAergic synapses are preserved in the IHFs in vitro. The frequency increase was prevented when IHFs were incubated for 1 day with TTX. TTX treatment affected neither the morphology of CA3 pyramidal neurones nor cell viability. The TTX effects were reproduced when IHFs were incubated in the presence of glutamatergic or GABAergic ionotropic receptor antagonists or in high divalent cationic medium. The present results indicate that the spontaneous synaptic activity generated by the developing hippocampus is a key player in the formation of functional GABAergic synapses, possibly via network events requiring both glutamatergic and GABAergic receptors.

Dark rearing alters the development of GABAergic transmission in visual cortex

We studied the role of sensory experience in the maturation of GABAergic circuits in the rat visual cortex. Between the time at which the eyes first open and the end of the critical period for experience-dependent plasticity, the total GABAergic input converging into layer II/III pyramidal cells increases threefold. We propose that this increase reflects changes in the number of quanta released by presynaptic axons. Here, we show that the developmental increase in GABAergic input is prevented in animals deprived of light since birth but not in animals deprived of light after a period of normal experience. Thus, sensory experience appears to play a permissive role in the maturation of intracortical GABAergic circuits.

Absence of long-term depression in the visual cortex of glutamic Acid decarboxylase-65 knock-out mice

Long-term depression (LTD) is widely considered a mechanism for experience-induced synaptic weakening in the brain. Recent in vivo studies on glutamic acid decarboxylase [GAD 65 (-/-)] knock-out mice indicates that GABAergic synaptic inhibition is also required for the normal weakening of deprived inputs in the visual cortex. To better understand how GABAergic inhibition might control plasticity, we assessed the status of synaptic inhibition and LTD in visual cortical slices of GAD 65 knock-out mice. We found the following: (1) the efficacy of GABAergic synapses during repetitive activation is reduced in GAD 65 (-/-) mice; (2) the induction of LTD is impaired in the visual cortex of GAD 65 (-/-) mice; and (3) chronic, but not acute, treatment with the benzodiazepine agonist diazepam restores LTD in GAD 65 (-/-) mice. These results suggest that a certain inhibitory tone is required for the induction of LTD in visual cortex. We propose that the lack of visual cortical LTD in GAD 65 (-/-) may account for the lack of experience-dependent plasticity in these mice.

Transition from GABAergic to glycinergic synaptic transmission in newly formed spinal networks

The role of glycinergic and GABAergic systems in mediating spontaneous synaptic transmission in newly formed neural networks was examined in motoneurons in the developing rat spinal cord. Properties of action potential-independent miniature inhibitory postsynaptic currents (mIPSCs) mediated by glycine and GABA(A) receptors (GlyR and GABA(A)R) were studied in spinal cord slices of 17- to 18-day-old embryos (E17-18) and 1- to 3-day-old postnatal rats (P1-3). mIPSC frequency and amplitude significantly increased after birth, while their decay time decreased. To determine the contribution of glycinergic and GABAergic synapses to those changes, GlyR- and GABA(A)R-mediated mIPSCs were isolated based on their pharmacological properties. Two populations of pharmacologically distinct mIPSCs were recorded in the presence of glycine or GABA(A) receptors antagonists: bicuculline-resistant, fast-decaying GlyR-mediated mIPSCs, and strychnine-resistant, slow-decaying GABA(A)R-mediated mIPSCs. The frequency of GABA(A)R-mediated mIPSCs was fourfold higher than that of GlyR-mediated mIPSCs at E17-18, indicating that GABAergic synaptic sites were functionally dominant at early stages of neural network formation. Properties of GABA(A)R-mediated mIPSC amplitude fluctuations changed from primarily unimodal skewed distribution at E17-18 to Gaussian mixtures with two to three discrete components at P1-3. A developmental shift from primarily long-duration GABAergic mIPSCs to short-duration glycinergic mIPSCs was evident after birth, when the frequency of GlyR-mediated mIPSCs increased 10-fold. This finding suggested that either the number of glycinergic synapses or the probability of vesicular glycine release increased during the period studied. The increased frequency of GlyR-mediated mIPSCs was associated with more than a twofold increase in their mean amplitude, and in the number of motoneurons in which mIPSC amplitude fluctuations were best fitted by multi-component Gaussian curves. A third subpopulation of mIPSCs was apparent in the absence of glycine and GABA(A) receptor antagonists: mIPSCs with both fast and slow decaying components. Based on their dual-component decay time and their suppression by either strychnine or bicuculline, we assumed that these were generated by the activation of co-localized postsynaptic glycine and GABA(A) receptors. The contribution of mixed glycine-GABA synaptic sites to the generation of mIPSCs did not change after birth. The developmental switch from predominantly long-duration GABAergic inhibitory synaptic currents to short-duration glycinergic currents might serve as a mechanism regulating neuronal excitation in the developing spinal networks.

Electrophysiological and morphological characterization of neurons within neocortical ectopias

Focal developmental abnormalities in neocortex, including ectopic collections of neurons in layer I (ectopias), have been associated with behavioral and neurological deficits. In this study, we used infrared differential interference contrast microscopy and whole cell patch-clamp to complete the first characterization of neurons within and surrounding neocortical ectopias. Current-clamp recordings revealed that neurons within ectopias display multiple types of action potential firing patterns, and biocytin labeling indicated that approximately 20% of the cells in neocortical ectopias can be classified as nonpyramidal cells and the rest as atypically oriented pyramidal cells. All cells had spontaneous excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents. Exhibitory postsynaptic currents consisted of both N-methyl-D-aspartate (NMDA) receptor-mediated and AMPA/kainate (A/K) receptor-mediated currents. The NMDA receptor-mediated component had decay time constants of 15.35 +/- 2.2 (SE) ms, while the A/K component had faster decay kinetics of 7.6 +/- 1.7 ms at -20 mV. GABA(A) receptor-mediated synaptic currents in ectopic cells reversed at potentials near the Cl- equilibrium potential and had decay kinetics of 16.65 +/- 1.3 ms at 0 mV. Furthermore we show that cells within ectopias receive direct excitatory and inhibitory input from adjacent normatopic cortex and can display a form of epileptiform activity.

Postsynaptic variability of firing in rat cortical neurons: the roles of input synchronization and synaptic NMDA receptor conductance

Neurons in the functioning cortex fire erratically, with highly variable intervals between spikes. How much irregularity comes from the process of postsynaptic integration and how much from fluctuations in synaptic input? We have addressed these questions by recording the firing of neurons in slices of rat visual cortex in which synaptic receptors are blocked pharmacologically, while injecting controlled trains of unitary conductance transients, to electrically mimic natural synaptic input. Stimulation with a Poisson train of fast excitatory (AMPA-type) conductance transients, to simulate independent inputs, produced much less variability than encountered in vivo. Addition of NMDA-type conductance to each unitary event regularized the firing but lowered the precision and reliability of spikes in repeated responses. Independent Poisson trains of GABA-type conductance transients (reversing at the resting potential), which simulated independent activity in a population of presynaptic inhibitory neurons, failed to increase timing variability substantially but increased the precision of responses. However, introduction of synchrony, or correlations, in the excitatory input, according to a nonstationary Poisson model, dramatically raised timing variability to in vivo levels. The NMDA phase of compound AMPA-NMDA events conferred a time-dependent postsynaptic variability, whereby the reliability and precision of spikes degraded rapidly over the 100 msec after the start of a synchronous input burst. We conclude that postsynaptic mechanisms add significant variability to cortical responses but that substantial synchrony of inputs is necessary to explain in vivo variability. We suggest that NMDA receptors help to implement a switch from precise firing to random firing during responses to concerted inputs.