Cortical Pyramidal Cells Research Articles

Selective Functional Interactions between Excitatory and Inhibitory Cortical Neurons and Differential Contribution to Persistent Activity of the Slow Oscillation

The neocortex depends upon a relative balance of recurrent excitation and inhibition for its operation. During spontaneous Up states, cortical pyramidal cells receive proportional barrages of excitatory and inhibitory synaptic potentials. Many of these synaptic potentials arise from the activity of nearby neurons, although the identity of these cells is relatively unknown, especially for those underlying the generation of inhibitory synaptic events. To address these fundamental questions, we developed an in vitro submerged slice preparation of the mouse entorhinal cortex that generates robust and regular spontaneous recurrent network activity in the form of the slow oscillation. By performing whole-cell recordings from multiple cell types identified with green fluorescent protein expression and electrophysiological and/or morphological properties, we show that distinct functional subpopulations of neurons exist in the entorhinal cortex, with large variations in contribution to the generation of balanced excitation and inhibition during the slow oscillation. The most active neurons during the slow oscillation are excitatory pyramidal and inhibitory fast spiking interneurons, receiving robust barrages of both excitatory and inhibitory synaptic potentials. Weak action potential activity was observed in stellate excitatory neurons and somatostatin-containing interneurons. In contrast, interneurons containing neuropeptide Y, vasoactive intestinal peptide, or the 5-hydroxytryptamine (serotonin) 3a receptor, were silent. Our data demonstrate remarkable functional specificity in the interactions between different excitatory and inhibitory cortical neuronal subtypes, and suggest that it is the large recurrent interaction between pyramidal neurons and fast spiking interneurons that is responsible for the generation of persistent activity that characterizes the depolarized states of the cortex.

Heterogeneity and independency of unitary synaptic outputs from hippocampal CA3 pyramidal cells

The variation of individual synaptic transmission impacts the dynamics of complex neural circuits. We performed whole-cell recordings from monosynaptically connected hippocampal neurons in rat organotypic slice cultures using a synapse mapping method. The amplitude of unitary excitatory postsynaptic current (uEPSC) varied from trial to trial and was independent of the physical distance between cell pairs. To investigate the source of the transmission variability, we obtained patch-clamp recordings from intact axons. Axonal action potentials (APs) were reliably transmitted throughout the axonal arbour and showed modest changes in width. In contrast, calcium imaging from presynaptic boutons revealed that the amplitude of AP-evoked calcium transients exhibited large variations both among different boutons at a given trial and among trials in a given bouton. These results suggest that a factor contributing to the uEPSC fluctuations is the variability in calcium dynamics at presynaptic terminals. Finally, we acquired triple whole-cell recordings from divergent circuit motifs with one presynaptic neuron projecting to two postsynaptic neurons. Consistent with the independency of calcium dynamics among axonal boutons, a series of uEPSC fluctuations was not correlated between the two postsynaptic cells, indicating that different synapses even from the same neuron act independently.We conclude that the intra-bouton and inter-bouton variability in AP-induced calcium dynamics determine the heterogeneity and independency of uEPSCs.

Sleep Slow-Wave Activity Regulates Cerebral Glycolytic Metabolism

Non-rapid eye movement sleep (NREMS) onset is characterized by a reduction in cerebral metabolism and an increase in slow waves, 1-4-Hz oscillations between relatively depolarized and hyperpolarized states in the cerebral cortex. The metabolic consequences of slow-wave activity (SWA) at the cellular level remain uncertain. We sought to determine whether SWA modulates the rate of glycolysis within the cerebral cortex. The real-time measurement of lactate concentration in the mouse cerebral cortex demonstrates that it increases during enforced wakefulness. In spontaneous sleep/wake cycles, lactate concentration builds during wakefulness and rapid eye movement sleep and declines during NREMS. The rate at which lactate concentration declines during NREMS is proportional to the magnitude of electroencephalographic (EEG) activity at frequencies of <10 Hz. The induction of 1-Hz oscillations, but not 10-Hz oscillations, in the electroencephalogram by optogenetic stimulation of cortical pyramidal cells during wakefulness triggers a decline in lactate concentration. We conclude that cerebral SWA promotes a decline in the rate of glycolysis in the cerebral cortex. These results demonstrate a cellular energetic function for sleep SWA, which may contribute to its restorative effects on brain function.

The Open Source Brain Initiative: enabling collaborative modelling in computational neuroscience

While an increasing number of biophysically detailed neuronal models (featuring (semi-) realistic morphologies and voltage and ligand gated conductances) are being shared across the community through resources like ModelDB, these usually only represent a snapshot of the model at the time of publication, in a format specific to the original simulator used. Models are constantly evolving however, to take account of new experimental findings and to address new research questions, both by the original modellers, and by other researchers who help provide quality control/debugging of original scripts and convert the model (components) for use in other simulators. This crucial part of the model life cycle is not well addressed with currently available infrastructure. The Open Source Brain (OSB) repository is being developed to provide a central location for researchers to collaboratively develop models which can be run across multiple simulators and can interact with the range of other applications in the NeuroML “ecosystem”. NeuroML [1] is a simulator independent language for expressing detailed single cell and network models, which is supported by an increasing number of applications for generating, visualising, simulating and analysing such models as well as by databases providing the base components (e.g. reconstructed morphologies, ion channels) for use in the models (http://www.neuroml. org/tool_support). The OSB repository differs from existing model databases which have traditionally concentrated on frozen, published models. The cell, ion channel, synapse and network models in this repository develop over time to ensure they reflect best practices in neurophysiological modelling and allow continuously improving, bug-free simulations. Multiple views of the model elements are available to encourage feedback from modellers, theoreticians and experimentalists. Links can be made to previous versions of the models in ModelDB, and deep links will be used to ensure cross referencing to other neuroinformatics resources such as NeuroMorpho and NeuroLex. The system is based around a Mercurial version control repository with models organised into projects illustrating a number of neurophysiologically relevant aspects of the cell and network behaviour. The history is recorded of all changes to each project by contributors who can be distributed worldwide. There is close integration with the application neuroConstruct [2], allowing the models to be examined with a 3D graphical user interface, and scripts automatically generated for use on a number of widely used neuronal simulators. A number of models are already available in the repository, including cell and network models from the cerebellum, detailed cortical and hippocampal pyramidal cell models and a 3D version of a single column thalamocortical network model [3]. While most of the models available are conversions of existing published models, some have been developed during original research projects using the tools and formats discussed here [4]. The repository is currently in alpha stage of development and is being tested with a small number of labs. The resource can be accessed at http://opensourcebrain.org:8080. This work has been primarily funded by the Wellcome Trust

Novel Allosteric Agonists of M1 Muscarinic Acetylcholine Receptors Induce Brain Region-Specific Responses That Correspond with Behavioral Effects in Animal Models

M(1) muscarinic acetylcholine receptors (mAChRs) represent a viable target for treatment of multiple disorders of the central nervous system (CNS) including Alzheimer's disease and schizophrenia. The recent discovery of highly selective allosteric agonists of M(1) receptors has provided a major breakthrough in developing a viable approach for the discovery of novel therapeutic agents that target these receptors. Here we describe the characterization of two novel M(1) allosteric agonists, VU0357017 and VU0364572, that display profound differences in their efficacy in activating M(1) coupling to different signaling pathways including Ca(2+) and β-arrestin responses. Interestingly, the ability of these agents to differentially activate coupling of M(1) to specific signaling pathways leads to selective actions on some but not all M(1)-mediated responses in brain circuits. These novel M(1) allosteric agonists induced robust electrophysiological effects in rat hippocampal slices, but showed lower efficacy in striatum and no measureable effects on M(1)-mediated responses in medial prefrontal cortical pyramidal cells in mice. Consistent with these actions, both M(1) agonists enhanced acquisition of hippocampal-dependent cognitive function but did not reverse amphetamine-induced hyperlocomotion in rats. Together, these data reveal that M(1) allosteric agonists can differentially regulate coupling of M(1) to different signaling pathways, and this can dramatically alter the actions of these compounds on specific brain circuits important for learning and memory and psychosis.

Stress-induced sensitization of cortical adrenergic receptors following a history of cannabinoid exposure

The cannabinoid receptor agonist, WIN 55,212-2, increases extracellular norepinephrine levels in the rat frontal cortex under basal conditions, likely via desensitization of inhibitory α2-adrenergic receptors located on norepinephrine terminals. Here, the effect of WIN 55,212-2 on stress-induced norepinephrine release was assessed in the medial prefrontal cortex (mPFC), in adult male Sprague–Dawley rats using in vivo microdialysis. Systemic administration of WIN 55,212-2 30min prior to stressor exposure prevented stress-induced cortical norepinephrine release induced by a single exposure to swim when compared to vehicle. To further probe cortical cannabinoid–adrenergic interactions, postsynaptic α2-adrenergic receptor (AR)-mediated responses were assessed in mPFC pyramidal neurons using electrophysiological analysis in an in vitro cortical slice preparation. We confirm prior studies showing that clonidine increases cortical pyramidal cell excitability and that this was unaffected by exposure to acute stress. WIN 55,212-2, via bath application, blocked postsynaptic α2-AR mediated responses in cortical neurons irrespective of exposure to stress. Interestingly, stress exposure prevented the desensitization of α2-AR mediated responses produced by a history of cannabinoid exposure. Together, these data indicate the stress-dependent nature of cannabinoid interactions via both pre- and postsynaptic ARs. In summary, microdialysis data indicate that cannabinoids restrain stress-induced cortical NE efflux. Electrophysiology data indicate that cannabinoids also restrain cortical cell excitability under basal conditions; however, stress interferes with these CB1–α2 AR interactions, potentially contributing to over-activation of pyramidal neurons in mPFC. Overall, cannabinoids are protective of the NE system and cortical excitability but stress can derail this protective effect, potentially contributing to stress-related psychopathology. These data add to the growing evidence of complex, stress-dependent modulation of monoaminergic systems by cannabinoids and support the potential use of cannabinoids in the treatment of stress-induced noradrenergic dysfunction.

Effects of Amyloid-β Plaque Proximity on the Axon Initial Segment of Pyramidal Cells

The output of cortical pyramidal cells reflects the balance between excitatory inputs of cortical and subcortical origin, and inhibitory inputs from distinct populations of cortical GABAergic interneurons, each of which selectively innervate different domains of neuronal pyramidal cells (i.e., dendrites, soma and axon initial segment [AIS]). In Alzheimer's disease (AD), the presence of amyloid-β (Aβ) plaques alters the synaptic input to pyramidal cells in a number of ways. However, the effects of Aβ plaques on the AIS have still not been investigated to date. This neuronal domain is involved in input integration, as well as action potential initiation and propagation, and it exhibits Ca2+- and activity-dependent structural plasticity. The AIS is innervated by GABAergic axon terminals from chandelier cells, which are thought to exert a strong influence on pyramidal cell output. In the AβPP/PS1 transgenic mouse model of AD, we have investigated the effects of Aβ plaques on the morphological and neurochemical features of the AIS, including the cisternal organelle, using immunocytochemistry and confocal microscopy, as well as studying the innervation of the AIS by chandelier cell axon terminals. There is a strong reduction in GABAergic terminals that appose AIS membrane surfaces that are in contact with Aβ plaques, indicating altered inhibitory synapsis at the AIS. Thus, despite a lack of gross structural alterations in the AIS, this decrease in GABAergic innervation may deregulate AIS activity and contribute to the hyperactivity of neurons in contact with Aβ plaques.

Possible mechanisms for the effects of neuromodulators on the perception of time intervals

We propose possible mechanisms for the effects of neuromodulators on the perception of time on the basis of our hypothesis that estimation of intervals between sensory stimuli depends on the beat frequency of a “internal clock,” which is inversely proportional to the latency of reentering excitation of the neocortex. These effects are based on the modification of the efficacy of excitatory inputs to the cortical neurons, as well as inputs from the cortex to the striatum, which are necessary for the disinhibition of thalamic neurons by the basal ganglia and subsequent excitation of the cortex. The character of influence is determined by the concentration of neuromodulators, the types of receptors activated by them on neurons of the striatum and cortex, and the modulation rules. According to the proposed mechanism, an increase in the frequency of the “internal clock” and overestimation of the duration of intervals may result from treatment with dopaminergic drugs, agonists of dopamine D1 and D2 receptors, and opioids and cannabinoids, which promote an increase in the dopamine concentration, as well as antagonists of adenosine A1 and A2A and muscarinic M2 receptors, whose activation facilitates disinhibition of the thalamus by the basal ganglia. Antagonists of A1, M2, and D2 receptors, which prevent depression of excitatory inputs to neocortical pyramidal cells, also can increase the frequency of the “internal clock.” The activation of a large number of D2 receptors on the cortical pyramidal cells, which results from a considerable increase in dopamine concentration, like activation of cannabinoid CB1 receptors, should promote a decrease in the frequency of the “internal clock” and the underestimation of interval duration. The activation of D2 and M2 receptors on the GABAergic interneurons of the cortex under conditions of strong inhibition of pyramidal neurons may increase the beat frequency of the “internal clock.” The proposed mechanism helps to understand the causes of errors in time perception in neurological diseases and to explain the discrepancies in the results of studies on the effects of neuromodulators on the estimation of time. The hypothesis may be experimentally examined by treatment of the striatum or neocortex with agonists and antagonists of various types of receptors and measurement of the drug-induced changes in the interval between the first and second peaks in the distribution of latencies of responses of cortical neurons to sensory stimulus.

Virus-mediated swapping of zolpidem-insensitive with zolpidem-sensitive GABA(A) receptors in cortical pyramidal cells.

Recently developed pharmacogenetic and optogenetic approaches, with their own advantages and disadvantages, have become indispensable tools in modern neuroscience. Here, we employed a previously described knock-in mouse line (GABAARγ277Ilox) in which the γ2 subunit of the GABAA receptor (GABAAR) was mutated to become zolpidem insensitive (γ277I) and used viral vectors to swap γ277I with wild-type, zolpidem-sensitive γ2 subunits (γ277F). The verification of unaltered density and subcellular distribution of the virally introduced γ2 subunits requires their selective labelling. For this we generated six N- and six C-terminal-tagged γ2 subunits, with which cortical cultures of GABAARγ2−/− mice were transduced using lentiviruses. We found that the N-terminal AU1 tag resulted in excellent immunodetection and unimpaired synaptic localization. Unaltered kinetic properties of the AU1-tagged γ2 (AU1γ277F) channels were demonstrated with whole-cell patch-clamp recordings of spontaneous IPSCs from cultured cells. Next, we carried out stereotaxic injections of lenti- and adeno-associated viruses containing Cre-recombinase and the AU1γ277F subunit (Cre-2A-AU1γ277F) into the neocortex of GABAARγ277Ilox mice. Light microscopic immunofluorescence and electron microscopic freeze-fracture replica immunogold labelling demonstrated the efficient immunodetection of the AU1 tag and the normal enrichment of the AU1γ277F subunits in perisomatic GABAergic synapses. In line with this, miniature and action potential-evoked IPSCs whole-cell recorded from transduced cells had unaltered amplitudes, kinetics and restored zolpidem sensitivity. Our results obtained with a wide range of structural and functional verification methods reveal unaltered subcellular distributions and functional properties of γ277I and AU1γ277F GABAARs in cortical pyramidal cells. This transgenic–viral pharmacogenetic approach has the advantage that it does not require any extrinsic protein that might endow some unforeseen alterations of the genetically modified cells. In addition, this virus-based approach opens up the possibility of modifying multiple cell types in distinct brain regions and performing alternative recombination-based intersectional genetic manipulations.

The Dendritic Density Field of a Cortical Pyramidal Cell

Much is known about the computation in individual neurons in the cortical column. Also, the selective connectivity between many cortical neuron types has been studied in great detail. However, due to the complexity of this microcircuitry its functional role within the cortical column remains a mystery. Some of the wiring behavior between neurons can be interpreted directly from their particular dendritic and axonal shapes. Here, I describe the dendritic density field (DDF) as one key element that remains to be better understood. I sketch an approach to relate DDFs in general to their underlying potential connectivity schemes. As an example, I show how the characteristic shape of a cortical pyramidal cell appears as a direct consequence of connecting inputs arranged in two separate parallel layers.

Quantitative comparison of novel GCaMP-type genetically encoded Ca2+ indicators in mammalian neurons

New variants of GCaMP-type genetically encoded Ca2+ indicators (GECIs) have been continuously developed and heavily used in many areas of biology including neuroscience. The latest subfamily called “GECOs” were developed with in vitro high-throughput screening, and shown to have novel spectral properties and/or improved fluorescent responses over their ancestor GCaMP3. The most critical parameter in evaluating performance in neurons, however, remains uncharacterized: the relationship between the GECI responses and the number of action potentials (APs). Here we analyzed the GECI responses to APs in cortical pyramidal cells of mouse acute brain slices. Unexpectedly, we found that none of the GECOs exhibited any improved performance over GCaMP3. Our results imply that careful validation is required for the accurate prediction of the actual performance of GECIs in mammalian neurons. We propose that appropriate guidelines for evaluating their efficacy should be established for the benefit of research community, given the rapidly expanding use of GECIs in neuroscience.

The Flatness of Bifurcations in 3D Dendritic Trees: An Optimal Design

The geometry of natural branching systems generally reflects functional optimization. A common property is that their bifurcations are planar and that daughter segments do not turn back in the direction of the parent segment. The present study investigates whether this also applies to bifurcations in 3D dendritic arborizations. This question was earlier addressed in a first study of flatness of 3D dendritic bifurcations by Uylings and Smit (1975), who used the apex angle of the right circular cone as flatness measure. The present study was inspired by recent renewed interest in this measure. Because we encountered ourselves shortcomings of this cone angle measure, the search for an optimal measure for flatness of 3D bifurcation was the second aim of our study. Therefore, a number of measures has been developed in order to quantify flatness and orientation properties of spatial bifurcations. All these measures have been expressed mathematically in terms of the three bifurcation angles between the three pairs of segments in the bifurcation. The flatness measures have been applied and evaluated to bifurcations in rat cortical pyramidal cell basal and apical dendritic trees, and to random spatial bifurcations. Dendritic and random bifurcations show significant different flatness measure distributions, supporting the conclusion that dendritic bifurcations are significantly more flat than random bifurcations. Basal dendritic bifurcations also show the property that their parent segments are generally aligned oppositely to the bisector of the angle between their daughter segments, resulting in “symmetrical” configurations. Such geometries may arise when during neuronal development the segments at a newly formed bifurcation are subjected to elastic tensions, which force the bifurcation into an equilibrium planar shape. Apical bifurcations, however, have parent segments oppositely aligned with one of the daughter segments. These geometries arise in the case of side branching from an existing apical main stem. The aligned “apical” parent and “apical” daughter segment form together with the side branch daughter segment already geometrically a flat configuration. These properties are clearly reflected in the flatness measure distributions. Comparison of the different flatness measures made clear that they all capture flatness properties in a different way. Selection of the most appropriate measure thus depends on the question of research. For our purpose of quantifying flatness and orientation of the segments, the dihedral angle β was found to be the most discriminative and applicable single measure. Alternatively, the parent elevation and azimuth angle formed an orthogonal pair of measures most clearly demonstrating the dendritic bifurcation “symmetry” properties.

Synaptically evoked Ca2+release from intracellular stores is not influenced by vesicular zinc in CA3 hippocampal pyramidal neurones

The co-release of neuromodulatory substances in combination with classic neurotransmitters such as glutamate and GABA from individual presynaptic nerve terminals has the capacity to dramatically influence synaptic efficacy and plasticity. At hippocampal mossy fibre synapses vesicular zinc is suggested to serve as a cotransmitter capable of regulating calcium release from internal stores in postsynaptic CA3 pyramidal cells. Here we investigated this possibility using combined intracellular ratiometric calcium imaging and patch-clamp recording techniques. In acute hippocampal slices a brief train of mossy fibre stimulation produced a large, delayed postsynaptic Ca(2+) wave that was spatially restricted to the proximal apical dendrites of CA3 pyramidal cells within stratum lucidum. This calcium increase was sensitive to intracellularly applied heparin indicating reliance upon release from internal stores and was triggered by activation of both group I metabotropic glutamate and NMDA receptors. Importantly, treatment of slices with the membrane-impermeant zinc chelator CaEDTA did not influence the synaptically evoked postsynaptic Ca(2+) waves. Moreover, mossy fibre stimulus evoked postsynaptic Ca(2+) signals were not significantly different between wild-type and zinc transporter 3 (ZnT3) knock-out animals. Considered together our data do not support a role for vesicular zinc in regulating mossy fibre evoked Ca(2+) release from CA3 pyramidal cell internal stores.

Differential effects of AMPA receptor potentiators and glycine reuptake inhibitors on antipsychotic efficacy and prefrontal glutamatergic transmission

The α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor positive allosteric modulators (AMPA-PAMs), Org 24448 and Org 26576, and the glycine transporter-1 (GlyT-1) inhibitor Org 25935 are developed for treatment of schizophrenia. Here we examined experimentally the ability of co-administration of these AMPA-PAMs or the GlyT-1 inhibitor to augment the antipsychotic activity and effect on cortical N-methyl-D: -aspartate (NMDA) receptor-mediated transmission of risperidone, olanzapine, or haloperidol. We examined antipsychotic efficacy using the conditioned avoidance response (CAR) test, extrapyramidal side effect liability using a catalepsy test, and cortical NMDA receptor-mediated glutamatergic transmission using intracellular electrophysiological recording technique in vitro. Both AMPA-PAMs enhanced the suppression of CAR induced by risperidone or olanzapine, and Org 24448 also enhanced the effect of haloperidol. In contrast, the GlyT-1 inhibitor did not cause any behaviorally significant effect in the CAR test. However, the GlyT-1 inhibitor, but not the AMPA-PAMs, produced a large facilitation of NMDA-induced currents. All three drugs potentiated the effect of risperidone but not haloperidol on these currents. The GlyT-1 inhibitor also facilitated the effect of olanzapine. All drugs potentiated the effect of risperidone on electrically stimulated excitatory postsynaptic potentials (EPSP) in cortical pyramidal cells, whereas only the GlyT inhibitor facilitated the effect of olanzapine. Our results suggest that the AMPA-PAMs, when compared to the GlyT-1 inhibitor, show differential effects in terms of augmentation of antipsychotic efficacy, particularly when combined with risperidone or olanzapine. Both AMPA-PAMs and the GlyT-1 inhibitor may also improve negative symptoms and cognitive impairments in schizophrenia, in particular when combined with risperidone.

Increase in cortical pyramidal cell excitability accompanies depression-like behavior in mice: a transcranial magnetic stimulation study.

Clinical evidence suggests that cortical excitability is increased in depressives. We investigated its cellular basis in a mouse model of depression. In a modified version of forced swimming (FS), mice were initially forced to swim for 5 consecutive days and then were treated daily with repetitive transcranial magnetic stimulation (rTMS) or sham treatment for the following 4 weeks without swimming. On day 2 through day 5, the mice manifested depression-like behaviors. The next and last FS was performed 4 weeks later, which revealed a 4 week maintenance of depression-like behavior in the sham mice. In slices from the sham controls, excitability in cingulate cortex pyramidal cells was elevated in terms of membrane potential and frequencies of spikes evoked by current injection. Depolarized resting potential was shown to depend on suppression of large conductance calcium-activated potassium (BK) channels. This BK channel suppression was confirmed by measuring spike width, which depends on BK channels. Chronic rTMS treatment during the 4 week period significantly reduced the depression-like behavior. In slices obtained from the rTMS mice, normal excitability and BK channel activity were recovered. Expression of a scaffold protein Homer1a was reduced by the FS and reversed by rTMS in the cingulate cortex. Similar recovery in the same behavioral, electrophysiological, and biochemical features was observed after chronic imipramine treatment. The present study demonstrated that manifestation and disappearance of depression-like behavior are in parallel with increase and decrease in cortical neuronal excitability in mice and suggested that regulation of BK channels by Homer1a is involved in this parallelism.

Pre‐ and postsynaptic effects of norepinephrine on γ‐aminobutyric acid‐mediated synaptic transmission in layer 2/3 of the rat auditory cortex

Noradrenergic terminals from the locus coeruleus release norepinephrine (NE) throughout most brain areas, including the auditory cortex, where they affect neural processing by modulating numerous cellular properties including the inhibitory γ-aminobutyric acid (GABA)ergic transmission. We recently demonstrated that NE affects GABAergic signaling onto cortical pyramidal cells in a complex manner. In this study, we used a combination of patch-clamp recording and immunohistochemical techniques to identify the synaptic site and the location of the adrenergic receptors involved in the modulation of GABAergic signaling in cortical layer 2/3 of the rat. Our results showed that NE increases the frequency of spike-independent miniature inhibitory postsynaptic currents (mIPSCs), as well as the probability of release of unitary inhibitory postsynaptic currents (IPSCs) obtained with patch-clamp pair-recordings. The pharmacology of mIPSCs and the identification of adrenergic receptors in neurons containing the GABAergic marker parvalbumin (PV) suggest that NE increases the presynaptic probability of GABA release by activating α(2) - and β-receptors on PV-positive neurons. On the contrary, bath-applied NE or phenylephrine, decreased the current mediated by pressure application of the GABA(A) -receptor agonist muscimol, as well as the amplitude-but not the frequency-of mIPSCs, indicating that activation of postsynaptic α(1) adrenoceptors reversibly depressed GABAergic currents. We speculate that while a generalized postsynaptic decrease of GABAergic inhibition might decrease the synaptic activation threshold for pyramidal neurons corresponding to an alert state, NE might promote perception and sensory binding by facilitating lateral inhibition as well as the production of γ-oscillations by a selective enhancement of perisomatic inhibition.

Homeostatic responses by surviving cortical pyramidal cells in neurodegenerative tauopathy

Cortical neuron death is prevalent by 9 months in rTg(tau(P301L))4510 tau mutant mice (TG) and surviving pyramidal cells exhibit dendritic regression and spine loss. We used whole-cell patch-clamp recordings to investigate the impact of these marked structural changes on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) of layer 3 pyramidal cells in frontal cortical slices from behaviorally characterized TG and non-transgenic (NT) mice at this age. Frontal lobe function of TG mice was intact following a short delay interval but impaired following a long delay interval in an object recognition test, and cortical atrophy and cell loss were pronounced. Surviving TG cells had significantly reduced dendritic diameters, total spine density, and mushroom spines, yet sEPSCs were increased and sIPSCs were unchanged in frequency. Thus, despite significant regressive structural changes, synaptic responses were not reduced in TG cells, indicating that homeostatic compensatory mechanisms occur during progressive tauopathy. Consistent with this idea, surviving TG cells were more intrinsically excitable than NT cells, and exhibited sprouting of filopodia and axonal boutons. Moreover, the neuropil in TG mice showed an increased density of asymmetric synapses, although their mean size was reduced. Taken together, these data indicate that during progressive tauopathy, cortical pyramidal cells compensate for loss of afferent input by increased excitability and establishment of new synapses. These compensatory homeostatic mechanisms may play an important role in slowing the progression of neuronal network dysfunction during neurodegenerative tauopathies.

Possible involvement of BDNF dysfunction in the synaptic loss caused by phencyclidine

Subchronic treatment with the N-methyl-d-aspartate (NMDA) antagonist phencyclidine (PCP) produces behavioral abnormalities in rodents which are considered a reliable pharmacological model of neurocognitive deficits in schizophrenia. Alterations in prefrontal neuronal firing after acute PCP administration have been observed, however enduring changes in prefrontal activity after subchronic PCP treatment have not been studied. To address this we have recorded cortical oscillations and unit responses in putative cortical pyramidal cells in subchronic PCP-treated rats (2 mg/kg twice daily for 7 days) under urethane anesthesia. We found that this regimen reduced theta oscillations in the medial prefrontal cortex. It further produced abnormal cortical synchronization in putative cortical pyramidal cells. These alterations in prefrontal cortex functioning may contribute to cognitive deficits seen in subchronic NMDA antagonist pre-treated animals in prefrontal-dependent tasks.

Impact of gamma-oscillatory inhibition on the signal transmission of a cortical pyramidal neuron

Networks of synchronized fast-spiking interneurons are thought to be key elements in the generation of gamma (γ) oscillations (30-80Hz) in the brain. We examined how such γ-oscillatory inhibition regulates the output of a cortical pyramidal cell. Specifically, we modeled a situation where a pyramidal cell receives inputs from γ-synchronized fast-spiking inhibitory interneurons. This model successfully reproduced several important aspects of a recent experimental result regarding the γ-inhibitory regulation of pyramidal cellular firing that is presumably associated with the sensation of whisker stimuli. Through an in-depth analysis of this model system, we show that there is an obvious rhythmic gating effect of the γ-oscillated interneuron networks on the pyramidal neuron's signal transmission. This effect is further illustrated by the interactions of this interneuron network and the pyramidal neuron. Prominent power in the γ frequency range can emerge provided that there are appropriate delays on the excitatory connections and inhibitory synaptic conductance between interneurons. These results indicate that interactions between excitation and inhibition are critical for the modulation of coherence and oscillation frequency of network activities.

Comparison of different neuron models to conductance-based post-stimulus time histograms obtained in cortical pyramidal cells using dynamic-clamp in vitro

A wide diversity of models have been proposed to account for the spiking response of central neurons, from the integrate-and-fire (IF) model and its quadratic and exponential variants, to multiple-variable models such as the Izhikevich (IZ) model and the well-known Hodgkin-Huxley (HH) type models. Such models can capture different aspects of the spiking response of neurons, but there is few objective comparison of their performance. In this article, we provide such a comparison in the context of well-defined stimulation protocols, including, for each cell, DC stimulation, and a series of excitatory conductance injections, arising in the presence of synaptic background activity. We use the dynamic-clamp technique to characterize the response of regular-spiking neurons from guinea-pig visual cortex by computing families of post-stimulus time histograms (PSTH), for different stimulus intensities, and for two different background activities (low- and high-conductance states). The data obtained are then used to fit different classes of models such as the IF, IZ, or HH types, which are constrained by the whole data set. This analysis shows that HH models are generally more accurate to fit the series of experimental PSTH, but their performance is almost equaled by much simpler models, such as the exponential or pulse-based IF models. Similar conclusions were also reached by performing partial fitting of the data, and examining the ability of different models to predict responses that were not used for the fitting. Although such results must be qualified by using more sophisticated stimulation protocols, they suggest that nonlinear IF models can capture surprisingly well the response of cortical regular-spiking neurons and appear as useful candidates for network simulations with conductance-based synaptic interactions.