Properties of excitatory synaptic connections mediated by the corpus callosum in the developing rat neocortex

Visualizing the triheteromeric N-methyl-D-aspartate receptor subunit composition

N-methyl-D-aspartate receptors (NMDARs) are one of three ligand-gated ionotropic channels that transduce the effects of neurotransmitter glutamate at excitatory synapses within the central nervous system. Their ability to influx Ca2+ into cells, unlike mature AMPA or kainate receptors, implicates them in a variety of processes ranging from synaptic plasticity to cell death. Many of the receptor's capabilities, including binding glutamate and regulating Ca2+ influx, have been attributed to their subunit composition, determined putatively using cell biology, electrophysiology and/or pharmacology. Here, we show that subunit composition of synaptic NMDARs can also be readily visualized in acute brain slices (rat) using highly specific antibodies directed against extracellular epitopes of the subunit proteins and high-resolution confocal microscopy. This has helped confirm the expression of triheteromeric t-NMDARs (containing GluN1, GluN2, and GluN3 subunits) at synapses for the first time and reconcile functional differences with diheteromeric d-NMDARs (containing GluN1 and GluN2 subunits) described previously. Even though structural information about individual receptors is still diffraction limited, fluorescently tagged receptor subunit puncta coalesce with precision at various magnifications and/or with the postsynaptic density (PSD-95) but not the presynaptic active zone marker Bassoon. These data are particularly relevant for identifying GluN3A-containing t-NMDARs that are highly Ca2+ permeable and whose expression at excitatory synapses renders neurons vulnerable to excitotoxicity and cell death. Imaging NMDAR subunit proteins at synapses not only offers firsthand insights into subunit composition to correlate function but may also help identify zones of vulnerability within brain structures underlying neurodegenerative diseases like Temporal Lobe Epilepsy.

Focal fMRI signal enhancement with implantable inductively coupled detectors

Despite extensive efforts to increase the signal-to-noise ratio (SNR) of fMRI images for brain-wide mapping, technical advances of focal brain signal enhancement are lacking, in particular, for animal brain imaging. Emerging studies have combined fMRI with fiber optic-based optogenetics to decipher circuit-specific neuromodulation from meso to macroscales. High-resolution fMRI is needed to integrate hemodynamic responses into cross-scale functional dynamics, but the SNR remains a limiting factor given the complex implantation setup of animal brains. Here, we developed a multimodal fMRI imaging platform with an implanted inductive coil detector. This detector boosts the tSNR of MRI images, showing a 2-3-fold sensitivity gain over conventional coil configuration. In contrast to the cryoprobe or array coils with limited spaces for implanted brain interface, this setup offers a unique advantage to study brain circuit connectivity with optogenetic stimulation and can be further extended to other multimodal fMRI mapping schemes.

An activity-dependent determinant of synapse elimination in the mammalian brain

To establish functional neural circuits in the brain, synaptic connections are refined by neural activity during development, where active connections are maintained and inactive ones are eliminated. However, the molecular signals that regulate synapse refinement remain to be elucidated. When we inactivate a subset of neurons in the mouse cingulate cortex, their callosal connections are eliminated through activity-dependent competition. Using this system, we identify JAK2 tyrosine kinase as a key regulator of inactive synapse elimination. We show that JAK2 is necessary and sufficient for elimination of inactive connections; JAK2 is activated at inactive synapses in response to signals from other active synapses; STAT1, a substrate of JAK2, mediates inactive synapse elimination; JAK2 signaling is critical for physiological refinement of synapses during normal development; and JAK2 regulates synapse refinement in multiple brain regions. We propose that JAK2 is an activity-dependent switch that serves as a determinant of inactive synapse elimination.

The Role of Glutamate in Language and Language Disorders - Evidence from ERP and Pharmacologic Studies

Current models of language processing do not address mechanisms at the neurotransmitter level, nor how pharmacologic agents may improve language function(s) in seemingly disparate disorders. L-Glutamate, the primary excitatory neurotransmitter in the human brain, is extensively involved in various higher cortical functions. We postulate that the physiologic role of L-Glutamate neurotransmission extends to the regulation of language access, comprehension, and production, and that disorders in glutamatergic transmission and circuitry contribute to the pathogenesis of neurodegenerative diseases and sporadic-onset language disorders such as the aphasic stroke syndromes. We start with a review of basic science data pertaining to various glutamate receptors in the CNS and ways that they may influence the physiological processes of language access and comprehension. We then focus on the dysregulation of glutamate neurotransmission in three conditions in which language dysfunction is prominent: Alzheimer's Disease, Fragile X-associated Tremor/Ataxia Syndrome, and Aphasic Stroke Syndromes. Finally, we review the pharmacologic and electrophysiologic (event related brain potential or ERP) data pertaining to the role glutamate neurotransmission plays in language processing and disorders.

Astrocyte-mediated switch in spike timing-dependent plasticity during hippocampal development

Presynaptic spike timing-dependent long-term depression (t-LTD) at hippocampal CA3-CA1 synapses is evident until the 3^rd postnatal week in mice, disappearing during the 4^th week. At more mature stages, we found that the protocol that induced t-LTD induced t-LTP. We characterized this form of t-LTP and the mechanisms involved in its induction, as well as that driving this switch from t-LTD to t-LTP. We found that this t-LTP is expressed presynaptically at CA3-CA1 synapses, as witnessed by coefficient of variation, number of failures, paired-pulse ratio and miniature responses analysis. Additionally, this form of presynaptic t-LTP does not require NMDARs but the activation of mGluRs and the entry of Ca²⁺ into the postsynaptic neuron through L-type voltage-dependent Ca²⁺ channels and the release of Ca²⁺ from intracellular stores. Nitric oxide is also required as a messenger from the postsynaptic neuron. Crucially, the release of adenosine and glutamate by astrocytes is required for t-LTP induction and for the switch from t-LTD to t-LTP. Thus, we have discovered a developmental switch of synaptic transmission from t-LTD to t-LTP at hippocampal CA3-CA1 synapses in which astrocytes play a central role and revealed a form of presynaptic LTP and the rules for its induction.

Presynaptic spike timing-dependent long-term depression at hippocampal CA3-CA1 synapses is evident until the third postnatal week in mice. The authors show that maturation beyond four weeks is associated with a switch to long-term potentiation in which astrocytes play a central role.

Synaptogenesis Stimulates a Proteasome-Mediated Ribosome Reduction in Axons

Ribosomes and a subset of cellular mRNAs are trafficked into axons of developing neurons. The axonal localization of translational machinery allows new proteins to be rapidly and locally synthesized during axonal growth and pathfinding. However, in mature neurons, axonal ribosomes are significantly reduced or even absent. The mechanism that elicits this removal is currently unknown. Here, we demonstrate that synapse formation is the trigger for ribosome reduction in mature axons. In vivo analysis shows that axonal ribosome levels decrease in rat brain at a developmental stage coincident with synapse formation. Next, we observe in vitro that different synaptogenic inducers trigger an overall decrease of ribosomal proteins and rRNA in the axons of spinal motor neurons. We further observe that this process is dependent on the ubiquitin-proteasome system but not on autophagy. Together, these data identify synaptogenesis as the long missing biological trigger that leads to ribosome disappearance during axonal maturation.

The mechanism behind the striking loss of ribosomes from axons during neuronal maturation is unknown. Using in vivo and in vitro models, including neuron-muscle co-cultures and combining biochemistry and imaging techniques, Costa et al. demonstrate that synapse formation triggers ribosome reduction by a mechanism involving the ubiquitin-proteasome system.

Mapping the Brain-Wide Network Effects by Optogenetic Activation of the Corpus Callosum

Optogenetically driven manipulation of circuit-specific activity enables causality studies, but its global brain-wide effect is rarely reported. Here, we applied simultaneous functional magnetic resonance imaging (fMRI) and calcium recording with optogenetic activation of the corpus callosum (CC) connecting barrel cortices (BC). Robust positive BOLD was detected in the ipsilateral BC due to antidromic activity, spreading to the ipsilateral motor cortex (MC), and posterior thalamus (PO). In the orthodromic target, positive BOLD was reliably evoked by 2 Hz light pulses, whereas 40 Hz light pulses led to reduced calcium, indicative of CC-mediated inhibition. This presumed optogenetic CC-mediated inhibition was further elucidated by pairing light pulses with whisker stimulation at varied interstimulus intervals. Whisker-induced positive BOLD and calcium signals were reduced at intervals of 50/100 ms. The calcium-amplitude-modulation-based correlation with whole-brain fMRI signal revealed that the inhibitory effects spread to contralateral BC, ipsilateral MC, and PO. This work raises the need for fMRI to elucidate the brain-wide network activation in response to optogenetic stimulation.

Loss of Cntnap2 Causes Axonal Excitability Deficits, Developmental Delay in Cortical Myelination, and Abnormal Stereotyped Motor Behavior

Contactin-associated protein-like 2 (Caspr2) is found at the nodes of Ranvier and has been associated with physiological properties of white matter conductivity. Genetic variation in CNTNAP2, the gene encoding Caspr2, has been linked to several neurodevelopmental conditions, yet pathophysiological effects of CNTNAP2 mutations on axonal physiology and brain myelination are unknown. Here, we have investigated mouse mutants for Cntnap2 and found profound deficiencies in the clustering of Kv1-family potassium channels in the juxtaparanodes of brain myelinated axons. These deficits are associated with a change in the waveform of axonal action potentials and increases in postsynaptic excitatory responses. We also observed that the normal process of myelination is delayed in Cntnap2 mutant mice. This later phenotype is a likely modulator of the developmental expressivity of the stereotyped motor behaviors that characterize Cntnap2 mutant mice. Altogether, our results reveal a mechanism linked to white matter conductivity through which mutation of CNTNAP2 may affect neurodevelopmental outcomes.

Cell-type-specific control of basolateral amygdala neuronal circuits via entorhinal cortex-driven feedforward inhibition

The basolateral amygdala (BLA) plays a vital role in associating sensory stimuli with salient valence information. Excitatory principal neurons (PNs) undergo plastic changes to encode this association; however, local BLA inhibitory interneurons (INs) gate PN plasticity via feedforward inhibition (FFI). Despite literature implicating parvalbumin expressing (PV+) INs in FFI in cortex and hippocampus, prior anatomical experiments in BLA implicate somatostatin expressing (Sst+) INs. The lateral entorhinal cortex (LEC) projects to BLA where it drives FFI. In the present study, we explored the role of interneurons in this circuit. Using mice, we combined patch clamp electrophysiology, chemogenetics, unsupervised cluster analysis, and predictive modeling and found that a previously unreported subpopulation of fast-spiking Sst+ INs mediate LEC→BLA FFI.

Spatio-temporal dynamics of neocortical presynaptic terminal development using multi-photon imaging of the corpus callosum in vivo

Within the developing central nervous system, the dynamics of synapse formation and elimination are insufficiently understood. It is ideal to study these processes in vivo, where neurons form synapses within appropriate behavioral and anatomical contexts. In vivo analysis is particularly important for long-range connections, since their development cannot be adequately studied in vitro. The corpus callosum (CC) represents a clinically-relevant long-range connection since several neurodevelopmental diseases involve CC defects. Here, we present a novel strategy for in vivo longitudinal and rapid time-lapse imaging of CC presynaptic terminal development. In postnatal mice, the time-course of CC presynaptic terminal formation and elimination was highly variable between axons or groups of axons. Young presynaptic terminals were remarkably dynamic - moving, dividing to generate more boutons, and merging to consolidate small terminals into large boutons. As synaptic networks matured, presynaptic mobility decreased. These rapid dynamics may be important for establishing initial synaptic contacts with postsynaptic partners, refining connectivity patterns or modifying synapse strength during development. Ultimately, this in vivo imaging approach will facilitate investigation of synapse development in other long-range connections and neurodevelopmental disease models.

How do we use in vitro models to understand epileptiform and ictal activity? A report of the TASK1-WG4 group of the ILAE/AES Joint Translational Task Force

In vitro brain tissue preparations allow the convenient and affordable study of brain networks and have allowed us to garner molecular, cellular, and electrophysiologic insights into brain function with a detail not achievable in vivo. Preparations from both rodent and human postsurgical tissue have been utilized to generate in vitro electrical activity similar to electrographic activity seen in patients with epilepsy. A great deal of knowledge about how brain networks generate various forms of epileptiform activity has been gained, but due to the multiple in vitro models and manipulations used, there is a need for a standardization across studies. Here, we describe epileptiform patterns generated using in vitro brain preparations, focusing on issues and best practices pertaining to recording, reporting, and interpretation of the electrophysiologic patterns observed. We also discuss criteria for defining in vitro seizure‐like patterns (i.e., ictal) and interictal discharges. Unifying terminologies and definitions are proposed. We suggest a set of best practices for reporting in vitro studies to favor both efficient across‐lab comparisons and translation to in vivo models and human studies.

Short-Term Synaptic Plasticity as a Mechanism for Sensory Timing

The ability to detect time intervals and temporal patterns is critical to some of the most fundamental computations the brain performs, including the ability to communicate and appraise a dynamically changing environment. Many of these computations take place on the scale of tens to hundreds of milliseconds. Electrophysiological evidence shows that some neurons respond selectively to duration, interval, rate, or order. Because the time constants of many time-varying neural and synaptic properties, including short-term synaptic plasticity (STP), are also in the range of tens to hundreds of milliseconds, they are strong candidates to underlie the formation of temporally selective neurons. Neurophysiological studies indicate that STP is indeed one of the mechanisms that contributes to temporal selectivity, and computational models demonstrate that neurons embedded in local microcircuits exhibit temporal selectivity if their synapses undergo STP. Converging evidence suggests that some forms of temporal selectivity emerge from the dynamic changes in the balance of excitation and inhibition imposed by STP.

Optogenetic investigation of the variable neurovascular coupling along the interhemispheric circuits

The interhemispheric circuit connecting the left and the right mammalian brain plays a key role in integration of signals from the left and the right side of the body. The information transfer is carried out by modulation of simultaneous excitation and inhibition. Hemodynamic studies of this circuit are inconsistent since little is known about neurovascular coupling of mixed excitatory and inhibitory signals. We investigated the variability in hemodynamic responses driven by the interhemispheric circuit during optogenetic and somatosensory activation. We observed differences in the neurovascular response based on the stimulation site - cell bodies versus distal projections. In half of the experiments, optogenetic stimulation of the cell bodies evoked a predominant post-synaptic inhibition in the other hemisphere, accompanied by metabolic oxygen consumption without coupled functional hyperemia. When the same transcallosal stimulation resulted in predominant post-synaptic excitation, the hemodynamic response was biphasic, consisting of metabolic dip followed by functional hyperemia. Optogenetic suppression of the postsynaptic excitation abolished the coupled functional hyperemia. In contrast, light stimulation at distal projections evoked consistently a metabolic response. Our findings suggest that functional hyperemia requires signals originating from the cell body and the hemodynamic response variability appears to reflect the balance between the post-synaptic excitation and inhibition.

Altered levels of the splicing factor muscleblind modifies cerebral cortical function in mouse models of myotonic dystrophy

Myotonic dystrophy (DM) is a progressive, multisystem disorder affecting skeletal muscle, heart, and central nervous system. In both DM1 and DM2, microsatellite expansions of CUG and CCUG RNA repeats, respectively, accumulate and disrupt functions of alternative splicing factors, including muscleblind (MBNL) proteins. Grey matter loss and white matter changes, including the corpus callosum, likely underlie cognitive and executive function deficits in DM patients. However, little is known how cerebral cortical circuitry changes in DM. Here, flavoprotein optical imaging was used to assess local and contralateral responses to intracortical motor cortex stimulation in DM-related mouse models. In control mice, brief train stimulation generated ipsilateral and contralateral homotopic fluorescence increases, the latter mediated by the corpus callosum. Single pulse stimulation produced an excitatory response with an inhibitory-like surround response mediated by GABA_A receptors. In a mouse model of DM2 (Mbnl2 KO), we observed prolonged and increased responsiveness to train stimulation and loss of the inhibition from single pulse stimulation. Conversely, mice overexpressing human MBNL1 (MBNL1-OE) exhibited decreased contralateral response to train stimulation and reduction of inhibitory-like surround to single pulse stimulation. Therefore, altering levels of two key DM-associated splicing factors modifies functions of local cortical circuits and contralateral responses mediated through the corpus callosum.

Callosal responses in a retrosplenial column

The axons forming the corpus callosum sustain the interhemispheric communication across homotopic cortical areas. We have studied how neurons throughout the columnar extension of the retrosplenial cortex integrate the contralateral input from callosal projecting neurons in cortical slices. Our results show that pyramidal neurons in layers 2/3 and the large, thick-tufted pyramidal neurons in layer 5B showed larger excitatory callosal responses than layer 5A and layer 5B thin-tufted pyramidal neurons, while layer 6 remained silent to this input. Feed-forward inhibitory currents generated by fast spiking, parvalbumin expressing  interneurons recruited by callosal axons mimicked the response size distribution of excitatory responses across pyramidal subtypes, being larger in those of superficial layers and in the layer 5B thick-tufted pyramidal cells. Overall, the combination of the excitatory and inhibitory currents evoked by callosal input had a strong and opposed effect in different layers of the cortex; while layer 2/3 pyramidal neurons were powerfully inhibited, the thick-tufted but not thin-tufted pyramidal neurons in layer 5 were strongly recruited. We believe that these results will help to understand the functional role of callosal connections in physiology and disease.

How to Properly Measure a Current-Voltage Relation?-Interpolation vs. Ramp Methods Applied to Studies of GABAA Receptors

The relation between current and voltage, I-V relation, is central to functional analysis of membrane ion channels. A commonly used method, since the introduction of the voltage-clamp technique, to establish the I-V relation depends on the interpolation of current amplitudes recorded at different steady voltages. By a theoretical computational approach as well as by experimental recordings from GABA_A-receptor mediated currents in mammalian central neurons, we here show that this interpolation method may give reversal potentials and conductances that do not reflect the properties of the channels studied under conditions when ion flux may give rise to concentration changes. Therefore, changes in ion concentrations may remain undetected and conclusions on changes in conductance, such as during desensitization, may be mistaken. In contrast, an alternative experimental approach, using rapid voltage ramps, enable I-V relations that much better reflect the properties of the studied ion channels.

Functional and structural specific roles of activity-driven BDNF within circuits formed by single spiny stellate neurons of the barrel cortex

Brain derived neurotrophic factor (BDNF) plays key roles in several neurodevelopmental disorders and actions of pharmacological treatments. However, it is unclear how specific BDNF’s effects are on different circuit components. Current studies have largely focused on the role of BDNF in modification of synaptic development. The precise roles of BDNF in the refinement of a functional circuit in vivo remain unclear. Val66Met polymorphism of BDNF may be associated with increased risk for cognitive impairments and is mediated at least in part by activity-dependent trafficking and/or secretion of BDNF. Using mutant mice that lacked activity-driven BDNF expression (bdnf-KIV), we previously reported that experience regulation of the cortical GABAergic network is mediated by activity-driven BDNF expression. Here, we demonstrate that activity-driven BDNF’s effects on circuits formed by the layer IV spiny stellate cells are highly specific. Structurally, dendritic but not axonal morphology was altered in the mutant. Physiologically, GABAergic but not glutamatergic synapses were severely affected. The effects on GABA transmission occurs via presynaptic alteration of calcium-dependent release probability. These results suggest that neuronal activity through activity-driven BDNF expression, can selectively regulate specific features of layer IV circuits in vivo. We postulate that the role of activity-dependent BDNF is to modulate the computational ability of circuits that relate to the gain control (i.e., feed-forward inhibition); whereas the basic wiring of circuits relevant to the sensory pathway is spared. Gain control modulation within cortical circuits has broad impact on cognitive processing and brain state-transitions. Cognitive behavior and mode is determined by brain states, thus the studying of circuit alteration by endogenous BDNF provides insights into the cellular and molecular mechanisms of diseases mediated by BDNF.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Stimulus-evoked potentials contribute to map the epileptogenic zone during stereo-EEG presurgical monitoring

Presurgical monitoring with intracerebral electrodes in patients with drug-resistant focal epilepsy represents a standard invasive procedure to localize the sites of seizures origin, defined as the epileptogenic zone (EZ). During presurgical evaluation, intracerebral single-pulse electrical stimulation (SPES) is performed to define the boundaries of eloquent areas and to evoke seizure-associated symptoms. Extensive intracranial exploration and stimulation generate a large dataset on brain connectivity that can be used to improve EZ detection and to understand the organization of the human epileptic brain. We developed a protocol to analyse field responses evoked by intracranial stimulation. Intracerebral recordings were performed with 105-162 recording sites positioned in fronto-temporal regions in 12 patients with pharmacoresistant focal epilepsy. Recording sites were used for bipolar SPES at 1 Hz. Reproducible early and late phases (<60 ms and 60-500 ms from stimulus artefact, respectively) were identified on averaged evoked responses. Phase 1 and 2 responses recorded at all and each recording sites were plotted on a 3D brain reconstructions. Based on connectivity properties, electrode contacts were primarily identified as receivers, mainly activators or bidirectional. We used connectivity patterns to construct networks and applied cluster partitioning to study the proprieties between potentials evoked/stimulated in different regions. We demonstrate that bidirectional connectivity during phase 1 is a prevalent feature that characterize contacts included in the EZ. This study shows that the application of an analytical protocol on intracerebral stimulus-evoked recordings provides useful information that may contribute to EZ detection and to the management of surgical-remediable epilepsies.

Target-cell-specific short-term plasticity in local circuits

Short-term plasticity (STP) denotes changes in synaptic strength that last up to tens of seconds. It is generally thought that STP impacts information transfer across synaptic connections and may thereby provide neurons with, for example, the ability to detect input coherence, to maintain stability and to promote synchronization. STP is due to a combination of mechanisms, including vesicle depletion and calcium accumulation in synaptic terminals. Different forms of STP exist, depending on many factors, including synapse type. Recent evidence shows that synapse dependence holds true even for connections that originate from a single presynaptic cell, which implies that postsynaptic target cell type can determine synaptic short-term dynamics. This arrangement is surprising, since STP itself is chiefly due to presynaptic mechanisms. Target-specific synaptic dynamics in addition imply that STP is not a bug resulting from synapses fatiguing when driven too hard, but rather a feature that is selectively implemented in the brain for specific functional purposes. As an example, target-specific STP results in sequential somatic and dendritic inhibition in neocortical and hippocampal excitatory cells during high-frequency firing. Recent studies also show that the Elfn1 gene specifically controls STP at some synapse types. In addition, presynaptic NMDA receptors have been implicated in synapse-specific control of synaptic dynamics during high-frequency activity. We argue that synapse-specific STP deserves considerable further study, both experimentally and theoretically, since its function is not well known. We propose that synapse-specific STP has to be understood in the context of the local circuit, which requires combining different scientific disciplines ranging from molecular biology through electrophysiology to computer modeling.

Binocular form deprivation influences the visual cortex

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors are considered to play a crucial role in synaptic plasticity in the developing visual cortex. In this study, we established a rat model of binocular form deprivation by suturing the rat binocular eyelids before eye-opening at postnatal day 14. During development, the decay time of excitatory postsynaptic currents mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors of normal rats became longer after eye-opening; however, the decay time did not change significantly in binocular form deprivation rats. The peak value in the normal group became gradually larger with age, but there was no significant change in the binocular form deprivation group. These findings indicate that binocular form deprivation influences the properties of excitatory postsynaptic currents mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the rat visual cortex around the end of the critical period, indicating that form stimulation is associated with the experience-dependent modification of neuronal synapses in the visual cortex.

Triheteromeric N-methyl-D-aspartate receptors differentiate synaptic inputs onto pyramidal neurons in somatosensory cortex: involvement of the GluN3A subunit

N-methyl-D-aspartate receptors (NMDARs) are glutamatergic by virtue of glutamate-binding GluN2 subunits and glycinergic by virtue of glycine-binding GluN1 and GluN3 subunits. The existence, location, and functional-significance of NMDARs containing both GluN2 and GluN3 subunits have as yet remained unelucidated. Here we report on the discovery and characterization of a novel type of NMDARs, found at layer (L)1/primary whisker-motor-cortex inputs onto L5 pyramidal neurons in somatosensory cortex, that are distinct in structure and function from conventional GluN2A-containing NMDARs at thalamic/striatal (Str) inputs onto the same neurons. These receptors had a threshold-like activation at hyperpolarized holding-potentials with strong outward rectification of their current-voltage relationships unlike any known GluN1/GluN2-containing NMDARs. Pharmacology revealed a triheteromeric-receptor with features common to glutamate-activated GluN1/GluN2-containing and glycine-activated GluN1/GluN3-containing diheteromeric NMDARs. However, unlike GluN1/GluN3 receptors, NMDARs at L1 inputs were activated by glutamate and blocked by d-AP5, Ca(2+)-permeable, and more efficient at integrating and potentiating EPSPs selectively over Str inputs during high-frequency stimulation while obviating the need for AMPAR-mediated depolarization.

Developmental shift of short-term synaptic plasticity in cortical organotypic slices

Although short-term synaptic plasticity (STP) is ubiquitous in neocortical synapses its functional role in neural computations is not well understood. Critical to elucidating the function of STP will be to understand how STP itself changes with development and experience. Previous studies have reported developmental changes in STP using acute slices. It is not clear, however, to what extent the changes in STP are a function of local ontogenetic programs or the result of the many different sensory and experience-dependent changes that accompany development in vivo. To address this question we examined the in vitro development of STP in organotypic slices cultured for up to 4 weeks. Paired recordings were performed in L5 pyramidal neurons at different stages of in vitro development. We observed a shift in STP in the form of a decrease in the paired-pulse ratio (PPR) (less depression) from the second to fourth week in vitro. This shift in STP was not accompanied by a change in initial excitatory postsynaptic potential (EPSP) amplitude. Fitting STP to a quantitative model indicated that the developmental shift is consistent with presynaptic changes. Importantly, despite the change in the PPR we did not observe changes in the time constant governing STP. Since these experiments were conducted in vitro our results indicate that the shift in STP does not depend on in vivo sensory experience. Although sensory experience may shape STP, we suggest that developmental shifts in STP are at least in part ontogenetically determined.

Propagation of epileptiform events across the corpus callosum in a cingulate cortical slice preparation

We report on a novel mouse in vitro brain slice preparation that contains intact callosal axons connecting anterior cingulate cortices (ACC). Callosal connections are demonstrated by the ability to regularly record epileptiform events between hemispheres (bilateral events). That the correlation of these events depends on the callosum is demonstrated by the bisection of the callosum in vitro. Epileptiform events are evoked with four different methods: (1) bath application of bicuculline (a GABA-A antagonist); (2) bicuculline+MK801 (an NMDA receptor antagonist), (3) a zero magnesium extracellular solution (0Mg); (4) focal application of bicuculline to a single cortical hemisphere. Significant increases in the number of epileptiform events, as well as increases in the ratio of bilateral events to unilateral events, are observed during bath applications of bicuculline, but not during applications of bicuculline+MK-801. Long ictal-like events (defined as events >20 seconds) are only observed in 0Mg. Whole cell patch clamp recordings of single neurons reveal strong feedforward inhibition during focal epileptiform events in the contralateral hemisphere. Within the ACC, we find differences between the rostral areas of ACC vs. caudal ACC in terms of connectivity between hemispheres, with the caudal regions demonstrating shorter interhemispheric latencies. The morphologies of many patch clamped neurons show callosally-spanning axons, again demonstrating intact callosal circuits in this in vitro preparation.

Variation in the NMDA receptor 2B subunit gene GRIN2B is associated with differential language lateralization

Variations in the N-methyl-d-aspartate receptor 2B subunit gene (GRIN2B) have been associated with schizophrenia, a psychiatric disorder associated with reduced left-hemispheric language dominance. Here, we investigated, whether different polymorphisms in GRIN2B influence language lateralization and handedness in healthy individuals. In a cohort of 424 genetically unrelated participants we found significant association between the synonymous GRIN2B variation rs1806201 and language lateralization assessed using the dichotic listening task. Individuals carrying the heterozygous CT genotype exhibited more pronounced left-hemispheric language dominance as compared to both homozygous CC and TT individuals. Such an association was not identified for handedness. These findings suggest that variation in NMDA-receptors contributes to the interindividual variability of language lateralization.

Midsagittal structural differences and sexual dimorphism of the corpus callosum in obsessive-compulsive disorder

Neurobiological abnormalities in various brain regions, including the orbitofrontal cortex, basal ganglia, and thalamus, have been found in patients with obsessive-compulsive disorder (OCD), and impairment in white matter connectivity in these regions has recently been suggested. To investigate structural connectivity in OCD, we used the midsagittal area and thickness to assess the morphology of the corpus callosum (CC), the largest connecting fiber tract in the human brain. Midsagittal magnetic resonance images of the CC were acquired from 69 adult patients with OCD and 69 matched normal controls. We calculated and compared the total area and the areas of five subregions of the CC as well as the distances between 200 equidistant points on the top and bottom of lines on the surface of the CC in the two groups. The absolute total area of the CC was significantly larger in OCD patients than in controls when brain size, age, gender, and IQ were controlled. Significant enlargements in CC1, CC2, and CC5 were seen in OCD patients before correction for multiple comparisons. The thickness of the caudal part of the splenium was greater in OCD patients than in controls. The analysis according to gender showed that only male OCD patients differed from male controls with respect to the area of the CC. These findings reflect structural abnormalities in the CC, and especially in the splenium, in adult patients with OCD. Abnormal interhemispheric connectivity, including the parietotemporal and occipital areas, may affect the pathophysiology of OCD. Sexual dimorphism in the CC of OCD patients should be considered.

Effects of trifluoromethylphenylpiperazine (TFMPP) on interhemispheric communication

BACKGROUND

'Party Pills' containing trifluoromethylphenylpiperazine (TFMPP) and benzylpiperazine are legally available in many countries and marketed as safe alternatives to other illicit substances such as methamphetamine and methylenedioxymethamphetamine (or Ecstasy). They have gained huge popularity around the world, especially amongst young adults. However, there is no information currently available describing the acute neurophysiological effects of these psychoactive drugs in humans. The purpose of this study is to investigate the effects of TFMPP on central information processing speed in humans.

METHODS

A randomised, double blind, placebo-controlled study using electroencephalography (EEG) was carried out to investigate the effects of TFMPP on interhemispheric transfer time (IHTT). Healthy, right-handed males (age: 25 ± 5.6 years) were given placebo (n = 15) or TFMPP (0.94 mg/kg, oral, n = 15) and tested both pre- and 2 h post-drug administration. High-density EEG recordings (128 channels which were re-referenced using an average reference to make 129 electrodes) were used to record event-related potentials. The N160 component was defined as the biggest negative peak in the range between 140 and 220 ms after the event. The IHHTs were analysed by deducting the N160 latency obtained in the contralateral hemisphere from the N160 latency obtained in the hemisphere ipsilateral to stimulus signal.

RESULTS

Statistical analysis using a split-plot design analysis of variance revealed that TFMPP significantly reduced the IHTT but did not affect reaction time. No statistically significant changes were observed in the placebo group.

CONCLUSIONS

This study is the first to report the neurophysiological effects of TFMPP in humans and suggests that TFMPP may affect transmitter systems involved in speeding of interhemispheric communication in the male brain.

Rules of engagement: factors that regulate activity-dependent synaptic plasticity during neural network development

Overproduction and pruning during development is a phenomenon that can be observed in the number of organisms in a population, the number of cells in many tissue types, and even the number of synapses on individual neurons. The sculpting of synaptic connections in the brain of a developing organism is guided by its personal experience, which on a neural level translates to specific patterns of activity. Activity-dependent plasticity at glutamatergic synapses is an integral part of neuronal network formation and maturation in developing vertebrate and invertebrate brains. As development of the rodent forebrain transitions away from an over-proliferative state, synaptic plasticity undergoes modification. Late developmental changes in synaptic plasticity signal the establishment of a more stable network and relate to pronounced perceptual and cognitive abilities. In large part, activation of glutamate-sensitive N-methyl-d-aspartate (NMDA) receptors regulates synaptic stabilization during development and is a necessary step in memory formation processes that occur in the forebrain. A developmental change in the subunits that compose NMDA receptors coincides with developmental modifications in synaptic plasticity and cognition, and thus much research in this area focuses on NMDA receptor composition. We propose that there are additional, equally important developmental processes that influence synaptic plasticity, including mechanisms that are upstream (factors that influence NMDA receptors) and downstream (intracellular processes regulated by NMDA receptors) from NMDA receptor activation. The goal of this review is to summarize what is known and what is not well understood about developmental changes in functional plasticity at glutamatergic synapses, and in the end, attempt to relate these changes to maturation of neural networks.

Inhibition by 5-HT of the synaptic responses evoked by callosal fibers on cortical neurons in the mouse

We have studied the modulation by 5-HT of the synaptic excitatory responses evoked by callosal fibers on cortical pyramidal neurons. We have used a mouse brain slice preparation that preserves the callosal fibers and allows their selective activation. EPSCs evoked by callosal stimulation (ccEPSCs) were recorded with patch electrodes from pyramidal neurons identified visually. We observed that 5-HT (10-40 μM) inhibited the ccEPSCs peak amplitude in 64% of the neurons; 5-HT had no effect in the remaining neurons. 5-HT also increased the frequency and amplitude of spontaneous EPSCs. This inhibition was accompanied with an increase in the coefficient of variation of the fluctuations of the ccEPSCs amplitude and with an increase in the ratio of the amplitudes of paired ccEPSCs. Agonists of 5-HT receptor subtypes 5-HT(1A) (8-OH-DPAT) and 5-HT(2A) (DOI) mimicked the effect of 5-HT; also, the effect of 8-OH-DPAT and DOI was blocked in the presence of specific blockers of 5-HT(1A) (WAY 100135) and 5-HT(2A) (MDL 11,939) receptors. Application of 5-HT did not change the amplitude of currents evoked by direct application of glutamate to neurons in which 5-HT decreased the amplitude of ccEPSC. The effects of 5-HT on ccEPSCs and on the synaptic currents evoked by intracortical stimulation were not correlated; this suggests that the effect of 5-HT was specific to particular synaptic inputs to a neuron. These results demonstrate the presynaptic modulation of the callosal synaptic responses by 5-HT and the implication of 5-HT(1A) and 5-HT(2A) receptors in this effect.

Presynaptic GABA(B) receptors regulate experience-dependent development of inhibitory short-term plasticity

Short-term changes in synaptic gain support information processing throughout the CNS, yet we know little about the developmental regulation of such plasticity. Here we report that auditory experience is necessary for the normal maturation of synaptic inhibitory short-term plasticity (iSTP) in the auditory cortex, and that presynaptic GABA(B) receptors regulate this development. Moderate or severe hearing loss was induced in gerbils, and iSTP was characterized by measuring inhibitory synaptic current amplitudes in response to repetitive stimuli. We reveal a profound developmental shift of iSTP from depressing to facilitating after the onset of hearing. Even moderate hearing loss prevented this shift. This iSTP change was mediated by a specific class of inhibitory interneurons, the low-threshold spiking cells. Further, using paired recordings, we reveal that presynaptic GABA(B) receptors at interneuron-pyramidal connections regulate iSTP in an experience-dependent manner. This novel synaptic mechanism may support the emergence of mature temporal processing in the auditory cortex.

Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational states

Following initial contact formation, glutamatergic synapses in cortical neurons undergo pronounced functional maturation. These maturational events, occurring both pre- and postsynaptically, have been well described in the developing hippocampus. In this paper, we characterized glutamatergic synapses in immature layer Vb pyramidal neurons of the mouse somatosensory cortex during early postnatal development. At postnatal day 7, a significant subpopulation of glutamatergic synapses exhibited a low release probability that was accompanied by strong paired-pulse facilitation of AMPA EPSCs (paired-pulse ratio C > or = 2). Increasing extracellular Ca(2+) concentration increased release probability and led to paired-pulse depression. During further postnatal development, these functionally immature synapses disappeared. As shown pharmacologically,these synapses expressed postsynaptic NMDA receptors containing NR2B subunits, while NMDA receptors with NR2A subunits were lacking. Taken together, a low release probability presynaptically was coupled to postsynaptic NR2B signaling. This subpopulation of neocortical synapses thus differed from the majority of synapses in the developing hippocampus, where high release probability is coupled to NR2B signaling. The novel type of functionally immature glutamatergic synapse described here might play an important role in early developmental synapse elimination and in the activity-dependent refinement of the neocortical synaptic microcircuitry.

Voltage-sensitive-dye imaging of microstimulation-evoked neural activity through intracortical horizontal and callosal connections in cat visual cortex

Recently, intrinsic signal optical imaging has been widely used as a routine procedure for visualizing cortical functional maps. We do not, however, have a well-established imaging method for visualizing cortical functional connectivity indicating spatio-temporal patterns of activity propagation in the cerebral cortex. In the present study, we developed a novel experimental setup for investigating the propagation of neural activities combining the intracortical microstimulation (ICMS) technique with voltage sensitive dye (VSD) imaging, and demonstrated the feasibility of this setup applying to the measurement of time-dependent intra- and inter-hemispheric spread of ICMS-evoked excitation in the cat visual cortices, areas 17 and 18. A microelectrode array for the ICMS was inserted with a specially designed easy-to-detach electrode holder around the 17/18 transition zones (TZs), where the left and right hemispheres were interconnected via the corpus callosum. The microelectrode array was stably anchored in agarose without any holder, which enabled us to visualize evoked activities even in the vicinity of penetration sites as well as in a wide recording region that covered a part of both hemispheres. The VSD imaging could successfully visualize ICMS-evoked excitation and subsequent propagation in the visual cortices contralateral as well as ipsilateral to the ICMS. Using the orientation maps as positional references, we showed that the activity propagation patterns were consistent with previously reported anatomical patterns of intracortical and interhemispheric connections. This finding indicates that our experimental system can serve for the investigation of cortical functional connectivity.

Altered development of glutamatergic synapses in layer V pyramidal neurons in NR3A knockout mice

Expression of the NMDA receptor (NMDAR) subunit NR3A reaches its highest level in layer V of the developing rodent cortex during the second postnatal week, a peak period of synaptogenesis. Incorporation of NR3A leads to the formation of non-canonical, Mg2+-insensitive NMDARs, but it is not known whether they participate in synaptic transmission and maturation. Here we show that in the second postnatal week, layer V pyramidal neurons in the somatosensory cortex of wild type (WT) mice exhibited evoked excitatory postsynaptic currents (eEPSCs) with 3- to 6-fold lower Mg2+ sensitivity than NR3A knockout (KO) mice and their reversal potential was approximately 2 mV more negative compared to KO mice consistent with decreased P(Ca) of NMDARs. Surprisingly, ablation of NR3A also led to a 20-fold reduction of the ratio of AMPAR- to NMDAR-mediated eEPSC amplitudes in KO mice. Insertion of AMPARs at the synapses of layer V pyramidal neurons appears to be facilitated by the expression of Mg2+-insensitive NMDARs. The data indicate that NR3A plays a significant role in the development of excitatory synapses in layer V of the developing neocortex.

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4-5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5-100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca(2+) control in the presynaptic terminal with a reduced Ca(2+) influx and/or a higher Ca(2+) buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.

Differential metabotropic glutamate receptor expression and modulation in two neocortical inhibitory networks

Taking advantage of transgenic mice with genetically labeled GABA-releasing interneurons, we examined the cell-specific patterns of mGluR expression in two broadly defined subtypes of inhibitory interneurons in layer IV of somatosensory cortex. Electrophysiological recording combined with application of specific agonists for specific mGluRs demonstrated different effects of mGluR activation in fast-spiking (FS) versus regular spiking nonpyramidal (RSNP) interneurons. Whereas activation of group I, II, and III mGluRs inhibited excitatory synaptic transmission in RSNP neurons predominantly via postsynaptic mechanisms, group I mGluR activation depolarized FS but not RSNP interneurons. Immunoreactivities of mGluR1, mGluR5, mGluR2/3, and mGluR8 exhibited different cellular expression patterns in the two groups of neurons that were not entirely consistent with physiological and pharmacological experiments. Taken together, our data indicate cell and circuit-specific roles for mGluRs in modulating inhibitory circuits in the somatosensory cortex. These results help to reinforce the concept that RSNP and FS cells represent morphologically, physiologically, and functionally distinct groups of interneurons. The results reported here help to increase our understanding of the roles of mGluRs in endogenous glutamatergic-induced plasticity of interneuronal networks.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Physiology and morphology of callosal projection neurons in mouse

In the mammalian neocortex, the corpus callosum serves as the major source of interhemispheric communication, composed of axons from callosal neurons located in supragranular (II/III) and infragranular (V/VI) layers. We sought to characterize the physiology and morphology of supragranular and infragranular callosal neurons in mice using retrograde tracers and whole-cell patch clamp recordings. Whole-cell patch clamp recordings were made from retrogradely labeled callosal neurons following unilateral injection of fluorescent latex microspheres in the contralateral sensory-motor cortex. Following recordings and biocytin dialysis, labeled neurons were reconstructed using computer-assisted camera lucida (Neurolucida) for morphological analyses. Whole-cell recordings revealed that callosal neurons in both supra- and infragranular layers display very similar intrinsic membrane properties and are characteristic regular-spiking neurons. Morphological features examined from biocytin-filled reconstructions as well as retrogradely BDA labeled cells did not reveal any differences. Analysis of spontaneous postsynaptic potentials from callosal neurons did reveal several differences including average amplitude, frequency, and decay time. These findings suggest that callosal neurons in both supra- and infragranular layers have similar phenotypes though belong to different local, intracortical networks.

Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced long-term depression at mossy fiber CA3 pyramid synapses

Many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors via changes in subunit composition. Concerning AMPA receptors (AMPARs), glutamate receptor 2 subunit (GluR2)-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons; however, accumulating evidence indicates that GluR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute at these synapses early in development. Here, we used a combination of imaging and electrophysiological recording techniques to investigate potential roles for CP-AMPARs at developing hippocampal mossy fiber-CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relationships, and partial philanthotoxin sensitivity of synaptic events. CP-AMPAR expression at MF-PYR synapses is transient, being limited to the first 3 postnatal weeks. Moreover, the expression of CP-AMPARs is regulated by the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing protein interacting with C kinase 1 (PICK1), because MF-PYR synapses in young PICK1 knock-out mice are philanthotoxin insensitive with linear I-V relationships. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced long-term depression (DiLTD), a postsynaptic form of MF-PYR plasticity observed only at young MF-PYR synapses. The selective depression of CP-AMPARs during DiLTD was evident as a loss of postsynaptic CP-AMPAR-mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents. Preferential targeting of CP-AMPARs during DiLTD is further supported by a lack of DiLTD in young PICK1 knock-out mice. Together, these findings indicate that the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window to observe DiLTD.

Long-term modifications in the strength of excitatory associative inputs in the piriform cortex

Afferent olfactory information, in vivo and in vitro, can be rapidly adapted to through a metabotropic glutamate receptor (mGluR)-mediated attenuation of synaptic strength. Specific cellular and synaptic mechanisms underlying olfactory learning and habituation at the cortical level remain unclear. Through whole-cell recording, excitatory postsynaptic currents (EPSCs) were obtained from piriform cortex (PC) principal cells. Using a coincidental, pre- and postsynaptic stimulation protocol, long-term depression (LTD) in synaptic strength was induced at associative, excitatory synapses onto layer II pyramidal neurons of the mouse (P15-27) PC. LTD was mimicked and occluded by mGluR agonists and blocked by nonselective mGluR antagonist (RS)-alpha-methyl-4-sulfonophenylglycine (MSPG) but not by N-methyl-D-aspartic acid (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV). Analysis of the paired-pulse ratio, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/NMDA current ratio, and spontaneous EPSCs indicate that electrically induced LTD was mediated predominantly by postsynaptic mechanisms. Additionally, presynaptic mGluRs were involved in agonist-mediated synaptic depression. Immunohistochemical analysis supports the presence of multiple subclasses of mGluRs throughout the PC, with large concentrations of several receptors present in layer II. These observations provide further evidence of activity-dependent, long-term modification of associative inputs and its underlying mechanisms. Cortical adaptation at associative synapses provides an additional link between cortical olfactory processing and subcortical centers that influence behavior.

Functional maturation of excitatory synapses in layer 3 pyramidal neurons during postnatal development of the primate prefrontal cortex

In the primate dorsolateral prefrontal cortex (DLPFC), the density of excitatory synapses decreases by 40-50% during adolescence. Although such substantial circuit refinement might underlie the adolescence-related maturation of working memory performance, its functional significance remains poorly understood. The consequences of synaptic pruning may depend on the properties of the eliminated synapses. Are the synapses eliminated during adolescence functionally immature, as is the case during early brain development? Or do maturation-independent features tag synapses for pruning? We examined excitatory synaptic function in monkey DLPFC during postnatal development by studying properties that reflect synapse maturation in rat cortex. In 3-month-old (early postnatal) monkeys, excitatory inputs to layer 3 pyramidal neurons had immature properties, including higher release probability, lower alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate (NMDA) ratio, and longer duration of NMDA-mediated synaptic currents, associated with greater sensitivity to the NMDA receptor subunit B (NR2B) subunit-selective antagonist ifenprodil. In contrast, excitatory synaptic inputs in neurons from preadolescent (15 months old) and adult (42 or 84 months old) monkeys had similar functional properties. We therefore conclude that the contribution of functionally immature synapses decreases significantly before adolescence begins. Thus, remodeling of excitatory connectivity in the DLPFC during adolescence may occur in the absence of widespread maturational changes in synaptic strength.

Channelrhodopsin-2-assisted circuit mapping of long-range callosal projections

The functions of cortical areas depend on their inputs and outputs, but the detailed circuits made by long-range projections are unknown. We show that the light-gated channel channelrhodopsin-2 (ChR2) is delivered to axons in pyramidal neurons in vivo. In brain slices from ChR2-expressing mice, photostimulation of ChR2-positive axons can be transduced reliably into single action potentials. Combining photostimulation with whole-cell recordings of synaptic currents makes it possible to map circuits between presynaptic neurons, defined by ChR2 expression, and postsynaptic neurons, defined by targeted patching. We applied this technique, ChR2-assisted circuit mapping (CRACM), to map long-range callosal projections from layer (L) 2/3 of the somatosensory cortex. L2/3 axons connect with neurons in L5, L2/3 and L6, but not L4, in both ipsilateral and contralateral cortex. In both hemispheres the L2/3-to-L5 projection is stronger than the L2/3-to-L2/3 projection. Our results suggest that laminar specificity may be identical for local and long-range cortical projections.

PKC and polyamine modulation of GluR2-deficient AMPA receptors in immature neocortical pyramidal neurons of the rat

AMPA receptors (AMPARs) mediate the bulk of fast synaptic excitation in the CNS. We have recently shown that AMPAR-dependent synaptic transmission in immature neocortical pyramidal neurons is mediated by GluR2-deficient receptors that can be modulated by intra- or extracellular polyamines (PAs). Phosphorylation of AMPARs, e.g. by PKC, can lead to enhanced excitation, and PAs are known to modulate PKC activity. Therefore, PAs and PKC might interact to influence AMPAR function. To test this hypothesis, we made whole cell recordings from immature (P12-14) layer V pyramidal neurons and assayed two measures of PA influence on synaptic AMPAR function - inward rectification and use-dependent unblock (UDU), with the latter assayed by differences in rectification between a pair of EPSCs evoked at short (50 ms) latencies. We have previously shown that EPSCs in immature pyramidal neurons displayed inward rectification, which was enhanced by intracellular spermine, as was UDU. Staurosporin (ST), a PKC inhibitor, reversed the effect of PA on rectification and UDU, suggesting that PKC modulates postsynaptic activation of AMPARs. Similarly, polyamine-dependent rectification of spontaneous EPSCs was reversed by treatment with ST or GFX109203X, a specific PKC inhibitor. Chelating intracellular Ca(2+) with BAPTA reproduced the effects of ST. In addition, PA immunoreactivity in layer V pyramidal neurons was reduced by PKC inhibition indicating that PKC activity influences PA metabolism. Taken together, these data support the involvement of postsynaptic PKC activation in both the inward rectification and UDU of EPSCs in immature rat cortex, and suggest an important mechanism by which excitatory synaptic transmission can be dynamically modulated by changes in either [Ca(2+)](i) or [PA](i).

Nonlinear neurovascular coupling in rat sensory cortex by activation of transcallosal fibers

Functional neuroimaging and normal brain function rely on the robust coupling between neural activity and cerebral blood flow (CBF), that is neurovascular coupling. We examined neurovascular coupling in rat sensory cortex in response to direct stimulation of transcallosal pathways, which allows examination of brain regions inaccessible to peripheral stimulation techniques. Using laser-Doppler flowmetry to record CBF and electrophysiologic recordings of local field potentials (LFPs), we show an exponential relation between CBF responses and summed LFP amplitudes. Hemodynamic responses were dependent on glutamate receptor activation. CNQX, an AMPA receptor blocker, strongly attenuated evoked CBF responses and LFP amplitudes at all stimulation frequencies. In comparison, N-methyl D-aspartate (NMDA) receptor blockade by MK801 attenuated CBF responses at high (>7 Hz) but not low (<7 Hz) stimulation frequencies, without affecting evoked LFP amplitudes. This shows the limitation of using LFP amplitudes as indicators of synaptic activity. 7-Nitroindazole, a neuronal nitric oxide synthase inhibitor, and indomethacin, a nonspecific cyclooxygenase inhibitor, attenuated the hemodynamic responses by 50%+/-1% and 48%+/-1%, respectively, without affecting LFP amplitudes. The data suggest that preserved activity of both AMPA and NMDA receptors is necessary for the full CBF response evoked by stimulation of rodent interhemispheric connections. AMPA receptor activation gives rise to a measurable LFP, but NMDA receptor activation does not. The lack of a measurable LFP from neural processes that contribute importantly to CBF may explain some of the difficulties in transforming extracellular current or voltage measurements to a hemodynamic response.

Modulation of epileptiform activity by glutamine and system A transport in a model of post-traumatic epilepsy

Epileptic activity arises from an imbalance in excitatory and inhibitory synaptic transmission. To determine if alterations in the metabolism of glutamate, the primary excitatory neurotransmitter, might contribute to epilepsy we directly and indirectly modified levels of glutamine, an immediate precursor of synaptically released glutamate, in the rat neocortical undercut model of hyperexcitability and epilepsy. We show that slices from injured cortex take up glutamine more readily than control slices, and an increased expression of the system A transporters SNAT1 and SNAT2 likely underlies this difference. We also examined the effect of exogenous glutamine on evoked and spontaneous activity and found that addition of physiological concentrations of glutamine to perfusate of slices isolated from injured cortex increased the incidence and decreased the refractory period of epileptiform potentials. By contrast, exogenous glutamine increased the amplitude of evoked potentials in normal cortex, but did not induce epileptiform potentials. Addition of physiological concentrations of glutamine to perfusate of slices isolated from injured cortex greatly increased abnormal spontaneous activity in the form of events resembling spreading depression, again while having no effect on slices from normal cortex. Interestingly, similar spreading depression like events were noted in control slices at supraphysiological levels of glutamine. In the undercut cortex addition of methylaminoisobutyric acid (MeAIB), an inhibitor of the system A glutamine transporters attenuated all physiological effects of added glutamine suggesting that uptake through these transporters is required for the effect of glutamine. Our findings support a role for glutamine transport through SNAT1 and/or SNAT2 in the maintenance of abnormal activity in this in vitro model of epileptogenesis and suggest that system A transport and glutamine metabolism are potential targets for pharmacological intervention in seizures and epilepsy.

Glutamate receptor expression and chronic glutamate toxicity in rat motor cortex

In addition to the loss of spinal motor neurons, amyotrophic lateral sclerosis (ALS) is also associated with degeneration of corticospinal layer V pyramidal neurons and decreased glutamate transport in the cortex. We characterized the glutamate receptors on corticospinal neurons in acutely isolated rat motor cortex slices and found that the synaptic inputs to the corticospinal layer V neurons had a lesser proportional contribution from NMDA receptors relative to AMPA receptors than did layer II/III pyramidal neurons. The synaptic I(AMPA) was also more inwardly rectified, indicating a greater Ca(2+)-permeable component, in layer V. In a cortical organotypic slice culture model, blockade of glutamate transporters elevated glutamate in the media and led to pyramidal neuron loss in both layers. The loss of layer V pyramidal neurons was attenuated by antagonists of AMPA/kainate or Ca(2+)-permeable AMPA receptors, suggesting their therapeutic potential in the protection of the motor cortex in ALS.

GABAergic and pyramidal neurons of deep cortical layers directly receive and differently integrate callosal input

We studied the involvement of deep cortical layer neurons in processing callosal information in the rat. We observed with electron microscopy that both parvalbumin (PV)-labeled profiles and unlabeled dendritic spines of deep cortical layer neurons receive synapses from the contralateral hemisphere. Stimulation of callosal fibers elicited monosynaptic excitatory postsynaptic currents in both layer VI pyramidal neurons and gamma-aminobutyric acidergic (GABAergic) interneurons immunopositive for the vesicular GABA transporter and PV. Pyramidal cells had intrinsic electrophysiological properties and synaptic responses with slow kinetics and a robust N-metyhl-D-aspartate (NMDA) component. In contrast, GABAergic interneurons had intrinsic membrane properties and synaptic responses with faster kinetics and a less pronounced NMDA component. Consistent with these results, the temporal integration of callosal input was effective over a significantly longer time window in pyramidal neurons compared with GABAergic interneurons. Interestingly, callosal stimulation did not evoke feedforward inhibition in all GABAergic interneurons and in the majority of pyramidal neurons tested. Furthermore, retrogradely labeled layer VI pyramidal neurons of the contralateral cortex responded monosynaptically to callosal stimulation, suggesting interconnectivity between callosally projecting neurons. The data show that pyramidal neurons and GABAergic interneurons of deep cortical layers receive interhemispheric information directly and have properties supporting their distinct roles.

Non-NMDA and NMDA receptors are involved in suprathreshold excitation of network of frog tectal neurons by a single retinal ganglion cell

NMDA receptors play an important functional role in the neuron excitability and plasticity. The conditions and consequences of their activation are of interest for many neuroscientists. This investigation was designed to explore an activation of the NMDA receptors of frog tectal neurons in vivo by a burst of spikes of individual retinotectal fiber. We show that: (1) the NMDA receptors of tectal neurons can be activated by an intense burst discharge of an individual ganglion cell (likely darkness detector) at physiological conditions. (2) Activation of the NMDA receptors is achieved, primarily, due to temporal summation and frequency facilitation of the fast non-NMDA synaptic potentials. However, it is very likely that spatial summation of the fast retinotectal synaptic potentials with excitatory synaptic potentials of recurrent connections contributes to elicit the NMDA response. (3) The activation of NMDA receptors causes a higher level of activity of tectal neuron network. The suprathreshold excitation of efferent tectal neurons is characteristic for this level. Therefore, the burst discharge of only single retinal ganglion cell can activate the tectobulbospinal tract and lead to the motor reaction.

Developmental remodelling of the lemniscal synapse in the ventral basal thalamus of the mouse

Synapse elimination occurs throughout the nervous system during development, and is essential for the formation of neural circuits. The mechanisms underlying synapse elimination in the brain, however, remain largely unknown. Using whole-cell patch-clamp recording in a slice preparation, we examined synaptic refinement at the somatosensory relay synapse (lemniscal synapse) in the ventral basal thalamus of the mouse during postnatal development. At 1 week old, each neuron in the ventral basal thalamus is innervated by multiple lemniscal fibres, as revealed by multiple increments of the synaptic response. By 16 days after birth (P16), the majority of neurons showed an all-or-none response, suggesting a single fibre innervation. In addition to synapse elimination, extensive modifications in synaptic properties occur during the second week after birth. The ratio of AMPA to NMDA component of the synaptic current tripled between P7 and P17. The decay constant of the NMDA component decreased by about 70% between P7 and P17; ifenprodil (3 microm) reduced the NMDA component by about 40% in neurons at P7-9, but was much less effective at P20-24. On the other hand, there was little change in the inward rectification of AMPA component between P11 and P24. Paired-pulse ratios, measured at -70 and +40 mV, were stable between P7 and P24. Whisker deprivation from P5 through P19 had no effect on the elimination or the maturation of the lemniscal synapse. These results suggest that the lemniscal synapse in the ventral basal thalamus undergoes extensive refinement during the second week, and that sensory experience has a rather limited role in this process.