Histone methylation mediated by NSD1 is required for the establishment and maintenance of neuronal identities

Appropriate histone modifications emerge as essential cell fate regulators of neuronal identities across neocortical areas and layers. Here we showed that NSD1, the methyltransferase for di-methylated lysine 36 of histone H3 (H3K36me2), controls both area and layer identities of the neocortex. Nsd1-ablated neocortex showed an area shift of all four primary functional regions and aberrant wiring of cortico-thalamic-cortical projections. Nsd1 conditional knockout mice displayed defects in spatial memory, motor learning, and coordination, resembling patients with the Sotos syndrome carrying NSD1 mutations. On Nsd1 loss, superficial-layer pyramidal neurons (PNs) progressively mis-expressed markers for deep-layer PNs, and PNs remained immature both morphologically and electrophysiologically. Loss of Nsd1 in postmitotic PNs causes genome-wide loss of H3K36me2 and re-distribution of DNA methylation, which accounts for diminished expression of neocortical layer specifiers but ectopic expression of non-neural genes. Together, H3K36me2 mediated by NSD1 is required for the establishment and maintenance of region- and layer-specific neocortical identities.

Single-compartment model of a pyramidal neuron, fitted to recordings with current and conductance injection

For single neuron models, reproducing characteristics of neuronal activity such as the firing rate, amplitude of spikes, and threshold potentials as functions of both synaptic current and conductance is a challenging task. In the present work, we measure these characteristics of regular spiking cortical neurons using the dynamic patch-clamp technique, compare the data with predictions from the standard Hodgkin-Huxley and Izhikevich models, and propose a relatively simple five-dimensional dynamical system model, based on threshold criteria. The model contains a single sodium channel with slow inactivation, fast activation and moderate deactivation, as well as, two fast repolarizing and slow shunting potassium channels. The model quantitatively reproduces characteristics of steady-state activity that are typical for a cortical pyramidal neuron, namely firing rate not exceeding 30 Hz; critical values of the stimulating current and conductance which induce the depolarization block not exceeding 80 mV and 3, respectively (both values are scaled by the resting input conductance); extremum of hyperpolarization close to the midpoint between spikes. The analysis of the model reveals that the spiking regime appears through a saddle-node-on-invariant-circle bifurcation, and the depolarization block is reached through a saddle-node bifurcation of cycles. The model can be used for realistic network simulations, and it can also be implemented within the so-called mean-field, refractory density framework.

Measuring Action Potential Propagation Velocity in Murine Cortical Axons

Measuring the action potential (AP) propagation velocity in axons is critical for understanding neuronal computation. This protocol describes the measurement of propagation velocity using a combination of somatic whole cell and axonal loose patch recordings in brain slice preparations. The axons of neurons filled with fluorescent dye via somatic whole-cell pipette can be targeted under direct optical control using the fluorophore-filled pipette. The propagation delays between the soma and 5-7 axonal locations can be obtained by analyzing the ensemble averages of 500-600 sweeps of somatic APs aligned at times of maximal rate-of-rise (dV/dtmax) and axonal action currents from these locations. By plotting the propagation delays against the distance, the location of the AP initiation zone becomes evident as the site exhibiting the greatest delay relative to the soma. Performing linear fitting of the delays obtained from sites both proximal and distal from the trigger zone allows the determination of the velocities of AP backward and forward propagation, respectively. Key features • Ultra-thin axons in cortical slices are targeted under direct optical control using the SBFI-filled pipette. • Dual somatic whole cell and axonal loose patch recordings from 5-7 axonal locations. • Ensemble averaging of 500-600 sweeps of somatic APs and axonal action currents. • Plotting the propagation delays against the distance enables the determination of the trigger zone's position and velocities of AP backward and forward propagation.

Simultaneous 3D cellular positioning and apical dendritic morphology of transgenic fluorescent mouse CA3 hippocampal pyramidal neurons

BACKGROUND

Pyramidal neurons throughout hippocampal CA3 are diverse in their dendritic morphology, and CA3 is not homogenous in its structure or function. Nonetheless, few structural studies have captured the precise 3D somatic position and the 3D dendritic morphology of CA3 pyramidal neurons simultaneously.

NEW METHOD

Here, we present a simple approach to reconstruct the apical dendritic morphology of CA3 pyramidal neurons using the transgenic fluorescent Thy1-GFP-M line. The approach simultaneously tracks the dorsoventral, tangential, and radial positions of reconstructed neurons within the hippocampus. It is especially designed for use with transgenic fluorescent mouse lines, which are commonly used in genetic studies of neuronal morphology and development.

RESULTS

We demonstrate how topographic and morphological data are captured from transgenic fluorescent mouse CA3 pyramidal neurons.

COMPARISON WITH EXISTING METHODS

There is no need to select and label CA3 pyramidal neurons with the transgenic fluorescent Thy1-GFP-M line. By taking transverse (not coronal) serial sections, we preserve fine dorsoventral, tangential, and radial somatic positioning of 3D-reconstructed neurons. Because CA2 is well defined by PCP4 immunohistochemistry, we use that technique here to to increase precision in defining tangential position along CA3.

CONCLUSIONS

We developed a method for simultaneously collecting precise somatic positioning as well as 3D morphological data among transgenic fluorescent mouse hippocampal pyramidal neurons. This fluorescent method should be compatible with many other transgenic fluorescent reporter lines and immunohistochemical methods, facilitating the capture of topographic and morphological data from a wide variety of genetic experiments in mouse hippocampus.

Strength of Excitatory Inputs to Layer 3 Pyramidal Neurons During Synaptic Pruning in the Monkey Prefrontal Cortex: Relevance for the Pathogenesis of Schizophrenia

BACKGROUND

In schizophrenia, layer 3 pyramidal neurons (L3PNs) of the dorsolateral prefrontal cortex exhibit deficits in markers of excitatory synaptic inputs that are thought to disrupt the patterns of neural network activity essential for cognitive function. These deficits are usually interpreted under Irwin Feinberg's hypothesis of altered synaptic pruning, which postulates that normal periadolescent pruning, thought to preferentially eliminate weak/immature synapses, is altered in schizophrenia. However, it remains unknown whether periadolescent pruning on L3PNs in the primate dorsolateral prefrontal cortex selectively eliminates weak excitatory synapses or uniformly eliminates excitatory synapses across the full distribution of synaptic strengths.

METHODS

To distinguish between these alternative models of synaptic pruning, we assessed the densities of dendritic spines, the site of most excitatory inputs to L3PNs, and the distributions of excitatory synaptic strengths in dorsolateral prefrontal cortex L3PNs from male and female monkeys across the periadolescent period of synaptic pruning. We used patch-clamp methods in acute brain slices to record miniature excitatory synaptic currents and intracellular filling with biocytin to quantify dendritic spines.

RESULTS

On L3PNs, dendritic spines exhibited the expected age-related decline in density, but mean synaptic strength and the shape of synaptic strength distributions remained stable with age.

CONCLUSIONS

The absence of age-related differences in mean synaptic strength and synaptic strength distributions supports the model of a uniform pattern of synaptic pruning across the full range of synaptic strengths. The implications of these findings for the pathogenesis and functional consequences of dendritic spine deficits in schizophrenia are discussed.

Adolescent alcohol exposure reduces dopamine 1 receptor modulation of prelimbic neurons projecting to the nucleus accumbens and basolateral amygdala

Binge drinking during adolescence is highly prevalent despite increasing evidence of its long-term impact on behaviors associated with modulation of behavioral flexibility by the medial prefrontal cortex (mPFC). In the present study, male and female rats underwent adolescent intermittent ethanol (AIE) exposure by vapor inhalation. After aging to adulthood, retrograde bead labelling and viral tagging were used to identify populations of neurons in the prelimbic region (PrL) of the mPFC that project to specific subcortical targets. Electrophysiological recording from bead-labelled neurons in PrL slices revealed that AIE did not alter the intrinsic excitability of PrL neurons that projected to either the NAc or the BLA. Similarly, recordings of spontaneous inhibitory and excitatory post-synaptic currents revealed no AIE-induced changes in synaptic drive onto either population of projection neurons. In contrast, AIE exposure was associated with a loss of dopamine receptor 1 (D1), but no change in dopamine receptor 2 (D2), modulation of evoked firing of both populations of projection neurons. Lastly, confocal imaging of proximal and apical dendritic tufts of viral-labelled PrL neurons that projected to the nucleus accumbens (NAc) revealed AIE did not alter the density of dendritic spines. Together, these observations provide evidence that AIE exposure results in disruption of D1 receptor modulation of PrL inputs to at least two major subcortical target regions that have been implicated in AIE-induced long-term changes in behavioral control.

Doublecortin mutation leads to persistent defects in the Golgi apparatus and mitochondria in adult hippocampal pyramidal cells

Human doublecortin (DCX) mutations are associated with severe brain malformations leading to aberrant neuron positioning (heterotopia), intellectual disability and epilepsy. DCX is a microtubule-associated protein which plays a key role during neurodevelopment in neuronal migration and differentiation. Dcx knockout (KO) mice show disorganized hippocampal pyramidal neurons. The CA2/CA3 pyramidal cell layer is present as two abnormal layers and disorganized CA3 KO pyramidal neurons are also more excitable than wild-type (WT) cells. To further identify abnormalities, we characterized Dcx KO hippocampal neurons at subcellular, molecular and ultrastructural levels. Severe defects were observed in mitochondria, affecting number and distribution. Also, the Golgi apparatus was visibly abnormal, increased in volume and abnormally organized. Transcriptome analyses from laser microdissected hippocampal tissue at postnatal day 60 (P60) highlighted organelle abnormalities. Ultrastructural studies of CA3 cells performed in P60 (young adult) and > 9 months (mature) tissue showed that organelle defects are persistent throughout life. Locomotor activity and fear memory of young and mature adults were also abnormal: Dcx KO mice consistently performed less well than WT littermates, with defects becoming more severe with age. Thus, we show that disruption of a neurodevelopmentally-regulated gene can lead to permanent organelle anomalies contributing to abnormal adult behavior.

Effect of 2400 MHz mobile phone radiation exposure on the behavior and hippocampus morphology in Swiss mouse model

Electromagnetic field exposure to the nervous system can cause neurological changes. The effects of extremely low-frequency electromagnetic fields, such as second-generation and third-generation radiation, have been studied in most studies. The current study aimed to explore fourth-generation cellular phone radiation on hippocampal morphology and behavior in mice. Swiss albino male mice (n = 30) were randomly categorized into 3 groups; control, 40 min, and 60 min exposure to 2400 MHz radiofrequency electromagnetic radiation (RF-EMR) daily for 60 days. The control mice were housed in the same environments but were not exposed to anything. Anxiety-like behaviors were tested using the elevated plus-maze. For histological and stereological examination, the brain was dissected from the cranial cavity. On Cresyl violet stained brain slices, the number of pyramidal neurons in the cornu ammonis of the hippocampus were counted. In exposed mice compared to control mice, a significant increase in anxiety-like behavior has been observed. Histological observations have shown many black and dark blue cytoplasmic cells with shrunken morphology degenerative alterations in the neuronal hippocampus in the radiation exposed mice. In the RF-EMR mouse hippocampus, stereological analyses revealed a significant decrease in pyramidal and granule neurons compared to controls. Our findings suggest that 2400-MHz RF-EMR cell phone radiation affects the structural integrity of the hippocampus, which would lead to behavioral changes such as anxiety. However, it alerts us to the possible long-term detrimental effects of exposure to RF-EMR.

Early derailment of firing properties in CA1 pyramidal cells of the ventral hippocampus in an Alzheimer's disease mouse model

Gradual decline in cognitive and non-cognitive functions are considered clinical hallmarks of Alzheimer's Disease (AD). Post-mortem autoptic analysis shows the presence of amyloid β deposits, neuroinflammation and severe brain atrophy. However, brain circuit alterations and cellular derailments, assessed in very early stages of AD, still remain elusive. The understanding of these early alterations is crucial to tackle defective mechanisms. In a previous study we proved that the Tg2576 mouse model of AD displays functional deficits in the dorsal hippocampus and relevant behavioural AD-related alterations. We had shown that these deficits in Tg2576 mice correlate with the precocious degeneration of dopamine (DA) neurons in the Ventral Tegmental Area (VTA) and can be restored by L-DOPA treatment. Due to the distinct functionality and connectivity of dorsal versus ventral hippocampus, here we investigated neuronal excitability and synaptic functionality in the ventral CA1 hippocampal sub-region of Tg2576 mice. We found an age-dependent alteration of cell excitability and firing in pyramidal neurons starting at 3 months of age, that correlates with reduced levels in the ventral CA1 of tyrosine hydroxylase - the rate-limiting enzyme of DA synthesis. Additionally, at odds with the dorsal hippocampus, we found no alterations in basal glutamatergic transmission and long-term plasticity of ventral neurons in 8-month old Tg2576 mice compared to age-matched controls. Last, we used computational analysis to model the early derailments of firing properties observed and hypothesize that the neuronal alterations found could depend on dysfunctional sodium and potassium conductances, leading to anticipated depolarization-block of action potential firing. The present study depicts that impairment of cell excitability and homeostatic control of firing in ventral CA1 pyramidal neurons is a prodromal feature in Tg2576 AD mice.

Subjective tinnitus: lesion-induced pathological central homeostasis remodeling

Subjective tinnitus is the most common type of tinnitus, which is the manifestation of pathological activities in the brain. It happens in a substantial portion of the general population and brings significant burden to the society. Severe subjective tinnitus can lead to depression and insomnia and severely affects patients' quality of life. However, due to poor understanding of its etiology and pathogenesis, treatment of subjective tinnitus remains challenging. In recent decades, a growing number of studies have shown that subjective tinnitus is related to lesion-induced neural plasticity of auditory and non-auditory central systems. This article reviews cellular mechanisms of neural plasticity in subjective tinnitus to provide further understanding of its pathogenesis.

Takeda G Protein-Coupled Receptor 5 Modulates Depression-like Behaviors via Hippocampal CA3 Pyramidal Neurons Afferent to Dorsolateral Septum

BACKGROUND

Takeda G protein-coupled receptor 5 (TGR5) is recognized as a promising target for type 2 diabetes and metabolic syndrome; its expression has been demonstrated in the brain and is thought to be neuroprotective. Here, we hypothesize that dysfunction of central TGR5 may contribute to the pathogenesis of depression.

METHODS

In well-established chronic social defeat stress (CSDS) and chronic restraint stress (CRS) models of depression, we investigated the functional roles of TGR5 in CA3 pyramidal neurons (PyNs) and underlying mechanisms of the neuronal circuit in depression (for in vivo studies, n = 10; for in vitro studies, n = 5-10) using fiber photometry; optogenetic, chemogenetic, pharmacological, and molecular profiling techniques; and behavioral tests.

RESULTS

Both CSDS and CRS most significantly reduced TGR5 expression of hippocampal CA3 PyNs. Genetic overexpression of TGR5 in CA3 PyNs or intra-CA3 infusion of INT-777, a specific agonist, protected against CSDS and CRS, exerting significant antidepressant-like effects that were mediated via CA3 PyN activation. Conversely, genetic knockout or TGR5 knockdown in CA3 facilitated stress-induced depression-like behaviors. Re-expression of TGR5 in CA3 PyNs rather than infusion of INT-777 significantly improved depression-like behaviors in Tgr5 knockout mice exposed to CSDS or CRS. Silencing and stimulation of CA3 PyNs→somatostatin-GABAergic (gamma-aminobutyric acidergic) neurons of the dorsolateral septum circuit bidirectionally regulated depression-like behaviors, and blockade of this circuit abrogated the antidepressant-like effects from TGR5 activation of CA3 PyNs.

CONCLUSIONS

These findings indicate that TGR5 can regulate depression via CA3 PyNs→somatostatin-GABAergic neurons of dorsolateral septum transmission, suggesting that TGR5 could be a novel target for developing antidepressants.

The effects of astaxanthin treatment on a rat model of Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder characterized by memory loss and dementia, could be a consequence of the abnormalities of cortical milieu, such as oxidative stress, inflammation, and/or accompanied with the aggregation of β-amyloid. The majority of AD patients are sporadic, late-onset AD, which predominantly occurs over 65 years of age. Our results revealed that the ferrous amyloid buthionine (FAB)-infused sporadic AD-like model showed deficits in spatial learning and memory and with apparent loss of choline acetyltransferase (ChAT) expression in medial septal (MS) nucleus. In hippocampal CA1 region, the loss of pyramidal neurons was accompanied with cholinergic fiber loss and neuroinflammatory responses including glial reaction and enhanced expression of inducible nitric oxide synthase (iNOS). Surviving hippocampal CA1 pyramidal neurons showed the reduction of dendritic spines as well. Astaxanthin (ATX), a potent antioxidant, reported to improve the outcome of oxidative-stress-related diseases. The ATX treatment in FAB-infused rats decreased neuroinflammation and restored the ChAT + fibers in hippocampal CA1 region and the ChAT expression in MS nucleus. It also partly recovered the spine loss on hippocampal CA1 pyramidal neurons and ameliorated the behavioral deficits in AD-like rats. From these data, we believed that the ATX can be a potential option for slowing the progression of Alzheimer's disease.

FMRP regulates the subcellular distribution of cortical dendritic spine density in a non-cell-autonomous manner

Fragile X syndrome (FXS) is the most common form of intellectual disability that arises from the dysfunction of a single gene-Fmr1. The main neuroanatomical correlate of FXS is elevated dendritic spine density on cortical pyramidal neurons, which has been modeled in Fmr1-/Y mice. However, the cell-autonomous contribution of Fmr1 on cortical dendritic spine density has not been assessed. Even less is known about the role of Fmr1 in heterozygous female mosaic mice, which are a putative model for human Fmr1 full mutation carriers (i.e., are heterozygous for the full Fmr1-silencing mutation). In this neuroanatomical study, spine density in cortical pyramidal neurons of Fmr1+/- and Fmr1-/Y mice was studied at multiple subcellular compartments, layers, and brain regions. Spine density in Fmr1+/- mice is higher than WT but lower than Fmr1-/Y. Not all subcellular compartments in layer V Fmr1+/- and Fmr1-/Y cortical pyramidal neurons are equally affected: the apical dendrite, a key subcellular compartment, is principally affected over basal dendrites. Within apical dendrites, spine density is differentially affected across branch orders. Finally, identification of FMRP-positive and FMRP-negative neurons within Fmr1+/- permitted the study of the cell-autonomous effect of Fmr1 on spine density. Surprisingly, layer V cortical pyramidal spine density between FMRP-positive and FMRP-negative neurons does not differ, suggesting that the regulation of the primary neuroanatomical defect of FXS-elevated spine density-is non-cell-autonomous.

Integrin β3 organizes dendritic complexity of cerebral cortical pyramidal neurons along a tangential gradient

Dysfunctional dendritic arborization is a key feature of many developmental neurological disorders. Across various human brain regions, basal dendritic complexity is known to increase along a caudal-to-rostral gradient. We recently discovered that basal dendritic complexity of layer II/III cortical pyramidal neurons in the mouse increases along a caudomedial-to-rostrolateral gradient spanning multiple regions, but at the time, no molecules were known to regulate that exquisite pattern. Integrin subunits have been implicated in dendritic development, and the subunit with the strongest associations with autism spectrum disorder and intellectual disability is integrin β3 (Itgb3). In mice, global knockout of Itgb3 leads to autistic-like neuroanatomy and behavior. Here, we tested the hypothesis that Itgb3 is required for increasing dendritic complexity along the recently discovered tangential gradient among layer II/III cortical pyramidal neurons. We targeted a subset of layer II/III cortical pyramidal neurons for Itgb3 loss-of-function via Cre-loxP-mediated excision of Itgb3. We tracked the rostrocaudal and mediolateral position of the targeted neurons and reconstructed their dendritic arbors. In contrast to controls, the basal dendritic complexity of Itgb3 mutant neurons was not related to their cortical position. Basal dendritic complexity of mutant and control neurons differed because of overall changes in branch number across multiple branch orders (primary, secondary, etc.), rather than any changes in the average length at those branch orders. Furthermore, dendritic spine density was related to cortical position in control but not mutant neurons. Thus, the autism susceptibility gene Itgb3 is required for establishing a tangential pattern of basal dendritic complexity among layer II/III cortical pyramidal neurons, suggesting an early role for this molecule in the developing brain.

LRP4 LDLα repeats of astrocyte enhance dendrite arborization of the neuron

The low-density lipoprotein receptor-related protein 4 (LRP4) is essential for inducing the neuromuscular junction (NMJ) formation in muscle fibers, and LRP4 plays a critical role in dendritic development and synaptogenesis in the central nervous system (CNS). As a single transmembrane protein, LRP4 contains an enormously sizeable extracellular domain (ECD), containing multiple LDLα repeats in the N-terminal of ECD. LRP4 only with extracellular domain acts as a similar mechanism of full-length LRP4 in muscles to stimulate acetylcholine receptor clustering. In this study, we elucidated that LDLα repeats of LRP4 maintained the body weight and survival rate. Dendritic branches of the pyramidal neurons in Lrp4-null mice with LRP4 LDLα repeats residue were more than in Lrp4-null mice without residual LRP4 domain. Supplement with conditioned medium from LRP4 LDLα overexpression cells, the primary culture pyramidal neurons achieved strong dendritic arborization ability. Besides, astrocytes with LRP4 LDLα repeats residue could promote pyramidal neuronal dendrite arborization in the primary co-cultured system. These observations signify that LRP4 LDLα repeats play a prominent underlying role in dendrite arborization.

The prefrontal cortex as a target for atypical antipsychotics in schizophrenia, lessons of neurodevelopmental animal models

Prefrontal cortex (PFC) inflammatory imbalance, oxidative/nitrosative stress (O/NS) and impaired neuroplasticity in schizophrenia are thought to have neurodevelopmental origins. Animal models are not only useful to test this hypothesis, they are also effective to establish a relationship among brain disturbances and behavior with the atypical antipsychotics (AAPs) effects. Here we review data of PFC post-mortem and in vivo neuroimaging, human induced pluripotent stem cells (hiPSC), and peripheral blood studies of inflammatory, O/NS, and neuroplasticity alterations in the disease as well as about their modulation by AAPs. Moreover, we reviewed the PFC alterations and the AAP mechanisms beyond their canonical antipsychotic action in four neurodevelopmental animal models relevant to the study of schizophrenia with a distinct approach in the generation of schizophrenia-like phenotypes, but all converge in O/NS and altered neuroplasticity in the PFC. These animal models not only reinforce the neurodevelopmental risk factor model of schizophrenia but also arouse some novel potential therapeutic targets for the disease including the reestablishment of the antioxidant response by the perineuronal nets (PNNs) and the nuclear factor erythroid 2-related factor (Nrf2) pathway, as well as the dendritic spine dynamics in the PFC pyramidal cells.

Memory retention in pyramidal neurons: a unified model of energy-based homo and heterosynaptic plasticity with homeostasis

The brain can learn new tasks without forgetting old ones. This memory retention is closely associated with the long-term stability of synaptic strength. To understand the capacity of pyramidal neurons to preserve memory under different tasks, we established a plasticity model based on the postsynaptic membrane energy state, in which the change in synaptic strength depends on the difference between the energy state after stimulation and the resting energy state. If the post-stimulation energy state is higher than the resting energy state, then synaptic depression occurs. On the contrary, the synapse is strengthened. Our model unifies homo- and heterosynaptic plasticity and can reproduce synaptic plasticity observed in multiple experiments, such as spike-timing-dependent plasticity, and cooperative plasticity with few and common parameters. Based on the proposed plasticity model, we conducted a simulation study on how the activation patterns of dendritic branches by different tasks affect the synaptic connection strength of pyramidal neurons. We further investigate the formation mechanism by which different tasks activate different dendritic branches. Simulation results show that compare to the classic plasticity model, the plasticity model we proposed can achieve a better spatial separation of different branches activated by different tasks in pyramidal neurons, which deepens our insight into the memory retention mechanism of brains.

Mechanisms of axon polarization in pyramidal neurons

Neurons are highly polarized cells that have specialized regions for synaptic input, the dendrites, and synaptic output, the axons. This polarity is critical for appropriate neural circuit formation and function. One of the central gaps in our knowledge is understanding how developing neurons initiate axon polarity. Given the critical nature of this polarity on neural circuit formation and function, neurons have evolved multiple mechanisms comprised of extracellular and intracellular cues that allow them to initiate and form axons. These mechanisms engage a variety of signaling cascades that provide positive and negative cues to ensure axon polarization. This review highlights our current knowledge of the molecular underpinnings of axon polarization in pyramidal neurons and their relevance to the development of the brain.

Ascending noradrenergic excitation from the locus coeruleus to the anterior cingulate cortex

Anterior cingulate cortex (ACC) plays important roles in sensory perception including pain and itch. Neurons in the ACC receive various neuromodulatory inputs from subcortical structures, including locus coeruleus noradrenaline (LC-NA) neurons. Few studies have been reported about synaptic and behavioral functions of LC-NA projections to the ACC. Using viral-genetic method (AAV-DIO-eYFP) on DBH-cre mice, we found that LC-NA formed synaptic connections to ACC pyramidal cells but not interneurons. This is further supported by the electron microscopic study showing NAergic fibers contact the presynaptic inputs and post-synaptic areas of the pyramidal cells. NA application produced both pre- and post-synaptic potentiation effects in ACC excitatory transmission in vivo and in vitro. Activation of LC-NA projection to the ACC by optogenetic method produced enhancement of excitatory transmission in vitro and induced scratching and behavioral sensitization for mechanical stimulation. Our results demonstrate that LC-NA projections enhance or facilitate brain responses to pain and itch by potentiating glutamatergic synaptic transmissions in the ACC.

Cux2 expression regulated by Lhx2 in the upper layer neurons of the developing cortex

The laminar structure, a unique feature of the mammalian cerebrum, is formed by a number of genes in a highly complex process. The pyramidal neurons that make up each layer of the cerebrum are functionally characterized by specific gene expressions. In particular, Cux1 and Cux2, which are specifically expressed in layer II-IV neurons, are known to regulate dendritic branching, spine morphology, and synapse formation. However, it is still unknown how their expression is regulated transcriptionally. Here we constructed Cux2-mCherry transgenic mice that reproduce the cortical layer II-IV-specific expression of Cux2, a member of the Cut/Cux/CDP family, using BAC transgenesis and a variety of coordinated cortical layer markers that are known to date. Our immunohistochemistry analysis shows that mCherry was expressed in cortical layer II-IV and the corpus callosum in the same way as endogenous Cux2 without ectopic expression. We also identified a region of 220 bp that is highly conserved in mammals and controls specific cerebral expression of Cux2, using comparative genome analysis and in vivo reporter assays. Furthermore, we confirm that Lhx2, whose expression in cortical layer II-IV is similar to that of the Cux2 enhancer, can act as a transcriptional activator. These results suggest that cortical layer II-IV expression of Cux2 can be regulated by the interaction of Cux2-E1 and Lhx2, and that their failure to co-regulate is associated with neurodevelopmental disorders such as autism and schizophrenia.

Calbindin immunostaining in the CA1 hippocampal pyramidal cell layer of the human and mouse: A comparative study

Immunostaining for calbindin (CB) is commonly used to label particular populations of neurons. Recently, it has been shown that the CA1 pyramidal cells in the mouse can be subdivided along the radial axis into superficial and deep pyramidal cells and that this segregation in the radial axis may represent a general principle of structural and functional organization of the hippocampus. One of the most widely used markers of the superficial pyramidal cells is CB. However, this laminar segregation of pyramidal cells has not been reported in the human CA1 using CB immunostaining. The problem is that the different pattern of CB immunostaining observed in the mouse compared to the human could be explained by technical features, of which one of the most important is the postmortem time (PT) delay typical of the brain tissue obtained from humans. In the present study, we have studied the influences of PT delays and fixation procedures and we found that the clear differences found between the CA1 of the human and mouse do not depend on the fixation, but represent actual species-specific differences. These remarkable differences between species should be taken into consideration when making interpretations in translational studies from mouse to human brains.

Biophysical reconstruction of the signal conduction underlying short-latency cortical evoked potentials generated by subthalamic deep brain stimulation

OBJECTIVE

Direct activation of the hyperdirect (HD) pathway has been linked to therapeutic benefit from subthalamic deep brain stimulation (DBS) for the treatment of Parkinson's disease (PD). We sought to quantify the axonal conduction biophysics of corticofugal axons directly stimulated by subthalamic DBS and reconcile those findings with short-latency cortical evoked potential (EP) results.

METHODS

We used a detailed computational model of human subthalamic DBS to quantify axonal activation and conduction. Signal propagation to cortex was evaluated for medium (5.7 µm), large (10.0 µm), and exceptionally large (15.0 µm) diameter corticofugal axons associated with either internal capsule (IC) fibers of passage or the HD pathway. We then compared the modeling results to human cortical EP measurements that have described an exceptionally fast component (EP0) occurring ~1 ms after the stimulus pulse, a fast component (EP1) at ~3 ms, and a slower component (EP2) at ~5 ms.

RESULTS

Subthalamic stimulation of the HD pathway with large and medium diameter axons propagated action potentials to cortex with timings that coincide with the EP1 and EP2 signals, respectively. Only direct activation of exceptionally large diameter fibers in the IC generated signals that could approach the EP0 timing. However, the action potential biophysics do not generally support the existence of a cortical EP less than 1.5 ms after DBS onset.

CONCLUSIONS

The EP1 and EP2 signals can be biophysically linked to antidromic activation of the HD pathway.

SIGNIFICANCE

Theoretical reconstruction of cortical EPs from subthalamic DBS demonstrate a convergence of anatomical, biophysical, and electrophysiological results.

Aging and HIV-1 alter the function of specific K+ channels in prefrontal cortex pyramidal neurons

The medial prefrontal cortex (mPFC) is a key regulator of neurocognition. The glutamatergic pyramidal neurons are the predominant component of neurons in the mPFC. Aging and HIV profoundly alter the structure and function of mPFC pyramidal neurons, including, but are not limited to, dysregulation of NMDA receptors and voltage-gated calcium channels. Here we assessed the impact of aging and in vivo HIV exposure on the functional activity (firing) of mPFC pyramidal neurons mediated by voltage-gated K⁺ (K_v) channels and inwardly-rectifying K⁺ (K_ir) channels using patch-clamp recording in rat brain slices ex vivo. We found that aging and HIV significantly affect firing in different manners by altering the activity of K_v and likely K_ir channels, associated with changes in membrane properties and the mRNA levels of specific K_v channels. Evoked firing was significantly decreased in mPFC neurons of older (12 month, 12 m) rats compared to younger (6/7 week, 6/7wk) rats, regardless of HIV status. In contrast, firing was significantly increased in neurons from Tg rats compared to non-Tg rats, regardless of age. Aging/HIV-induced alterations in firing were mediated by dysfunctional K_v channels and K_ir channels, which exhibit significant changes in their activity and/or expression induced by aging and HIV exposure in vivo. Collectively, these novel findings demonstrate that aging is associated with a significant decline of mPFC neuronal activity; while long-term HIV exposure in vivo could drive mPFC neurons from over-activation to loss of firing, which could ultimately exacerbate the decline of mPFC neuronal activity.

Cyto-nuclear shuttling of afadin is required for rapid estradiol-mediated modifications of histone H3

Estrogens have been shown to rapidly regulate local signalling at synapses and within the nucleus. The result of these signalling events is to rapidly modulate synapse structure and function, as well as epigenetic mechanisms including histone modifications. Ultimately these mechanisms are thought to contribute to long-lasting changes in neural circuitry, and thus influence cognitive functions such as learning and memory. However, the mechanisms by which estrogen-mediated local synaptic and nuclear signalling events are coordinated are not well understood. In this study we have found that the scaffold protein afadin, (also known as AF-6), undergoes a bi-directional trafficking to both synaptic and nuclear compartment in response to acute 17β-estradiol (estradiol) treatment, in mixed sex neuronal cultures derived from fetal cortex. Interestingly, nuclear accumulation of afadin was coincidental with an increase in the phosphorylation of histone H3 at serine 10 (H3S10p). This epigenetic modification is associated with the remodeling of chromatin into an open euchromatin state, allowing for transcriptional activation and related learning and memory processes. Critically, the cyto-nuclear trafficking of afadin was required for estradiol-dependent H3S10p. We further determined that nuclear accumulation of afadin is sufficient to induce phosphorylation of the mitogentic kinases ERK1/2 (pERK1/2) within the nucleus. Moreover, nuclear pERK1/2 was required for estradiol-dependent H3S10p. Taken together, we propose a model whereby estradiol induces the bi-directional trafficking of afadin to synaptic and nuclear sub-compartments. Within the nucleus, afadin is required for increased pERK1/2 which in turn is required for H3S10p. Therefore this represents a mechanism through which estrogens may be able to coordinate both synaptic and nucleosomal events within the same neuronal population.

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17β-estradiol targets afadin to membrane and nuclear subcompartments.

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Histone H3 is rapidly phosphorylated by 17β-estradiol.

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Histone H3 phosphorylation by 17β-estradiol requires afadin nuclear accumulation.

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17β-estradiol-mediated ERK1/2 activation is required for histone H3 phosphorylation.

Thermogenetic stimulation of single neocortical pyramidal neurons transfected with TRPV1-L channels

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of single neurons using thermosensitive cation channels and IR stimulation. The main advantage of IR stimulation compared to conventional visible light optogenetics is the depth of penetration (up to millimeters). Due to physiological limitations, thermogenetic molecular tools for mammalian brain stimulation remain poorly developed. Here, we tested the possibility of employment of this new technique for stimulation of neocortical neurons. The method is based on activation gating of TRPV1-L channels selectively expressed in specific cells. Pyramidal neurons of layer 2/3 of neocortex were transfected at an embryonic stage using a pCAG expression vector and electroporation in utero. Depolarization and spiking responses of TRPV1L+ pyramidal neurons to IR radiation were recorded electrophysiologically in acute brain slices of adult animals with help of confocal visualization. As TRPV1L-expressing neurons are not sensitive to visible light, there were no limitations of the use of this technique with conventional fluorescence imaging. Our experiments demonstrated that the TRPV1-L+ pyramidal neurons preserve their electrical excitability in acute brain slices, while IR radiation can be successfully used to induce single neuronal depolarization and spiking at near physiological temperatures. Obtained results provide important information for adaptation of thermogenetic technology to mammalian brain studies in vivo.

Presynaptic Effects of N-Methyl-D-Aspartate Receptors Enhance Parvalbumin Cell-Mediated Inhibition of Pyramidal Cells in Mouse Prefrontal Cortex

BACKGROUND

Testing hypotheses regarding the role of N-methyl-D-aspartate receptor (NMDAR) hypofunction in schizophrenia requires understanding the mechanisms of NMDAR regulation of prefrontal cortex (PFC) circuit function. NMDAR antagonists are thought to produce pyramidal cell (PC) disinhibition. However, inhibitory parvalbumin-positive basket cells (PVBCs) have modest NMDAR-mediated excitatory drive and thus are unlikely to participate in NMDAR antagonist-mediated disinhibition. Interestingly, recent studies demonstrated that presynaptic NMDARs enhance transmitter release at central synapses. Thus, if presynaptic NMDARs enhance gamma-aminobutyric acid release at PVBC-to-PC synapses, they could participate in NMDAR-dependent PC disinhibition. Here, we examined whether presynaptic NMDAR effects could modulate gamma-aminobutyric acid release at PVBC-to-PC synapses in mouse PFC.

METHODS

Using whole-cell recordings from synaptically connected pairs in mouse PFC, we determined whether NMDA or NMDAR antagonist application affects PVBC-to-PC inhibition in a manner consistent with a presynaptic mechanism.

RESULTS

NMDAR activation enhanced by ∼40% the synaptic current at PVBC-to-PC pairs. This effect was consistent with a presynaptic mechanism given that it was 1) observed with postsynaptic NMDARs blocked by intracellular MK801, 2) associated with a lower rate of transmission failures and a higher transmitter release probability, and 3) blocked by intracellular MK801 in the PVBC. NMDAR antagonist application did not affect the synaptic currents in PVBC-to-PC pairs, but it reduced the inhibitory currents elicited in PCs with simultaneous glutamate release by extracellular stimulation.

CONCLUSIONS

We demonstrate that NMDAR activation enhances PVBC-to-PC inhibition in a manner consistent with presynaptic mechanisms, and we suggest that the functional impact of this presynaptic effect depends on the activity state of the PFC network.

Action potential initiation, propagation, and cortical invasion in the hyperdirect pathway during subthalamic deep brain stimulation

BACKGROUND

High frequency (∼130 Hz) deep brain stimulation (DBS) of the subthalamic region is an established clinical therapy for the treatment of late stage Parkinson's disease (PD). Direct modulation of the hyperdirect pathway, defined as cortical layer V pyramidal neurons that send an axon collateral to the subthalamic nucleus (STN), has emerged as a possible component of the therapeutic mechanisms. However, numerous questions remain to be addressed on the basic biophysics of hyperdirect pathway stimulation.

OBJECTIVE

Quantify action potential (AP) initiation, propagation, and cortical invasion in hyperdirect neurons during subthalamic stimulation.

METHODS

We developed an anatomically and electrically detailed computational model of hyperdirect neuron stimulation with explicit representation of the stimulating electric field, axonal response, AP propagation, and synaptic transmission.

RESULTS

We found robust AP propagation throughout the complex axonal arbor of the hyperdirect neuron. Even at therapeutic DBS frequencies, stimulation induced APs could reach all of the intracortical axon terminals with ∼100% fidelity. The functional result of this high frequency axonal driving of the thousands of synaptic connections made by each directly stimulated hyperdirect neuron is a profound synaptic suppression that would effectively disconnect the neuron from the cortical circuitry.

CONCLUSIONS

The synaptic suppression hypothesis integrates the fundamental biophysics of electrical stimulation, axonal transmission, and synaptic physiology to explain a generic mechanism of DBS.

The TrkB agonist 7,8-dihydroxyflavone changes the structural dynamics of neocortical pyramidal neurons and improves object recognition in mice

BDNF and its receptor TrkB have important roles in neurodevelopment, neural plasticity, learning, and memory. Alterations in TrkB expression have been described in different CNS disorders. Therefore, drugs interacting with TrkB, specially agonists, are promising therapeutic tools. Among them, the recently described 7,8-dihydroxyflavone (DHF), an orally bioactive compound, has been successfully tested in animal models of these diseases. Recent studies have shown the influence of this drug on the structure of pyramidal neurons, specifically on dendritic spine density. However, there is no information yet on how DHF may alter the structural dynamics of these neurons (i.e., real-time study of the addition/elimination of dendritic spines and axonal boutons). To gain knowledge on these effects of DHF, we have performed a real-time analysis of spine and axonal dynamics in pyramidal neurons of barrel cortex, using cranial windows and 2-photon microscopy during a chronic oral treatment with this drug. After confirming TrkB expression in these neurons, we found that DHF increased the gain rates of spines and axonal boutons, as well as improved object recognition memory. These results help to understand how the activation of the BDNF-TrkB system can improve basic behavioral tasks through changes in the structural dynamics of pyramidal neurons. Moreover, they highlight DHF as a promising therapeutic vector for certain brain disorders in which this system is altered.

Towards a circuit-level understanding of hippocampal CA1 dysfunction in Alzheimer's disease across anatomical axes

The hippocampus has been a primary region of study with regards to synaptic and functional changes in Alzheimer’s disease (AD) due to its involvement in early stages, specifically area CA1. However, most work in this area has treated CA1 as a homogeneous structure comprised of uniform neural circuits. Yet, there is a plethora of evidence that CA1 varies in its structure and function across anatomical axes. Here I review the heterogeneity of the functional and circuit architecture of hippocampal area CA1 across three primary anatomical axes. I also summarize evidence that AD differentially affects these subregions, as well as hypotheses as to why this may occur.

Translaminar circuits formed by the pyramidal cells in the superficial layers of cat visual cortex

Pyramidal cells in the superficial layers of the neocortex provide a major excitatory projection to layer 5, which contains the pyramidal cells that project to subcortical motor-related targets. Both structurally and functionally rather little is known about this interlaminar pathway, especially in higher mammals. Here, we made sparse ultrastructural reconstructions of the projection to layer 5 of three pyramidal neurons from layer 3 in cat V1 whose morphology, physiology, and synaptic connections with layers 2 and 3 were known. The dominant targets of the 74 identified synapses in layer 5 were the dendritic spines of pyramidal cells. The fractions of target spiny dendrites were 59, 61, and 84% for the three cells, with the remaining targets being dendrites of smooth neurons. These fractions were similar to the distribution of targets of unlabeled asymmetric synapses in the surrounding neuropil. Serial section reconstructions revealed that the target dendrites were heterogenous in morphology, indicating that different cell types are innervated. This new evidence indicates that the descending projection from the superficial layer pyramidal cells does not simply drive the output pyramidal cells that project to cortical and subcortical targets, but participates in the complex circuitry of the deep cortical layers.

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.

The brain-penetrant 5-HT7 receptor agonist LP-211 reduces the sensory and affective components of neuropathic pain

Neuropathic pain is a debilitating pathological condition of high clinical relevance. Changes in neuronal excitability in the anterior cingulate cortex (ACC) play a central role in the negative emotional and affective aspects of chronic pain. We evaluated the effects of LP-211, a new serotonin-receptor-type-7 (5-HT₇R) agonist that crosses the blood-brain barrier, on ACC neurons in a mouse model of neuropathic pain. LP-211 reduced synaptic integration in layer 5 pyramidal neurons, which was enhanced in neuropathic pain due to a dysfunction of dendritic hyperpolarization-activated-and-cyclic-nucleotide-regulated (HCN) channels. Acute injection of LP-211 had an analgesic effect, increasing the mechanical withdrawal threshold in neuropathic animals, which was partially mediated by an action in the ACC. Additionally, the acute application of LP-211 blocked the switch in the place escape/avoidance behavior induced by noxious stimuli. Thus systemic treatment with a 5-HT₇R agonist leads to modulation of the ACC, which dampens sensory and affective aspects of chronic pain.

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Anterior cingulate cortex contributes to the emotional/affective distress in chronic pain.

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Dysfunction of HCN channels increase cellular excitability in chronic pain.

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LP-211 is a brain-penetrant 5-HT₇ receptor agonist that enhances HCN channel function.

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LP-211 alleviates the sensory and affective/emotional pain behavior in neuropathic animals.

Functional perturbation of forebrain principal neurons reveals differential effects in novel and well-learned tasks

Neural circuits in mammalian brains consist of large numbers of different cell types having different functional properties. To better understand the separate roles of individual neuron types in specific aspects of spatial learning and memory, we perturbed the function of principal neurons in vivo during maze performance or in hippocampal slices during recording of evoked excitatory synaptic potentials. Transgenic mice expressing the Drosophila allatostatin receptor (AlstR) in cortical and hippocampal pyramidal cells were tested on an elevated plus maze, in a Y-maze, and in the Morris water maze. Relative to a control cohort, AlstR-positive mice treated with allatostatin exhibited no difference in open arm dwell time on the elevated plus maze or total number of arm entries in a Y-maze, but displayed reduced spontaneous alternation. When animals received massed or spaced training trials in the Morris water maze, and the peptide was delivered prior to an immediate probe, no effects on performance were observed. When the peptide was delivered during a probe trial performed 24h after seven days of spaced training, allatostatin delivery to AlstR positive mice enhanced direct navigation to the escape platform. Combined, these results suggest that cortical and hippocampal pyramidal neurons are required during spatial decision-making in a novel environment and compete with other neural systems after extended training in a long-term reference memory task. In hippocampal slices collected from AlstR positive animals, allatostatin delivery produced frequency dependent alterations in the Schaffer collateral fiber volley (attenuated accommodation at 100Hz) and excitatory postsynaptic potential (attenuated facilitation at 5Hz). Combined, the neural and behavioral discoveries support the involvement of short-term plasticity of Schaffer collateral axons and synapses during exploration of a novel environment and during initial orientation to a goal in a well-learned setting.

A biological blueprint for the axons of superficial layer pyramidal cells in cat primary visual cortex

Pyramidal cells in the superficial layers of neocortex of higher mammals form a lateral network of axon clusters known as the 'daisy' network. The role of these axon clusters remains speculative and we still lack a comprehensive quantitative description of the single neurons forming the daisy or their heterogeneity. We filled intracellularly 50 superficial layer pyramidal neurons in the cat primary visual cortex and reconstructed the axonal tree and their synaptic boutons in 3D. Individual bouton clusters were identified using an objective mean-shift algorithm. By parameterizing the morphology of these 50 axonal trees and the 217 bouton clusters they formed, we were able to extract one set of relatively constant parameters and another set of variable parameters. Both sets combined allowed us to outline a comprehensive biological blueprint of superficial layer pyramidal neurons. Overall, our detailed analysis supports the hypothesis that pyramidal neurons use their lateral clusters to combine differential contextual cues, required for context-dependent processing of natural scenes.

Synaptic connections formed by patchy projections of pyramidal cells in the superficial layers of cat visual cortex

The present study is the first to describe quantitatively the patterns of synaptic connections made by the patchy network of pyramidal cell axons in the superficial layers of cat V1 in relation to the orientation map. Intrinsic signal imaging of the orientation map was combined with 3D morphological reconstructions of physiologically-characterized neurons at light and electron microscope levels. A Similarity Index (SI) expressed the similarity of the orientation domain of a given bouton cluster to that of its parent dendritic tree. Six pyramidal cells whose axons had a wide range of SIs were examined. Boutons were sampled from five local and five distal clusters, and from the linear segments that link the clusters. The synaptic targets were reconstructed by serial section electron microscopy. Of the 233 synapses examined, 182 synapses were formed with spiny neurons, the remainder with smooth neurons. The proportion of smooth neurons that were synaptic targets varied greatly (from 0 to 50%) between the cluster samples, but was not correlated with the SI. The postsynaptic density sizes were similar for synapses in local and distal clusters, regardless of their SI. This heterogeneity in the synaptic targets of single cells within the superficial layers is a network feature well-suited for context-dependent processing.

Chronic nicotine differentially affects murine transcriptome profiling in isolated cortical interneurons and pyramidal neurons

Background

Nicotine is known to differentially regulate cortical interneuron and pyramidal neuron activities in the neocortex, while the underlying molecular mechanisms have not been well studied. In this study, RNA-sequencing was performed in acutely isolated cortical somatostatin (Sst)- positive interneurons and pyramidal neurons (Thy1) from mice treated with systemic nicotine for 14 days. We assessed the differentially expressed genes (DEGs) by nicotine in Sst- or Thy1- neurons, respectively, and then compared DEGs between Sst- and Thy1- neurons in the absence and presence of nicotine.

Results

In Sst-neurons, the DEGs by nicotine were associated with glycerophospholipid and nicotinate and nicotinamide metabolism; while in Thy1-neurons those related to immune response and purine and pyrimidine metabolisms were affected. Under basal condition, the DEGs between Sst- and Thy1- neurons were frequently associated with signal transduction, phosphorylation and potassium channel regulation. However, some new DEGs between Sst- and Thy1- neurons were found after nicotine, the majority of which belong to mitochondrial respiratory chain complex.

Conclusions

Nicotine differentially affected subset of genes in Sst- and Thy1- neurons, which might contribute to the distinct effect of nicotine on interneuron and pyramidal neuron activities. Meanwhile, the altered transcripts associated with mitochondrial activity were found between interneurons and pyramidal neurons after chronic nicotine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3593-x) contains supplementary material, which is available to authorized users.

Chloride conducting light activated channel GtACR2 can produce both cessation of firing and generation of action potentials in cortical neurons in response to light

Optogenetics is a powerful technique in neuroscience that provided a great success in studying the brain functions during the last decade. Progress of optogenetics crucially depends on development of new molecular tools. Light-activated cation-conducting channelrhodopsin2 was widely used for excitation of cells since the emergence of optogenetics. In 2015 a family of natural light activated chloride channels GtACR was identified which appeared to be a very promising tool for using in optogenetics experiments as a cell silencer. Here we examined properties of GtACR2 channel expressed in the rat layer 2/3 pyramidal neurons by means of in utero electroporation. We have found that despite strong inhibition the light stimulation of GtACR2-positive neurons can surprisingly lead to generation of action potentials, presumably initiated in the axonal terminals. Thus, when using the GtACR2 in optogenetics experiments, its ability to induce action potentials should be taken into account. Our results also open an interesting possibility of using the GtACR2 both as cell silencer and cell activator in the same experiment varying the pattern of light stimulation.

Streptozotocin inhibits synaptic transmission and edaravone attenuates streptozotocin-induced electrophysiological changes in CA1 pyramidal neurons of rat hippocampal slices

The purpose of this study was to investigate the acute and chronic effects of streptozotocin (STZ) upon synaptic transmission and the effects of edaravone (EDA, a free radical scavenger) on STZ-induced electrophysiological changes in CA1 pyramidal neurons of rat hippocampal slices. To accomplish this goal, spontaneous excitatory postsynaptic current (sEPSC), miniature excitatory postsynaptic current (mEPSC), spontaneous inhibitory postsynaptic current (sIPSC) and miniature inhibitory postsynaptic current (mIPSC) were recorded within hippocampal slices using whole-cell patch clamp techniques. The results showed that the amplitudes and frequencies of sEPSC, mEPSC, sIPSC and mIPSC were inhibited by 1000μM STZ, while treatment of EDA (1000μM) attenuated these STZ-induced changes. The degree of these neurotoxic effects of STZ and effects of EDA increased as a function of drug duration as assessed at 2, 4 or 8h of exposure. Taken together, our results demonstrate that STZ induces neurotoxicity within these hippocampal slices through its capacity to alter synaptic transmission and these STZ-induced alterations in electrophysiological responses are attenuated by EDA.

The Planar Cell Polarity Transmembrane Protein Vangl2 Promotes Dendrite, Spine and Glutamatergic Synapse Formation in the Mammalian Forebrain

The transmembrane protein Vangl2, a key regulator of the Wnt/planar cell polarity (PCP) pathway, is involved in dendrite arbor elaboration, dendritic spine formation and glutamatergic synapse formation in mammalian central nervous system neurons. Cultured forebrain neurons from Vangl2 knockout mice have simpler dendrite arbors, fewer total spines, less mature spines and fewer glutamatergic synapse inputs on their dendrites than control neurons. Neurons from mice heterozygous for a semidominant Vangl2 mutation have similar but not identical phenotypes, and these phenotypes are also observed in Golgi-stained brain tissue from adult mutant mice. Given increasing evidence linking psychiatric pathophysiology to these subneuronal sites and structures, our findings underscore the relevance of core PCP proteins including Vangl2 to the underlying biology of major mental illnesses and their treatment.

Combined chronic blockade of hyper-active L-type calcium channels and NMDA receptors ameliorates HIV-1 associated hyper-excitability of mPFC pyramidal neurons

Human Immunodeficiency Virus type 1 (HIV-1) infection induces neurological and neuropsychological deficits, which are associated with dysregulation of the medial prefrontal cortex (mPFC) and other vulnerable brain regions. We evaluated the impact of HIV infection in the mPFC and the therapeutic potential of targeting over-active voltage-gated L-type Ca(2+) channels (L-channel) and NMDA receptors (NMDAR), as modeled in HIV-1 transgenic (Tg) rats. Whole-cell patch-clamp recording was used to assess the membrane properties and voltage-sensitive Ca(2+) potentials (Ca(2+) influx) in mPFC pyramidal neurons. Neurons from HIV-1 Tg rats displayed reduced rheobase, spike amplitude and inwardly-rectifying K(+) influx, increased numbers of action potentials, and a trend of aberrant firing compared to those from non-Tg control rats. Neuronal hyper-excitation was associated with abnormally-enhanced Ca(2+) influx (independent of NMDAR), which was eliminated by acute L-channel blockade. Combined chronic blockade of over-active L-channels and NMDARs with open-channel blockers abolished HIV effects on spiking, aberrant firing and Ca(2+) potential half-amplitude duration, though not the reduced inward rectification. In contrast, individual chronic blockade of over-active L-channels or NMDARs did not alleviate HIV-induced mPFC hyper-excitability. These studies demonstrate that HIV alters mPFC neuronal activity by dysregulating membrane excitability and Ca(2+) influx through the L-channels. This renders these neurons more susceptible and vulnerable to excitatory stimuli, and could contribute to HIV-associated neuropathogenesis. Combined targeting of over-active L-channels/NMDARs alleviates HIV-induced dysfunction of mPFC pyramidal neurons, emphasizing a potential novel therapeutic strategy that may effectively decrease HIV-induced Ca(2+) dysregulation in the mPFC.

RNA-seq analysis of developing olfactory bulb projection neurons

Transmission of olfactory information to higher brain regions is mediated by olfactory bulb (OB) projection neurons, the mitral and tufted cells. Although mitral/tufted cells are often characterized as the OB counterpart of cortical projection neurons (also known as pyramidal neurons), they possess several unique morphological characteristics and project specifically to the olfactory cortices. Moreover, the molecular networks contributing to the generation of mitral/tufted cells during development are largely unknown. To understand the developmental patterns of gene expression in mitral/tufted cells in the OB, we performed transcriptome analyses targeting purified OB projection neurons at different developmental time points with next-generation RNA sequencing (RNA-seq). Through these analyses, we found 1202 protein-coding genes that are temporally differentially-regulated in developing projection neurons. Among them, 388 genes temporally changed their expression level only in projection neurons. The data provide useful resource to study the molecular mechanisms regulating development of mitral/tufted cells. We further compared the gene expression profiles of developing mitral/tufted cells with those of three cortical projection neuron subtypes, subcerebral projection neurons, corticothalamic projection neurons, and callosal projection neurons, and found that the molecular signature of developing olfactory projection neuron bears resemblance to that of subcerebral neurons. We also identified 3422 events that change the ratio of splicing isoforms in mitral/tufted cells during maturation. Interestingly, several genes expressed a novel isoform not previously reported. These results provide us with a broad perspective of the molecular networks underlying the development of OB projection neurons.

The pervasive reduction of GABA-mediated synaptic inhibition of principal neurons in the hippocampus during status epilepticus

The goal of this study was to determine whether there are region-specific or time-dependent changes in GABA-mediated synaptic inhibition of principal neurons in the hippocampus during in vivo status epilepticus. Standard whole cell patch clamp electrophysiological techniques were used to characterize miniature inhibitory postsynaptic currents (mIPSCs) in recordings from the principal neurons (PNs) of the dentate gyrus, CA1, and CA3 in acutely-obtained hippocampal slices from control and lithium/pilocarpine-induced status epilepticus(SE)-treated animals. The reduction in mIPSC amplitude was pervasive across the 3 regions examined in hippocampal slices obtained after 60 min (late) or just 15 min after the onset of SE. The mIPSC frequency was reduced in all 3 regions after 60 min and 2 regions (dentate, CA1) after 15 min. These findings lend further support to the hypothesis that a rapid modification of the postsynaptic GABAA receptor population leads to a widespread decline in GABA-mediated inhibition that, in part, contributes to both the self-sustaining nature of SE and to the decrease in the efficacy of benzodiazepines.

The rostral migratory stream generates hippocampal CA1 pyramidal-like neurons in a novel organotypic slice co-culture model

The mouse subventricular zone (SVZ) generates large numbers of neuroblasts, which migrate in a distinct pathway, the rostral migratory stream (RMS), and replace specific interneurons in the olfactory bulb (OB). Here, we introduce an organotypic slice culture model that directly connects the RMS to the hippocampus as a new destination. RMS neuroblasts widely populate the hippocampus and undergo cellular differentiation. We demonstrate that RMS cells give rise to various neuronal subtypes and, surprisingly, to CA1 pyramidal neurons. Pyramidal neurons are typically generated before birth and are lost in various neurological disorders. Hence, this unique slice culture model enables us to investigate their postnatal genesis under defined in vitro conditions from the RMS, an unanticipated source for hippocampal pyramidal neurons.

Contribution of CA3 and CA1 pyramidal neurons to the tonic α7 nAChR-dependent glutamatergic input to CA1 pyramidal neurons

The Schaffer collaterals are among the major glutamatergic inputs to CA1 pyramidal neurons, the primary output of the hippocampus, which also receive sparse recurrent inputs from pyramidal neurons in the CA1 field. Although tonically active α7 nicotinic acetylcholine receptors (nAChRs) have been shown to sustain spontaneous glutamate transmission to CA1 pyramidal neurons in hippocampal slices under resting conditions, it remains to be determined whether these receptors are those expressed by CA3 or CA1 pyramidal neurons. This study was designed to test the hypothesis that the CA3 field of the hippocampus is a significant source of α7 nAChR-sustained glutamatergic transmission to CA1 pyramidal neurons. To this end, spontaneous excitatory postsynaptic currents (EPSCs) were recorded from CA1 and CA3 pyramidal neurons in intact rat hippocampal slices as well as from CA1 pyramidal neurons in CA3-ablated slices under various experimental conditions. Surgical removal of the CA3 region from the slices reduced by 20% the frequency of spontaneous EPSCs recorded from CA1 pyramidal neurons. This finding is in agreement with the concept that the CA3 field contributes significantly to the maintenance of spontaneous glutamatergic synaptic activity in CA1 pyramidal neurons. In addition, the α7 nAChR antagonist methyllycaconitine (MLA, 10nM) reduced the frequency of spontaneous EPSCs recorded from CA1 pyramidal neurons by 30% in intact slices and 12% in CA3-ablated slices. Taken together, these results demonstrate that tonically active α7 nAChRs in CA3 pyramidal neurons and/or in the Mossy fibers that innervate the CA3 pyramidal neurons do in fact contribute to the maintenance of glutamatergic synaptic activity in CA1 pyramidal neurons of hippocampal slices under resting conditions.

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 (30-80 Hz) 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.

Modulation of dendritic spine development and plasticity by BDNF and vesicular trafficking: fundamental roles in neurodevelopmental disorders associated with mental retardation and autism

The process of axonal and dendritic development establishes the synaptic circuitry of the central nervous system (CNS) and is the result of interactions between intrinsic molecular factors and the external environment. One growth factor that has a compelling function in neuronal development is the neurotrophin brain-derived neurotrophic factor (BDNF). BDNF participates in axonal and dendritic differentiation during embryonic stages of neuronal development, as well as in the formation and maturation of dendritic spines during postnatal development. Recent studies have also implicated vesicular trafficking of BDNF via secretory vesicles, and both secretory and endosomal trafficking of vesicles containing synaptic proteins, such as neurotransmitter and neurotrophin receptors, in the regulation of axonal and dendritic differentiation, and in dendritic spine morphogenesis. Several genes that are either mutated or deregulated in neurodevelopmental disorders associated with mental retardation have now been identified, and several mouse models of these disorders have been generated and characterized. Interestingly, abnormalities in dendritic and synaptic structure are consistently observed in human neurodevelopmental disorders associated with mental retardation, and in mouse models of these disorders as well. Abnormalities in dendritic and synaptic differentiation are thought to underlie altered synaptic function and network connectivity, thus contributing to the clinical outcome. Here, we review the roles of BDNF and vesicular trafficking in axonal and dendritic differentiation in the context of dendritic and axonal morphological impairments commonly observed in neurodevelopmental disorders associated with mental retardation.