Cell type specific connections from primary motor to primary somatosensory cortex.

Anatomical studies have shown that primary somatosensory (S1) and primary motor (M1) cortices are reciprocally connected. The pathway from primary motor cortex (M1) to primary somatosensory cortex (S1) is thought to influence activity in S1 by conveying a general modulatory signal and/or a copy of the motor command. In these studies, we investigated M1 synaptic inputs to S1 by injecting an AAV virus containing channelrhodopsin-2 and a fluorescent tag into M1. Consistent with previous results, we found labeling of M1 axons within S1 that was most robust in the deep layers and in L1. We recorded in vitro from excitatory neurons and two classes of inhibitory interneurons, fast-spiking and somatostatin-expressing inhibitory interneurons. All three cell types had a high probability of receiving direct excitatory M1 input, with both excitatory and inhibitory cells in L4 being the least likely to receive input from M1. Disynaptic inhibition was observed frequently, indicating that M1 recruits substantial inhibition within S1. A subpopulation of pyramidal neurons in layers 5 and 6 received especially strong input from M1, suggesting M1 differentially contacts classes of pyramidal neurons, such as those projecting to different sensorimotor centers at cortical and subcortical levels. We tested this hypothesis by combining optogenetic techniques to specifically label M1 synaptic inputs to S1 and retrograde tracing to identify specific populations of projection neurons in infragranular layers of S1. We determined that both the intrinsic properties and the magnitude of M1 input to an S1 pyramidal neuron is highly dependent on its projection target. Overall, our results suggest that activation of M1 evokes within S1 a general increase in excitatory and inhibitory synaptic activity that could contribute in a layer-specific manner to state-dependent changes in S1. Our results further indicate that M1 may specifically engage subcircuits within S1 in order to differentially regulate particular downstream cortical and subcortical processing centers

Dendritic NMDA receptors in parvalbumin neurons enable strong and stable neuronal assemblies

Parvalbumin-expressing (PV+) GABAergic interneurons mediate feedforward and feedback inhibition and have a key role in gamma oscillations and information processing. The importance of fast synaptic recruitment, action potential initiation and repolarization, and rapid synchronous GABA release by PV+ cells is well established. In contrast, the functional significance of PV+ cell NMDA receptors (NMDARs), which generate relatively slow postsynaptic currents, is unclear. Underlining their importance, several studies implicate PV+ cell NMDAR disruption in impaired network function and circuit pathologies. Here, we show that dendritic NMDARs underlie supralinear integration of feedback excitation from local pyramidal neurons onto mouse CA1 PV+ cells. Furthermore, by incorporating NMDARs at feedback connections onto PV+ cells in spiking networks, we show that these receptors enable cooperative recruitment of PV+ interneurons, strengthening and stabilising principal cell assemblies. Failure of this phenomenon provides a parsimonious explanation for cognitive and sensory gating deficits in pathologies with impaired PV+ NMDAR signalling

Sensory information processing in mouse barrel cortex

Processazione dell'informazione sensoriale nella "barrel cortex" di top

Normal And Epilepsy-Associated Pathologic Function Of The Dentate Gyrus

The dentate gyrus plays critical roles both in cognitive processing and in regulating propagation of pathological, synchronous activity through the limbic system. The cellular and circuit mechanisms underlying these diverse functions overlap extensively. At the cellular level, the intrinsic properties of dentate granule cells combine to make these neurons fundamentally reluctant to activate, one of their hallmark traits. At the circuit level, the dentate gyrus is one of the more heavily inhibited regions of the brain, with powerful feedforward and feedback GABAergic inhibition dominating responses to afferent activation. In pathologic states such as epilepsy, disease-associated alterations within the dentate gyrus combine to compromise this circuit’s regulatory properties, culminating in a collapse of its normal function. Through the use of dynamic circuit imaging and electrophysiological brain slice recordings, pharmacology, immunohistochemistry, and a pilocarpine model of epilepsy, I characterize the emergence of dentate granule cell firing properties during brain development and then examine how the circuit’s normal activation properties become corrupted as epilepsy develops. I find that, in the perinatal brain, dentate granule cells activate in large numbers. As animals mature, these cells become less excitable and activate in extremely sparse populations in a precise, repeatable, frequency-dependent manner. This sparse activation is mediated by local circuit inhibition and not by alterations in afferent innervation of granule cells. Later, in a pilocarpine model of epilepsy, I demonstrate that normally sparse granule cell activation is massively enhanced during both epilepsy development and expression. This augmentation in excitability is mediated primarily by local disinhibition, and the mechanistic cause of this compromised inhibitory function varies over time following epileptogenic injury. My results implicate a reduction in chloride ion extrusion as a mechanism compromising inhibitory function and contributing to granule cell hyperactivation specifically during early epilepsy development. In contrast, we demonstrate that sparse dentate granule cell activation in chronically epileptic mice is rescued by glutamine application, implicating compromised GABA synthesis as a mechanism of disinhibition in chronic epilepsy. We conclude that compromised feedforward inhibition within the local circuit is the predominant mediator of the massive dentate gyrus circuit hyperactivation evident in animals during and following epilepsy development

Properties and function of somatostatin-containing inhibitory interneurons in the somatosensory cortex of the mouse

GABAergic inhibitory interneurons play a pivotal role in balancing neuronal activity in the neocortex. They can be classified into different classes according to their variable morphological, electrophysiological, and neurochemical properties, including two major groups: parvalbumin-containing (PV+), fast-spiking (FS) cells and somatostatin-containing (SOM+) cells. Using transgenic mice, we identified two subgroups, distinct by all criteria, of SOM+ cells in the somatosensory (barrel) cortex of the mouse, one (called X94) in layer 4 and 5B, and the other one (X98) in deep layers (Ma et al., 2006). We found that X98 cells were calbindin-expressing (CB+), infragranular, layer 1--targeting Martinotti cells, and had a propensity to fire low-threshold calcium spikes, whereas X94 cells did not express CB, targeted mostly layer 4, discharged in stuttering pattern and with quasi fast-spiking properties. In the barrel cortex, it was previously shown that SOM+ cells mediate disynaptic inhibition in supragranular and infragranular layers. However, the roles of layer 4 SOM+ cells remain largely unknown. We used dual whole-cell recording to elucidate the synaptic circuits in layer 4 and the function of layer 4 SOM+ cells during cortical network activities. We found that layer 4 X94 SOM+ cells received strongly facilitating excitatory input and generated relatively slow rising inhibitory postsynaptic currents (IPSCs) compared to those evoked by FS cells. Strikingly, our data showed that SOM+ cells mediated strong synaptic inhibition of FS cells with connection probability greater than 90% in layer 4, but received very little reciprocal inhibition from FS cells, and no reciprocal inhibition from other SOM+ cells. Moreover, 100% of recorded SOM+-SOM+ cell pairs were electrically coupled with higher coupling ratio compared to that of electrically coupled FS cell pairs. In order to examine the functions of SOM+ cells, we applied 0 Mg2+ artificial cerebrospinal fluid (ACSF) to induce episodes of cortical network activity and observed that, during episodes of network activity, SOM+ cells fired robustly and synchronously, and produced strong inhibition of regular-spiking (RS) excitatory cells and inhibitory FS cells, especially the latter. Taken together, our data reveal that SOM+ cells in the barrel cortex can be sub-divided into different subtypes, and that layer 4 SOM+ cells exert a powerful inhibitory effect during high frequency network activity

Hippocampal GABAergic inhibitory interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies

Dendritic integration in olfactory bulb granule cells: Thresholds for lateral inhibition and role of active conductances upon 3D multi-site photostimulation of spines using a holographic projector module

The inhibitory axonless olfactory bulb granule cells (OB GCs) form reciprocal dendrodendritic synapses with mitral and tufted cells (MCs and TCs) via large spines, mediating recurrent and lateral inhibition. Rat GC dendrites are excitable by local Na⁺ spine spikes and global Ca²⁺- and Na⁺-spikes. Since reaching global threshold potentials also represents the onset of lateral inhibition, the goal of my work was to investigate the exact transition from local to global signalling: How many spines, in which position and distribution on the dendritic tree have to be activated to trigger global spikes and what are the molecular key players, i.e. which ion channels are involved. In the first part of this study we have integrated a holographic projector into the existing commercial two-photon (2P) Galvanometer-based 2D laser scanning microscope with an uncaging unit (Uncaging: Activation of photolabile biologically inactive derivatives of neurotransmitters by photolysis), which allows the simultaneous photostimulation of several spines in three dimensions (3D) in acute brain slices. Patterned 2P photolysis via holographic illumination is a powerful method to investigate neuronal function because of its capability to emulate multiple synaptic inputs in three dimensions (3D) simultaneously. However, like any optical system, holographic projectors have a finite space-bandwidth product that restricts the spatial range of patterned illumination or field-of-view (FOV) for a desired resolution. Such trade-off between holographic FOV and resolution restricts the coverage within a limited domain of the neuron’s dendritic tree to perform highly resolved patterned 2P photolysis on individual spines. Here, we integrate a holographic projector into a commercial 2P galvanometer-based 2D scanning microscope with an uncaging unit and extend the accessible holographic FOV by using the galvanometer scanning mirrors to reposition the holographic FOV arbitrarily across the imaging FOV. The projector system utilizes the microscope’s built-in imaging functions. Stimulation positions can be selected from within an acquired 3D image stack (the volume of interest, VOI) and the holographic projector then generates 3D illumination patterns with multiple uncaging foci. The imaging FOV of our system is 800×800 μm² within which a holographic VOI of 70×70×70 μm³ can be chosen at arbitrary positions and also moved during experiments without moving the sample. We describe the design and alignment protocol as well as the custom software plugin that controls the 3D positioning of stimulation sites. We demonstrate the neurobiological application of the system by simultaneously uncaging glutamate at multiple spines within dendritic domains and consequently observing summation of postsynaptic potentials at the soma, eventually resulting in APs. At the same time, it is possible to perform 2P Ca²⁺ imaging in 2D in the dendrite and thus to monitor synaptic Ca²⁺ entry in selected spines and also local regenerative events such as dendritic APs. In the second part of this study we applied the system to study dendritic integration in GCs. Less than 10 coactive reciprocal spines were sufficient to generate diverse regional and global signals that also included local dendritic Ca²⁺- and Na⁺-spikes (D-spikes). Individual spines could sense the respective signal transitions as increments in Ca²⁺ entry. Dendritic integration was mostly linear until a few spines below global Na⁺-spike threshold, where often D-spikes set in. NMDARs strongly contributed to active integration, whereas morphological parameters barely mattered. In summary, thresholds for GC-mediated bulbar lateral inhibition are low

Hippocampal gabaergic inhibitory interneurons

This is the author accepted manuscript. The final version is available from American Physiological Society via the DOI in this record In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage-and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.National Institute of Child Health and Human Developmen

Characterization of the neocortical network excitability with multielectrode arrays: alterations in migraine and epilepsy mouse models.

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