Back-Propagation of Physiological Action Potential Output in Dendrites of Slender-Tufted L5A Pyramidal Neurons

Pyramidal neurons of layer 5A are a major neocortical output type and clearly distinguished from layer 5B pyramidal neurons with respect to morphology, in vivo firing patterns, and connectivity; yet knowledge of their dendritic properties is scant. We used a combination of whole-cell recordings and Ca2+ imaging techniques in vitro to explore the specific dendritic signaling role of physiological action potential patterns recorded in vivo in layer 5A pyramidal neurons of the whisker-related ‘barrel cortex’. Our data provide evidence that the temporal structure of physiological action potential patterns is crucial for an effective invasion of the main apical dendrites up to the major branch point. Both the critical frequency enabling action potential trains to invade efficiently and the dendritic calcium profile changed during postnatal development. In contrast to the main apical dendrite, the more passive properties of the short basal and apical tuft dendrites prevented an efficient back-propagation. Various Ca2+ channel types contributed to the enhanced calcium signals during high-frequency firing activity, whereas A-type K+ and BKCa channels strongly suppressed it. Our data support models in which the interaction of synaptic input with action potential output is a function of the timing, rate and pattern of action potentials, and dendritic location

Pathophysiology of information processing in neocortical dendrites in Fragile X Syndrome

Le Syndrome de l’X Fragile (SXF) est la forme héréditaire de retard mental la plus fréquente et la cause la mieux caractérisée de troubles du spectre autistique (TSA). Elle est causée par une mutation causant l’inactivation du gène Fmr1 (codant pour la protéine FMRP). La sensibilité accrue aux stimuli sensoriels est une caractéristique importante du SXF et des TSA, mais les mécanismes sous-jacents sont encore mal compris. Nous avons constaté que la suppression du gène Fmr1 entrainait une hyperexcitabilité sensorielle dans le modèle murin du SXF. Les souris Fmr1KO nécessitaient significativement moins d'informations tactiles pour l'exploration haptique, et les représentations évoquées par les informations tactiles provenant des vibrisses dans le cortex somatosensoriel primaire (S1) se propageaient à une vitesse plus élevée chez les souris Fmr1KO par rapport aux souris témoins sauvages.Au niveau cellulaire, il a été montré que les ARNm de plusieurs sous-unités de canaux ioniques (par exemple HCN1, KCNMA1) jouant un rôle clé dans le traitement de l'information dendritique / neuronale étaient des cibles de la protéine FMRP (Liao et al, 2008; Darnell et al, 2011). Sur la base de ces observations, nous avons étudié les canalopathies comme une caractéristique importante du SXF. Nous avons testé de possibles dysfonctionnement des canaux ioniques, et leurs conséquences sur le traitement de l'information dendritique dans les neurones pyramidaux du néocortex de la couche 5 chez les souris Fmr1KO, en utilisant une combinaison d’approches électrophysiologiques et d’imagerie calcique bi-photonique. Nos résultats ont montré que les dendrites des neurones pyramidaux du S1 étaient hyperexcitables, facilitant ainsi le couplage des entrées d’information synaptique à la génération de potentiel d'action en sortie dans les neurones. Cette altération était, au moins en partie, attribuable à un dysfonctionnement des canaux Ih et BKCa et a été partiellement restaurée par l'activation pharmacologique des canaux BKCa. Ces résultats plaident en faveur d'un rôle nouveau et crucial des canalopathies dans l'expression de l'hyperexcitabilité sensorielle dans le SXF.Fragile X Syndrome (FXS) is the most common form of inherited mental retardation syndrome and most well characterized cause of Autism Spectrum Disorders (ASD), and it is caused by a silencing mutation of the gene Fmr1 (encoding the protein FMRP). Increased sensitivity to sensory stimuli is a prominent feature of FXS and ASD, but its underlying mechanisms are poorly understood. We found that deletion of the Fmr1 gene results in somatosensory hyper-excitability in a mouse model for FXS. Fmr1 knockout (Fmr1KO) mice required significantly less tactile information for haptic exploration, and touch-evoked whisker representations in the primary somatosensory cortex (S1) spread with increased velocity in Fmr1KO mice compared to wild-type control. At the cellular level, it has been shown that the mRNAs of several ion channel subunits (e.g. HCN1, KCNMA1) playing key roles in dendritic/neuronal information processing are regulated by FMRP (Liao et al., 2008; Darnell et al., 2011). Based on these observations, we investigated channelopathies as a prominent feature of FXS. We probed ion channel dysfunction, and its consequence for dendritic information processing in neocortical pyramidal neurons of layer 5 in Fmr1KO mice, using a combination of electrophysiological and 2-photon calcium imaging approaches. Our results showed that dendrites of S1 pyramidal neurons were hyper-excitable, facilitating the coupling of synaptic input to the generation of action potential output in these neurons. This defect was, at least in part, attributable to a dysfunction of Ih channels and BKCa channels and was partially rescued by pharmacological activation of BKCa channels. These findings argue for a novel and critical role for channelopathies in the expression of sensory hyper-excitability in FXS

Pathophysiology of information processing in neocortical dendrites in Fragile X Syndrome

Le Syndrome de l’X Fragile (SXF) est la forme héréditaire de retard mental la plus fréquente et la cause la mieux caractérisée de troubles du spectre autistique (TSA). Elle est causée par une mutation causant l’inactivation du gène Fmr1 (codant pour la protéine FMRP). La sensibilité accrue aux stimuli sensoriels est une caractéristique importante du SXF et des TSA, mais les mécanismes sous-jacents sont encore mal compris. Nous avons constaté que la suppression du gène Fmr1 entrainait une hyperexcitabilité sensorielle dans le modèle murin du SXF. Les souris Fmr1KO nécessitaient significativement moins d'informations tactiles pour l'exploration haptique, et les représentations évoquées par les informations tactiles provenant des vibrisses dans le cortex somatosensoriel primaire (S1) se propageaient à une vitesse plus élevée chez les souris Fmr1KO par rapport aux souris témoins sauvages.Au niveau cellulaire, il a été montré que les ARNm de plusieurs sous-unités de canaux ioniques (par exemple HCN1, KCNMA1) jouant un rôle clé dans le traitement de l'information dendritique / neuronale étaient des cibles de la protéine FMRP (Liao et al, 2008; Darnell et al, 2011). Sur la base de ces observations, nous avons étudié les canalopathies comme une caractéristique importante du SXF. Nous avons testé de possibles dysfonctionnement des canaux ioniques, et leurs conséquences sur le traitement de l'information dendritique dans les neurones pyramidaux du néocortex de la couche 5 chez les souris Fmr1KO, en utilisant une combinaison d’approches électrophysiologiques et d’imagerie calcique bi-photonique. Nos résultats ont montré que les dendrites des neurones pyramidaux du S1 étaient hyperexcitables, facilitant ainsi le couplage des entrées d’information synaptique à la génération de potentiel d'action en sortie dans les neurones. Cette altération était, au moins en partie, attribuable à un dysfonctionnement des canaux Ih et BKCa et a été partiellement restaurée par l'activation pharmacologique des canaux BKCa. Ces résultats plaident en faveur d'un rôle nouveau et crucial des canalopathies dans l'expression de l'hyperexcitabilité sensorielle dans le SXF.Fragile X Syndrome (FXS) is the most common form of inherited mental retardation syndrome and most well characterized cause of Autism Spectrum Disorders (ASD), and it is caused by a silencing mutation of the gene Fmr1 (encoding the protein FMRP). Increased sensitivity to sensory stimuli is a prominent feature of FXS and ASD, but its underlying mechanisms are poorly understood. We found that deletion of the Fmr1 gene results in somatosensory hyper-excitability in a mouse model for FXS. Fmr1 knockout (Fmr1KO) mice required significantly less tactile information for haptic exploration, and touch-evoked whisker representations in the primary somatosensory cortex (S1) spread with increased velocity in Fmr1KO mice compared to wild-type control. At the cellular level, it has been shown that the mRNAs of several ion channel subunits (e.g. HCN1, KCNMA1) playing key roles in dendritic/neuronal information processing are regulated by FMRP (Liao et al., 2008; Darnell et al., 2011). Based on these observations, we investigated channelopathies as a prominent feature of FXS. We probed ion channel dysfunction, and its consequence for dendritic information processing in neocortical pyramidal neurons of layer 5 in Fmr1KO mice, using a combination of electrophysiological and 2-photon calcium imaging approaches. Our results showed that dendrites of S1 pyramidal neurons were hyper-excitable, facilitating the coupling of synaptic input to the generation of action potential output in these neurons. This defect was, at least in part, attributable to a dysfunction of Ih channels and BKCa channels and was partially rescued by pharmacological activation of BKCa channels. These findings argue for a novel and critical role for channelopathies in the expression of sensory hyper-excitability in FXS

Graded control of climbing-fiber-mediated plasticity and learning by inhibition in the cerebellum

Purkinje cell dendrites convert excitatory climbing fiber input into signals that instruct plasticity and motor learning. Modulation of instructive signaling may increase the range in which learning is encoded, yet the mechanisms that allow for this are poorly understood. We found that optogenetic activation of molecular layer interneurons (MLIs) that inhibit Purkinje cells suppressed climbing-fiber-evoked dendritic Ca2+ spiking. Inhibitory suppression of Ca2+ spiking depended on the level of MLI activation and influenced the induction of associative synaptic plasticity, converting climbing-fiber-mediated potentiation of parallel fiber-evoked responses into depression. In awake mice, optogenetic activation of floccular climbing fibers in association with head rotation produced an adaptive increase in the vestibulo-ocular reflex (VOR). However, when climbing fibers were co-activated with MLIs, adaptation occurred in the opposite direction, decreasing the VOR. Thus, MLIs can direct a continuous spectrum of plasticity and learning through their influence on Purkinje cell dendritic Ca2+ signaling.MOE (Min. of Education, S’pore

Dendritic channelopathies contribute to neocortical and sensory hyperexcitability in Fmr1 −/y mice

International audienceHypersensitivity in response to sensory stimuli and neocortical hyperexcitability are prominent features of Fragile X Syndrome (FXS) and autism spectrum disorders, but little is known about the dendritic mechanisms underlying these phenomena. We found that the primary somatosensory neocortex (S1) was hyperexcited in response to tactile sensory stimulation in Fmr1(-/y) mice. This correlated with neuronal and dendritic hyperexcitability of S1 pyramidal neurons, which affect all major aspects of neuronal computation, from the integration of synaptic input to the generation of action potential output. Using dendritic electrophysiological recordings, calcium imaging, pharmacology, biochemistry and a computer model, we found that this defect was, at least in part, attributable to the reduction and dysfunction of dendritic h- and BKCa channels. We pharmacologically rescued several core hyperexcitability phenomena by targeting BKCa channels. Our results provide strong evidence pointing to the utility of BKCa channel openers for the treatment of the sensory hypersensitivity aspects of FXS

Chemogenetic suppression of molecular layer inhibition using DREADDs.

<p><b>(A)</b> MLIs in a <i>c-kit</i>^{<i>IRES-Cre</i>} mouse transduced with the DREADD hM4Di by Cre-dependent AAV. <b>(B)</b> Cartoon depicting the PC recording configuration. MLIs in a region of high hM4Di expression were induced to fire by extracellular electrical stimulation in the molecular layer. <b>(C)</b> Average IPSCs recorded in a PC in control and after application of CNO. Summary plot on the right shows the effect of DREADD activation on the peak IPSC amplitude in PCs (*P = 0.035, paired t-test).<b>(D)</b> MLIs in a <i>c-kit</i>^{<i>IRES-Cre</i>} mouse transduced with a bicistronic, Cre-dependent AAV containing both ChR2 and hM4Di. <b>(E)</b> Cartoon depicting the PC recording configuration. MLIs were stimulated using blue light directed towards the molecular layer. <b>(F)</b> Similar to panel C but using optogenetic (3.3 mW/mm²) rather than electrical stimulation to drive MLI inhibition onto PCs (*P = 0.005, paired t-test).</p

<i>c-kit</i>^{<i>IRES-Cre</i>} mice allow for MLI targeting in mature mice.

<p><b>(A)</b> Schematic of <i>c-kit</i>^{<i>IRES-Cre</i>} mouse generation. ML, molecular layer; PCL, Purkinje cell layer; GL, granule cell layer. <b>(B)</b> Reporter protein expression in a <i>c-kit</i>^{<i>IRES-Cre</i>}:Ai14 (<i>lox</i>-P flanked, Rosa26-tdTomato) mouse perfused at PND 38. This cross will report the entire history of Cre recombinase activity including during embryogenesis. <b>(C)</b> The cerebella from <i>c-kit</i>^{<i>IRES-Cre</i>} mice injected with Cre-reporter virus at PND 0 or PND 64. Tissue was prepared from animals 27 and 14 days after injection, respectively. Arrow points to a labeled PC in PND 0-injected cerebellum. Note the absence of Cre activity in PCs in mature animals. Scale bar in lower right applies to all images.</p

MLI transduction independent of viral promoter.

<p><b>(A)</b> Tissue from <i>c-kit</i>^{<i>IRES-Cre</i>} mice injected with AAVs (≥PND 30) using either the EF1α or hSyn promoter. Scale bars apply to all images of equivalent size. Dashed red and blue lines indicate the pia and the PCL/GL boundary, respectively. <b>(B)</b> On the left, an image from <i>c-kit</i>^{<i>IRES-Cre</i>} mice injected with AAVs using CAG promoter and co-labeled with PV antibody (low magnification view of GFP label is shown above). Arrow marks PV+ cell without GFP. On the right, quantification of GFP and PV labeling across identified cells in the molecular layer (n = 8 sections from 2 mice). <b>(C)</b> The average intensity profile (with SEM) of fluorescence in the across cortical layers from <i>c-kit</i>^{<i>IRES-Cre</i>} mice injected with Cre-reporter AAVs using either hSyn (n = 6 sections from 2 mice), CAG (n = 5 sections from 2 mice), or EF1α (n = 3 sections from 2 mice) as promoters. Note that the ML fluorescence borders the GL due to the basket cell pinceaux. <b>(D)</b> Histogram of fluorescence intensity distributions for individual cells identified from images used for panel C.</p

Sparse non-MLI targeting in <i>c-kit</i>^{<i>IRES-Cre</i>} mice.

<p><b>(A)</b> Fluorescence image from a <i>c-kit</i>^{<i>IRES-Cre</i>} mouse injected with Cre-reporter virus showing sparse Cre-activity in GoCs as indicated by co-labeling for mGluR2/3. Arrowheads point to co-labeled cells. <b>(B)</b> Same as above but using an Olig2 antibody to label oligodendrocytes. Note that sparse, co-labeled cells are found in both the molecular layer (top two arrows) as well as the GrC layer (bottom two arrows).</p