Functional Properties of Dendritic Gap Junctions in Cerebellar Golgi Cells.

The strength and variability of electrical synaptic connections between GABAergic interneurons are key determinants of spike synchrony within neuronal networks. However, little is known about how electrical coupling strength is determined due to the inaccessibility of gap junctions on the dendritic tree. We investigated the properties of gap junctions in cerebellar interneurons by combining paired somato-somatic and somato-dendritic recordings, anatomical reconstructions, immunohistochemistry, electron microscopy, and modeling. By fitting detailed compartmental models of Golgi cells to their somato-dendritic voltage responses, we determined their passive electrical properties and the mean gap junction conductance (0.9 nS). Connexin36 immunofluorescence and freeze-fracture replica immunogold labeling revealed a large variability in gap junction size and that only 18% of the 340 channels are open in each plaque. Our results establish that the number of gap junctions per connection is the main determinant of both the strength and variability in electrical coupling between Golgi cells

Axo-axonic cells in neuropsychiatric disorders: a systematic review

Imbalance between excitation and inhibition in the cerebral cortex is one of the main theories in neuropsychiatric disorder pathophysiology. Cortical inhibition is finely regulated by a variety of highly specialized GABAergic interneuron types, which are thought to organize neural network activities. Among interneurons, axo-axonic cells are unique in making synapses with the axon initial segment of pyramidal neurons. Alterations of axo-axonic cells have been proposed to be implicated in disorders including epilepsy, schizophrenia and autism spectrum disorder. However, evidence for the alteration of axo-axonic cells in disease has only been examined in narrative reviews. By performing a systematic review of studies investigating axo-axonic cells and axo-axonic communication in epilepsy, schizophrenia and autism spectrum disorder, we outline convergent findings and discrepancies in the literature. Overall, the implication of axo-axonic cells in neuropsychiatric disorders might have been overstated. Additional work is needed to assess initial, mostly indirect findings, and to unravel how defects in axo-axonic cells translates to cortical dysregulation and, in turn, to pathological states

Maturation morpho-fonctionnelle de la synapse fibre moussue/cellule pyramidale de CA3 dans l’hippocampe

The formation of synapses follows different steps including synaptogenesis and maturation. Thesedifferent steps depend on coordinated pre- and post-synaptic assembly. Pre-synaptic proteins andionotropic glutamate receptors play a central role in these processes. During my thesis, I have beeninterested in the implication of the presynaptic protein Bassoon in the maturation of the hippocampalmossy fiber to CA3 pyramidal cell glutamatergic synapses. This synapse constitutes an attractivemodel for the study of synaptic maturation because it follows several steps of defined morphologicaland functional maturation. Bassoon in one of the first protein present at newly formed synapticcontacts. By electrophysiological approaches, we showed that Bassoon is important for theorganization of the active zone during the first two postnatal weeks.Kainate receptors play an important role in the regulation of network activity during postnataldevelopment. However, the impact of kainate receptors activation on synaptic maturation is less known.I showed a delay in functional maturation of mossy fiber synapses in mice deficient for the GluK2subunit of kainate receptors (GluK2-/-). To know if this delay is correlated to morphological alterations ofthis synapse, we setup in vivo lentiviral infections of membrane fluorescent protein (YFP) in mousepups (P1-P2). Using confocal microscopy and 3D reconstruction, we described the morphologicalmaturation of mossy fiber synapses. We were able to correlate functional and morphological maturationand our results also showed an impairment in the formation of mossy fiber synapses in GluK2-/-.Together, these data reveal the importance of synaptic activity and of the coordination of pre- and postsynaptic assembly during synaptic maturation.Les synapses se forment selon plusieurs étapes comprenant la stabilisation des contacts nouvellementformés et leur maturation. Ces différentes étapes dépendent d’une mise en place coordonnée entre laterminaison pré- et postsynaptique. Les protéines composant la présynapse et les récepteursionotropiques du glutamate ont des rôles clés dans ces processus. Lors de ma thèse, je me suisintéressé à l’implication de la protéine présynaptique Bassoon lors de la maturation des synapsesglutamatergiques entre les fibres moussues et les cellules pyramidales de CA3 dans l’hippocampe.Cette synapse constitue un modèle attractif pour l’étude de la maturation synaptique car elle suit desétapes de maturation morphologique et fonctionnelle bien définies. Bassoon est une des premièresprotéines se mettant en place au niveau des contacts synaptiques nouvellement formés. Par desapproches électrophysiologiques, nous avons montré que la protéine Bassoon était importante pourl’organisation du site de libération de neurotransmetteur durant les deux premières semaines de viepost-natale chez la souris.Les récepteurs kaïnate jouent un rôle important dans la régulation de l’activité de réseau au cours dudéveloppement post-natal. Cependant l’impact de l’activation de ces récepteurs sur la maturationsynaptique est peu connu. J’ai pu mettre en évidence un délai dans la maturation fonctionnelle de lasynapse fibre moussue/cellule pyramidale de CA3 chez les souris déficientes pour la sous-unité GluK2des récepteurs kaïnate (GluK2-/-). Afin de comprendre si ce délai de maturation fonctionnelle est corréléà un retard dans la maturation morphologique de cette synapse, nous avons mis en place desinfections de lentivirus codant pour une protéine membranaire fluorescente (YFP) chez le souriceaunouveau-né (P1-P2). A l’aide de microscopie confocale et de reconstruction en 3D, nous avons ainsi pudécrire la maturation morphologique de la synapse fibre moussue/cellule pyramidale de CA3. Cela m’aégalement permis de corréler la maturation fonctionnelle à la maturation morphologique et mesrésultats montrent également un retard dans la mise en place des synapses chez les souris GluK2-/-.L’ensemble de cette étude révèle l’importance de l’activité synaptique et de la coordination entre miseen place de la pré- et de la postsynapse au cours de la maturation synaptique

Cerebellar granule cell axons support high-dimensional representations

International audienceIn classical theories of cerebellar cortex, high-dimensional sensorimotor representations are used to separate neuronal activity patterns, improving associative learning and motor performance. Recent experimental studies suggest that cerebellar granule cell (GrC) population activity is low-dimensional. To examine sensorimotor representations from the point of view of downstream Purkinje cell ‘decoders’, we used three-dimensional acousto-optic lens two-photon microscopy to record from hundreds of GrC axons. Here we show that GrC axon population activity is high dimensional and distributed with little fine-scale spatial structure during spontaneous behaviors. Moreover, distinct behavioral states are represented along orthogonal dimensions in neuronal activity space. These results suggest that the cerebellar cortex supports high-dimensional representations and segregates behavioral state-dependent computations into orthogonal subspaces, as reported in the neocortex. Our findings match the predictions of cerebellar pattern separation theories and suggest that the cerebellum and neocortex use population codes with common features, despite their vastly different circuit structures

Axo-axonic cells in neuropsychiatric disorders: a systematic review

Imbalance between excitation and inhibition in the cerebral cortex is one of the main theories in neuropsychiatric disorder pathophysiology. Cortical inhibition is finely regulated by a variety of highly specialized GABAergic interneuron types, which are thought to organize neural network activities. Among interneurons, axo-axonic cells are unique in making synapses with the axon initial segment of pyramidal neurons. Alterations of axo-axonic cells have been proposed to be implicated in disorders including epilepsy, schizophrenia and autism spectrum disorder. However, evidence for the alteration of axo-axonic cells in disease has only been examined in narrative reviews. By performing a systematic review of studies investigating axo-axonic cells and axo-axonic communication in epilepsy, schizophrenia and autism spectrum disorder, we outline convergent findings and discrepancies in the literature. Overall, the implication of axo-axonic cells in neuropsychiatric disorders might have been overstated. Additional work is needed to assess initial, mostly indirect findings, and to unravel how defects in axo-axonic cells translates to cortical dysregulation and, in turn, to pathological states

Glutamate-Bound NMDARs Arising from <i>In Vivo</i>-like Network Activity Extend Spatio-temporal Integration in a L5 Cortical Pyramidal Cell Model

<div><p><i>In vivo</i>, cortical pyramidal cells are bombarded by asynchronous synaptic input arising from ongoing network activity. However, little is known about how such ‘background’ synaptic input interacts with nonlinear dendritic mechanisms. We have modified an existing model of a layer 5 (L5) pyramidal cell to explore how dendritic integration in the apical dendritic tuft could be altered by the levels of network activity observed <i>in vivo</i>. Here we show that asynchronous background excitatory input increases neuronal gain and extends both temporal and spatial integration of stimulus-evoked synaptic input onto the dendritic tuft. Addition of fast and slow inhibitory synaptic conductances, with properties similar to those from dendritic targeting interneurons, that provided a ‘balanced’ background configuration, partially counteracted these effects, suggesting that inhibition can tune spatio-temporal integration in the tuft. Excitatory background input lowered the threshold for NMDA receptor-mediated dendritic spikes, extended their duration and increased the probability of additional regenerative events occurring in neighbouring branches. These effects were also observed in a passive model where all the non-synaptic voltage-gated conductances were removed. Our results show that glutamate-bound NMDA receptors arising from ongoing network activity can provide a powerful spatially distributed nonlinear dendritic conductance. This may enable L5 pyramidal cells to change their integrative properties as a function of local network activity, potentially allowing both clustered and spatially distributed synaptic inputs to be integrated over extended timescales.</p></div

Stimulation of a dendritic branch triggers regenerative potentials in neighbouring branches during background synaptic input.

<p>(<b>A</b>) Apical tuft with inset showing branch 11 (red) stimulated with 30 synaptic inputs (5 ms window) and branch 12 (blue) that receives no stimulus evoked input. Inset: enlarged tuft region with overlapping branches removed for clarity. (<b>B</b>) Membrane voltage of all 28 terminal apical branches during activation of branch 11 (red trace) in the presence (upper trace) and absence (lower trace) of distributed background synaptic activity from 900 excitatory inputs. Asterisks denote additional regenerative events triggered in branches 12 and 13 in the presence of background activity. (<b>C</b>) Voltage in branch 11 (red) and branch 12 (blue) with (solid lines) and without (dashed lines) background activity. (<b>D</b>) Average number of additional regenerative events triggered in neighbouring branches (identified using a 13 mV increase above level observed in the absence of background input) during different levels of background excitatory input (10 trials per branch, per condition).</p

Background excitatory input extends the duration of NMDAR spikes.

<p>(<b>A1–4</b>) Dendritic NMDAR spikes triggered in different terminal branches (30 synapses; 100 trials) in the absence (control) and presence of 900, 1200 and 1500 background excitatory synapses. Single trials (grey) and average (solid colour). (<b>B1</b>) Average NMDAR spikes in (A) overlaid. (<b>B2</b>) Average decay time (37% of peak) of NMDAR spikes in (A). (<b>C1</b>) Cumulative distributions of decay times for control and different levels of background input. (<b>C2</b>) NMDAR spike decay time distribution in the absence (black) and presence (blue) of 1500 background synapses. (<b>D</b>) Fractional increase in average NMDAR spike decay time for excitatory background synapses containing AMPAR/NMDARs (blue) and equivalent dendritic depolarization obtained with background AMPAR-only synapses (green) or current injection (red). (<b>E</b>) Cumulative distributions of NMDAR spike decay times during depolarization mediated by AMPAR-only (green) and mixed AMPAR/NMDAR (blue) background synaptic input.</p

Background excitatory input reduces the number of nearly synchronously stimulated branches required to trigger action potentials.

<p>(<b>A</b>) Somatic voltage response to the activation of 4 apical branches stimulated with 30 nearly synchronous synapses each, in the absence (black) and presence (blue) of 1500 background excitatory synapses distributed on the apical tuft. (<b>B</b>) Probability of triggering action potentials (P(AP)) versus number of stimulated branches, in the absence (black open circles) and presence of different levels of background excitation (blue filled circles). Lines show fits to a sigmoid function. Grey line and circles show results for 15 synaptic inputs per branch and 1500 background excitatory synapses.</p