Parallel Computational Subunits in Dentate Granule Cells Generate Multiple Place Fields

A fundamental question in understanding neuronal computations is how dendritic events influence the output of the neuron. Different forms of integration of neighbouring and distributed synaptic inputs, isolated dendritic spikes and local regulation of synaptic efficacy suggest that individual dendritic branches may function as independent computational subunits. In the present paper, we study how these local computations influence the output of the neuron. Using a simple cascade model, we demonstrate that triggering somatic firing by a relatively small dendritic branch requires the amplification of local events by dendritic spiking and synaptic plasticity. The moderately branching dendritic tree of granule cells seems optimal for this computation since larger dendritic trees favor local plasticity by isolating dendritic compartments, while reliable detection of individual dendritic spikes in the soma requires a low branch number. Finally, we demonstrate that these parallel dendritic computations could contribute to the generation of multiple independent place fields of hippocampal granule cells

Dendritic integration in hippocampal dentate gyrus granule cells

Hippocampal granule cells are critical relay stations to transfer spatial information from the entorhinal cortex into the hippocampus proper. Therefore, the integrative properties of the small-caliber granule cell dendrites were examined in this thesis, using a combination of dual somato-dendritic patch-clamp recordings and two-photon glutamate uncaging. These experiments revealed unusual integrative properties that differ substantially from other principal neurons. Due to a strong dendritic voltage attenuation, the impact of individual synapses on granule cell output is low. At the same time, integration is linear, only weakly affected by input synchrony, and is independent of the spatial location of input sites. These integrative properties can enhance contrast in the generation of place-specific firing maps from entorhinal inputs and contribute to the sparse representation of space in the dentate gyrus

Fast-spiking parvalbumin^+ GABAergic interneurons: From cellular design to microcircuit function

The success story of fast-spiking, parvalbumin-positive (PV+) GABAergic interneurons (GABA, γ-aminobutyric acid) in the mammalian central nervous system is noteworthy. In 1995, the properties of these interneurons were completely unknown. Twenty years later, thanks to the massive use of subcellular patch-clamp techniques, simultaneous multiple-cell recording, optogenetics, in vivo measurements, and computational approaches, our knowledge about PV+ interneurons became more extensive than for several types of pyramidal neurons. These findings have implications beyond the “small world” of basic research on GABAergic cells. For example, the results provide a first proof of principle that neuroscientists might be able to close the gaps between the molecular, cellular, network, and behavioral levels, representing one of the main challenges at the present time. Furthermore, the results may form the basis for PV+ interneurons as therapeutic targets for brain disease in the future. However, much needs to be learned about the basic function of these interneurons before clinical neuroscientists will be able to use PV+ interneurons for therapeutic purposes

Slow Inhibition and Inhibitory Recruitment in the Hippocampal Dentate Gyrus

L’hippocampe joue un rôle central dans la navigation spatiale, la mémoire et l’organisation spatio-temporelle des souvenirs. Ces fonctions sont maintenues par la capacité du gyrus denté (GD) de séparation des patrons d'activité neuronales. Le GD est situé à l’entrée de la formation hippocampique où il reconnaît la présence de nouveaux motifs parmi la densité de signaux afférant arrivant par la voie entorhinale (voie perforante). Le codage parcimonieux est la marque distinctive du GD. Ce type de codage est le résultat de la faible excitabilité intrinsèque des cellules granulaires (CGs) en combinaison avec une inhibition locale prédominante. En particulier, l’inhibition de type « feedforward » ou circuit inhibiteur antérograde, est engagée par la voie perforante en même temps que les CGs. Ainsi les interneurones du circuit antérograde fournissent des signaux GABAergique aux CGs de manière presque simultanée qu’elles reçoivent les signaux glutamatergiques. Cette thèse est centrée sur l’étude des interactions entre ces signaux excitateurs de la voie entorhinale et les signaux inhibiteurs provenant des interneurones résidant dans le GD et ceci dans le contexte du codage parcimonieux et le patron de décharge en rafale caractéristique des cellules granulaires. Nous avons adressé les relations entre les projections entorhinales et le réseau inhibitoire antérograde du GD en faisant des enregistrements électrophysiologiques des CG pendant que la voie perforante est stimulée de manière électrique ou optogénétique. Nous avons découvert un nouvel mécanisme d’inhibition qui apparait à délais dans les CGs suite à une stimulation dans les fréquences gamma. Ce mécanisme induit une hyperpolarisation de longue durée (HLD) et d’une amplitude prononce. Cette longue hyperpolarisation est particulièrement prolongée et dépasse la durée d’autres types d’inhibition transitoire lente décrits chez les CGs. L’induction de HLD crée une fenêtre temporaire de faible excitabilité suite à laquelle le patron de décharge des CGs et l’intégration d’autres signaux excitateurs sont altérés de manière transitoire. Nous avons donc conclu que l’activité inhibitrice antérograde joue un rôle central dans les processus de codage dans le GD. Cependant, alors qu’il existe une multitude d’études décrivant les interneurones qui font partie de ce circuit inhibiteur, la question de comment ces cellules sont recrutées par la voie entorhinale reste quelque peu explorée. Pour apprendre plus à ce sujet, nous avons enregistré des interneurones résidant iii dans la couche moléculaire du GD tout en stimulant la voie perforante de manière optogénétique. Cette méthode de stimulation nous a permis d’induire la libération de glutamate endogène des terminales entorhinales et ainsi d’observer le recrutement purement synaptique d’interneurones. De manière surprenante, les résultats de cette expérience démontrent un faible taux d’activation des interneurones, accompagné d’un tout aussi faible nombre total de potentiels d’action émis en réponse à la stimulation même à haute fréquence. Ce constat semble contre-intuitif étant donné qu’en générale on assume qu’une forte activité inhibitrice est requise pour le maintien du codage parcimonieux. Tout de même, l’analyse des patrons de décharge des interneurones qui ont été activés a fait ressortir la prééminence de trois grands types: décharge précoce, retardée ou régulière par rapport le début des pulses lumineux. Les résultats obtenus durant cette thèse mettent la lumière sur l’important conséquences fonctionnelles des interactions synaptique et polysynaptique de nature transitoire dans les réseaux neuronaux. Nous aimerions aussi souligner l’effet prononcé de l’inhibition à court terme du type prolongée sur l’excitabilité des neurones et leurs capacités d’émettre des potentiels d’action. De plus que cet effet est encore plus prononcé dans le cas de HLD dont la durée dépasse souvent la seconde et altère l’intégration d’autres signaux arrivants simultanément. Donc on croit que les effets de HLD se traduisent au niveau du réseaux neuronal du GD comme une composante cruciale pour le codage parcimonieux. En effet, ce type de codage semble être la marque distinctive de cette région étant donné que nous avons aussi observé un faible niveau d’activation chez les interneurones. Cependant, le manque d’activité accrue du réseau inhibiteur antérograde peut être compensé par le maintien d’un gradient GABAergique constant à travers le GD via l’alternance des trois modes de décharges des interneurones. En conclusion, il semble que le codage parcimonieux dans le GD peut être préservé même en absence d’activité soutenue du réseau inhibiteur antérograde et ceci grâce à des mécanismes alternatives d’inhibition prolongée à court terme.The hippocampus is implicated in spatial navigation, the generation and recall of memories, as well as their spatio-temporal organization. These functions are supported by the processes of pattern separation that occurs in the dentate gyrus (DG). Situated at the entry of the hippocampal formation, the DG is well placed to detect and sort novelty patterns amongst the high-density excitatory signals that arrive via the entorhinal cortex (EC). A hallmark of the DG is sparse encoding that is enabled by a combination of low intrinsic excitability of the principal cells and local inhibition. Feedforward inhibition (FFI) is recruited directly by the EC and simultaneously with the granule cells (GCs). Therefore, FFI provides fast GABA release and shapes input integration at the millisecond time scale. This thesis aimed to investigate the interplay of entorhinal excitatory signals with GCs and interneurons, from the FFI in the DG, in the framework of sparse encoding and GC’s characteristic burst firing. We addressed the long-range excitation – local inhibitory network interactions using electrophysiological recordings of GCs – while applying an electrical or optogenetic stimulation of the perforant path (PP) in the DG. We discovered and described a novel delayed-onset inhibitory post synaptic potential (IPSP) in GCs, following PP stimulation in the gamma frequency range. Most importantly, the IPSP was characterized by a large amplitude and prolonged decay, outlasting previously described slow inhibitory events in GCs. The long-lasting hyperpolarization (LLH) caused by the slow IPSPs generates a low excitability time window, alters the GCs firing pattern, and interferes with other stimuli that arrive simultaneously. FFI is therefore a key player in the computational processes that occurs in the DG. However, while many studies have been dedicated to the description of the various types of the interneurons from the FFI, the question of how these cells are synaptically recruited by the EC remains not entirely elucidated. We tackled this problem by recording from interneurons in the DG molecular layer during PP-specific optogenetic stimulation. Light-driven activation of the EC terminals enabled a purely synaptic recruitment of interneurons via endogenous glutamate release. We found that this method of stimulation recruits only a subset of interneurons. In addition, the total number of action potentials (AP) was surprisingly low even at high frequency stimulation. This result is counterintuitive, as strong and persistent inhibitory signals are assumed to restrict GC v activation and maintain sparseness. However, amongst the early firing interneurons, late and regular spiking patterns were clearly distinguishable. Interestingly, some interneurons expressed LLH similar to the GCs, arguing that it could be a commonly used mechanism for regulation of excitability across the hippocampal network. In summary, we show that slow inhibition can result in a prolonged hyperpolarization that significantly alters concurrent input’s integration. We believe that these interactions contribute to important computational processes such as sparse encoding. Interestingly, sparseness seems to be the hallmark of the DG, as we observed a rather low activation of the interneuron network as well. However, the alternating firing of ML-INs could compensate the lack of persistent activity by the continuous GABA release across the DG. Taken together these results offer an insight into a mechanism of feedforward inhibition serving as a sparse neural code generator in the DG

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

Steep, Spatially Graded Recruitment of Feedback Inhibition by Sparse Dentate Granule Cell Activity

The dentate gyrus of the hippocampus is thought to subserve important physiological functions, such as 'pattern separation'. In chronic temporal lobe epilepsy, the dentate gyrus constitutes a strong inhibitory gate for the propagation of seizure activity into the hippocampus proper. Both examples are thought to depend critically on a steep recruitment of feedback inhibition by active dentate granule cells. Here, I used two complementary experimental approaches to quantitatively investigate the recruitment of feedback inhibition in the dentate gyrus. I showed that the activity of approximately 4% of granule cells suffices to recruit maximal feedback inhibition within the local circuit. Furthermore, the inhibition elicited by a local population of granule cells is distributed non-uniformly over the extent of the granule cell layer. Locally and remotely activated inhibition differ in several key aspects, namely their amplitude, recruitment, latency and kinetic properties. Finally, I show that net feedback inhibition facilitates during repetitive stimulation. Taken together, these data provide the first quantitative functional description of a canonical feedback inhibitory microcircuit motif. They establish that sparse granule cell activity, within the range observed in-vivo, steeply recruits spatially and temporally graded feedback inhibition

Morphological Correlates Of Synaptic Plasticity After Long Term Potentiation In The Rat Hippocampus

Changes in synapse and neuronal morphology have been reported in the rat hippocampal formation after the induction of long-term potentiation (LTP) of the perforant path, although few studies have investigated such parameters in the maintenance phase of L-LTP. Moreover, the results of investigations of synaptic and neuronal morphometry changes after LTP have varied and this could be due to the methods of analysis employed, the choice of stimulation protocol and or whether an in vitro or in vivo study. This in vivo investigation applied unbiased stereological methods to examine the morphology and morphometry of perforant path-granule cell synapses, in the dentate gyrus, after the induction of LTP. Two controls were employed, the contralateral hemisphere of each animal and the inner molecular layer, where the medial perforant path has little synaptic input. Many previous studies of the first 6Omin post tetanisation have used high frequency stimulation (HFS) to induce LTP however, in this study – to determine whether changes in morphology were due to LTP per se - potentiation was induced by theta burst stimulation (TBS). 45min after the induction of LTP there were no significant differences, between hemispheres, in the mean numerical density (Nv) of axodendritic or axospinous asymmetric synapses, or the mean number of synapses per neuron in the middle molecular layer (MML) of the dentate gyrus. There were no significant differences, between potentiated and non-potentiated tissue, in the Nvs of those asymmetric synapses with perforated or concave profiles. Neither were significant differences following LTP demonstrated in the size of the postsynaptic densities of these synaptic subtypes or the volume density of apposition zone (AZ) area (Sv) of individual, or all, asymmetric axospinous synapses. However, there was a trend towards larger perforated synapses in the potentiated hemisphere and, in both hemispheres, concave and perforated synapses were larger than average. In the inner molecular layer (IML), there were no differences except for a significant decrease in the total AZ volume density in the potentiated hemisphere. This would suggest that any morphological modifications taking place in the induction phase of L-LTP may be restricted to a fraction of synapses in the MML, although perforated synapses appear to be involved. The second part of this study examined morphological correlates 24h after the induction of LTP with TBS and HFS. In the MML after induction of LTP with TBS there were significant increases in the Nv of asymmetric axodendritic synapses and the mean number of axodendritic synapses per neuron. There was an increase in the Nv of axospinous synapses and in the mean number of axospinous synapses per neuron that was not significant. This was reflected in significant increases in the total AZ Sv and in the frequency of macular synapses in the potentiated hemisphere. 24h post tetanisation with HFS, there was a significant difference in the Nv of axospinous synapses in the MML of the potentiated compared to the contralateral hemisphere. There were also significant differences in the frequency of synapses with perforated and concave profiles. There were no significant differences in synaptic morphometric parameters, between hemispheres, in the IML after either of the stimulating regimes. Results from the three animals in each group showing the greatest degree of potentiation, were pooled and demonstrated significant differences in the Nv and mean number of axospinous synapses per neuron. There was also a significant difference in the number of synapses with concave profiles but this was replicated in the IML. The effects of these morphological changes, after LTP induction, on the cellular mechanisms involved and on synaptic efficacy are discussed, and possible reasons for the variable pattern of morphology after different stimulating protocols is considered

Ablation of BAF170 in developing and postnatal dentate gyrus affects neural stem cell proliferation, differentiation, and learning

The BAF chromatin remodeling complex plays an essential role in brain development. However its function in postnatal neurogenesis in hippocampus is still unknown. Here, we show that in postnatal dentate gyrus (DG), the BAF170 subunit of the complex is expressed in radial glial-like (RGL) progenitors and in cell types involved in subsequent steps of adult neurogenesis including mature astrocytes. Conditional deletion of BAF170 during cortical late neurogenesis as well as during adult brain neurogenesis depletes the pool of RGL cells in DG, and promotes terminal astrocyte differentiation. These derangements are accompanied by distinct behavioral deficits, as reflected by an impaired accuracy of place responding in the Morris water maze test, during both hidden platform as well as reversal learning. Inducible deletion of BAF170 in DG during adult brain neurogenesis resulted in mild spatial learning deficits, having a more pronounced effect on spatial learning during the reversal test. These findings demonstrate involvement of BAF170-dependent chromatin remodeling in hippocampal neurogenesis and cognition and suggest a specific role of adult neurogenesis in DG in adaptive behavior