Cortical oscillations implement a backbone for sampling-based computation in spiking neural networks

Brains need to deal with an uncertain world. Often, this requires visiting multiple interpretations of the available information or multiple solutions to an encountered problem. This gives rise to the so-called mixing problem: since all of these "valid" states represent powerful attractors, but between themselves can be very dissimilar, switching between such states can be difficult. We propose that cortical oscillations can be effectively used to overcome this challenge. By acting as an effective temperature, background spiking activity modulates exploration. Rhythmic changes induced by cortical oscillations can then be interpreted as a form of simulated tempering. We provide a rigorous mathematical discussion of this link and study some of its phenomenological implications in computer simulations. This identifies a new computational role of cortical oscillations and connects them to various phenomena in the brain, such as sampling-based probabilistic inference, memory replay, multisensory cue combination and place cell flickering.Comment: 30 pages, 11 figure

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

Top-down and bottom-up control of drug-induced sleep and anaesthesia

In recent decades, research has unravelled fascinating detail about the molecular mechanisms underpinning pharmacologic loss of consciousness (LOC). However, the systems-level mechanisms are far less clear. Recent genetic approaches, however, enable unprecedented dissection on neural pathways, and they are paving a way for this line of research. The focus of this thesis is to investigate the neuroanatomical substrates of commonly used drugs which reversibly render us unconscious. Zolpidem is a positive allosteric modulator (PAM) of the GABAA receptor which binds to the benzodiazepine (BZ) site. Because zolpidem binds 1-3,,2 containing GABAA receptors, which are widespread, it acts virtually everywhere. We do not know if zolpidem causes sleep by enhancing GABAergic inhibition throughout the entire brain, or if the therapeutic sleep-inducing property depends upon specific brain circuitry. 2I77 mice are devoid of zolpidem-sensitivity. But, zolpidem-sensitivity can be restored selectively in brain regions, enabling dissection of the circuitry involved in zolpidem’s effect. To isolate the therapeutic effect of zolpidem we deleted GABAA-2I77-subunits and replaced them with GABAA-2F77-subunits in HDC neurons or frontal-cortex in isolation. We were able to selectively restore zolpidem-sensitivity in target neurons. This conferred zolpidem-enhanced IPSCs locally. Compared with wild-type mice and zolpidem-insensitive 2I77lox mice, we found that GABAA-2F77 receptors in either HDC-neurons or frontal cortex alone were enough to rescue the majority of zolpidem-mediated sleep. The response in HDC-2F77 mice was similar to that of an H1-receptor antagonist. By producing a null effect in a negative-control area – the superior colliculus – we show that HDC neurons and the frontal cortex are both substrates involved in zolpidem-mediated sleep. We also investigated the role of synaptic-inhibition onto corticothalamic-neurons in anaesthetic-induced LOC and sleep-wake. To do this, we genetically ablated 2-subunits from layer-6 corticothalamic-cells by crossing Ntsr1-Cre mice with GABAA-2I77lox mice. We found this reduced isoflurane sensitivity, but left sleep-wake behaviours virtually unaffected.Open Acces

Investigations into GABAB receptor surface stability and molecular interactions.

Whereas most G-protein-coupled receptors (GPCRs) are monomeric in structure, gamma-Aminobutyric acid type B (GABAB) receptors are heterodimers of two seven transmembrane protein subunits, GABAB(1) and GABAB(2). GABAB receptor function is dependent upon the co-expression of both these proteins which, when individually expressed, are devoid of receptor activity. Desensitisation of cell surface receptors allows tissues to rapidly adjust their response to agonist. A conserved mechanism ensures that GPCR signalling is closely followed by desensitisation. This entails the phosphorylation of activated receptors enabling interaction with arrestin proteins and subsequent internalisation. It is not known if heterodimeric GABAB receptors employ this method of desensitisation. Data presented here result from experiments to determine whether GABAB receptor cell surface stability is controlled in a similar manner to that of other GPCRs. Also documented is the study of a putative interaction between the protein kinase AMPK and the GABAB(1) subunit. Initial whole cell labelling studies demonstrated that both GABAB(1) and GABAB(2) are basally phosphorylated. Dissimilar to other GPCRs, agonist did not increase levels of phosphorylation and this remained true upon overexpression of G protein receptor kinases. Because GABAB receptors lacked the internalisation promoting increase in phosphorylation upon agonist, it was predicted that they might have enhanced surface stability. This was confirmed in heterologous systems where GABAB receptors did not demonstrably internalise after agonist application even when arrestins were overexpressed. GABAB receptors in cultured cortical neurones showed a similar lack of internalisation in response to agonist. Biotinylation of neuronal surface receptors demonstrated that GABAB receptors reside for an unusually long time at the plasma membrane. Chronic agonist decreased the surface receptor half-life, but this did not correlate with an increase in internalised receptor. Interestingly, chronic agonist did not significantly reduce total receptor protein levels, suggesting GABAB receptors may not downregulate. Protein kinase A (PKA) stimulation, both exogenously and through intracellular pathways, counteracted the agonist-induced degradation and demonstrated that this particular kinase can control GABAB receptor surface numbers. Protection from degradation was correlated with increased phosphorylation at serine 892 within GABAB(2), a residue previously demonstrated to be a PKA substrate. Subsequent experiments were carried out to identify kinases capable of phosphorylating GABAB(1). Affinity purification assays isolated a kinase from brain able to interact with and phosphorylate a twenty amino acid stretch of the carboxy-terminal domain of GABAB(1). Yeast two-hybrid studies identified the catalytic subunit of AMPK as a putative interacting protein with GABAB(1). AMPK was found to phosphorylate GABAB(1) at serines 917 and 923 within the carboxy-terminal domain. Phosphorylation of serine 917 was further confirmed with a phospho-specific antibody raised to this residue. AMPK affinity purifies with GABAB(1) carboxy-terminal domain GST fusion proteins and also co-immunoprecipitates with GABAB receptors from brain. Preliminary investigations indicate that AMPK activation increases surface GABAB receptor levels in cortical neurones and may affect the protein protein interactions of GABAB(1). The results presented in this thesis suggest GABAB receptors are highly stable at the neuronal surface. The activation of PKA and AMPK may be mechanisms by which neurones are able to regulate plasma membrane GABAB receptors

The Role of the E3 Ubiquitin Ligases Nedd4-1 and Nedd4-2 in Synaptic Transmission and Plasticity

Nervenzellen sind hochspezialisierte Zellen, die an Synapsen miteinander verbunden sind, was die Übertragung von neuronalen Informationen erlaubt. Die Entwicklung von Synapsen und die Informationsverarbeitung und Gedächtnisbildung bei reifen Synapsen erfordert eine dynamische Umorganisation von neuronalen Netzwerken. Das beinhaltet die Bildung und Entfernung von Synapsen, Umsatz von synaptischen Proteinen und die Veränderung und Anpassung von synaptischer Erregungsübertragung. U. a. Ubiquitinierung, als regulatorische, posttranslationale Modifikation von Proteinen, könnte eine entscheidende Rolle für solche komplexe, synaptische Umorganisationen spielen. Nedd4-1, eine HECT-Typ E3 Ubiquitin Ligase, reguliert und fördert die Entwicklung von Nervenzellfortsätzen durch die Ubiquitinierung von Rap2. Um die Bedeutung von Nedd4-abhänginger Ubiquitinierung im entwickelten Gehirn zu untersuchen, wurden Mausmodelle generiert und analysiert, in denen Nedd4-1 und dessen nächstes Homolog Nedd4-2, speziell in Nervenzellen ausgeschaltet wurde. Ich habe herausgefunden, dass Nedd4-1 und Nedd4-2 wichtige regulatorische Proteine für die neuronale Morphogenese und die synaptische Plastizität, insbesondere die Aufrechterhaltung von LTP, darstellen. Desweiteren habe ich festgestellt, dass Synaptopodin (SYNPO), ein Prolin-reiches, Aktin-assoziiertes Protein, von Nedd4-1 und Nedd4-2 in vitro ubiquitiniert wird. Dieses Ergebnis deutet daraufhin, dass SYNPO in dem Mechanismus eine Rolle spielt, durch den Nedd4-1 und Nedd4-2 LTP aufrechterhalten. Diese Studie wirft ein neues Licht auf die funktionelle Rolle von Nedd4-abhänginger Ubiquitinierung bei höheren Funktionen des Gehirns von Säugetieren sowie der neuronalen Entwicklung

Work Toward a Theory of Brain Function

This dissertation reports research from 1971 to the present, performed in three parts. The first part arose from unilateral electrical stimulation of motivational/reward pathways in the lateral hypothalamus and brain stem of “split-brain” cats, in which the great cerebral commissures were surgically divided. This showed that motivation systems in split-brain animals exert joint influence upon learning in both of the divided cerebral hemispheres, in contrast to the separation of cognitive functions produced by commissurotomy. However, attempts to identify separate signatures of electrocortical activity associated with the diffuse motivational/alerting effects and those of the cortically lateralised processes failed to achieve this goal, and showed that an adequate model of cerebral information processing was lacking. The second part describes how this recognition of inadequacy led into computer simulations of large populations of cortical neurons – work which slowly led my colleagues and me to successful explanations of mechanisms for cortical synchrony and oscillation, and of evoked potentials and the global EEG. These results complemented the work of overseas groups led by Nunez, by Freeman, by Lopes da Silva and others, but also differed from the directions taken by these workers in certain important respects. It became possible to conceive of information transfer in the active cortex as a series of punctuated synchronous equilibria of signal exchange among cortical neurons – equilibria reached repeatedly, with sequential perturbations of the neural activity away from equilibrium caused by exogenous inputs and endogenous pulse-bursting, thus forming a basis for cognitive sequences. The third part reports how the explanation of synchrony gave rise to a new theory of the regulation of embryonic cortical growth and the emergence of mature functional connections. This work was based upon very different assumptions, and reaches very different conclusions, to that of pioneers of the field such as Hubel and Wiesel, whose ideas have dominated cortical physiology for more than fifty years. In conclusion, findings from all the stages of this research are linked together, to show they provide a sketch of the working brain, fitting within and helping to unify wider contemporary concepts of brain function

Unravelling Molecular Genetic Causes and Disease Mechanisms in Landau Kleffner Syndrome

The cost of epilepsy to an individual lies not just in the burden of having recurrent seizures but also in the condition’s neurodevelopmental, cognitive, psychological and social co-morbidities. Presently, our understanding of the pathophysiological mechanisms underlying epilepsy and its neurocognitive co-morbidities remains severely limited, translating to our current lack of targeted treatment options. This PhD study aims to better understand the pathophysiological mechanisms underlying epilepsy and its neurocognitive co-morbidities through the clinical and molecular genetic study of a cohort of patients with Landau Kleffner syndrome (LKS), an epilepsy syndrome characterised by seizures, and neurodevelopmental regression in the form of loss of speech and language skills. Patients were recruited from a database of children referred for LKS at Great Ormond Street Hospital’s Developmental Epilepsy Clinic. Clinical data was extracted through case note review. As mutations in GRIN2A, a gene encoding the N2A subunit of the Nmethyl-D-Aspartate (NMDA) receptor have previously been described in 8-20% of individuals with LKS and related disorders, recruited individuals were screened for GRIN2A mutations via Sanger Sequencing and multiplex-ligation probe amplification. Functional investigations exploring gene/protein expression, protein localisation and channel function were carried out on missense GRIN2A mutations identified. Individuals who screened negative for GRIN2A variants underwent whole exome sequencing or whole genome sequencing to identify novel genes associated with LKS. This study has drawn conclusions that LKS is a neurodevelopmental disorder and clinical features influencing prognosis include age at onset of regression, non-verbal intelligence, and the presence of motor difficulties. GRIN2A mutations are likely to lead to LKS through overall NMDA receptor loss of function effects. Nonetheless, LKS may be a complex disorder with multi-factorial or oligogenic aetiology. Lastly, the long term potentiation pathway, important for learning and memory mechanisms, features strongly in the pathogenesis of LKS