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Cajal–Retzius cells (CRs) are a class of transient neurons in the mammalian cortex that play a critical role in cortical development. Neocortical CRs undergo almost complete elimination in the first two postnatal weeks in rodents and the persistence of CRs during postnatal life has been detected in pathological conditions related to epilepsy. However, it is unclear whether their persistence is a cause or consequence of these diseases. To decipher the molecular mechanisms involved in CR death, we investigated the contribution of the PI3K/AKT/mTOR pathway as it plays a critical role in cell survival. We first showed that this pathway is less active in CRs after birth before massive cell death. We also explored the spatio-temporal activation of both AKT and mTOR pathways and reveal area-specific differences along both the rostro–caudal and medio–lateral axes. Next, using genetic approaches to maintain an active pathway in CRs, we found that the removal of either PTEN or TSC1, two negative regulators of the pathway, lead to differential CR survivals, with a stronger effect in the Pten model. Persistent cells in this latter mutant are still active. They express more Reelin and their persistence is associated with an increase in the duration of kainate-induced seizures in females. Altogether, we show that the decrease in PI3K/AKT/mTOR activity in CRs primes these cells to death by possibly repressing a survival pathway, with the mTORC1 branch contributing less to the phenotype.

Astroglial networks mediated by gap junction channels contribute to neurotransmission and promote neuronal coordination. Connexin 30, one of the two main astroglial gap junction forming protein, alters at the behavioral level the reactivity of mice to novel environment and at the synaptic level excitatory transmission. However, the role and function of Cx30 at the neuronal network level remain unclear. We thus investigated whether Cx30 regulates neuronal population bursts and associated convulsive behavior. We found in vivo that Cx30 is upregulated by kainate‐induced seizures and that it regulates in turn the severity of associated behavioral seizures. Using electrophysiology ex vivo, we report that Cx30 regulates aberrant network activity via control of astroglial glutamate clearance independently of gap‐junction mediated biochemical coupling. Altogether, our results indicate that astroglial Cx30 is an important player in orchestrating neuronal network activity.

Dysregulation of γ‐aminobutyric acidergic (GABAergic) transmission has been reported in lesional acquired epilepsies (gliomas, hippocampal sclerosis). We investigated its involvement in a developmental disorder, human focal cortical dysplasia (FCD), focusing on chloride regulation driving GABAergic signals. Methods In vitro recordings of 47 human cortical acute slices from 11 pediatric patients who received operations for FCD were performed on multielectrode arrays. GABAergic receptors and chloride regulators were pharmacologically modulated. Immunostaining for chloride cotransporter KCC2 and interneurons were performed on recorded slices to correlate electrophysiology and expression patterns. Results FCD slices retain intrinsic epileptogenicity. Thirty‐six of 47 slices displayed spontaneous interictal discharges, along with a pattern specific to the histological subtypes. Ictal discharges were induced in proepileptic conditions in 6 of 8 slices in the areas generating spontaneous interictal discharges, with a transition to seizure involving the emergence of preictal discharges. Interictal discharges were sustained by GABAergic signaling, as a GABAA receptor blocker stopped them in 2 of 3 slices. Blockade of NKCC1 Cl− cotransporters further controlled interictal discharges in 9 of 12 cases, revealing a Cl− dysregulation affecting actions of GABA. Immunohistochemistry highlighted decreased expression and changes in KCC2 subcellular localization and a decrease in the number of GAD67‐positive interneurons in regions generating interictal discharges. Interpretation Altered chloride cotransporter expression and changes in interneuron density in FCD may lead to paradoxical depolarization of pyramidal cells. Spontaneous interictal discharges are consequently mediated by GABAergic signals, and targeting chloride regulation in neurons may be considered for the development of new antiepileptic drugs. Sin financiación 9.037 JCR (2019) Q1, 9/204 Clinical Neurology, 18/271 Neurosciences 4.900 SJR (2019) Q1, 6/378 Neurology (clinical), 1/164 Neurology No data IDR 2019 UEM

International audience; Objective: Dysregulation of GABAergic transmission has been reported in lesional acquired epilepsies (gliomas, hippocampal sclerosis). We investigated its involvement in a developmental disorder, human Focal Cortical Dysplasia, focusing on chloride regulation driving GABAergic signals.Methods: In vitro recordings of 47 human cortical acute slices from 11 pediatric patients operated from a Focal Cortical Dysplasia were performed on Multi Electrode Array. GABAergic receptors and chloride regulators were pharmacologically modulated. Immunostaining for chloride co-transporter KCC2 and interneurons were performed on recorded slices to correlate electrophysiology and expression patterns. Results: Focal Cortical Dysplasia slices retain intrinsic epileptogenicity. 36/47 slices displayed spontaneous interictal discharges, along with a pattern specific to the histological subtypes. Ictal discharges were induced in proepileptic conditions in 6/8 slices in the areas generating spontaneous interictal discharges, with a transition to seizure involving the emergence of preictal discharges. Interictal discharges were sustained by GABAergic signaling as a GABAA receptors blocker stopped them in 2/3 slices. Blockade of NKCC1 Cl-co-transporters further controlled interictal discharges in 9/12 cases, revealing a Cl-dysregulation affecting actions of GABA. Immunohistochemistry highlighted decreased expression and changes in KCC2 sub-cellular localization and a decrease in the number of GAD67-positive interneurons in regions generating interictal discharges. Interpretation: Altered chloride cotransporters expression and changes in interneuron density in FCD may lead to paradoxical depolarization of pyramidal cells. Spontaneous interictal discharges are consequently mediated by GABAergic signals and targeting chloride regulation in neurons may be considered for the development of new antiepileptic drugs.

International audience

Dopaminergic neurons in the VTA send projections to different forebrain structures, forming a complex crucial neuromodulatory system. Prefrontal, orbitofrontal and cingulated cortices receive the most marked innervation from the VTA; projections from the VTA to the medial prefrontal cortex (mPFC) constitute a portion of the mesocortical dopamine system. Much of this connectivity is bidirectional. Furthermore, the mesocortical dopamine system is involved in a great variety of brain functions, such as working memory, attention selection and memory retrieval, because of its interconnections with brain areas processing external information as well as internal information (Miller et al., 2002). Dopaminergic afferents from the mesencephalon seem to have a crucial role in the normal development of the PFC as well as in the regulation of neuronal activity in this brain area (Lewis et al., 1998). In fact, an abnormal maturation and alterations of the mesocortical projections to the PFC have been suggested to be involved in the development of psychostimulant-induced sensitization and in the pathophysiology of several disorders, such as schizophrenia, addiction or attention- deficit/hyperactivity disorder. The analysis of the formation and development of functional connections in the Central Nervous System needs suitable model systems to understand the basic, functional mechanisms (Hofmann et al., 2004). We have reconstructed parts of the mesocorticolimbic dopaminergic system using the model of organotypic co-cultures of tissue slices from the VTA/SN-complex and the PFC. In fact, organotypic brain slices are closer to the in vivo situation than cell cultures and they represent the most intact culture system for studying cortex function in isolation (Gähwiler et al., 1997; Karpiak and Plenz, 2002; Stewart and Plenz, 2008); furthermore, by combining organotypic cultures with microelectrode array (MEA) recordings, it is possible to simultaneously and repeatedly perform extracellular, non-invasive, multi-site recordings from electrodes covering the whole preparation, and to monitor long- term processes of neurite outgrowth and development and synapse formation at a functional level (Egert et al., 1998; Beggs and Plenz, 2003 and 2004; Hofmann et al., 2004). The aim of the present study was to characterize the developmental features of the VTA/SN-PFC co-cultures maintained on multielectrode array platforms, in order to study the functionality of the neuronal projections which have been demonstrated to grow in vitro between the two areas of the cultures (Franke et al., 2003; Heine et al., 2007). Furthermore, we validated the system as an useful pharmacological tool to study the effects of different neuromodulatory substances.