GABA tonic currents and glial cells are altered during epileptogenesis in a mouse model of Dravet syndrome

Dravet Syndrome (DS) is a rare autosomic encephalopathy with epilepsy linked to Na_v1.1 channel mutations and defective GABAergic signaling. Effective therapies for this syndrome are lacking, urging a better comprehension of the mechanisms involved. In a recognized mouse model of DS, we studied GABA tonic current, a form of inhibition largely neglected in DS, in brain slices from developing mice before spontaneous seizures are reported. In neurons from the temporal cortex (TeCx) and CA1 region, GABA tonic current was reduced in DS mice compared to controls, while in the entorhinal cortex (ECx) it was not affected. In this region however allopregnanonole potentiation of GABA tonic current was reduced in DS mice, suggesting altered extrasynaptic GABA_A subunits. Using THIP as a selective agonist, we found reduced δ subunit mediated tonic currents in ECx of DS mice. Unexpectedly in the dentate gyrus (DG), a region with high δ subunit expression, THIP-evoked currents in DS mice were larger than in controls. An immunofluorescence study confirmed that δ subunit expression was reduced in ECx and increased in DG of DS mice. Finally, considering the importance of neuroinflammation in epilepsy and neurodevelopmental disorders, we evaluated classical markers of glia activation. Our results show that DS mice have increased Iba1 reactivity and GFAP expression in both ECx and DG, compared to controls. Altogether we report that before spontaneous seizures, DS mice develop significant alterations of GABA tonic currents and glial cell activation. Understanding all the mechanisms involved in these alterations during disease maturation and progression may unveil new therapeutic targets.

Interneuron-specific signaling evokes distinctive somatostatin-mediated responses in adult cortical astrocytes

The signaling diversity of GABAergic interneurons to post-synaptic neurons is crucial to generate the functional heterogeneity that characterizes brain circuits. Whether this diversity applies to other brain cells, such as the glial cells astrocytes, remains unexplored. Using optogenetics and two-photon functional imaging in the adult mouse neocortex, we here reveal that parvalbumin- and somatostatin-expressing interneurons, two key interneuron classes in the brain, differentially signal to astrocytes inducing weak and robust GABA_B receptor-mediated Ca²⁺ elevations, respectively. Furthermore, the astrocyte response depresses upon parvalbumin interneuron repetitive stimulations and potentiates upon somatostatin interneuron repetitive stimulations, revealing a distinguished astrocyte plasticity. Remarkably, the potentiated response crucially depends on the neuropeptide somatostatin, released by somatostatin interneurons, which activates somatostatin receptors at astrocytic processes. Our study unveils, in the living brain, a hitherto unidentified signaling specificity between interneuron subtypes and astrocytes opening a new perspective into the role of astrocytes as non-neuronal components of inhibitory circuits.

Unaltered Network Activity and Interneuronal Firing During Spontaneous Cortical Dynamics In Vivo in a Mouse Model of Severe Myoclonic Epilepsy of Infancy

Severe myoclonic epilepsy of infancy (SMEI) is associated with loss of function of the SCN1A gene encoding the Na_V1.1 sodium channel isoform. Previous studies in Scn1a^−/+ mice during the pre-epileptic period reported selective reduction in interneuron excitability and proposed this as the main pathological mechanism underlying SMEI. Yet, the functional consequences of this interneuronal dysfunction at the circuit level in vivo are unknown. Here, we investigated whether Scn1a^−/+ mice showed alterations in cortical network function. We found that various forms of spontaneous network activity were similar in Scn1a^−/+ during the pre-epileptic period compared with wild-type (WT) in vivo. Importantly, in brain slices from Scn1a^−/+ mice, the excitability of parvalbumin (PV) and somatostatin (SST) interneurons was reduced, epileptiform activity propagated more rapidly, and complex synaptic changes were observed. However, in vivo, optogenetic reduction of firing in PV or SST cells in WT mice modified ongoing network activities, and juxtasomal recordings from identified PV and SST interneurons showed unaffected interneuronal firing during spontaneous cortical dynamics in Scn1a^−/+ compared with WT. These results demonstrate that interneuronal hypoexcitability is not observed in Scn1a^−/+ mice during spontaneous activities in vivo and suggest that additional mechanisms may contribute to homeostatic rearrangements and the pathogenesis of SMEI.

The inhibitory neurotransmitter GABA evokes long-lasting Ca(2+) oscillations in cortical astrocytes

Studies over the last decade provided evidence that in a dynamic interaction with neurons glial cell astrocytes contribut to fundamental phenomena in the brain. Most of the knowledge on this derives, however, from studies monitoring the astrocyte Ca²⁺ response to glutamate. Whether astrocytes can similarly respond to other neurotransmitters, including the inhibitory neurotransmitter GABA, is relatively unexplored. By using confocal and two photon laser‐scanning microscopy the astrocyte response to GABA in the mouse somatosensory and temporal cortex was studied. In slices from developing (P15‐20) and adult (P30‐60) mice, it was found that in a subpopulation of astrocytes GABA evoked somatic Ca²⁺ oscillations. This response was mediated by GABA_B receptors and involved both G_i/o protein and inositol 1,4,5‐trisphosphate (IP₃) signalling pathways. In vivo experiments from young adult mice, revealed that also cortical astrocytes in the living brain exibit GABA_B receptor‐mediated Ca²⁺ elevations. At all astrocytic processes tested, local GABA or Baclofen brief applications induced long‐lasting Ca²⁺ oscillations, suggesting that all astrocytes have the potential to respond to GABA. Finally, in patch‐clamp recordings it was found that Ca²⁺ oscillations induced by Baclofen evoked astrocytic glutamate release and slow inward currents (SICs) in pyramidal cells from wild type but not IP₃R2^−/− mice, in which astrocytic GABA_B receptor‐mediated Ca²⁺ elevations are impaired. These data suggest that cortical astrocytes in the mouse brain can sense the activity of GABAergic interneurons and through their specific recruitment contribut to the distinct role played on the cortical network by the different subsets of GABAergic interneurons. GLIA 2016;64:363–373

A brain slice experimental model to study the generation and the propagation of focally-induced epileptiform activity

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We propose a model of seizure-like events in temporal cortex slices from young mice.

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In the model local NMDA stimulations induce multiple seizure-like events.

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Electrophysiological, Ca²⁺ imaging and optogenetics are combined in the model.

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Seizures generation and propagation are separately studied.

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The role of specific cell population on these events can be accurately analyzed.

The early cellular events that in a brain network lead to seizure generation and govern seizure propagation are probably based on different cellular mechanisms. Experimental models in which these events can be separately studied would contribute to improve our understanding of epilepsy. We recently described an in vitro model in entorhinal cortex slices from young rats in which focal seizure-like discharges (SLDs) can be induced in spatially defined regions and at predictable times by local NMDA applications performed in the presence of 4-amimopyridine (4-AP) and low extracellular Mg²⁺. Through the use of single-dual cell patch-clamp and field potential recordings, and Ca²⁺ imaging from large ensembles of neurons, interneurons and astrocytes, we here extend this model to entorhinal and temporal cortex slices of rat and mouse brain, providing evidence that multiple SLDs exhibiting the typical tonic–clonic discharge pattern can be also evoked in these cortical regions by successive NMDA applications. Importantly, the temporal cortex is more accessible to viral vector injections than the entorhinal cortex: this makes it feasible in the former region the selective expression in inhibitory interneurons or principal neurons of genetically encoded Ca²⁺ indicators (GECI) or light-gated opsins. In this model, an optogenetic approach allows to activate specific neuronal types at spatially defined locations, i.e., the focus or the propagating region, and at precise time, i.e., before or during SLD. The NMDA/4-AP model can, therefore, represent a valuable tool to gain insights into the role of specific cell populations in seizure generation, propagation and cessation.

Educational program of organ donation and transplantation at medical school

A favorable attitude of health professionals to organ donation can positively influence the decision of families of potential donors. By increasing health professionals knowledge about donation and transplantation and qualifying them to disseminate information, education has produced a positive response to increase the insufficient number of donors. Educating students early in their careers may become crucial in this setting. In order to supply the necessary information about the process of donation and transplantation, a medical school in association with the Hospital Transplant Coordination Department created an educational program of organ donation and transplantation. This course is intended for medical, biomedical, and nutrition students. The objective of our program is to supply basic knowledge about organ donation and transplantation to students of medicine, nutrition, and biomedicine and to enhance their commitment to this process. Each semester, 50 to 90 students are enrolled in the course, which involves a total of 25 hours. Various aspects are approached such as brain death, donor management, political and legal aspects of donation, and skin, lung, bone marrow, heart, pancreas, liver, and kidney transplantation. Between March 2006 and June 2007, three courses were carried out and 200 students were trained. The students evaluated the course and rated it as excellent, concluding that it contributed to their education. Their attitude toward organ donation and transplantation was strongly positive at the end of the course. This project aims to educate and stimulate students in the process of organ donation and transplantation and should be implemented in other medical schools.