Clinical and experimental evidence indicates that inflammatory processes contribute to the pathophysiology of epilepsy, but underlying mechanisms remain mostly unknown. Using immunohistochemistry for CD45 (common leukocyte antigen) and CD3 (T-lymphocytes), we show here microglial activation and infiltration of leukocytes in sclerotic tissue from patients with mesial temporal lobe epilepsy (TLE), as well as in a model of TLE (intrahippocampal kainic acid injection), characterized by spontaneous, nonconvulsive focal seizures. Using specific markers of lymphocytes, microglia, macrophages, and neutrophils in kainate-treated mice, we investigated with pharmacological and genetic approaches the contribution of innate and adaptive immunity to kainate-induced inflammation and neurodegeneration. Furthermore, we used EEG analysis in mutant mice lacking specific subsets of lymphocytes to explore the significance of inflammatory processes for epileptogenesis. Blood-brain barrier disruption and neurodegeneration in the kainate-lesioned hippocampus were accompanied by sustained ICAM-1 upregulation, microglial cell activation, and infiltration of CD3 ϩ T-cells. Moreover, macrophage infiltration was observed, selectively in the dentate gyrus where prominent granule cell dispersion was evident. Unexpectedly, depletion of peripheral macrophages by systemic clodronate liposome administration affected granule cell survival. Neurodegeneration was aggravated in kainate-lesioned mice lacking T-and B-cells (RAG1-knock-out), because of delayed invasion by Gr-1 ϩ neutrophils. Most strikingly, these mutant mice exhibited early onset of spontaneous recurrent seizures, suggesting a strong impact of immunemediated responses on network excitability. Together, the concerted action of adaptive and innate immunity triggered locally by intrahippocampal kainate injection contributes seizure-suppressant and neuroprotective effects, shedding new light on neuroimmune interactions in temporal lobe epilepsy.

In the adult mammalian brain, neurogenesis occurs in the olfactory bulb (OB) and in the dentate gyrus (DG) of the hippocampus. Several studies have shown that multiple stages of neurogenesis are regulated by GABAergic transmission with precise spatio-temporal selectivity, and involving mechanisms common to both systems or specific only to one. In the subgranular zone (SGZ) of the DG, GABA neurotransmitter, released by a specific population of interneurons, regulates stem cell quiescence and neuronal cell fate decisions. Similarly, in the subventricular zone (SVZ), OB neuroblast production is modulated by ambient GABA. Ambient GABA, acting on extrasynaptic GABAA receptors (GABAAR), is also crucial for proper adult-born granule cell (GC) maturation and synaptic integration in the OB as well as in the DG. Throughout adult-born neuron development, various GABA receptors and receptor subunits play specific roles. Previous work has demonstrated that adult-born GCs in both the OB and the DG show a time window of increased plasticity in which adult-born cells are more prone to modification by external stimuli. One mechanism that controls this “critical period” is GABAergic modulation. Indeed, depleting the main phasic GABAergic inputs in adult-born neurons results in dramatic effects, such as reduction of spine density and dendritic branching in adult-born OB GCs. In this review, we systematically compare the role of GABAergic transmission in the regulation of adult neurogenesis between the OB and the hippocampus, focusing on the role of GABA in modulating plasticity and critical periods of adult-born neuron development. Finally, we discuss signaling pathways that might mediate some of the deficits observed upon targeted deletion of postsynaptic GABAARs in adult-born neurons.

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Alterations of neuronal activity due to changes in GABAA receptors (GABAAR) mediating tonic inhibition influence different hippocampal functions. Gabra5-null mice and α5 subunit(H105R) knock-in mice exhibit signs of hippocampal dysfunction, but are capable of improved performance in several learning and memory tasks. Accordingly, alleviating abnormal GABAergic tonic inhibition in the hippocampal formation by selective α5-GABAAR modulators represents a possible therapeutic approach for several intellectual deficit disorders. Adult neurogenesis in the dentate gyrus is an important facet of hippocampal plasticity; it is regulated by tonic GABAergic transmission, as shown by deficits in proliferation, migration, and dendritic development of adult-born neurons in Gabra4-null mice. Here, we investigated the contribution of α5-GABAARs to granule cell development, using retroviral vectors expressing eGFP for labeling precursor cells in the subgranular zone. Global α5-GABAAR knockout (α5-KO) mice showed no alterations in migration and morphological development of eGFP-positive granule cells. However, upregulation of α1 subunit-immunoreactivity was observed in the hippocampal formation and cerebral cortex. In contrast, partial gene inactivation in α5-heterozygous (α5-het) mice, as well as single-cell deletion of Gabra5 in newborn granule cells from α5-floxed mice, caused severe alterations of migration and dendrite development. In α5-het mice, retrovirally-mediated overexpression of Cdk5 resulted in normal migration and dendritic branching, suggesting that Cdk5 cooperates with α5-GABAARs to regulate neuronal development. These results show that minor imbalance of α5-GABAAR-mediated transmission may have major consequences for neuronal plasticity; and call for caution upon chronic therapeutic use of negative allosteric modulators acting at these receptors.

In adult rodent olfactory bulb, GABAergic signaling regulates migration, differentiation, and synaptic integration of newborn granule cells (GCs), migrating from the subventricular zone. Here we show that these effects depend on the formation of a postsynaptic scaffold organized by gephyrin-the main scaffolding protein of GABAergic synapses, which anchors receptors and signaling molecules to the postsynaptic density-and are regulated by the phosphorylation status of gephyrin. Using lentiviral vectors to selectively transfect adult-born GCs, we observed that overexpression of the phospho-deficient gephyrin mutant eGFP-gephyrin(S270A), which facilitates the formation of supernumerary GABAergic synapses in vitro, favors dendritic branching and the formation of transient GABAergic synapses on spines, identified by the presence of 2-GABAA Rs. In contrast, overexpression of the dominant-negative eGFP-gephyrin(L2B) (a chimera that is enzymatically active but clustering defective), curtailed dendritic growth, spine formation, and long-term survival of GCs, pointing to the essential role of gephyrin cluster formation for its function. We could exclude any gephyrin overexpression artifacts, as GCs infected with eGFP-gephyrin were comparable to those infected with eGFP alone. The opposite effects induced by the two gephyrin mutant constructs indicate that the gephyrin scaffold at GABAergic synapses orchestrates signaling cascades acting on the cytoskeleton to regulate neuronal growth and synapse formation. Specifically, gephyrin phosphorylation emerges as a novel mechanism regulating morphological differentiation and long-term survival of adult-born olfactory bulb neurons. 2 AbstractIn adult rodent olfactory bulb, GABAergic signaling regulates migration, differentiation, and synaptic integration of newborn granule cells (GCs), migrating from the subventricular zone.Here, we show that these effects depend on the formation of a postsynaptic scaffold organized by gephyrin -the main scaffolding protein of GABAergic synapses, which anchors receptors and signaling molecules to the postsynaptic density -and are regulated by the phosphorylation status of gephyrin. Using lentiviral vectors to selectively transfect adult-born GCs, we observed that overexpression of the phospho-deficient gephyrin mutant eGFPgephyrin(S270A), which facilitates the formation of supernumerary GABAergic synapses in vitro, favors dendritic branching and formation of transient GABAergic synapses on spines, identified by the presence of 2-GABA A Rs. In contrast, overexpression of the dominantnegative eGFP-gephyrin(L2B) (a chimera that is enzymatically active but clustering defective), curtailed dendritic growth, spine formation, and long-term survival of GCs, pointing to the essential role of gephyrin cluster formation for its function. We could exclude any gephyrin overexpression artefacts, as GCs infected with eGFP-gephyrin were comparable to those infected with eGFP alone. The opposite effects induced by the two gephyrin mutant constructs indi...