Cell-type specific inhibitory plasticity in subicular pyramidal cells

The balance between excitation and inhibition is essential to the proper function of cortical circuits. To maintain this balance during dynamic network activity, modulation of the strength of inhibitory synapses is a central requirement. In this study, we aimed to characterize perisomatic inhibition and its plasticity onto pyramidal cells (PCs) in the subiculum, the main output region of the hippocampus. We performed whole-cell patch-clamp recordings from the two main functional PC types, burst (BS) and regular spiking (RS) neurons in acute rat hippocampal slices and applied two different extracellular high-frequency stimulation paradigms: non-associative (presynaptic stimulation only) and associative stimulation (concurrent pre-and postsynaptic stimulation) to induce plasticity. Our results revealed cell type-specific differences in the expression of inhibitory plasticity depending on the induction paradigm: While associative stimulation caused robust inhibitory plasticity in both cell types, non-associative stimulation produced long-term potentiation in RS, but not in BS PCs. Analysis of paired-pulse ratio, variance of IPSPs, and postsynaptic Ca2+ buffering indicated a dominant postsynaptic calcium-dependent signaling and expression of inhibitory plasticity in both PC types. This divergence in inhibitory plasticity complements a stronger inhibition and a higher intrinsic excitability in RS as compared to BS neurons, suggesting differential involvement of the two PC types during network activation and information processing in the subiculum.Peer Reviewe

Interferon-γ acutely augments inhibition of neocortical layer 5 pyramidal neurons

BACKGROUND: Interferon-γ (IFN-γ, a type II IFN) is present in the central nervous system (CNS) under various conditions. Evidence is emerging that, in addition to its immunological role, IFN-γ modulates neuronal morphology, function, and development in several brain regions. Previously, we have shown that raising levels of IFN-β (a type I IFN) lead to increased neuronal excitability of neocortical layer 5 pyramidal neurons. Because of shared non-canonical signaling pathways of both cytokines, we hypothesized a similar neocortical role of acutely applied IFN-γ. METHODS: We used semi-quantitative RT-PCR, immunoblotting, and immunohistochemistry to analyze neuronal expression of IFN-γ receptors and performed whole-cell patch-clamp recordings in layer 5 pyramidal neurons to investigate sub- and suprathreshold excitability, properties of hyperpolarization-activated cyclic nucleotide-gated current (Ih), and inhibitory neurotransmission under the influence of acutely applied IFN-γ. RESULTS: We show that IFN-γ receptors are present in the membrane of rat's neocortical layer 5 pyramidal neurons. As expected from this and the putative overlap in IFN type I and II alternative signaling pathways, IFN-γ diminished Ih, mirroring the effect of type I IFNs, suggesting a likewise activation of protein kinase C (PKC). In contrast, IFN-γ did neither alter subthreshold nor suprathreshold neuronal excitability, pointing to augmented inhibitory transmission by IFN-γ. Indeed, IFN-γ increased electrically evoked inhibitory postsynaptic currents (IPSCs) on neocortical layer 5 pyramidal neurons. Furthermore, amplitudes of spontaneous IPSCs and miniature IPSCs were elevated by IFN-γ, whereas their frequency remained unchanged. CONCLUSIONS: The expression of IFN-γ receptors on layer 5 neocortical pyramidal neurons together with the acute augmentation of inhibition in the neocortex by direct application of IFN-γ highlights an additional interaction between the CNS and immune system. Our results strengthen our understanding of the role of IFN-γ in neocortical neurotransmission and emphasize its impact beyond its immunological properties, particularly in the pathogenesis of neuropsychiatric disorders

Loss of long-term potentiation at hippocampal output synapses in experimental temporal lobe epilepsy

Patients suffering from temporal lobe epilepsy (TLE) show severe problems in hippocampus dependent memory consolidation. Memory consolidation strongly depends on an intact dialog between the hippocampus and neocortical structures. Deficits in hippocampal signal transmission are known to provoke disturbances in memory formation. In the present study, we investigate changes of synaptic plasticity at hippocampal output structures in an experimental animal model of TLE. In pilocarpine-treated rats, we found suppressed long-term potentiation (LTP) in hippocampal and parahippocampal regions such as the subiculum and the entorhinal cortex (EC). Subsequently we focused on the subiculum, serving as the major relay station between the hippocampus proper and downstream structures. In control animals, subicular pyramidal cells express different forms of LTP depending on their intrinsic firing pattern. In line with our extracellular recordings, we could show that LTP could only be induced in a minority of subicular pyramidal neurons. We demonstrate that a well-characterized cAMP-dependent signaling pathway involved in presynaptic forms of LTP is perturbed in pilocarpine-treated animals. Our findings suggest that in TLE, disturbances of synaptic plasticity may influence the information flow between the hippocampus and the neocortex

Modulating hippocampal output in the pilocarpine model of epilepsy by beta-adrenoceptor activation

Experimentelle Modelle und aktuelle Studien legen nahe, dass epileptische Anfälle mit Störungen des adrenergen Systems im Gehirns einhergehen. Noradrenalin, welches an beta-adrenerge Rezeptoren bindet, ist für hippokampale Plastizität sowie für das hippokampale Lernen und Gedächtnis von großer Bedeutung. Die vorliegende Arbeit untersucht epilepsieinduzierte Veränderungen der noradrenergen Steuerung von hippokampalen Ausgangssignalen. Gezielt werden die funktionellen Konsequenzen synaptischer Plastizität, hervorgerufen durch beta-adrenerge Rezeptoraktivierung an CA1-Subiculum Synapsen, für die neuronale Signaltransduktion zwischen Hippocampus und parahippokampal Regionen in einem Tiermodell für Epilepsie untersucht. Wir kombinieren elektrophysiologische Methoden (Single-Cell- und Multi-Elektroden-Array Ableitungen) um zu zeigen, dass die Aktivierung von beta-adrenergen Rezeptoren eine zellspezifische Form der Langzeit-Potenzierung in subiculären Pyramidenzellen induziert und eine Verstärkung der Konnektivität zwischen Subiculum und Presubiculum, beziehungsweise Subiculum und entorhinalen Cortex nach sich zieht. Bei Tieren, die mit dem Parasympathomimetikum Pilocarpin behandelten wurden, ist die beta-adrenerge Modulation zwischen dem Hippocampus und verschiedenen parahippokampal Zielstrukturen beeinträchtigt. Die gestörte polysynaptische Transmission zwischen CA1, dem Subiculum und parahippokampalen Zielstrukturen resultiert in einer Abnahme der Langzeit-Potenzierung im Presubiculum, wohingegen die Transmission zum medialen EC intakt bleibt. Diese Beeinträchtigung der beta-Adrenorezeptor abhängigen Modulation der Informationsübertragung vom Hippocampus zu seine Zielstrukturen können zu hippocampalen Defiziten, wie Gedächtnis- und Stimmungsstörungen beitragen, die häufig bei Patienten mit Temporallappen-Epilepsie beobachtet werden.Experimental models and previous studies suggest that seizures are accompanied by disturbances in the beta-adrenergic (beta-AR) system of the brain. Norepinephrine acting via beta-ARs plays a major role in hippocampal plasticity and hippocampus-dependent learning and memory. To elucidate seizure-associated alterations in the norepinephrine-dependent encoding of hippocampal output, the present study investigates the functional consequences of the beta-AR mediated synaptic plasticity at CA1-subiculum synapses for the transduction of hippocampal output to the parahippocampal region in an animal model of epilepsy. Using combined electrophysiological (single-cell and multi-electrode array recordings) approaches, we show that activation of beta-AR induces a cell-specific form of long-term potentiation in subicular pyramidal cells that may allow a strengthening of target-specific connectivity to the presubiculum and entorhinal cortex (EC). In pilocarpine-treated animals, the beta-AR-mediated modulation of functional connectivity between the hippocampus and distinct parahippocampal target str uctures is disturbed. The attenuated long-term potentiation is associated with a disturbed polysynaptic transmission from the CA1, via the subiculum to the presubiculum, but with a preserved transmission to the medial EC. The impairment in the beta-AR-dependent modulation of information transfer from the hippocampus to its target structures may contribute to hippocampus-dependent deficits like memory impairments and mood disorders which are often observed in patients with temporal lobe epilepsy

Minimizing shrinkage of acute brain slices using metal spacers during histological embedding

The morphological structure of neurons provides the basis for their functions and is a major focus of contemporary neuroscience studies. Intracellular staining of single cells in acute slices is a well-established approach, offering high-resolution information on neuronal morphology, complementing their physiology. Despite major technical advances, however, a common histological artifact often precludes precise morphological analysis: shrinkage of the sampled tissue after embedding for microscopy. Here, we describe a new approach using a metal spacer, sandwiched between two coverslips to reduce shrinkage of whole-mount slice preparations during embedding with aqueous mounting medium under a coverslip. This approach additionally allows imaging the slices from both sides to obtain better quality images of deeper structures. We demonstrate that the use of this spacer system can efficiently and stably reduce the shrinkage of slices, whereas conventional embedding methods without spacer or with agar spacer cause severe, progressive shrinkage after embedding. We further show that the shrinkage of slices is not uniform and artifacts in morphology and anatomical parameters produced cannot be compensated using linear correction algorithms. Our study, thus, emphasizes the importance of preventing the deformation of slice preparations and offers an effective means for reducing shrinkage and associated artifacts during embedding

Hilar somatostatin interneurons contribute to synchronized GABA activity in an in vitro epilepsy model.

Epilepsy is a disorder characterized by excessive synchronized neural activity. The hippocampus and surrounding temporal lobe structures appear particularly sensitive to epileptiform activity. Somatostatin (SST)-positive interneurons within the hilar region have been suggested to gate hippocampal activity, and therefore may play a crucial role in the dysregulation of hippocampal activity. In this study, we examined SST interneuron activity in the in vitro 4-aminopyridine (4-AP) model of epilepsy. We employed a multi-disciplinary approach, combining extracellular multi-electrode array (MEA) recordings with patch-clamp recordings and optical imaging using a genetically encoded calcium sensor. We observed that hilar SST interneurons are strongly synchronized during 4-AP-induced local field potentials (LFPs), as assayed by Ca(2+) imaging as well as juxtacellular or intracellular recording. SST interneurons were particularly responsive to GABA-mediated LFPs that occurred in the absence of ionotropic glutamatergic transmission. Our results present evidence that the extensive synchronized activity of SST-expressing interneurons contribute to the generation of GABAergic LFPs in an in vitro model of temporal lobe seizures

Common genetic variation in the Angelman syndrome imprinting centre affects the imprinting of chromosome 15

Angelman syndrome (AS) is a rare neurogenetic imprinting disorder caused by the loss of function of UBE3A. In ~3-5% of AS patients, the disease is due to an imprinting defect (ID). These patients lack DNA methylation of the maternal SNRPN promotor so that a large SNRPN sense/UBE3A antisense transcript (SNHG14) is expressed, which silences UBE3A. In very rare cases, the ID is caused by a deletion of the AS imprinting centre (AS-IC). To search for sequence alterations, we sequenced this region in 168 patients without an AS-IC deletion, but did not detect any sequence alteration. However, the AS-IC harbours six common variants (five single nucleotide variants and one TATG insertion/deletion variant), which constitute five common haplotypes. To determine if any of these haplotypes is associated with an increased risk for an ID, we investigated 119 informative AS-ID trios with the transmission disequilibrium test, which is a family-based association test that measures the over-transmission of an allele or haplotype from heterozygous parents to affected offspring. By this we observed maternal over-transmission of haplotype H-AS3 (p = 0.0073). Interestingly, H-AS3 is the only haplotype that includes the TATG deletion allele. We conclude that this haplotype and possibly the TATG deletion, which removes a SOX2 binding site, increases the risk for a maternal ID and AS. Our data strengthen the notion that the AS-IC is important for establishing and/or maintaining DNA methylation at the SNRPN promotor and show that common genetic variation can affect genomic imprinting

Image_1_Cell-type specific inhibitory plasticity in subicular pyramidal cells.TIFF

The balance between excitation and inhibition is essential to the proper function of cortical circuits. To maintain this balance during dynamic network activity, modulation of the strength of inhibitory synapses is a central requirement. In this study, we aimed to characterize perisomatic inhibition and its plasticity onto pyramidal cells (PCs) in the subiculum, the main output region of the hippocampus. We performed whole-cell patch-clamp recordings from the two main functional PC types, burst (BS) and regular spiking (RS) neurons in acute rat hippocampal slices and applied two different extracellular high-frequency stimulation paradigms: non-associative (presynaptic stimulation only) and associative stimulation (concurrent pre-and postsynaptic stimulation) to induce plasticity. Our results revealed cell type-specific differences in the expression of inhibitory plasticity depending on the induction paradigm: While associative stimulation caused robust inhibitory plasticity in both cell types, non-associative stimulation produced long-term potentiation in RS, but not in BS PCs. Analysis of paired-pulse ratio, variance of IPSPs, and postsynaptic Ca2+ buffering indicated a dominant postsynaptic calcium-dependent signaling and expression of inhibitory plasticity in both PC types. This divergence in inhibitory plasticity complements a stronger inhibition and a higher intrinsic excitability in RS as compared to BS neurons, suggesting differential involvement of the two PC types during network activation and information processing in the subiculum.</p