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Changes in neural network homeostasis trigger neuropsychiatric symptoms

The mechanisms that regulate the strength of synaptic transmission and intrinsic neuronal excitability are well characterized; however, the mechanisms that promote disease-causing neural network dysfunction are poorly defined. We generated mice with targeted neuron type-specific expression of a gain-of-function variant of the neurotransmitter receptor for glycine (GlyR) that is found in hippocampectomies from patients with temporal lobe epilepsy. In this mouse model, targeted expression of gain-of-function GlyR in terminals of glutamatergic cells or in parvalbumin-positive interneurons persistently altered neural network excitability. The increased network excitability associated with gain-of-function GlyR expression in glutamatergic neurons resulted in recurrent epileptiform discharge, which provoked cognitive dysfunction and memory deficits without affecting bidirectional synaptic plasticity. In contrast, decreased network excitability due to gain-of-function GlyR expression in parvalbumin-positive interneurons resulted in an anxiety phenotype, but did not affect cognitive performance or discriminative associative memory. Our animal model unveils neuron type-specific effects on cognition, formation of discriminative associative memory, and emotional behavior in vivo. Furthermore, our data identify a presynaptic disease-causing molecular mechanism that impairs homeostatic regulation of neural network excitability and triggers neuropsychiatric symptoms.

Partial Disinhibition Is Required for Transition of Stimulus-Induced Sharp Wave–Ripple Complexes Into Recurrent Epileptiform Discharges in Rat Hippocampal Slices

Sharp wave-ripple complexes (SPW-Rs) in the intact rodent hippocampus are characterized by slow field potential transients superimposed by close to 200-Hz ripple oscillations. Similar events have been recorded in hippocampal slices where SPW-Rs occur spontaneously or can be induced by repeated application of high-frequency stimulation, a standard protocol for induction of long-lasting long-term potentiation. Such stimulation is reminiscent of protocols used to induce kindling epilepsy and ripple oscillations may be predictive of the epileptogenic zone in temporal lobe epilepsy. In the present study, we investigated the relation between recurrent epileptiform discharges (REDs) and SPW-Rs by studying effects of partial removal of inhibition. In particular, we compared the effects of nicotine, low-dose bicuculline methiodide (BMI), and elevated extracellular potassium concentration ([K(+)](o)) on induced SPW-Rs. We show that nicotine dose-dependently transformed SPW-Rs into REDs. This transition was associated with reduced inhibitory conductance in CA3 pyramidal cells. Similar results were obtained from slices where the GABAergic conductance was reduced by application of low concentrations of BMI (1-2 μM). In contrast, sharp waves were diminished by phenobarbital. Elevating [K(+)](o) from 3 to 8.5 mM did not transform SPW-Rs into REDs but significantly increased their incidence and amplitude. Under these conditions, the equilibrium potential for inhibition was shifted in depolarizing direction, whereas inhibitory conductance was significantly increased. Interestingly, the propensity of elevated [K(+)](o) to induce seizure-like events was reduced in slices where SPW-Rs had been induced. In conclusion, recruitment of inhibitory cells during SPW-Rs may serve as a mechanism by which hyperexcitation and eventually seizure generation might be prevented.

Long-Lasting Increase of Corticosterone After Fear Memory Reactivation: Anxiolytic Effects and Network Activity Modulation in the Ventral Hippocampus

Pathological fear and anxiety can be studied, in rodents, with fear conditioning and exposure to reminder cues. These paradigms are thought to critically involve the ventral hippocampus, which also serves as key site of glucocorticoid action in the brain. Here, we demonstrate a long-lasting reduction of kainate-induced gamma oscillations in slice preparations of the ventral hippocampal area CA3, 30 days after a single fear conditioning training. Reduction of gamma power was sensitive to corticosterone application and associated with a decrease in glucocorticoid and mineralocorticoid receptor mRNA expression across strata of the ventral hippocampal CA3. A fear reactivation session 24 h after the initial conditioning normalized receptor expression levels and attenuated the corticosterone-mediated recovery of gamma oscillations. It moreover increased both baseline and stimulus-induced corticosterone plasma levels and evoked a generalization of fear memory to the background context. Reduced ventral hippocampal gamma oscillation in both fear reactivated and non-reactivated mice were associated with a decrease of anxiety-like behavior in an elevated plus maze. Taking advantage of the circadian fluctuation in corticosterone, we demonstrated the association of high endogenous basal corticosterone plasma concentrations during morning hours with reduced anxiety-like behavior in fear reactivated mice. The anxiolytic effect of the hormone was verified with local applications to the ventral hippocampus. Our data suggest that corticosterone acting on ventral hippocampal network activity has anxiolytic-like effects following fear exposure, highlighting its potential therapeutic value for anxiety disorders.

The GAD65 knock out mouse – a model for GABAergic processes in fear- and stress-induced psychopathology

The γ‐amino butyric acid (GABA) synthetic enzyme glutamic acid decarboxylase (GAD)65 is critically involved in the activity‐dependent regulation of GABAergic inhibition in the central nervous system. It is also required for the maturation of the GABAergic system during adolescence, a phase that is critical for the development of several neuropsychiatric diseases. Mice bearing a null mutation of the GAD65 gene develop hyperexcitability of the amygdala and hippocampus, and a phenotype of increased anxiety and pathological fear memory reminiscent of posttraumatic stress disorder. Although genetic association of GAD65 in human has not yet been reported, these findings are in line with observations of reduced GABAergic function in these brain regions of anxiety disorder patients. The particular value of GAD65(−/−) mice thus lies in modeling the effects of reduced GABAergic function in the mature nervous system. The expression of GAD65 and a second GAD isozyme, GAD67, are differentially regulated in response to stress in limbic brain areas suggesting that by controlling GABAergic inhibition these enzymes determine the vulnerability for the development of pathological anxiety and other stress‐induced phenotypes. In fact, we could recently show that GAD65 haplodeficiency, which results in delayed postnatal increase of GABA levels, provides resilience to juvenile‐stress‐induced anxiety to GAD65(+/−) mice thus foiling the increased fear and anxiety in homozygous GAD65(−/−) mice.

Identification of Parvalbumin Interneurons as Cellular Substrate of Fear Memory Persistence

Parvalbumin-positive (PV) basket cells provide perisomatic inhibition in the cortex and hippocampus and control generation of memory-related network activity patterns, such as sharp wave ripples (SPW-R). Deterioration of this class of fast-spiking interneurons has been observed in neuropsychiatric disorders and evidence from animal models suggests their involvement in the acquisition and extinction of fear memories. Here, we used mice with neuron type-targeted expression of the presynaptic gain-of-function glycine receptor RNA variant GlyR α3L185L to genetically enhance the network activity of PV interneurons. These mice showed reduced extinction of contextual fear memory but normal auditory cued fear memory. They furthermore displayed increase of SPW-R activity in area CA3 and CA1 and facilitated propagation of this particular network activity pattern, as determined in ventral hippocampal slice preparations. Individual freezing levels during extinction and SPW-R propagation were correlated across genotypes. The same was true for parvalbumin immunoreactivity in the ventral hippocampus, which was generally augmented in the GlyR mutant mice and correlated with individual freezing levels. Together, these results identify PV interneurons as critical cellular substrate of fear memory persistence and associated SPW-R activity in the hippocampus. Our findings may be relevant for the identification and characterization of physiological correlates for posttraumatic stress and anxiety disorders.

Adenosine A1 receptor-mediated suppression of carbamazepine-resistant seizure-like events in human neocortical slices.

The need for alternative pharmacologic strategies in treatment of epilepsies is pressing for about 30% of patients with epilepsy who do not experience satisfactory seizure control with present treatments. In temporal lobe epilepsy (TLE) even up to 80% of patients are pharmacoresistant, and surgical resection of the ictogenic tissue is only possible for a minority of TLE patients. In this study we investigate purinergic modulation of drug‐resistant seizure‐like events (SLEs) in human temporal cortex slices.

Corticosterone and corticotropin-releasing factor acutely facilitate gamma oscillations in the hippocampus in vitro.

Stressful experiences do not only cause peripheral changes in stress hormone levels, but also affect central structures such as the hippocampus, implicated in spatial orientation, stress evaluation, and learning and memory. It has been suggested that formation of memory traces is dependent on hippocampal gamma oscillations observed during alert behaviour and rapid eye movement sleep. Furthermore, during quiescent behaviour, sharp wave‐ripple (SW‐R) activity emerges. These events provide a temporal window during which reactivation of memory ensembles occur. We hypothesized that stress‐responsive modulators, such as corticosterone (CORT), corticotropin‐releasing factor (CRF) and the neurosteroid 3α, 21‐dihydroxy‐5α‐pregnan‐20‐one (THDOC) are able to modulate gamma oscillations and SW‐Rs. Using in vitro hippocampal slices, we studied acute and subacute (2 h) impact of these agents on gamma oscillations in area cornu ammonis 3 of the ventral hippocampus induced by acetylcholine (10 μm) combined with physostigmine (2 μm). CORT increased the gamma oscillations in a dose‐dependent fashion. This effect was mediated by glucocorticoid receptors. Likewise, CRF augmented gamma oscillations via CRF type 1 receptor. Lastly, THDOC was found to diminish cholinergic gamma oscillations in a dose‐dependent manner. Neither CORT, CRF nor THDOC modulated gamma power when pre‐applied for 1 h, 2 h before the induction of gamma oscillations. Interestingly, stress‐related neuromodulators had rather mild effects on spontaneous SW‐R compared with their effects on gamma oscillations. These data suggest that the alteration of hippocampal gamma oscillation strength in vitro by stress‐related agents is an acute process, permitting fast adaptation to new attention‐requiring situations in vivo.

Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience

Graphical abstract Figure. No Caption available. HighlightsChildhood stress is an important risk factor for posttraumatic stress disorder.Juvenile stress in rodents models human childhood stress.Juvenile stress induces long‐term alterations in GABAergic neurotransmission.Some GABAergic changes may relate to allostasis rather than behavioral impairments.Behavioral profiling tools allow to dissect resilience‐ and pathology‐related changes. ABSTRACT ALBRECHT, A., MÜLLER, I., ARDI, Z., ÇALIŞKAN, G., GRUBER, D., IVENS, S., SEGAL, M., BEHR, J., HEINEMANN, U., STORK, O., and RICHTER‐LEVIN, G. Neurobiological consequences of juvenile stress: A GABAergic perspective on risk and resilience. NEUROSCI BIOBEHAV REV XXX‐XXX, 2016. – Childhood adversity is among the most potent risk factors for developing mood and anxiety disorders later in life. Therefore, understanding how stress during childhood shapes and rewires the brain may optimize preventive and therapeutic strategies for these disorders. To this end, animal models of stress exposure in rodents during their post‐weaning and pre‐pubertal life phase have been developed. Such ‘juvenile stress’ has a long‐lasting impact on mood and anxiety‐like behavior and on stress coping in adulthood, accompanied by alterations of the GABAergic system within core regions for the stress processing such as the amygdala, prefrontal cortex and hippocampus. While many regionally diverse molecular and electrophysiological changes are observed, not all of them correlate with juvenile stress‐induced behavioral disturbances. It rather seems that certain juvenile stress‐induced alterations reflect the system’s attempts to maintain homeostasis and thus promote stress resilience. Analysis tools such as individual behavioral profiling may allow the association of behavioral and neurobiological alterations more clearly and the dissection of alterations related to the pathology from those related to resilience.

Differential effects of blockade of ERG channels on gamma oscillations and excitability in rat hippocampal slices

Agents such as sertindole and astemizole affect heart action by inducing long‐QT syndrome, suggesting that apart from their neuronal actions through histamine receptors, 5‐HT2 serotonin receptors and D2 dopamine receptors they also affect ether‐a‐go‐go channels and particularly ether‐a‐go‐go‐related (ERG) potassium (K+) channels, comprising the Kv11.1, Kv11.2 and Kv11.3 voltage‐gated potassium currents. Changes in ERG K+ channel expression and activity have been reported and may be linked to schizophrenia [Huffaker, S.J., Chen, J., Nicodemus, K.K., Sambataro, F., Yang, F., Mattay, V., Lipska, B.K., Hyde, T.M., Song, J., Rujescu, D., Giegling, I., Mayilyan, K., Proust, M.J., Soghoyan, A., Caforio, G., Callicott, J.H., Bertolino, A., Meyer‐Lindenberg, A., Chang, J., Ji, Y., Egan, M.F., Goldberg, T.E., Kleinman, J.E., Lu, B. & Weinberger DR. (2009). Nat. Med., 15, 509–518; Shepard, P.D., Canavier, C.C. & Levitan, E.S. (2007). Schizophr Bull., 33, 1263–1269]. We have previously shown that histamine H1 blockers augment gamma oscillations (γ) which are thought to be involved in cognition and storage of information. These effects were particularly pronounced for γ induced by acetylcholine. Here we have compared neuronal effects of three agents which interfere with ERG K+ channels. We found that astemizole and sertindole, but not the Kv11 channel blocker E4031, augmented γ induced by acetylcholine in hippocampal slices. Kainate‐induced γ were only affected by astemizole. Evoked responses induced by stratum radiatum stimulation in area CA1 revealed that only E4031 augmented stimulus‐induced synaptic potentials and neuronal excitability. Our findings suggest that Kv11 channels are involved in neuronal excitability without clear effects on γ and that the effect of astemizole is related to actions on H1 receptors.

Histaminergic modulation of acetylcholine-induced γ oscillations in rat hippocampus.

We investigated the interaction between ambient histamine and acetylcholine by studying &ggr; oscillations in rat hippocampus, induced by bath application of acetylcholine (10 µM combined with 2 µM physostigmine). The power of &ggr; was significantly increased by the H1 antagonist, fexofenadine, and H2 receptor agonist, dimaprit, and reduced by the H2 receptor antagonist, cimetidine. These effects suggest an interference with ambient histamine. Depletion of histamine from their fibers by hypoxia and blockade of histamine uptake resulted in loss of the fexofenadine-mediated and cimetidine-mediated effects on acetylcholine-induced &ggr;. We conclude that acetylcholine can cause histamine release from histaminergic fibers and thereby can influence attentional states by augmenting &ggr;. This effect is likely due to activation of H2 receptors by histamine and thereby might contribute to the previously described enhancement of working memory.