Spike-wave discharges in absence epilepsy: segregation of electrographic components reveals distinct pathways of seizure activity.

KEY POINTS: The major electrophysiological hallmarks of absence seizures are spike and wave discharges (SWDs), consisting of a sharp spike component and a slow wave component. In a widely accepted scheme, these components are functionally coupled and reflect an iterative progression of neuronal excitation during the spike and post-excitatory silence during the wave. In a genetic rat model of absence epilepsy, local pharmacological inhibition of the centromedian thalamus (CM) selectively suppressed the spike component, leaving self-contained waves in epidural recordings. Thalamic inputs induced activity in cortical microcircuits underlying the spike component, while intracortical oscillations generated the wave component. Based on these findings, we propose a model in which oscillatory waves provide adequate time windows for integration of thalamocortical inputs and feedback responses during generation of a synchronized SWD. ABSTRACT: Spike and wave discharges (SWDs) are the electrographic hallmark of absence seizures and the major diagnostic criterion for childhood absence epilepsy (CAE). In a widely accepted scheme, the alternating sequence of spikes and waves reflects an iterative progression of neuronal excitation during the spike component and post-excitatory silence during the wave component. Here we challenge this view by showing that these two components are not necessarily coupled. In a genetic rat model of CAE, self-contained waves occurred in motor cortex in synchrony with SWDs in the somatosensory system during blockade of afferent input from the thalamus. Current-source density analyses of multi-site local field potentials (LFPs) revealed layer-specific activity, in which thalamic inputs induced a sequence of cellular-synaptic events underlying the spike component, while intracortical oscillations generated the wave component. These findings indicate novel principles of SWDs, where oscillatory cortical waves provide adequate time windows for integration of thalamocortical inputs and feedback responses during generation of seizure activity.

Environmental enrichment and physical exercise revert behavioral and electrophysiological impairments caused by reduced adult neurogenesis.

It is well known that adult neurogenesis occurs in two distinct regions, the subgranular zone of the dentate gyrus and the subventricular zone along the walls of the lateral ventricles. Until now, the contribution of these newly born neurons to behavior and cognition is still uncertain. The current study tested the functional impacts of diminished hippocampal neurogenesis on emotional and cognitive functions in transgenic Gfap-tk mice. Our results showed that anxiety-related behavior evaluated both in the elevated plus maze as well as in the open field, social interaction in the sociability test, and spatial working memory in the spontaneous alternation test were not affected. On the other hand, recognition and emotional memory in the object recognition test and contextual fear conditioning, and hippocampal long-term potentiation were impaired in transgenic mice. Furthermore, we evaluated whether environmental enrichment together with physical exercise could improve or even restore the level of adult neurogenesis, as well as the behavioral functions. Our results clearly demonstrated that environmental enrichment together with physical exercise successfully elevated the overall number of progenitor cells and young neurons in the dentate gyrus of transgenic mice. Furthermore, it led to a significant improvement in object recognition memory and contextual fear conditioning, and reverted impairments in hippocampal long-term potentiation. Thus, our results confirm the importance of adult neurogenesis for learning and memory processes and for hippocampal circuitry in general. Environmental enrichment and physical exercise beneficially influenced adult neurogenesis after it had been disrupted and most importantly recovered cognitive functions and long-term potentiation. © 2016 Wiley Periodicals, Inc.

The quality of cortical network function recovery depends on localization and degree of axonal demyelination.

Myelin loss is a severe pathological hallmark common to a number of neurodegenerative diseases, including multiple sclerosis (MS). Demyelination in the central nervous system appears in the form of lesions affecting both white and gray matter structures. The functional consequences of demyelination on neuronal network and brain function are not well understood. Current therapeutic strategies for ameliorating the course of such diseases usually focus on promoting remyelination, but the effectiveness of these approaches strongly depends on the timing in relation to the disease state. In this study, we sought to characterize the time course of sensory and behavioral alterations induced by de- and remyelination to establish a rational for the use of remyelination strategies. By taking advantage of animal models of general and focal demyelination, we tested the consequences of myelin loss on the functionality of the auditory thalamocortical system: a well-studied neuronal network consisting of both white and gray matter regions. We found that general demyelination was associated with a permanent loss of the tonotopic cortical organization in vivo, and the inability to induce tone-frequency-dependent conditioned behaviors, a status persisting after remyelination. Targeted, focal lysolecithin-induced lesions in the white matter fiber tract, but not in the gray matter regions of cortex, were fully reversible at the morphological, functional and behavioral level. These findings indicate that remyelination of white and gray matter lesions have a different functional regeneration potential, with the white matter being able to regain full functionality while cortical gray matter lesions suffer from permanently altered network function. Therefore therapeutic interventions aiming for remyelination have to consider both region- and time-dependent strategies.

Short-term depression of gap junctional coupling in reticular thalamic neurons of absence epileptic rats.

KEY POINTS: Gap junctional electrical coupling between neurons of the reticular thalamic nucleus (RTN) is critical for hypersynchrony in the thalamo-cortical network. This study investigates the role of electrical coupling in pathological rhythmogenesis in RTN neurons in a rat model of absence epilepsy. Rhythmic activation resulted in a Ca(2+) -dependent short-term depression (STD) of electrical coupling between pairs of RTN neurons in epileptic rats, but not in RTN of a non-epileptic control strain. Pharmacological blockade of gap junctions in RTN in vivo induced a depression of seizure activity. The STD of electrical coupling represents a mechanism of Ca(2+) homeostasis in RTN aimed to counteract excessive synchronization. ABSTRACT: Neurons in the reticular thalamic nucleus (RTN) are coupled by electrical synapses, which play a major role in regulating synchronous activity. This study investigates electrical coupling in RTN neurons from a rat model of childhood absence epilepsy, genetic absence epilepsy rats from Strasbourg (GAERS), compared with a non-epileptic control (NEC) strain, to assess the impact on pathophysiological rhythmogenesis. Whole-cell recordings were obtained from pairs of RTN neurons of GAERS and NEC in vitro. Coupling was determined by injection of hyperpolarizing current steps in one cell and monitoring evoked voltage responses in both activated and coupled cell. The coupling coefficient (cc) was compared under resting condition, during pharmacological interventions and repeated activation using a series of current injections. The effect of gap junctional coupling on seizure expression was investigated by application of gap junctional blockers into RTN of GAERS in vivo. At resting conditions, cc did not differ between GAERS and NEC. During repeated activation, cc declined in GAERS but not in NEC. This depression in cc was restored within 25 s and was prevented by intracellular presence of BAPTA in the activated but not in the coupled cell. Local application of gap junctional blockers into RTN of GAERS in vivo resulted in a decrease of spike wave discharge (SWD) activity. Repeated activation results in a short-term depression (STD) of gap junctional coupling in RTN neurons of GAERS, depending on intracellular Ca(2+) mechanisms in the activated cell. As blockage of gap junctions in vivo results in a decrease of SWD activity, the STD observed in GAERS is considered a compensatory mechanism, aimed to dampen SWD activity.

Neuronal correlates of sustained fear in the anterolateral part of the bed nucleus of stria terminalis.

As part of the extended amygdala network, the bed nucleus of the stria terminalis (BNST) was shown to be critically involved in processing sustained fear responses to diffuse and unpredictable threats. However, neuronal activity patterns in relation to sustained components of the fear response remain elusive, so far. We used a fear training paradigm with unpredictable pairing of conditioned and unconditioned stimuli allowing distinction between phasic and sustained components of conditioned fear, and recorded single units in the anterolateral part of the BNST (BNSTal) in freely behaving mice. An objective, non-biased cluster-analysis was performed for each identified single unit on specific waveform-, activity-, stimulus-dependent and LFP-related parameters. The analysis revealed three distinct neuronal subpopulations of biphasic-, sustained fear on- and fear off-neurons. Results show that activities of biphasic- and sustained fear on-neurons temporally coincide with the shift from phasic to sustained components of the fear response. Presentation of non-conditioned auditory stimuli resulted in a variety of neuronal responses in BNSTal with no indication of biphasic response profiles. It is suggested that fear conditioning sharpens neuronal response profiles in BNSTal with biphasic-cells signaling phasic and sustained fear. These results confirm the pivotal role of BNST in processing sustained fear on the neuronal level, thereby complementing pharmacological experimental animal and human imaging data.

Seizure Prediction in Genetic Rat Models of Absence Epilepsy: Improved Performance through Multiple-Site Cortico-Thalamic Recordings Combined with Machine Learning.

Seizure prediction is the grand challenge of epileptology. However, effort was devoted to prediction of focal seizures, while generalized seizures were regarded as stochastic events. Long-lasting local field potential (LFP) recordings containing several hundred generalized spike and wave discharges (SWDs), acquired at eight locations in the cortico-thalamic system of absence epileptic rats, were iteratively analyzed in all possible combinations of either two or three recording sites, by a wavelet-based algorithm, calculating the product of the wavelet-energy signaling increases in synchronicity. Sensitivity and false alarm rate of prediction were compared between various combinations, and wavelet spectra of true and false positive predictions were fed to a random forest machine learning algorithm to further differentiate between them. Wavelet analysis of intracortical and cortico-thalamic LFP traces showed a significantly smaller number of false alarms compared with intrathalamic combinations, while predictions based on recordings in Layers IV, V, and VI of the somatosensory-cortex significantly outreached all other combinations in terms of prediction sensitivity. In 24-h out-of-sample recordings of nine Genetic Absence Epilepsy Rats from Strasbourg (GAERS), containing diurnal fluctuations of SWD occurrence, classification of true and false positives by the trained random forest further reduced the false alarm rate by 71%, although at some trade-off between false alarms and sensitivity of prediction, as reflected in relatively low F1 score values. Results provide support for the cortical-focus theory of absence epilepsy and allow the conclusion that SWDs are predictable to some degree. The latter paves the way for the development of closed-loop SWD prediction-prevention systems. Suggestions for a possible translation to human data are outlined.

Seizure-related activity of intralaminar thalamic neurons in a genetic model of absence epilepsy.

Absence seizures are characterized by bilateral spike-and-wave discharges (SWDs) in thalamo-cortical circuits. In view of clinical studies indicating a critical involvement of intralaminar thalamic nuclei, we thought it timely to characterize the specific role and activity patterns of the respective neurons. Electrocorticographic (ECoG), intracellular, and unit activity recordings were performed in vivo from intralaminar thalamic neurons of the centrolateral (CL) and the paracentral (PC) thalamic nucleus in an established genetic rat model of absence epilepsy (WAG/Rij). Neurons in PC are depolarized to produce tonic series of action potentials at seizure-free episodes, and are rhythmically silenced concomitant with SWDs in a spike-locked manner. Rebound from spike-locked inhibition is associated with a transient increase in action potential activity. Neurons in CL possess a relatively negative membrane potential with overall low electrogenic activity at seizure-free episodes and generate burst-like discharges during SWDs that are locked to the decaying phase of the spike component on the ECoG. The SWD-locked membrane responses reverse close to the presumed chloride equilibrium potential, indicating GABA(A) receptor-mediated inhibitory postsynaptic potentials (IPSPs), with cell-type specific differences in polarity. In PC neurons, hyperpolarizing IPSPs result in spike-locked silencing of tonic firing and rebound burst discharges, while in CL neurons, IPSPs are depolarizing and trigger low-threshold burst firing likely mediated by a t-type Ca(2+) conductance. These data show a unique pattern of rhythmic SWD-locked IPSPs in PC and CL associated with paroxysms apt to impose a transient dysfunctional state to thalamo-striato-prefrontocortical networks during absence seizures.

Impaired extinction of fear and maintained amygdala-hippocampal theta synchrony in a mouse model of temporal lobe epilepsy.

PURPOSE: The relationship between epilepsy and fear has received much attention. However, seizure-modulated fear and physiologic or structural correlates have not been examined systematically, and the underlying basics of network levels remain unclear to date. Therefore, this project was set up to characterize the neurophysiologic basis of seizure-related fear and the contribution of the amygdala-hippocampus system. METHODS: The experimental strategy was composed of the following steps: (1) use of the mouse pilocarpine model of temporal lobe epilepsy (TLE); (2) behavioral analyses of anxiety states in the elevated plus maze test, light-dark avoidance test, and Pavlovian fear conditioning; and (3) probing neurophysiologic activity patterns in amygdala-hippocampal circuits in freely behaving mice. RESULTS: Our results displayed no significant differences in basic anxiety levels comparing mice that developed spontaneous recurrent seizures (SRS) and controls. Furthermore, conditioned fear memory retrieval was not influenced in SRS mice. However, during fear memory extinction, SRS mice showed an extended freezing behavior and a maintained amygdala-hippocampal theta frequency synchronization compared to controls. DISCUSSION: These results indicate specific alterations in conditioned fear behavior and related neurophysiologic activities in the amygdala-hippocampal network contributing to impaired fear memory extinction in mice with TLE. Clinically, the nonextinguished fear memories may well contribute to the experience of fear in patients with TLE.

Single-cell profiling of CNS border compartment leukocytes reveals that B cells and their progenitors reside in non-diseased meninges.

The CNS is ensheathed by the meninges and cerebrospinal fluid, and recent findings suggest that these CNS-associated border tissues have complex immunological functions. Unlike myeloid lineage cells, lymphocytes in border compartments have yet to be thoroughly characterized. Based on single-cell transcriptomics, we here identified a highly location-specific composition and expression profile of tissue-resident leukocytes in CNS parenchyma, pia-enriched subdural meninges, dura mater, choroid plexus and cerebrospinal fluid. The dura layer of the meninges contained a large population of B cells under homeostatic conditions in mice and rats. Murine dura B cells exhibited slow turnover and long-term tissue residency, and they matured in experimental neuroinflammation. The dura also contained B lineage progenitors at the pro-B cell stage typically not found outside of bone marrow, without direct influx from the periphery or the skull bone marrow. This identified the dura as an unexpected site of B cell residence and potentially of development in both homeostasis and neuroinflammation.

Deficiency of the 65 kDa isoform of glutamic acid decarboxylase impairs extinction of cued but not contextual fear memory.

Extinction procedures are clinically relevant for reducing pathological fear, and the mechanisms of fear regulation are a subject of intense research. The amygdala, hippocampus, and prefrontal cortex (PFC) have all been suggested to be key brain areas in extinction of conditioned fear. GABA has particularly been implicated in extinction learning, and the 65 kDa isoform of glutamic acid decarboxylase (GAD65) may be important in elevating GABA levels in response to environmental signals. Extinction of conditioned fear was examined in Gad65(-/-) mice while recording local field potentials from the amygdala, hippocampus, and PFC simultaneously while monitoring behavior. Gad65(-/-) mice showed generalization of cued fear, as reported previously, and impaired extinction of cued fear, such that fear remained high across extinction training. This endurance in cued fear was associated with theta frequency synchronization between the amygdala and hippocampus. Extinction of contextual fear, however, was unaltered in Gad65(-/-) mice when compared with wild-type littermates. The data imply that GAD65 plays a critical role in regulating cued fear responses during extinction learning and that, during this process, GABAergic signaling is involved in modulating synchronized activity between the amygdala and hippocampus. In view of the more pronounced effect on cued versus contextual fear extinction, these influences may rely more on GABAergic mechanisms in the amygdala.

Alteration of NMDA receptor-mediated synaptic interactions in the lateral amygdala associated with seizure activity in a mouse model of chronic temporal lobe epilepsy.

PURPOSE: Because results from both animal models and human temporal lobe epilepsy (TLE) have pointed to synaptic network alterations in the amygdala, we have tested the hypothesis that glutamatergic transmission in the lateral amygdala (LA) is critically involved. METHODS: Using the pilocarpine mouse model, LA slices were prepared ex vivo in the recurrent phase of TLE (Pilo group), and LA projection neurons (PNs) were recorded using patch-clamp techniques. Intrinsic and synaptic properties of LA PNs were analyzed and compared with those in age-matched saline-injected controls. RESULTS: Only mild changes were observed in intrinsic properties of LA PNs, whereas both spontaneous excitatory postsynaptic currents (sEPSCs) and miniature EPSCs (mEPSCs) were significantly increased in Pilo as compared to saline controls. This difference was sensitive to AP5, but persisted during action of NBQX, indicating mediation by N-methyl-d-aspartate (NMDA) receptors. Moreover, these changes were associated with an increase in frequency but not amplitude of mEPSCs, indicative of a contribution of presynaptic mechanisms. DISCUSSION: In conclusion, dynamic changes seem to occur in glutamatergic transmission within the amygdala during TLE, to which a functional upregulation of presynaptic NMDA receptors in LA PNs makes a significant contribution.

Stimulation of 5-HT receptors in anterodorsal BNST guides fear to predictable and unpredictable threat.

Through pharmacological manipulation of the serotonergic (5-Hydroxytryptamin, 5-HT) system, combined with behavioral analysis, we tested the hypothesis that fear responses to predictable and unpredictable threat are regulated through stimulation of 5-HT receptors (5-HT-R) in the anterodorsal section of the bed nucleus of the stria terminalis (adBNST). Local adBNST application of 5-HT1A-R antagonist WAY100635 and 5-HT1B-R antagonist NAS-181 before fear retrieval enhanced freezing, 24 h after predictable fear conditioning. In contrast, increased fear responses to unpredictable threat were blocked by 5-HT1A-R agonist Buspirone (given before conditioning or retrieval) and 5-HT1B-R agonist CP-94253 (applied before training). Prolonged fear responses were also blocked by local application of the 5-HT2A-R antagonist R-96544 before fear retrieval, and conversely, local application of the 5-HT2A-R agonist NBOH-2C-CN hydrochloride before fear retrieval enhanced freezing 24 h after predictable conditioning, indicating augmented fear responses. Activation of inhibitory 5-HT1A- or 5-HT1B-Rs and the blockade of the excitatory 5-HT2A-R before unpredictable fear conditioning significantly reduced freezing during retrieval. The results from this study suggest that modulation of inhibitory 5-HT1A/1B-R and/or excitatory 5-HT2A-R activity in the adBNST may represent potential targets for the development of new treatment strategies in anxiety disorders. In addition, this study supports the validity and reliability of the mouse model of modulated fear to predictable and unpredictable threats to study mechanisms of fear and anxiety in combination with pharmacological manipulations.

Mice with a targeted disruption of the Cl-/HCO3- exchanger AE3 display a reduced seizure threshold.

Neuronal activity results in significant pH shifts in neurons, glia, and interstitial space. Several transport mechanisms are involved in the fine-tuning and regulation of extra- and intracellular pH. The sodium-independent electroneutral anion exchangers (AEs) exchange intracellular bicarbonate for extracellular chloride and thereby lower the intracellular pH. Recently, a significant association was found with the variant Ala867Asp of the anion exchanger AE3, which is predominantly expressed in brain and heart, in a large cohort of patients with idiopathic generalized epilepsy. To analyze a possible involvement of AE3 dysfunction in the pathogenesis of seizures, we generated an AE3-knockout mouse model by targeted disruption of Slc4a3. AE3-knockout mice were apparently healthy, and neither displayed gross histological and behavioral abnormalities nor spontaneous seizures or spike wave complexes in electrocorticograms. However, the seizure threshold of AE3-knockout mice exposed to bicuculline, pentylenetetrazole, or pilocarpine was reduced, and seizure-induced mortality was significantly increased compared to wild-type littermates. In the pyramidal cell layer of the hippocampal CA3 region, where AE3 is strongly expressed, disruption of AE3 abolished sodium-independent chloride-bicarbonate exchange. These findings strongly support the hypothesis that AE3 modulates seizure susceptibility and, therefore, are of significance for understanding the role of intracellular pH in epilepsy.

Making translation work: Harmonizing cross-species methodology in the behavioural neuroscience of Pavlovian fear conditioning.

Translational neuroscience bridges insights from specific mechanisms in rodents to complex functions in humans and is key to advance our general understanding of central nervous function. A prime example of translational research is the study of cross-species mechanisms that underlie responding to learned threats, by employing Pavlovian fear conditioning protocols in rodents and humans. Hitherto, evidence for (and critique of) these cross-species comparisons in fear conditioning research was based on theoretical viewpoints. Here, we provide a perspective to substantiate these theoretical concepts with empirical considerations of cross-species methodology. This meta-research perspective is expected to foster cross-species comparability and reproducibility to ultimately facilitate successful transfer of results from basic science into clinical applications.

Impairment of frequency-specific responses associated with altered electrical activity patterns in auditory thalamus following focal and general demyelination.

Multiple sclerosis is characterized by intermingled episodes of de- and remyelination and the occurrence of white- and grey-matter damage. To mimic the randomly distributed pathophysiological brain lesions observed in MS, we assessed the impact of focal white and grey matter demyelination on thalamic function by directing targeted lysolecithin-induced lesions to the capsula interna (CI), the auditory cortex (A1), or the ventral medial geniculate nucleus (vMGN) in mice. Pathophysiological consequences were compared with those of cuprizone treatment at different stages of demyelination and remyelination. Combining single unit recordings and auditory stimulation in freely behaving mice revealed changes in auditory response profile and electrical activity pattern in the thalamus, depending on the region of the initial insult and the state of remyelination. Cuprizone-induced general demyelination significantly diminished vMGN neuronal activity and frequency-specific responses. Targeted lysolecithin-induced lesions directed either to A1 or to vMGN revealed a permanent impairment of frequency-specific responses, an increase in latency of auditory responses and a reduction in occurrence of burst firing in vMGN neurons. These findings indicate that demyelination of grey matter areas in the thalamocortical system permanently affects vMGN frequency specificity and the prevalence of bursting in the auditory thalamus.

Protective potential of dimethyl fumarate in a mouse model of thalamocortical demyelination.

Alterations in cortical cellular organization, network functionality, as well as cognitive and locomotor deficits were recently suggested to be pathological hallmarks in multiple sclerosis and corresponding animal models as they might occur following demyelination. To investigate functional changes following demyelination in a well-defined, topographically organized neuronal network, in vitro and in vivo, we focused on the primary auditory cortex (A1) of mice in the cuprizone model of general de- and remyelination. Following myelin loss in this model system, the spatiotemporal propagation of incoming stimuli in A1 was altered and the hierarchical activation of supra- and infragranular cortical layers was lost suggesting a profound effect exerted on neuronal network level. In addition, the response latency in field potential recordings and voltage-sensitive dye imaging was increased following demyelination. These alterations were accompanied by a loss of auditory discrimination abilities in freely behaving animals, a reduction of the nuclear factor-erythroid 2-related factor-2 (Nrf-2) protein in the nucleus in histological staining and persisted during remyelination. To find new strategies to restore demyelination-induced network alteration in addition to the ongoing remyelination, we tested the cytoprotective potential of dimethyl fumarate (DMF). Therapeutic treatment with DMF during remyelination significantly modified spatiotemporal stimulus propagation in the cortex, reduced the cognitive impairment, and prevented the demyelination-induced decrease in nuclear Nrf-2. These results indicate the involvement of anti-oxidative mechanisms in regulating spatiotemporal cortical response pattern following changes in myelination and point to DMF as therapeutic compound for intervention.

T-current related effects of antiepileptic drugs and a Ca2+ channel antagonist on thalamic relay and local circuit interneurons in a rat model of absence epilepsy.

Channel blocking, anti-oscillatory, and anti-epileptic effects of clinically used anti-absence substances (ethosuximide, valproate) and the T-type Ca2+ current (IT) blocker mibefradil were tested by analyzing membrane currents in acutely isolated local circuit interneurons and thalamocortical relay (TC) neurons, slow intrathalamic oscillations in brain slices, and spike and wave discharges (SWDs) occurring in vivo in Wistar Albino Glaxo rats from Rijswijk (WAG/Rij). Substance effects in vitro were compared between WAG/Rij and a non-epileptic control strain, the ACI rats. Ethosuximide (ETX) and valproate were found to block IT in acutely isolated thalamic neurons. Block of IT by therapeutically relevant ETX concentrations (0.25-0.75 mM) was stronger in WAG/Rij, although the maximal effect at saturating concentrations (>or=10 mM) was stronger in ACI. Ethosuximide delayed the onset of the low threshold Ca2+ spike (LTS) of neurons recorded in slice preparations. Mibefradil (>or=2 microM) completely blocked IT and the LTS, dampened evoked thalamic oscillations, and attenuated SWDs in vivo. Computational modeling demonstrated that the complete effect of ETX can be replicated by a sole reduction of IT. However, the necessary degree of IT reduction was not induced by therapeutically relevant ETX concentrations. A combined reduction of IT, the persistent sodium current, and the Ca2+ activated K+ current resulted in an LTS alteration resembling the experimental observations. In summary, these results support the hypothesis of IT reduction as part of the mechanism of action of anti-absence drugs and demonstrate the ability of a specific IT antagonist to attenuate rhythmic burst firing and SWDs.

Theta resynchronization during reconsolidation of remote contextual fear memory.

We have recently demonstrated high theta-phase synchronization between the lateral amygdala and CA1 area of the hippocampus during retrieval of long-term (1 day) fear memory, and not during short-term (2 h) or remote memory retrieval (30 days). These results indicated that the amygdalo-hippocampal interaction reflects a dynamic change of ensemble activities related to various stages of fear memory storage. In this study, we investigated theta activity during the reconsolidation of a remote contextual fear memory by re-exposing animals to the shock context 30 days after training. Consistent with our previous results, high theta synchronization was no longer apparent during re-exposure to the shock context, but was significantly higher 1 day after context re-exposure. These data indicate that the reconsolidation of remote contextual fear memory includes changes in ensemble activities between the lateral amygdala and CA1.

Distinct state anxiety after predictable and unpredictable fear training in mice.

Sustained fear paradigms in rodents have been developed to monitor states of anxious apprehension and to model situations in patients suffering from long-lasting anxiety disorders. A recent report describes a fear conditioning paradigm, allowing distinction between phasic and sustained states of conditioned fear in non-restrained mice. However, so far no prospective studies have yet been conducted to elucidate whether induction of phasic or sustained fear can affect states of anxiety. Here, we used CS (conditioned stimulus) and US (unconditioned stimulus) pairing with predictable and unpredictable timing to induce phasic and sustained fear in mice. State anxiety during various fear response components was assessed using the elevated plus-maze test. Training with unpredictable CS-US timing resulted in CS-evoked sustained components of fear (freezing), while predictable CS-US timing resulted in rapid decline. Data suggested the influence of training procedure on state anxiety which is dependent on progression of conditioned fear during fear memory retrieval. Animals trained with unpredictable CS-US timing showed an unchanged high anxiety state throughout behavioral observation. In contrast, mice trained with predictable CS-US timing showed anxiolytic-like behavior 3 min after CS onset, which was accompanied by a fast decline of the fear conditioned response (freezing). Further systematic studies are needed to validate the phasic/sustained fear model in rodents as translational model for anxiety disorders in humans.

Impact of Life History on Fear Memory and Extinction.

Behavioral profiles are strongly shaped by an individual's whole life experience. The accumulation of negative experiences over lifetime is thought to promote anxiety-like behavior in adulthood ("allostatic load hypothesis"). In contrast, the "mismatch hypothesis" of psychiatric disease suggests that high levels of anxiety-like behavior are the result of a discrepancy between early and late environment. The aim of the present study was to investigate how different life histories shape the expression of anxiety-like behavior and modulate fear memory. In addition, we aimed to clarify which of the two hypotheses can better explain the modulation of anxiety and fear. For this purpose, male mice grew up under either adverse or beneficial conditions during early phase of life. In adulthood they were further subdivided in groups that either matched or mismatched the condition experienced before, resulting in four different life histories. The main results were: (i) Early life benefit followed by late life adversity caused decreased levels of anxiety-like behavior. (ii) Accumulation of adversity throughout life history led to impaired fear extinction learning. Late life adversity as compared to late life benefit mainly affected extinction training, while early life adversity as compared to early life benefit interfered with extinction recall. Concerning anxiety-like behavior, the results do neither support the allostatic load nor the mismatch hypothesis, but rather indicate an anxiolytic effect of a mismatched early beneficial and later adverse life history. In contrast, fear memory was strongly affected by the accumulation of adverse experiences over the lifetime, therefore supporting allostatic load hypothesis. In summary, this study highlights that anxiety-like behavior and fear memory are differently affected by specific combinations of adverse or beneficial events experienced throughout life.