Highly specific neuron loss preserves lateral inhibitory circuits in the dentate gyrus of kainate-induced epileptic rats

Effect of adult-born immature granule cells on pattern separation in the hippocampal dentate gyrus

Young immature granule cells (imGCs) appear via adult neurogenesis in the hippocampal dentate gyrus (DG). In comparison to mature GCs (mGCs) (born during development), the imGCs exhibit two competing distinct properties such as high excitability (increasing activation degree) and low excitatory innervation (reducing activation degree). We develop a spiking neural network for the DG, incorporating both the mGCs and the imGCs. The mGCs are well known to perform "pattern separation" (i.e., a process of transforming similar input patterns into less similar output patterns) to facilitate pattern storage in the hippocampal CA3. In this paper, we investigate the effect of the young imGCs on pattern separation of the mGCs. The pattern separation efficacy (PSE) of the mGCs is found to vary through competition between high excitability and low excitatory innervation of the imGCs. Their PSE becomes enhanced (worsened) when the effect of high excitability is higher (lower) than the effect of low excitatory innervation. In contrast to the mGCs, the imGCs are found to perform "pattern integration" (i.e., making association between dissimilar patterns). Finally, we speculate that memory resolution in the hippocampal CA3 might be optimally maximized via mixed cooperative encoding through pattern separation and pattern integration.

Pentylenetetrazole kindling induces dynamic changes in GAD65 expression in hippocampal somatostatin interneurons

INTRODUCTION

One of the mechanisms of epileptgenesis is impairment of inhibitory neural circuits. Several studies have compared neural changes among subtypes of gamma-aminobutyric acid-related (GABAergic) neurons after acquired epileptic seizure. However, it is unclear that GABAergic neural modifications that occur during acquisition process of epileptic seizure.

METHODS

Male rats were injected with pentylenetetrazole (PTZ kindling: n = 30) or saline (control: n = 15) every other day to observe the development of epileptic seizure stages. Two time points were identified: the point at which seizures were most difficult to induce, and the point at which seizures were most easy to induce. The expression of GABAergic neuron-related proteins in the hippocampus was immunohistochemically compared among GABAergic subtypes at each of these time points.

RESULTS

Bimodal changes in seizure stages were observed in response to PTZ kindling. The increase of seizure stage was transiently suppressed after 8 or 10 injections, and then progressed again by the 16th injection. Based on these results, we defined 10 injections as a short-term injection period during which seizures are less likely to occur, and 20 injections as a long-term injection period during which continuous seizures are likely to occur. The immunohistochemical analysis showed that hippocampal glutamic acid decarboxylase 65 (GAD65) expression was increased after short-term kindling but unchanged after long-term kindling. Increased GAD65 expression was limited to somatostatin-positive (SOM+) cells among several GABAergic subtypes. By contrast, GAD, GABA, GABAAR α1, GABABR1, and VGAT cells showed no change following short- or long-term PTZ kindling.

CONCLUSION

PTZ kindling induces bimodal changes in the epileptic seizure stage. Seizure stage is transiently suppressed after short-term PTZ injection with GAD65 upregulation in SOM+ cells. The seizure stage is progressed again after long-term PTZ injection with GAD65 reduction to baseline level.

Maladaptation of dentate gyrus mossy cells mediates contextual discrimination deficit after traumatic stress

Fear overgeneralization is a maladaptive response to traumatic stress that is associated with the inability to discriminate between threat and safety contexts, a hallmark feature of post-traumatic stress disorder (PTSD). However, the neural mechanisms underlying this deficit remain unclear. Here, we show that traumatic stress exposure impairs contextual discrimination between threat and safety contexts in the learned helplessness (LH) model. Mossy cells (MCs) in the dorsal hippocampus are suppressed in response to traumatic stress. Bidirectional manipulation of MC activity in the LH model reveals that MC inhibition is causally linked to impaired contextual discrimination. Mechanistically, MC inhibition increases the number of active granule cells in a given context, significantly overlapping context-specific ensembles. Our study demonstrates that maladaptive inhibition of MCs after traumatic stress is a substantial mechanism underlying fear overgeneralization with contextual discrimination deficit, suggesting a potential therapeutic target for cognitive symptoms of PTSD.

Reclusive chandeliers: Functional isolation of dentate axo-axonic cells after experimental status epilepticus

Axo-axonic cells (AACs) provide specialized inhibition to the axon initial segment (AIS) of excitatory neurons and can regulate network output and synchrony. Although hippocampal dentate AACs are structurally altered in epilepsy, physiological analyses of dentate AACs are lacking. We demonstrate that parvalbumin neurons in the dentate molecular layer express PTHLH, an AAC marker, and exhibit morphology characteristic of AACs. Dentate AACs show high-frequency, non-adapting firing but lack persistent firing in the absence of input and have higher rheobase than basket cells suggesting that AACs can respond reliably to network activity. Early after pilocarpine-induced status epilepticus (SE), dentate AACs receive fewer spontaneous excitatory and inhibitory synaptic inputs and have significantly lower maximum firing frequency. Paired recordings and spatially localized optogenetic stimulation revealed that SE reduced the amplitude of unitary synaptic inputs from AACs to granule cells without altering reliability, short-term plasticity, or AIS GABA reversal potential. These changes compromised AAC-dependent shunting of granule cell firing in a multicompartmental model. These early post-SE changes in AAC physiology would limit their ability to receive and respond to input, undermining a critical brake on the dentate throughput during epileptogenesis.

Bioactive Compounds and Their Influence on Postnatal Neurogenesis

Since postnatal neurogenesis was revealed to have significant implications for cognition and neurological health, researchers have been increasingly exploring the impact of natural compounds on this process, aiming to uncover strategies for enhancing brain plasticity. This review provides an overview of postnatal neurogenesis, neurogenic zones, and disorders characterized by suppressed neurogenesis and neurogenesis-stimulating bioactive compounds. Examining recent studies, this review underscores the multifaceted effects of natural compounds on postnatal neurogenesis. In essence, understanding the interplay between postnatal neurogenesis and natural compounds could bring novel insights into brain health interventions. Exploiting the therapeutic abilities of these compounds may unlock innovative approaches to enhance cognitive function, mitigate neurodegenerative diseases, and promote overall brain well-being.

RECLUSIVE CHANDELIERS: FUNCTIONAL ISOLATION OF DENTATE AXO-AXONIC CELLS AFTER EXPERIMENTAL STATUS EPILEPTICUS

Axo-axonic cells (AACs) provide specialized inhibition to the axon initial segment (AIS) of excitatory neurons and can regulate network output and synchrony. Although hippocampal dentate AACs are structurally altered in epilepsy, physiological analyses of dentate AACs are lacking. We demonstrate that parvalbumin neurons in the dentate molecular layer express PTHLH, an AAC marker, and exhibit morphology characteristic of AACs. Dentate AACs show high-frequency, non-adapting firing but lack persistent firing in the absence of input and have higher rheobase than basket cells suggesting that AACs can respond reliably to network activity. Early after pilocarpine-induced status epilepticus (SE), dentate AACs receive fewer spontaneous excitatory and inhibitory synaptic inputs and have significantly lower maximum firing frequency. Paired recordings and spatially localized optogenetic stimulation revealed that SE reduced the amplitude of unitary synaptic inputs from AACs to granule cells without altering reliability, short-term plasticity, or AIS GABA reversal potential. These changes compromised AAC-dependent shunting of granule cell firing in a multicompartmental model. These early post-SE changes in AAC physiology would limit their ability to receive and respond to input, undermining a critical brake on the dentate throughput during epileptogenesis.

Oscillations in the dentate gyrus as a tool for the performance of the hippocampal functions: Healthy and epileptic brain

The dentate gyrus (DG) is part of the hippocampal formation and is essential for important cognitive processes such as navigation and memory. The oscillatory activity of the DG network is believed to play a critical role in cognition. DG circuits generate theta, beta, and gamma rhythms, which participate in the specific information processing performed by DG neurons. In the temporal lobe epilepsy (TLE), cognitive abilities are impaired, which may be due to drastic alterations in the DG structure and network activity during epileptogenesis. The theta rhythm and theta coherence are especially vulnerable in dentate circuits; disturbances in DG theta oscillations and their coherence may be responsible for general cognitive impairments observed during epileptogenesis. Some researchers suggested that the vulnerability of DG mossy cells is a key factor in the genesis of TLE, but others did not support this hypothesis. The aim of the review is not only to present the current state of the art in this field of research but to help pave the way for future investigations by highlighting the gaps in our knowledge to completely appreciate the role of DG rhythms in brain functions. Disturbances in oscillatory activity of the DG during TLE development may be a diagnostic marker in the treatment of this disease.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Deep brain stimulation of the anterior nuclei of the thalamus can alleviate seizure severity and induce hippocampal GABAergic neuronal changes in a pilocarpine-induced epileptic mouse brain

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) has been widely used as an effective treatment for refractory temporal lobe epilepsy. Despite its promising clinical outcome, the exact mechanism of how ANT-DBS alleviates seizure severity has not been fully understood, especially at the cellular level. To assess effects of DBS, the present study examined electroencephalography (EEG) signals and locomotor behavior changes and conducted immunohistochemical analyses to examine changes in neuronal activity, number of neurons, and neurogenesis of inhibitory neurons in different hippocampal subregions. ANT-DBS alleviated seizure activity, abnormal locomotor behaviors, reduced theta-band, increased gamma-band EEG power in the interictal state, and increased the number of neurons in the dentate gyrus (DG). The number of parvalbumin- and somatostatin-expressing inhibitory neurons was recovered to the level in DG and CA1 of naïve mice. Notably, BrdU-positive inhibitory neurons were increased. In conclusion, ANT-DBS not only could reduce the number of seizures, but also could induce neuronal changes in the hippocampus, which is a key region involved in chronic epileptogenesis. Importantly, our results suggest that ANT-DBS may lead to hippocampal subregion-specific cellular recovery of GABAergic inhibitory neurons.

Disynaptic effect of hilar cells on pattern separation in a spiking neural network of hippocampal dentate gyrus

We study the disynaptic effect of the hilar cells on pattern separation in a spiking neural network of the hippocampal dentate gyrus (DG). The principal granule cells (GCs) in the DG perform pattern separation, transforming similar input patterns into less-similar output patterns. In our DG network, the hilus consists of excitatory mossy cells (MCs) and inhibitory HIPP (hilar perforant path-associated) cells. Here, we consider the disynaptic effects of the MCs and the HIPP cells on the GCs, mediated by the inhibitory basket cells (BCs) in the granular layer; MC → BC → GC and HIPP → BC → GC. The MCs provide disynaptic inhibitory input (mediated by the intermediate BCs) to the GCs, which decreases the firing activity of the GCs. On the other hand, the HIPP cells disinhibit the intermediate BCs, which leads to increasing the firing activity of the GCs. In this way, the disynaptic effects of the MCs and the HIPP cells are opposite. We investigate change in the pattern separation efficacy by varying the synaptic strength K ( BC , X ) [from the pre-synaptic X (= MC or HIPP) to the post-synaptic BC]. Thus, sparsity for the firing activity of the GCs is found to improve the efficacy of pattern separation, and hence the disynaptic effects of the MCs and the HIPP cells on the pattern separation become opposite ones. In the combined case when simultaneously changing both K ( BC , MC ) and K ( BC , HIPP ) , as a result of balance between the two competing disynaptic effects of the MCs and the HIPP cells, the efficacy of pattern separation is found to become the highest at their original default values where the activation degree of the GCs is the lowest. We also note that, while the GCs perform pattern separation, sparsely synchronized rhythm is found to appear in the population of the GCs. Hence, we examine quantitative association between population and individual firing behaviors in the sparsely synchronized rhythm and pattern separation. They are found to be strongly correlated. Consequently, the better the population and individual firing behaviors in the sparsely synchronized rhythm are, the more pattern separation efficacy becomes enhanced.

Seizure-induced strengthening of a recurrent excitatory circuit in the dentate gyrus is proconvulsant

Epilepsy is a devastating brain disorder for which effective treatments are very limited. There is growing interest in early intervention, which requires a better mechanistic understanding of the early stages of this disorder. While diverse brain insults can lead to epileptic activity, a common cellular mechanism relies on uncontrolled recurrent excitatory activity. In the dentate gyrus, excitatory mossy cells (MCs) project extensively onto granule cells (GCs) throughout the hippocampus, thus establishing a recurrent MC-GC-MC excitatory loop. MCs are implicated in temporal lobe epilepsy, a common form of epilepsy, but their role during initial seizures (i.e., before the characteristic MC loss that occurs in late stages) is unclear. Here, we show that initial seizures acutely induced with an intraperitoneal kainic acid (KA) injection in adult mice, a well-established model that leads to experimental epilepsy, not only increased MC and GC activity in vivo but also triggered a brain-derived neurotrophic factor (BDNF)-dependent long-term potentiation (LTP) at MC-GC excitatory synapses. Moreover, in vivo induction of MC-GC LTP using MC-selective optogenetic stimulation worsened KA-induced seizures. Conversely, Bdnf genetic removal from GCs, which abolishes LTP, and selective MC silencing were both anticonvulsant. Thus, initial seizures are associated with MC-GC synaptic strengthening, which may promote later epileptic activity. Our findings reveal a potential mechanism of epileptogenesis that may help in developing therapeutic strategies for early intervention.

Hyperexcitability: From Normal Fear to Pathological Anxiety and Trauma

Hyperexcitability in fear circuits is suggested to be important for development of pathological anxiety and trauma from adaptive mechanisms of fear. Hyperexcitability is proposed to be due to acquired sensitization in fear circuits that progressively becomes more severe over time causing changing symptoms in early and late pathology. We use the metaphor and mechanisms of kindling to examine gains and losses in function of one excitatory and one inhibitory neuropeptide, corticotrophin releasing factor and somatostatin, respectively, to explore this sensitization hypothesis. We suggest amygdala kindling induced hyperexcitability, hyper-inhibition and loss of inhibition provide clues to mechanisms for hyperexcitability and progressive changes in function initiated by stress and trauma.

Population and individual firing behaviors in sparsely synchronized rhythms in the hippocampal dentate gyrus

We investigate population and individual firing behaviors in sparsely synchronized rhythms (SSRs) in a spiking neural network of the hippocampal dentate gyrus (DG). The main encoding granule cells (GCs) are grouped into lamellar clusters. In each GC cluster, there is one inhibitory (I) basket cell (BC) along with excitatory (E) GCs, and they form the E-I loop. Winner-take-all competition, leading to sparse activation of the GCs, occurs in each GC cluster. Such sparsity has been thought to enhance pattern separation performed in the DG. During the winner-take-all competition, SSRs are found to appear in each population of the GCs and the BCs through interaction of excitation of the GCs with inhibition of the BCs. Sparsely synchronized spiking stripes appear successively with the population frequencyf p ( = 13.1 Hz) in the raster plots of spikes. We also note that excitatory hilar mossy cells (MCs) control the firing activity of the GC-BC loop by providing excitation to both the GCs and the BCs. SSR also appears in the population of MCs via interaction with the GCs (i.e., GC-MC loop). Population behaviors in the SSRs are quantitatively characterized in terms of the synchronization measures. In addition, we investigate individual firing activity of GCs, BCs, and MCs in the SSRs. Individual GCs exhibit random spike skipping, leading to a multi-peaked inter-spike-interval histogram, which is well characterized in terms of the random phase-locking degree. In this case, population-averaged mean-firing-rate (MFR) < f i ( GC ) > is less than the population frequency f p . On the other hand, both BCs and MCs show "intrastripe" burstings within stripes, together with random spike skipping. Thus, the population-averaged MFR ⟨ f i ( X ) ⟩ ( X = MC and BC) is larger than f p , in contrast to the case of the GCs. MC loss may occur during epileptogenesis. With decreasing the fraction of the MCs, changes in the population and individual firings in the SSRs are also studied. Finally, quantitative association between the population/individual firing behaviors in the SSRs and the winner-take-all competition is discussed.

Anxiety and hippocampal neuronal activity: Relationship and potential mechanisms

The hippocampus has been implicated in modulating anxiety. It interacts with a variety of brain regions, both cortical and subcortical areas regulating emotion and stress responses, including prefrontal cortex, amygdala, hypothalamus, and the nucleus accumbens, to adjust anxiety levels in response to a variety of stressful conditions. Growing evidence indicates that anxiety is associated with increased neuronal excitability in the hippocampus, and alterations in local regulation of hippocampal excitability have been suggested to underlie behavioral disruptions characteristic of certain anxiety disorders. Furthermore, studies have shown that some anxiolytics can treat anxiety by altering the excitability and plasticity of hippocampal neurons. Hence, identifying cellular and molecular mechanisms and neural circuits that regulate hippocampal excitability in anxiety may be beneficial for developing targeted interventions for treatment of anxiety disorders particularly for the treatment-resistant cases. We first briefly review a role of the hippocampus in fear. We then review the evidence indicating a relationship between the hippocampal activity and fear/anxiety and discuss some possible mechanisms underlying stress-induced hippocampal excitability and anxiety-related behavior.

Anticonvulsive and Neuroprotective Effects of Eupafolin in Rats Are Associated with the Inhibition of Glutamate Overexcitation and Upregulation of the Wnt/β-Catenin Signaling Pathway

Several plant compounds have been found to possess neuroactive properties. The aim of this study was to investigate the anticonvulsant effect of eupafolin, a major active component extracted from Salvia plebeia, a herb used in traditional medicine for its anti-inflammatory properties. To this end, we assessed the anticonvulsant effects of eupafolin in rats intraperitoneally (i.p.) injected with kainic acid (KA) to elucidate this mechanism. Treatment with eupafolin (i.p.) for 30 min before KA administration significantly reduced behavioral and electrographic seizures induced by KA, similar to carbamazepine (i.p.), a widely used antiepileptic drug. Eupafolin treatment also significantly decreased KA seizure-induced neuronal cell death and glutamate elevation in the hippocampus. In addition, eupafolin notably reversed KA seizure-induced alterations in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluR2, glutamate decarboxylase 67 (GAD67, GABAergic enzyme), and Wnt signaling-related proteins, including porcupine, Wnt1, phosphorylated-glycogen synthase kinase-3β, β-catenin, and Bcl-2 in the hippocampus. Furthermore, the increased level of Dickkopf-related protein 1 (Dkk-1, a Wnt signaling antagonist) and the decreased level of Disheveled1 (Dvl-1, a Wnt signaling activator) in the hippocampus of KA-treated rats were reversed by eupafolin. This study provides evidence of the anticonvulsant and neuroprotective properties of eupafolin and of the involvement of regulation of glutamate overexcitation and Wnt signaling in the mechanisms of these properties. These findings support the benefits of eupafolin in treating epilepsy.

An aqueous extract of Khaya senegalensis (Desv.) A. Juss. (Meliaceae) prevents seizures and reduces anxiety in kainate-treated rats: modulation of GABA neurotransmission, oxidative stress, and neuronal loss in the hippocampus

Ethnopharmacological relevance

Temporal lobe epilepsy is the most common form of drug-resistant epilepsy. Therefore, medicinal plants provide an alternative source for the discovery of new antiepileptic drugs.

Aim of the study

This study was aimed at investigating the antiepileptic- and anxiolytic-like effects of an aqueous extract of Khaya senegalensis (K. senegalensis) in kainate-treated rats.

Methods

Seventy-two rats received a single dose of kainate (12 mg/kg) intraperitoneally. Those that exhibited two hours of status epilepticus were selected and monitored for the first spontaneous seizure. Then, animals that developed seizures were divided into 6 groups of 8 rats each and treated twice daily for 14 days as follows: negative control group received per os (p.o.) distilled water (10 ml/kg); two positive control groups received either sodium valproate (300 mg/kg, p.o.) or phenobarbital (20 mg/kg, p.o.); and three test groups received different doses of the extract (50, 100, and 200 mg/kg, p.o.). In addition, a group of 8 normal rats (normal control group) received distilled water (10 ml/kg, p.o.). During the treatment period, the animals were video-monitored 12 h/day for behavioral seizures. At the end of the treatment period, animals were subjected to elevated plus-maze and open field tests. Thereafter, rats were euthanized for the analysis of γ-aminobutyric acid (GABA) concentration, oxidative stress status, and neuronal loss in the hippocampus.

Results

The aqueous extract of K. senegalensis significantly reduced spontaneous recurrent seizures (generalized tonic-clonic seizures) and anxiety-like behavior compared to the negative control group. These effects were more marked than those of sodium valproate or phenobarbital. Furthermore, the extract significantly increased GABA concentration, alleviated oxidative stress, and mitigated neuronal loss in the dentate gyrus of the hippocampus.

Conclusion

These findings suggest that the aqueous extract of K. senegalensis possesses antiepileptic- and anxiolytic-like effects. These effects were greater than those of sodium valproate or phenobarbital, standard antiepileptic drugs. Furthermore, these effects are accompanied by neuromodulatory and antioxidant activities that may be related to their behavioral effects. These data justify further studies to identify the bioactive molecules present in the extract for possible future therapeutic development and to unravel their mechanisms of action.

Dynamical origin for winner-take-all competition in a biological network of the hippocampal dentate gyrus

We consider a biological network of the hippocampal dentate gyrus (DG). Computational models suggest that the DG would be a preprocessor for pattern separation (i.e., a process transforming a set of similar input patterns into distinct nonoverlapping output patterns) which could facilitate pattern storage and retrieval in the CA3 area of the hippocampus. The main encoding cells in the DG are the granule cells (GCs) which receive the input from the entorhinal cortex (EC) and send their output to the CA3. We note that the activation degree of GCs is very low (∼5%). This sparsity has been thought to enhance the pattern separation. We investigate the dynamical origin for winner-take-all (WTA) competition which leads to sparse activation of the GCs. The whole GCs are grouped into lamellar clusters. In each cluster, there is one inhibitory (I) basket cell (BC) along with excitatory (E) GCs. There are three kinds of external inputs into the GCs: the direct excitatory EC input; the indirect feedforward inhibitory EC input, mediated by the HIPP (hilar perforant path-associated) cells; and the excitatory input from the hilar mossy cells (MCs). The firing activities of the GCs are determined via competition between the external E and I inputs. The E-I conductance ratio R_{E-I}^{(con)}^{*} (given by the time average of the ratio of the external E to I conductances) may represent well the degree of such external E-I input competition. It is thus found that GCs become active when their R_{E-I}^{(con)}^{*} is larger than a threshold R_{th}^{*}, and then the mean firing rates of the active GCs are strongly correlated with R_{E-I}^{(con)}^{*}. In each cluster, the feedback inhibition from the BC may select the winner GCs. GCs with larger R_{E-I}^{(con)}^{*} than the threshold R_{th}^{*} survive, and they become winners; all the other GCs with smaller R_{E-I}^{(con)}^{*} become silent. In this way, WTA competition occurs via competition between the firing activity of the GCs and the feedback inhibition from the BC in each cluster. Finally, we also study the effects of MC death and adult-born immature GCs on the WTA competition.

Aberrant hippocampal mossy fibers in temporal lobe epilepsy target excitatory and inhibitory neurons

OBJECTIVE

The pathoanatomical correlate of temporal lobe epilepsy is hippocampal sclerosis, characterized by selective neuronal death of mossy cells in the hilus and of pyramidal cells in cornu ammonis 1. Although granule cells survive, they lose mossy cells as a target and redirect their axons (mossy fibers) backward into the molecular cell layer. It has been assumed that this process results in excitatory circuits. We therefore examined whether sprouted mossy fibers form synaptic connection not only with excitatory granule cells but also with inhibitory interneurons, such as basket cells.

METHODS

Resected hippocampal specimens of patients with hippocampal sclerosis were compared to controls of patients with extrahippocampal lesions with only mild sclerosis. Mossy fibers were traced with Neurobiotin or labeled against synaptoporin; inhibitory interneurons were labeled against parvalbumin. Synapses were examined with electron microscopy, labeled with γ-aminobutyric acid immunogold.

RESULTS

Sprouted mossy fibers of epileptic hippocampi innervate not only excitatory granule cells but also inhibitory parvalbuminergic interneurons. Despite neuronal death in hippocampal sclerosis, the axonal plexus of inhibitory parvalbuminergic interneurons surrounding the granule cells is preserved. Connections of sprouted mossy fibers and inhibitory axon terminals were quantified, showing that the number of inhibitory axon terminals significantly exceeds the number of sprouted excitatory mossy fiber terminals (.03 boutons/µm vs. .11 boutons/µm; p < .001).

SIGNIFICANCE

Although no definite conclusions regarding the function of our findings may be derived from this anatomical study, the observed aberrant connectivity might lead to an increased inhibition and synchronization of granule cells, because the preserved inhibitory interneurons show an additional innervation through sprouted mossy fibers. This might result in the instability of a previously balanced network.

Alterations of Neuronal Dynamics as a Mechanism for Cognitive Impairment in Epilepsy

Epilepsy is commonly associated with cognitive and behavioral deficits that dramatically affect the quality of life of patients. In order to identify novel therapeutic strategies aimed at reducing these deficits, it is critical first to understand the mechanisms leading to cognitive impairments in epilepsy. Traditionally, seizures and epileptiform activity in addition to neuronal injury have been considered to be the most significant contributors to cognitive dysfunction. In this review we however highlight the role of a new mechanism: alterations of neuronal dynamics, i.e. the timing at which neurons and networks receive and process neural information. These alterations, caused by the underlying etiologies of epilepsy syndromes, are observed in both animal models and patients in the form of abnormal oscillation patterns in unit firing, local field potentials, and electroencephalogram (EEG). Evidence suggests that such mechanisms significantly contribute to cognitive impairment in epilepsy, independently of seizures and interictal epileptiform activity. Therefore, therapeutic strategies directly targeting neuronal dynamics rather than seizure reduction may significantly benefit the quality of life of patients.

Operations Research Methods for Estimating the Population Size of Neuron Types

Understanding brain computation requires assembling a complete catalog of its architectural components. Although the brain is organized into several anatomical and functional regions, it is ultimately the neurons in every region that are responsible for cognition and behavior. Thus, classifying neuron types throughout the brain and quantifying the population sizes of distinct classes in different regions is a key subject of research in the neuroscience community. The total number of neurons in the brain has been estimated for multiple species, but the definition and population size of each neuron type are still open questions even in common model organisms: the so called "cell census" problem. We propose a methodology that uses operations research principles to estimate the number of neurons in each type based on available information on their distinguishing properties. Thus, assuming a set of neuron type definitions, we provide a solution to the issue of assessing their relative proportions. Specifically, we present a three-step approach that includes literature search, equation generation, and numerical optimization. Solving computationally the set of equations generated by literature mining yields best estimates or most likely ranges for the number of neurons in each type. While this strategy can be applied towards any neural system, we illustrate its usage on the rodent hippocampus.

The Complement Regulator Susd4 Influences Nervous-System Function and Neuronal Morphology in Mice

The SUSD4 (Sushi domain-containing protein 4) gene encodes a complement inhibitor that is frequently deleted in 1q41q42 microdeletion syndrome, a multisystem congenital disorder that includes neurodevelopmental abnormalities. To understand SUSD4's role in the mammalian nervous system, we analyzed Susd4 knockout (KO) mice. Susd4 KO mice exhibited significant defects in motor performance and significantly higher levels of anxiety-like behaviors. Susd4 KO brain had abnormal “hairy” basket cells surrounding Purkinje neurons within the cerebellum and significantly reduced dendritic spine density in hippocampal pyramidal neurons. Neurons and oligodendrocyte lineage cells of wild-type mice were found to express Susd4 mRNA. Protein expression of the complement component C1q was increased in the brains of Susd4 KO mice. Our data indicate that SUSD4 plays an important role in neuronal functions, possibly via the complement pathway, and that SUSD4 deletion may contribute to the nervous system abnormalities in patients with 1q41q42 deletions.

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Susd4 is expressed in neurons and oligodendrocyte lineage cells

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Susd4 knockout mice have abnormal hippocampal and cerebellar neuronal morphologies

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Susd4 knockout mice exhibit anxiety-like behaviors and impaired motor function

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Susd4 knockout mice have elevated brain levels of the complement component C1q

Modeling of Brain-Like Concept Coding with Adulthood Neurogenesis in the Dentate Gyrus

Mammalian brains respond to new concepts via a type of neural coding termed “concept coding.” During concept coding, the dentate gyrus (DG) plays a vital role in pattern separation and pattern integration of concepts because it is a brain region with substantial neurogenesis in adult mammals. Although concept coding properties of the brain have been extensively studied by experimental work, modeling of the process to guide both further experimental studies and applications such as natural language processing is scarce. To model brain-like concept coding, we built a spiking neural network inspired by adulthood neurogenesis in the DG. Our model suggests that neurogenesis may facilitate integration of closely related concepts and separation of less relevant concepts. Such pattern agrees with the previous experimental observations in classification tasks and place cells in the hippocampus. Therefore, our simulation provides insight for future experimental studies on the neural coding difference between perception and cognition. By presenting 14 contexts each containing 4 concepts to the network, we found that neural responses of the DG changed dynamically as the context repetition time increased and were eventually consistent with the category organization of humans. Thus, our work provides a new framework of word representation for the construction of brain-like knowledge map.

Aging and Central Auditory Disinhibition: Is It a Reflection of Homeostatic Downregulation or Metabolic Vulnerability?

Aging-related changes have been identified at virtually every level of the central auditory system. One of the most common findings across these nuclei is a loss of synaptic inhibition with aging, which has been proposed to be at the heart of several aging-related changes in auditory cognition, including diminished speech perception in complex environments and the presence of tinnitus. Some authors have speculated that downregulation of synaptic inhibition is a consequence of peripheral deafferentation and therefore is a homeostatic mechanism to restore excitatory/inhibitory balance. As such, disinhibition would represent a form of maladaptive plasticity. However, clinical data suggest that deafferentation-related disinhibition tends to occur primarily in the aged brain. Therefore, aging-related disinhibition may, in part, be related to the high metabolic demands of inhibitory neurons relative to their excitatory counterparts. These findings suggest that both deafferentation-related maladaptive plastic changes and aging-related metabolic factors combine to produce changes in central auditory function. Here, we explore the arguments that downregulation of inhibition may be due to homeostatic responses to diminished afferent input vs. metabolic vulnerability of inhibitory neurons in the aged brain. Understanding the relative importance of these mechanisms will be critical for the development of treatments for the underlying causes of aging-related central disinhibition.

Delayed audiogenic seizure development in a genetic rat model is associated with overactivation of ERK1/2 and disturbances in glutamatergic signaling

Krushinsky-Molodkina (KM) rats genetically prone to audiogenic seizure are characterized by age-dependent expression of audiogenic seizures (AGS). It is known that the critical period of enhanced seizure susceptibility in rodents occurs at 2nd-3rd weeks of postnatal development. However, KM rats do not express AGS at this time-point, but start to demonstrate a stable AGS only after the age of 3 months. We hypothesized that this delay in AGS susceptibility in KM rats is genetically determined and may depend on some alterations in the development of the hippocampal glutamatergic system during the early postnatal period. We analyzed the expression and activity of seizure-related proteins, such as vesicular glutamate transporter 2 (VGLUT2), extracellular signal-regulated kinases 1 and 2 (ERK1/2), synapsin I, and NR2B subunit of the N-methyl-d-aspartate (NMDA) receptor (NR2B) in the hippocampus of KM rats during postnatal development. A significantly higher activity of ERK1/2 in KM rats was observed at 14th, 30th, and 60th days of postnatal development (P14, P30, P60) in comparison with control Wistar rats of the corresponding ages, while in adult (P120) KM rats it was at the same level with Wistar rats. Despite the increased activity of ERK1/2 at P14 and P30, the phosphorylation of synapsin I at Ser62/67 was significantly lower in the hippocampus of KM rats than in Wistar rats of the same ages; however, at P60 and P120, the phosphorylation of synapsin I was enhanced. Our data also revealed the increase of VGLUT2 and NR2B expression at P14, which dramatically decreased at the later stages. Our data indicate that a genetically determined increase in ERK1/2 kinase activity during postnatal ontogenesis in KM rats may be associated with the disturbances in synthesis and activity of the proteins, which are responsible for glutamatergic transmission in the KM rat hippocampus during the seizure susceptibility development.

Function of local circuits in the hippocampal dentate gyrus-CA3 system

Anatomical observations, theoretical work and lesioning experiments have supported the idea that the CA3 in the hippocampus is important for encoding, storage and retrieval of memory while the dentate gyrus (DG) is important for the pattern separation of the incoming inputs from the entorhinal cortex. Study of the presumed function of the dentate gyrus in pattern separation has been hampered by the lack of reliable methods to identify different excitatory cell types in the DG. Recent papers have identified different cell types in the DG, in awake behaving animals, with more reliable methods. These studies have revealed each cell type's spatial representation as well as their involvement in pattern separation. Moreover, chronic electrophysiological recording from sleeping and waking animals also provided more insights into the operation of the DG-CA3 system for memory encoding and retrieval. This article will review the local circuit architectures and physiological properties of the DG-CA3 system and discuss how the local circuit in the DG-CA3 may function, incorporating recent physiological findings in the DG-CA3 system.

Temporal lobe epilepsy affects spatial organization of entorhinal cortex connectivity

Evidence for structural connectivity patterns within the medial temporal lobe derives primarily from postmortem histological studies. In humans and nonhuman primates, the parahippocampal gyrus (PHg) is subdivided into parahippocampal (PHc) and perirhinal (PRc) cortices, which receive input from distinct cortical networks. Likewise, their efferent projections to the entorhinal cortex (ERc) are distinct. The PHc projects primarily to the medial ERc (M-ERc). The PRc projects primarily to the lateral portion of the ERc (L-ERc). Both M-ERc and L-ERc, via the perforant pathway, project to the dentate gyrus and hippocampal (HC) subfields. Until recently, these neural circuits could not be visualized in vivo. Diffusion tensor imaging algorithms have been developed to segment gray matter structures based on probabilistic connectivity patterns. However, these algorithms have not yet been applied to investigate connectivity in the temporal lobe or changes in connectivity architecture related to disease processes. In this study, this segmentation procedure was used to classify ERc gray matter based on PRc, ERc, and HC connectivity patterns in 7 patients with temporal lobe epilepsy (TLE) without hippocampal sclerosis (mean age, 14.86 ± 3.34 years) and 7 healthy controls (mean age, 23.86 ± 2.97 years). Within samples paired t-tests allowed for comparison of ERc connectivity between epileptogenic and contralateral hemispheres. In healthy controls, there were no significant within-group differences in surface area, volume, or cluster number of ERc connectivity-defined regions (CDR). Likewise, in line with histology results, ERc CDR in the control group were well-organized, uniform, and segregated via PRc/PHc afferent and HC efferent connections. Conversely, in TLE, there were significantly more PRc and HC CDR clusters in the epileptogenic than the contralateral hemisphere. The surface area of the PRc CDR was greater, and that of the HC CDRs was smaller, in the epileptogenic hemisphere as well. Further, there was no clear delineation between M-ERc and L-ERc connectivity with PRc, PHc or HC in TLE. These results suggest a breakdown of the spatial organization of PHg-ERc-HC connectivity in TLE. Whether this breakdown is the cause or result of epileptic activity remains an exciting research question.

Advances in understanding hilar mossy cells of the dentate gyrus

Hilar mossy cells (MCs) of the dentate gyrus (DG) distinguish the DG from other hippocampal subfields (CA1-3) because there are two glutamatergic cell types in the DG rather than one. Thus, in the DG, the main cell types include glutamatergic granule cells (GCs) and MCs, whereas in CA1-3, the only glutamatergic cell type is the pyramidal cell. In contrast to GCs, MCs are different in morphology, intrinsic electrophysiological properties, afferent input and axonal projections, so their function is likely to be very different from GCs. Why are MCs necessary to the DG? In past studies, the answer has been unclear because MCs not only excite GCs directly but also inhibit them disynaptically, by exciting GABAergic neurons that project to GCs. Results of new studies are discussed that shed light on this issue. These studies take advantage of recently available transgenic mice with Cre recombinase expression mostly in MCs and techniques such as optogenetics and DREADDs (designer receptors exclusively activated by designer drugs). The recent studies also address in vivo behavioral functions of MCs. Some of the results support past hypotheses whereas others suggest new conceptualizations of how the MCs contribute to DG circuitry and function. While substantial progess has been made, additional research is still needed to clarify the characteristics and functions of these unique cells.

Cannabidiol exerts antiepileptic effects by restoring hippocampal interneuron functions in a temporal lobe epilepsy model

Background and Purpose

A non‐psychoactive phytocannabinoid, cannabidiol (CBD), shows promising results as an effective potential antiepileptic drug in some forms of refractory epilepsy. To elucidate the mechanisms by which CBD exerts its anti‐seizure effects, we investigated its effects at synaptic connections and on the intrinsic membrane properties of hippocampal CA1 pyramidal cells and two major inhibitory interneurons: fast spiking, parvalbumin (PV)‐expressing and adapting, cholecystokinin (CCK)‐expressing interneurons. We also investigated whether in vivo treatment with CBD altered the fate of CCK and PV interneurons using immunohistochemistry.

Experimental Approach

Electrophysiological intracellular whole‐cell recordings combined with neuroanatomy were performed in acute brain slices of rat temporal lobe epilepsy in in vivo (induced by kainic acid) and in vitro (induced by Mg²⁺‐free solution) epileptic seizure models. For immunohistochemistry experiments, CBD was administered in vivo (100 mg·kg⁻¹) at zero time and 90 min post status epilepticus, induced with kainic acid.

Key Results

Bath application of CBD (10 μM) dampened excitability at unitary synapses between pyramidal cells but enhanced inhibitory synaptic potentials elicited by fast spiking and adapting interneurons at postsynaptic pyramidal cells. Furthermore, CBD restored impaired membrane excitability of PV, CCK and pyramidal cells in a cell type‐specific manner. These neuroprotective effects of CBD were corroborated by immunohistochemistry experiments that revealed a significant reduction in atrophy and death of PV‐ and CCK‐expressing interneurons after CBD treatment.

Conclusions and Implications

Our data suggest that CBD restores excitability and morphological impairments in epileptic models to pre‐epilepsy control levels through multiple mechanisms to reinstate normal network function.

Chronic agomelatine treatment prevents comorbid depression in the post-status epilepticus model of acquired epilepsy through suppression of inflammatory signaling

Inflammatory signal molecules are suggested to be involved in the mechanism underlying comorbid depression in epilepsy. In the present study, we tested the hypothesis that the novel antidepressant agomelatine, a potent melatonin MT₁ and MT₂ receptor agonist and serotonin 5HT_2C receptor antagonist, can prevent depressive symptoms developed during the chronic epileptic phase by suppressing an inflammatory response. Chronic treatment with agomelatine (40 mg/kg, i.p.) was initiated an hour after the kainate acid (KA)-induced status epilepticus (SE) and maintained for a period of 10 weeks in Wistar rats. Registration of spontaneous motor seizures was performed through a video (24 h/day) and EEG monitoring. Antidepressant activity of agomelatine was explored in the splash test, sucrose preference test (SPT) and forced swimming test (FST) while anxiolytic effect was observed through the novelty suppression-feeding test (NSFT) during chronic phase in epileptic rats. The frequency of motor seizures detected by video and EEG recording did not differ between vehicle and Ago group. Rats with registered spontaneous motor seizures showed symptoms typical for depressive behavior that included decreased grooming, anhedonia during the dark period and hopeless-like behavior. Epileptic rats exhibited also anxiety with novelty-induced hypophagia. This behavioral deficit correlated with increased signal markers of inflammation (plasma levels of interleukin (IL)-1β and activated glia in brain), while plasma corticosterone levels were not changed. Agomelatine treatment during epileptogenesis exerted a clear antidepressant effect by suppressing all behavioral hallmarks, reducing plasma IL-1β levels and preventing microgliosis and astrogliosis in specific limbic regions. The present results suggest that agomelatine treatment starting after SE can provide an effective therapy of comorbid depression in chronic epileptic condition through suppression of inflammatory signaling.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Disrupted Co-activation of Interneurons and Hippocampal Network after Focal Kainate Lesion

GABAergic interneurons are known to control activity balance in physiological conditions and to coordinate hippocampal networks during cognitive tasks. In temporal lobe epilepsy interneuron loss and consecutive network imbalance could favor pathological hypersynchronous epileptic discharges. We tested this hypothesis in mice by in vivo unilateral epileptogenic hippocampal kainate lesion followed by in vitro recording of extracellular potentials and patch-clamp from GFP-expressing interneurons in CA3, in an optimized recording chamber. Slices from lesioned mice displayed, in addition to control synchronous events, larger epileptiform discharges. Despite some ipsi/contralateral and layer variation, interneuron density tended to decrease, average soma size to increase. Their membrane resistance decreased, capacitance increased and contralateral interneuron required higher current intensity to fire action potentials. Examination of synchronous discharges of control and larger amplitudes, revealed that interneurons were biased to fire predominantly with the largest population discharges. Altogether, these observations suggest that the overall effect of reactive cell loss, hypertrophy and reduced contralateral excitability corresponds to interneuron activity tuning to fire with larger population discharges. Such cellular and network mechanisms may contribute to a runaway path toward epilepsy.

Properties of afterdischarges from electrical stimulation in patients with epilepsy

OBJECTIVE

To investigate the properties of afterdischarges (ADs) from intracerebral electrical stimulation (ES) in patients with epilepsy who underwent stereotactic electroencephalography (SEEG) and determine the relationship between epileptogenic zone (EZ) or irritative zone (IZ) and ADs.

METHODS

We retrospectively analyzed 10 patients with intractable epilepsy who underwent SEEG. ESs were delivered following the given parameters: bipolar, biphasic, 50Hz, 0.2ms pulse duration, 0.5-10mA. The properties of ADs were documented, including their incidence, location, threshold, morphology and evolution.

RESULTS

A total of 213 ADs (5%) were elicited by 4701 trains of ES. Stimulation through contacts implanted in the hippocampus (59%) generally evoked more ADs than contacts elsewhere (19%). AD thresholds for hippocampal stimulation were significantly lower than those for stimulation in grey matter. Polyspikes (58%) were the most common AD morphology. Evolution occurred more commonly with sequential spikes (47%) than with other AD morphologies (14%). There was no significant correlation between the location of ADs and EZ. However, ADs were significantly more frequently localized to IZ than areas outside IZ (P<0.05).

CONCLUSIONS

There seemed to be a lack of correlation between the location of ADs and EZ. However, ADs were more likely to be elicited in IZ.

Rabies tracing of birthdated dentate granule cells in rat temporal lobe epilepsy

OBJECTIVE

To understand how monosynaptic inputs onto adult-born dentate granule cells (DGCs) are altered in experimental mesial temporal lobe epilepsy (mTLE) and whether their integration differs from early-born DGCs that are mature at the time of epileptogenesis.

METHODS

A dual-virus tracing strategy combining retroviral birthdating with rabies virus-mediated putative retrograde trans-synaptic tracing was used to identify and compare presynaptic inputs onto adult-born and early-born DGCs in the rat pilocarpine model of mTLE.

RESULTS

Our results demonstrate that hilar ectopic DGCs preferentially synapse onto adult-born DGCs after pilocarpine-induced status epilepticus (SE), whereas normotopic DGCs synapse onto both adult-born and early-born DGCs. We also find that parvalbumin^- and somatostatin^- interneuron inputs are greatly diminished onto early-born DGCs after SE. However, somatostatin^- interneuron inputs onto adult-born DGCs are maintained, likely due to preferential sprouting. Intriguingly, CA3 pyramidal cell backprojections that specifically target adult-born DGCs arise in the epileptic brain, whereas axons of interneurons and pyramidal cells in CA1 appear to sprout across the hippocampal fissure to preferentially synapse onto early-born DGCs.

INTERPRETATION

These data support the presence of substantial hippocampal circuit remodeling after an epileptogenic insult that generates prominent excitatory monosynaptic inputs, both local recurrent and widespread feedback loops, onto DGCs. Both adult-born and early-born DGCs are targets of new inputs from other DGCs as well as from CA3 and CA1 pyramidal cells after pilocarpine treatment, changes that likely contribute to epileptogenesis in experimental mTLE. Ann Neurol 2017;81:790-803.

Acute restraint stress decreases c-fos immunoreactivity in hilar mossy cells of the adult dentate gyrus

Although a great deal of information is available about the circuitry of the mossy cells (MCs) of the dentate gyrus (DG) hilus, their activity in vivo is not clear. The immediate early gene c-fos can be used to gain insight into the activity of MCs in vivo, because c-fos protein expression reflects increased neuronal activity. In prior work, it was identified that control rats that were perfusion-fixed after removal from their home cage exhibited c-fos immunoreactivity (ir) in the DG in a spatially stereotyped pattern: ventral MCs and dorsal granule cells (GCs) expressed c-fos protein (Duffy et al., Hippocampus 23:649-655, 2013). In this study, we hypothesized that restraint stress would alter c-fos-ir, because MCs express glucocorticoid type 2 receptors and the DG is considered to be involved in behaviors related to stress or anxiety. We show that acute restraint using a transparent nose cone for just 10 min led to reduced c-fos-ir in ventral MCs compared to control rats. In these comparisons, c-fos-ir was evaluated 30 min after the 10 min-long period of restraint, and if evaluation was later than 30 min c-fos-ir was no longer suppressed. Granule cells (GCs) also showed suppressed c-fos-ir after acute restraint, but it was different than MCs, because the suppression persisted for over 30 min after the restraint. We conclude that c-fos protein expression is rapidly and transiently reduced in ventral hilar MCs after a brief period of restraint, and suppressed longer in dorsal GCs.

Dendrites of dentate gyrus granule cells contribute to pattern separation by controlling sparsity

The hippocampus plays a key role in pattern separation, the process of transforming similar incoming information to highly dissimilar, nonverlapping representations. Sparse firing granule cells (GCs) in the dentate gyrus (DG) have been proposed to undertake this computation, but little is known about which of their properties influence pattern separation. Dendritic atrophy has been reported in diseases associated with pattern separation deficits, suggesting a possible role for dendrites in this phenomenon. To investigate whether and how the dendrites of GCs contribute to pattern separation, we build a simplified, biologically relevant, computational model of the DG. Our model suggests that the presence of GC dendrites is associated with high pattern separation efficiency while their atrophy leads to increased excitability and performance impairments. These impairments can be rescued by restoring GC sparsity to control levels through various manipulations. We predict that dendrites contribute to pattern separation as a mechanism for controlling sparsity. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Interplay between excitability type and distributions of neuronal connectivity determines neuronal network synchronization

While the interplay between neuronal excitability properties and global properties of network topology is known to affect network propensity for synchronization, it is not clear how detailed characteristics of these properties affect spatiotemporal pattern formation. Here we study mixed networks, composed of neurons having type I and/or type II phase response curves, with varying distributions of local and random connections and show that not only average network properties, but also the connectivity distribution statistics, significantly affect network synchrony. Namely, we study networks with fixed networkwide properties, but vary the number of random connections that nodes project. We show that varying node excitability (type I vs type II) influences network synchrony most dramatically for systems with long-tailed distributions of the number of random connections per node. This indicates that a cluster of even a few highly rewired cells with a high propensity for synchronization can alter the degree of synchrony in the network as a whole. We show this effect generally on a network of coupled Kuramoto oscillators and investigate the impact of this effect more thoroughly in pulse-coupled networks of biophysical neurons.

Infiltrating monocytes promote brain inflammation and exacerbate neuronal damage after status epilepticus

The generalized seizures of status epilepticus (SE) trigger a series of molecular and cellular events that produce cognitive deficits and can culminate in the development of epilepsy. Known early events include opening of the blood-brain barrier (BBB) and astrocytosis accompanied by activation of brain microglia. Whereas circulating monocytes do not infiltrate the healthy CNS, monocytes can enter the brain in response to injury and contribute to the immune response. We examined the cellular components of innate immune inflammation in the days following SE by discriminating microglia vs. brain-infiltrating monocytes. Chemokine receptor 2 (CCR2(+)) monocytes invade the hippocampus between 1 and 3 d after SE. In contrast, only an occasional CD3(+) T lymphocyte was encountered 3 d after SE. The initial cellular sources of the chemokine CCL2, a ligand for CCR2, included perivascular macrophages and microglia. The induction of the proinflammatory cytokine IL-1β was greater in FACS-isolated microglia than in brain-invading monocytes. However, Ccr2 knockout mice displayed greatly reduced monocyte recruitment into brain and reduced levels of the proinflammatory cytokine IL-1β in hippocampus after SE, which was explained by higher expression of the cytokine in circulating and brain monocytes in wild-type mice. Importantly, preventing monocyte recruitment accelerated weight regain, reduced BBB degradation, and attenuated neuronal damage. Our findings identify brain-infiltrating monocytes as a myeloid-cell subclass that contributes to neuroinflammation and morbidity after SE. Inhibiting brain invasion of CCR2(+) monocytes could represent a viable method for alleviating the deleterious consequences of SE.

Interneuron Progenitor Transplantation to Treat CNS Dysfunction

Due to the inadequacy of endogenous repair mechanisms diseases of the nervous system remain a major challenge to scientists and clinicians. Stem cell based therapy is an exciting and viable strategy that has been shown to ameliorate or even reverse symptoms of CNS dysfunction in preclinical animal models. Of particular importance has been the use of GABAergic interneuron progenitors as a therapeutic strategy. Born in the neurogenic niches of the ventral telencephalon, interneuron progenitors retain their unique capacity to disperse, integrate and induce plasticity in adult host circuitries following transplantation. Here we discuss the potential of interneuron based transplantation strategies as it relates to CNS disease therapeutics. We also discuss mechanisms underlying their therapeutic efficacy and some of the challenges that face the field.

Neuronal Circuitry Mechanisms Regulating Adult Mammalian Neurogenesis

The adult mammalian brain is a dynamic structure, capable of remodeling in response to various physiological and pathological stimuli. One dramatic example of brain plasticity is the birth and subsequent integration of newborn neurons into the existing circuitry. This process, termed adult neurogenesis, recapitulates neural developmental events in two specialized adult brain regions: the lateral ventricles of the forebrain. Recent studies have begun to delineate how the existing neuronal circuits influence the dynamic process of adult neurogenesis, from activation of quiescent neural stem cells (NSCs) to the integration and survival of newborn neurons. Here, we review recent progress toward understanding the circuit-based regulation of adult neurogenesis in the hippocampus and olfactory bulb.

The enigmatic mossy cell of the dentate gyrus

Mossy cells comprise a large fraction of the cells in the hippocampal dentate gyrus, suggesting that their function in this region is important. They are vulnerable to ischaemia, traumatic brain injury and seizures, and their loss could contribute to dentate gyrus dysfunction in such conditions. Mossy cell function has been unclear because these cells innervate both glutamatergic and GABAergic neurons within the dentate gyrus, contributing to a complex circuitry. It has also been difficult to directly and selectively manipulate mossy cells to study their function. In light of the new data generated using methods to preferentially eliminate or activate mossy cells in mice, it is timely to ask whether mossy cells have become any less enigmatic than they were in the past.

Organization and control of epileptic circuits in temporal lobe epilepsy

When studying the pathological mechanisms of epilepsy, there are a seemingly endless number of approaches from the ultrastructural level-receptor expression by EM-to the behavioral level-comorbid depression in behaving animals. Epilepsy is characterized as a disorder of recurrent seizures, which are defined as "a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain" (Fisher et al., 2005). Such abnormal activity typically does not occur in a single isolated neuron; rather, it results from pathological activity in large groups-or circuits-of neurons. Here we choose to focus on two aspects of aberrant circuits in temporal lobe epilepsy: their organization and potential mechanisms to control these pathological circuits. We also look at two scales: microcircuits, ie, the relationship between individual neurons or small groups of similar neurons, and macrocircuits, ie, the organization of large-scale brain regions. We begin by summarizing the large body of literature that describes the stereotypical anatomical changes in the temporal lobe-ie, the anatomical basis of alterations in microcircuitry. We then offer a brief introduction to graph theory and describe how this type of mathematical analysis, in combination with computational neuroscience techniques and using parameters obtained from experimental data, can be used to postulate how microcircuit alterations may lead to seizures. We then zoom out and look at the changes which are seen over large whole-brain networks in patients and animal models, and finally we look to the future.

Hilar somatostatin interneuron loss reduces dentate gyrus inhibition in a mouse model of temporal lobe epilepsy

OBJECTIVE

In patients with temporal lobe epilepsy, seizures usually start in the hippocampus, and dentate granule cells are hyperexcitable. Somatostatin interneurons are a major subpopulation of inhibitory neurons in the dentate gyrus, and many are lost in patients and animal models. However, surviving somatostatin interneurons sprout axon collaterals and form new synapses, so the net effect on granule cell inhibition remains unclear.

METHODS

The present study uses optogenetics to activate hilar somatostatin interneurons and measure the inhibitory effect on dentate gyrus perforant path-evoked local field potential responses in a mouse model of temporal lobe epilepsy.

RESULTS

In controls, light activation of hilar somatostatin interneurons inhibited evoked responses up to 40%. Epileptic pilocarpine-treated mice exhibited loss of hilar somatostatin interneurons and less light-induced inhibition of evoked responses.

SIGNIFICANCE

These findings suggest that severe epilepsy-related loss of hilar somatostatin interneurons can overwhelm the surviving interneurons' capacity to compensate by sprouting axon collaterals.

Dentate cannabinoid-sensitive interneurons undergo unique and selective strengthening of mutual synaptic inhibition in experimental epilepsy

Altered inhibition is a salient feature of hippocampal network reorganization in epilepsy. Hippocampal pyramidal cells and dentate granule cells show specific reduction in cannabinoid receptor type 1 (CB1R)-sensitive GABAergic inputs in experimental epilepsy. In the dentate gyrus, CB1Rs regulate synaptic release from accommodating interneurons (AC-INs) with adapting firing characteristics and axonal projections in the molecular layer, but not from fast-spiking basket cells (FS-BCs). However, it is not known whether the intrinsic physiology and synaptic inhibition of AC-INs responsible for CB1R-sensitive inhibition is altered in epilepsy. Using the pilocarpine-induced status epilepticus (SE) model of epilepsy, we find that the basic physiological characteristics of AC-INs in epileptic rats are not different from age-matched controls. In paired interneuronal recordings, the amplitude of unitary inhibitory synaptic currents (uIPSCs) between AC-INs doubled after SE. Non-stationary noise analysis revealed that the post-SE strengthening of synapses between AC-INs resulted from an increase in postsynaptic receptors. Baseline synaptic release and CB1R antagonist enhancement of release at synapses between AC-INs were not different between control and post-SE rats. Additionally, uIPSC amplitude in FS-BCs to AC-INs pairs was unchanged after SE indicating input-specific microcircuit alterations in inhibitory inputs to AC-INs. At the network level, AC-INs showed no reduction in spontaneous and miniature inhibitory synaptic current (sIPSC or mIPSC) frequency or amplitude after SE. However, AC-IN mIPSC amplitude was persistently enhanced in post-SE and epileptic rats. CB1R agonist reduced the amplitude and suppressed a greater proportion of sIPSCs in AC-INs from post-SE and epileptic rats demonstrating a novel, cell-type specific increase in CB1R-sensitive inhibition of AC-INs after SE. This unique post-SE strengthening of inhibition between AC-INs could lead to activity-dependent suppression of AC-IN firing and compromise dentate CB1R-sensitive inhibition in epilepsy.

Potential implications of a monosynaptic pathway from mossy cells to adult-born granule cells of the dentate gyrus

The dentate gyrus (DG) is important to many aspects of hippocampal function, but there are many aspects of the DG that are incompletely understood. One example is the role of mossy cells (MCs), a major DG cell type that is glutamatergic and innervates the primary output cells of the DG, the granule cells (GCs). MCs innervate the GCs as well as local circuit neurons that make GABAergic synapses on GCs, so the net effect of MCs on GCs – and therefore the output of the DG – is unclear. Here we first review fundamental information about MCs and the current hypotheses for their role in the normal DG and in diseases that involve the DG. Then we review previously published data which suggest that MCs are a source of input to a subset of GCs that are born in adulthood (adult-born GCs). In addition, we discuss the evidence that adult-born GCs may support the normal inhibitory ‘gate’ functions of the DG, where the GCs are a filter or gate for information from the entorhinal cortical input to area CA3. The implications are then discussed in the context of seizures and temporal lobe epilepsy (TLE). In TLE, it has been suggested that the DG inhibitory gate is weak or broken and MC loss leads to insufficient activation of inhibitory neurons, causing hyperexcitability. That idea was called the “dormant basket cell hypothesis.” Recent data suggest that loss of normal adult-born GCs may also cause disinhibition, and seizure susceptibility. Therefore, we propose a reconsideration of the dormant basket cell hypothesis with an intervening adult-born GC between the MC and basket cell and call this hypothesis the “dormant immature granule cell hypothesis.”

A Million-Plus Neuron Model of the Hippocampal Dentate Gyrus: Critical Role for Topography in Determining Spatiotemporal Network Dynamics

GOAL

This paper describes a million-plus granule cell compartmental model of the rat hippocampal dentate gyrus, including excitatory, perforant path input from the entorhinal cortex, and feedforward and feedback inhibitory input from dentate interneurons.

METHODS

The model includes experimentally determined morphological and biophysical properties of granule cells, together with glutamatergic AMPA-like EPSP and GABAergic GABAA-like IPSP synaptic excitatory and inhibitory inputs, respectively. Each granule cell was composed of approximately 200 compartments having passive and active conductances distributed throughout the somatic and dendritic regions. Modeling excitatory input from the entorhinal cortex was guided by axonal transport studies documenting the topographical organization of projections from subregions of the medial and lateral entorhinal cortex, plus other important details of the distribution of glutamatergic inputs to the dentate gyrus. Information contained within previously published maps of this major hippocampal afferent were systematically converted to scales that allowed the topographical distribution and relative synaptic densities of perforant path inputs to be quantitatively estimated for inclusion in the current model.

RESULTS

Results showed that when medial and lateral entorhinal cortical neurons maintained Poisson random firing, dentate granule cells expressed, throughout the million-cell network, a robust nonrandom pattern of spiking best described as a spatiotemporal "clustering." To identify the network property or properties responsible for generating such firing "clusters," we progressively eliminated from the model key mechanisms, such as feedforward and feedback inhibition, intrinsic membrane properties underlying rhythmic burst firing, and/or topographical organization of entorhinal afferents.

CONCLUSION

Findings conclusively identified topographical organization of inputs as the key element responsible for generating a spatiotemporal distribution of clustered firing. These results uncover a functional organization of perforant path afferents to the dentate gyrus not previously recognized: topography-dependent clusters of granule cell activity as "functional units" or "channels" that organize the processing of entorhinal signals. This modeling study also reveals for the first time how a global signal processing feature of a neural network can evolve from one of its underlying structural characteristics.

Surviving mossy cells enlarge and receive more excitatory synaptic input in a mouse model of temporal lobe epilepsy

Numerous hypotheses of temporal lobe epileptogenesis have been proposed, and several involve hippocampal mossy cells. Building on previous hypotheses we sought to test the possibility that after epileptogenic injuries surviving mossy cells develop into super-connected seizure-generating hub cells. If so, they might require more cellular machinery and consequently have larger somata, elongate their dendrites to receive more synaptic input, and display higher frequencies of miniature excitatory synaptic currents (mEPSCs). To test these possibilities pilocarpine-treated mice were evaluated using GluR2-immunocytochemistry, whole-cell recording, and biocytin-labeling. Epileptic pilocarpine-treated mice displayed substantial loss of GluR2-positive hilar neurons. Somata of surviving neurons were 1.4-times larger than in controls. Biocytin-labeled mossy cells also were larger in epileptic mice, but dendritic length per cell was not significantly different. The average frequency of mEPSCs of mossy cells recorded in the presence of tetrodotoxin and bicuculline was 3.2-times higher in epileptic pilocarpine-treated mice as compared to controls. Other parameters of mEPSCs were similar in both groups. Average input resistance of mossy cells in epileptic mice was reduced to 63% of controls, which is consistent with larger somata and would tend to make surviving mossy cells less excitable. Other intrinsic physiological characteristics examined were similar in both groups. Increased excitatory synaptic input is consistent with the hypothesis that surviving mossy cells develop into aberrantly super-connected seizure-generating hub cells, and soma hypertrophy is indirectly consistent with the possibility of axon sprouting. However, no obvious evidence of hyperexcitable intrinsic physiology was found. Furthermore, similar hypertrophy and hyper-connectivity has been reported for other neuron types in the dentate gyrus, suggesting mossy cells are not unique in this regard. Thus, findings of the present study reveal epilepsy-related changes in mossy cell anatomy and synaptic input but do not strongly support the hypothesis that mossy cells develop into seizure-generating hub cells.

Post-status epilepticus treatment with the cannabinoid agonist WIN 55,212-2 prevents chronic epileptic hippocampal damage in rats

Repeated seizures are often associated with development of refractory chronic epilepsy, the most common form of which is temporal lobe epilepsy. G-protein-coupled cannabinoid receptors (CB1 and CB2 receptors) regulate neuronal excitability and have been shown to mediate acute anticonvulsant effects of cannabinoids in animal models. However, the potential of cannabinoids to prevent chronic neuronal damage and development of epilepsy remains unexplored. We hypothesized that treatment with a CB receptor agonist after an episode of status epilepticus--but before development of spontaneous recurrent seizures--might prevent the development of functional changes that lead to chronic epilepsy. Using the rat pilocarpine model, a therapeutic approach was simulated by administering the CB agonist, WIN 55,212-2 after an episode of status epilepticus. Epileptic behavior was monitored during development of spontaneous recurrent seizures for up to 6 months. Histology, neurochemistry, redox status and NMDA receptor subunit expression were assessed at 6 months after pilocarpine-induced seizures. Sub-acute treatment with WIN 55,212-2 (for 15 days starting 24h after PILO injection) dramatically attenuated the severity, duration and frequency of spontaneous recurrent seizures. Further, in contrast to vehicle-treated animals, hippocampi from WIN 55,212-2-treated animals showed: normal thiol redox state, normal NR2A and NR2B subunit expression, preservation of GABAergic neurons and prevention of abnormal proliferation of GABAergic progenitors. This study shows for the first time that, after a known inciting event, treatment with a compound targeting CB receptors has the potential to prevent the epileptogenic events that result in chronic epileptic damage.

Persistent reduction of hippocampal glutamine synthetase expression after status epilepticus in immature rats

Mesiotemporal sclerosis (MTS), the most frequent form of drug-resistant temporal lobe epilepsy, often develops after an initial precipitating injury affecting the immature brain. To analyse early processes in epileptogenesis we used the juvenile pilocarpine model to study status epilepticus (SE)-induced changes in expression of key components in the glutamate-glutamine cycle, known to be affected in MTS patients. SE was induced by Li(+) /pilocarpine injection in 21-day-old rats. At 2-19 weeks after SE hippocampal protein expression was analysed by immunohistochemistry and neuron damage by FluoroJade staining. Spontaneous seizures occurred in at least 44% of animals 15-18 weeks after SE. As expected in this model, we did not observe loss of principal hippocampal neurons. Neuron damage was most pronounced in the hilus, where we also detected progressive loss of parvalbumin-positive GABAergic interneurons. Hilar neuron loss (or end-folium sclerosis), a common feature in patients with MTS, was accompanied by a progressively decreased glutamine synthetase (GS)-immunoreactivity from 2 (-15%) to 19 weeks (-33.5%) after SE. Immunoreactivity for excitatory amino-acid transporters, vesicular glutamate transporter 1 and glial fibrillary acidic protein was unaffected. Our data show that SE elicited in 21-day-old rats induces a progressive reduction in hilar GS expression without affecting other key components of the glutamate-glutamine cycle. Reduced expression of glial enzyme GS was first detected 2 weeks after SE, and thus clearly before spontaneous recurrent seizures occurred. These results support the hypothesis that reduced GS expression is an early event in the development of hippocampal sclerosis in MTS patients and emphasize the importance of astrocytes in early epileptogenesis.