Status epilepticus enhances tonic GABA currents and depolarizes GABA reversal potential in dentate fast-spiking basket cells

δ-containing GABA A receptors on parvalbumin interneurons modulate neuronal excitability and network dynamics in the mouse medial prefrontal cortex

In medial prefrontal cortex (mPFC), fast-spiking parvalbumin (PV) interneurons regulate excitability and microcircuit oscillatory activity important for cognition. Although PV interneurons inhibit pyramidal neurons, they themselves express δ subunits of GABAA receptors important for slow inhibition. However, the specific contribution of δ-containing GABAA receptors to the function of PV interneurons in mPFC is unclear. We explored cellular, synaptic, and local-circuit activity in PV interneurons and pyramidal neurons in mouse mPFC after selectively deleting δ subunits in PV interneurons (cKO mice). In current-clamp recordings, cKO PV interneurons exhibited a higher frequency of action potentials and higher input resistance than wild type (WT) PV interneurons. Picrotoxin increased firing and GABA decreased firing in WT PV interneurons but not in cKO PV interneurons. The δ-preferring agonist THIP reduced spontaneous inhibitory postsynaptic currents in WT pyramidal neurons but not in cKO pyramidal neurons. In WT slices, depolarizing the network with 400 nM kainate increased firing of pyramidal neurons but had little effect on PV interneuron firing. By contrast, in cKO slices kainate recruited PV interneurons at the expense of pyramidal neurons. At the population level, kainate induced broadband increases in local field potentials in WT but not cKO slices. These results on cells and the network can be understood through increased excitability of cKO PV interneurons. In summary, our study demonstrates that δ-containing GABAA receptors in mPFC PV interneurons play a crucial role in regulating their excitability and the phasic inhibition of pyramidal neurons, elucidating intricate mechanisms governing cortical circuitry.

Significance statement

By selectively deleting δ-containing GABAA receptors in PV interneurons, we demonstrate the importance of these receptors on PV interneuron excitability, synaptic inhibition of pyramidal neurons, and circuit function.

Enhancing GABAergic Tonic Inhibition Reduces Seizure-Like Activity in the Neonatal Mouse Hippocampus and Neocortex

Approximately one-third of neonatal seizures do not respond to first-line anticonvulsants, including phenobarbital, which enhances phasic inhibition. Whether enhancing tonic inhibition decreases seizure-like activity in the neonate when GABA is mainly depolarizing at this age is unknown. We evaluated if increasing tonic inhibition using THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, gaboxadol], a δ-subunit-selective GABAA receptor agonist, decreases seizure-like activity in neonatal C57BL/6J mice (postnatal day P5-8, both sexes) using acute brain slices. Whole-cell patch-clamp recordings showed that THIP enhanced GABAergic tonic inhibitory conductances in layer V neocortical and CA1 pyramidal neurons and increased their rheobase without altering sEPSC characteristics. Two-photon calcium imaging demonstrated that enhancing the activity of extrasynaptic GABAARs decreased neuronal firing in both brain regions. In the 4-aminopyridine and the low-Mg2+ model of pharmacoresistant seizures, THIP reduced epileptiform activity in the neocortex and CA1 hippocampal region of neonatal and adult brain slices in a dose-dependent manner. We conclude that neocortical layer V and CA1 pyramidal neurons have tonic inhibitory conductances, and when enhanced, they reduce neuronal firing and decrease seizure-like activity. Therefore, augmenting tonic inhibition could be a viable approach for treating neonatal seizures.

Impacted spike frequency adaptation associated with reduction of KCNQ2/3 exacerbates seizure activity in temporal lobe epilepsy

Numerous epilepsy-related genes have been identified in recent decades by unbiased genome-wide screens. However, the available druggable targets for temporal lobe epilepsy (TLE) remain limited. Furthermore, a substantial pool of candidate genes potentially applicable to TLE therapy awaits further validation. In this study, we reveal the significant role of KCNQ2 and KCNQ3, two M-type potassium channel genes, in the onset of seizures in TLE. Our investigation began with a quantitative analysis of two publicly available TLE patient databases to establish a correlation between seizure onset and the downregulated expression of KCNQ2/3. We then replicated these pathological changes in a pilocarpine seizure mouse model and observed a decrease in spike frequency adaptation due to the affected M-currents in dentate gyrus granule neurons. In addition, we performed a small-scale simulation of the dentate gyrus network and confirmed that the impaired spike frequency adaptation of granule cells facilitated epileptiform activity throughout the network. This, in turn, resulted in prolonged seizure duration and reduced interictal intervals. Our findings shed light on an underlying mechanism contributing to ictogenesis in the TLE hippocampus and suggest a promising target for the development of antiepileptic drugs.

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.

Antiepileptogenic and neuroprotective effect of mefloquine after experimental status epilepticus

Acquired temporal lobe epilepsy (TLE) characterized by spontaneous recurrent seizures (SRS) and hippocampal inhibitory neuron dysfunction is often refractory to current therapies. Gap junctional or electrical coupling between inhibitory neurons has been proposed to facilitate network synchrony and intercellular molecular exchange suggesting a role in both seizures and neurodegeneration. While gap junction blockers can limit acute seizures, whether blocking neuronal gap junctions can modify development of chronic epilepsy has not been examined. This study examined whether mefloquine, a selective blocker of Connexin 36 gap junctions which are well characterized in inhibitory neurons, can limit epileptogenesis and related cellular and behavioral pathology in a model of acquired TLE. A single, systemic dose of mefloquine administered early after pilocarpine-induced status epilepticus (SE) in rat reduced both development of SRS and behavioral co-morbidities. Immunostaining for interneuron subtypes identified that mefloquine treatment likely reduced delayed inhibitory neuronal loss after SE. Uniquely, parvalbumin expressing neurons in the hippocampal dentate gyrus appeared relatively resistant to early cell loss after SE. Functionally, whole cell patch clamp recordings revealed that mefloquine treatment preserved inhibitory synaptic drive to projection neurons one week and one month after SE. These results demonstrate that mefloquine, a drug already approved for malaria prophylaxis, is potentially antiepileptogenic and can protect against progressive interneuron loss and behavioral co-morbidities of epilepsy.

Antagonism of P2X7 receptors enhances lorazepam action in delaying seizure onset in an in vitro model of status epilepticus

Approximately 30% of patients with status epilepticus (SE) become refractory to two or more antiseizure medications (ASMs). There is thus a real need to identify novel targets against which to develop new ASMs for treating this clinical emergency. Among purinergic receptors, the ionotropic ATP-gated P2X7 receptor (P2X7R) has received attention as a potential ASM target. This study evaluated the effect of the selective P2X7R antagonist A740003 on acute seizures in the dentate gyrus (DG) of hippocampal brain slices, where P2X7Rs are highly expressed, with a view to establishing the potential of P2X7R antagonists as a therapy or adjunct with lorazepam (LZP) in refractory SE. Extracellular electrophysiological recordings were made from the DG of male mouse hippocampal slices. Spontaneous seizure-like events (SLEs) were induced by removing extracellular Mg2+ and sequentially adding the K+ channel blocker 4-aminopyridine and the adenosine A1 receptor antagonist 8-cyclopentyltheophylline, during which the early and late application of A740003 and/or lorazepam was evaluated. Our study revealed that, in the absence of changes in mRNA for P2X7Rs or inflammatory markers, P2X7R antagonism did not reduce the frequency of SLEs. However, A740003 in conjunction with LZP delayed the onset of seizures. Furthermore, our results support the need for employing LZP before seizures become refractory during SE as delayed application of LZP increased seizure frequency. These studies reveal possible sites of intervention that could have a positive impact in patients with high risk of suffering SE.

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.

Excitatory/inhibitory balance in epilepsies and neurodevelopmental disorders: Depolarizing γ-aminobutyric acid as a common mechanism

Epilepsy is one of the most common neurological disorders. Although many factors contribute to epileptogenesis, seizure generation is mostly linked to hyperexcitability due to alterations in excitatory/inhibitory (E/I) balance. The common hypothesis is that reduced inhibition, increased excitation, or both contribute to the etiology of epilepsy. Increasing evidence shows that this view is oversimplistic, and that increased inhibition through depolarizing γ-aminobutyric acid (GABA) similarly contributes to epileptogenisis. In early development, GABA signaling is depolarizing, inducing outward Cl- currents due to high intracellular Cl- concentrations. During maturation, the mechanisms of GABA action shift from depolarizing to hyperpolarizing, a critical event during brain development. Altered timing of this shift is associated with both neurodevelopmental disorders and epilepsy. Here, we consider the different ways that depolarizing GABA contributes to altered E/I balance and epileptogenesis, and discuss that alterations in depolarizing GABA could be a common denominator underlying seizure generation in neurodevelopmental disorders and epilepsies.

Degeneracy in epilepsy: multiple routes to hyperexcitable brain circuits and their repair

Due to its complex and multifaceted nature, developing effective treatments for epilepsy is still a major challenge. To deal with this complexity we introduce the concept of degeneracy to the field of epilepsy research: the ability of disparate elements to cause an analogous function or malfunction. Here, we review examples of epilepsy-related degeneracy at multiple levels of brain organisation, ranging from the cellular to the network and systems level. Based on these insights, we outline new multiscale and population modelling approaches to disentangle the complex web of interactions underlying epilepsy and to design personalised multitarget therapies.

Long-Term Effects of Moderate Concussive Brain Injury During Adolescence on Synaptic and Tonic GABA Currents in Dentate Granule Cells and Semilunar Granule Cells

Progressive physiological changes in the hippocampal dentate gyrus circuits following traumatic brain injury (TBI) contribute to temporal evolution of neurological sequelae. Although early posttraumatic changes in dentate synaptic and extrasynaptic GABA currents have been reported, and whether they evolve over time and remain distinct between the two projection neuron classes, granule cells and semilunar granule cells, have not been evaluated. We examined long-term changes in tonic GABA currents and spontaneous inhibitory postsynaptic currents (sIPSCs) and in dentate projection neurons 3 months after moderate concussive fluid percussion injury (FPI) in adolescent rats. Granule cell tonic GABA current amplitude remained elevated up to 1 month after FPI, but decreased to levels comparable with age-matched controls by 3 months postinjury. Granule cell sIPSC frequency, which we previously reported to be increased 1 week after FPI, remained higher than in age-matched controls at 1 month and was significantly reduced 3 months after FPI. In semilunar granule cells, tonic GABA current amplitude and sIPSC frequency were not different from controls 3 months after FPI, which contrast with decreases observed 1 week after injury. The switch in granule cell inhibitory inputs from early increase to subsequent decrease could contribute to the delayed emergence of cognitive deficits and seizure susceptibility after brain injury.

Dysregulation of REV-ERBα impairs GABAergic function and promotes epileptic seizures in preclinical models

To design potentially more effective therapies, we need to further understand the mechanisms underlying epilepsy. Here, we uncover the role of Rev-erbα in circadian regulation of epileptic seizures. We first show up-regulation of REV-ERBα/Rev-erbα in brain tissues from patients with epilepsy and a mouse model. Ablation or pharmacological modulation of Rev-erbα in mice decreases the susceptibility to acute and chronic seizures, and abolishes diurnal rhythmicity in seizure severity, whereas activation of Rev-erbα increases the animal susceptibility. Rev-erbα ablation or antagonism also leads to prolonged spontaneous inhibitory postsynaptic currents and elevated frequency in the mouse hippocampus, indicating enhanced GABAergic signaling. We also identify the transporters Slc6a1 and Slc6a11 as regulators of Rev-erbα-mediated clearance of GABA. Mechanistically, Rev-erbα promotes the expressions of Slc6a1 and Slc6a11 through transcriptional repression of E4bp4. Our findings propose Rev-erbα as a regulator of synaptic function at the crosstalk between pathways regulating the circadian clock and epilepsy.

Traumatic Brain Injury Diminishes Feedforward Activation of Parvalbumin-Expressing Interneurons in the Dentate Gyrus

Traumatic brain injury (TBI) is associated with aberrant network hyperexcitability in the dentate gyrus (DG). GABA_Aergic parvalbumin-expressing interneurons (PV-INs) in the DG regulate network excitability with strong, perisomatic inhibition, although the posttraumatic effects on PV-IN function after TBI are not well understood. In this study, we investigated physiological alterations in PV-INs one week after mild lateral fluid percussion injury (LFPI) in mice. PV-IN cell loss was observed in the dentate hilus after LFPI, with surviving PV-INs showing no change in intrinsic membrane properties. Whole-cell voltage clamp recordings in PV-INs revealed alterations in both EPSCs and IPSCs (EPSCs/IPSCs). Evoked EPSCs (eEPSCs) in PV-INs from perforant path electrical stimulation were diminished after injury but could be recovered with application of a GABA_A-receptor antagonist. Furthermore, current-clamp recordings using minimal perforant path stimulation demonstrated a decrease in evoked PV-IN action potentials (APs) after LFPI, which could be restored by blocking GABA_Aergic inhibition. Together, these findings suggest that injury alters synaptic input onto PV-INs, resulting in a net inhibitory effect that reduces feedforward PV-IN activation in the DG. Decreased PV-IN activation suggests a potential mechanism of DG network hyperexcitability contributing to hippocampal dysfunction after TBI.

Dendritic morphology and inhibitory regulation distinguish dentate semilunar granule cells from granule cells through distinct stages of postnatal development

Semilunar granule cells (SGCs) have been proposed as a morpho-functionally distinct class of hippocampal dentate projection neurons contributing to feedback inhibition and memory processing in juvenile rats. However, the structural and physiological features that can reliably classify granule cells (GCs) from SGCs through postnatal development remain unresolved. Focusing on postnatal days 11-13, 28-42, and > 120, corresponding with human infancy, adolescence, and adulthood, we examined the somato-dendritic morphology and inhibitory regulation in SGCs and GCs to determine the cell-type specific features. Unsupervised cluster analysis confirmed that morphological features reliably distinguish SGCs from GCs irrespective of animal age. SGCs maintain higher spontaneous inhibitory postsynaptic current (sIPSC) frequency than GCs from infancy through adulthood. Although sIPSC frequency in SGCs was particularly enhanced during adolescence, sIPSC amplitude and cumulative charge transfer declined from infancy to adulthood and were not different between GCs and SGCs. Extrasynaptic GABA current amplitude peaked in adolescence in both cell types and was significantly greater in SGCs than in GCs only during adolescence. Although GC input resistance was higher than in SGCs during infancy and adolescence, input resistance decreased with developmental age in GCs, while it progressively increased in SGCs. Consequently, GCs' input resistance was significantly lower than SGCs in adults. The data delineate the structural features that can reliably distinguish GCs from SGCs through development. The results reveal developmental differences in passive membrane properties and steady-state inhibition between GCs and SGCs which could confound their use in classifying the cell types.

Goldilocks Zone of Ictal Onset: Partially Recovered Synapses Provide the Kindling to Fuel Ictal Activity

A Proposed Mechanism for Spontaneous Transitions Between Interictal and Ictal Activity

Jacob T, Lillis KP, Wang Z, Swiercz W, Rahmati N, Staley KJ. J Neurosci. 2019;39(3):557-575. doi:10.1523/JNEUROSCI.0719-17.2018. Epub 2018 Nov 16. PMID: 30446533

Epileptic networks are characterized by 2 outputs: brief interictal spikes and rarer, more prolonged seizures. Although either output state is readily modeled in silico and induced experimentally, the transition mechanisms are unknown, in part because no models exhibit both output states spontaneously. In silico, small-world neural networks were built using single-compartment neurons whose physiological parameters were derived from dual whole-cell recordings of pyramidal cells in organotypic hippocampal slice cultures that were generating spontaneous seizure-like activity. In silico, neurons were connected by abundant local synapses and rare long-distance synapses. Activity-dependent synaptic depression and gradual recovery delimited synchronous activity. Full synaptic recovery engendered interictal population spikes that spread via long-distance synapses. When synaptic recovery was incomplete, postsynaptic neurons required coincident activation of multiple presynaptic terminals to reach firing threshold. Only local connections were sufficiently dense to spread activity under these conditions. This coalesced network activity into traveling waves whose velocity varied with synaptic recovery. Seizures were comprised of sustained traveling waves that were similar to those recorded during experimental and human neocortical seizures. Sustained traveling waves occurred only when wave velocity, network dimensions, and the rate of synaptic recovery enabled wave reentry into previously depressed areas at precisely ictogenic levels of synaptic recovery. Wide-field, cellular resolution GCamP7b calcium imaging demonstrated similar initial patterns of activation in the hippocampus, although the anatomical distribution of traveling waves of synaptic activation was altered by the pattern of synaptic connectivity in the organotypic hippocampal cultures.

GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit

KEY POINTS

The lateral superior olive (LSO), a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABAB Rs modulate Ca2+ influx into medial nucleus of the trapezoid body axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABAA Rs. RNA sequencing revealed highly abundant δ subunits, which are characteristic of extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter.

ABSTRACT

GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole-cell patch-clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABAA receptors (GABAA Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABAA Rs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca2+ influx into MNTB axon terminals following activation of presynaptic GABAB Rs. GABAB R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABAA Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABAA R agonist. RNA sequencing showed high mRNA levels for the δ-subunit in the LSO. Moreover, GABA transporters GAT-1 and GAT-3 appear to control extracellular GABA. Finally, we show an age-dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABAA Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

Lack of β-amyloid cleaving enzyme-1 (BACE1) impairs long-term synaptic plasticity but enhances granule cell excitability and oscillatory activity in the dentate gyrus in vivo

BACE1 is a β-secretase involved in the cleavage of amyloid precursor protein and the pathogenesis of Alzheimer's disease (AD). The entorhinal cortex and the dentate gyrus are important for learning and memory, which are affected in the early stages of AD. Since BACE1 is a potential target for AD therapy, it is crucial to understand its physiological role in these brain regions. Here, we examined the function of BACE1 in the dentate gyrus. We show that loss of BACE1 in the dentate gyrus leads to increased granule cell excitability, indicated by enhanced efficiency of synaptic potentials to generate granule cell spikes. The increase in granule cell excitability was accompanied by prolonged paired-pulse inhibition, altered network gamma oscillations, and impaired synaptic plasticity at entorhinal-dentate synapses of the perforant path. In summary, this is the first detailed electrophysiological study of BACE1 deletion at the network level in vivo. The results suggest that BACE1 is important for normal dentate gyrus network function. This has implications for the use of BACE1 inhibitors as therapeutics for AD therapy, since BACE1 inhibition could similarly disrupt synaptic plasticity and excitability in the entorhinal-dentate circuitry.

Locally Reducing KCC2 Activity in the Hippocampus is Sufficient to Induce Temporal Lobe Epilepsy

Mesial temporal lobe epilepsy (mTLE) is the most common form of epilepsy, believed to arise in part from compromised GABAergic inhibition. The neuronal specific K+/Cl- co-transporter 2 (KCC2) is a critical determinant of the efficacy of GABAergic inhibition and deficits in its activity are observed in mTLE patients and animal models of epilepsy. To test if reductions of KCC2 activity directly contribute to the pathophysiology of mTLE, we locally ablated KCC2 expression in a subset of principal neurons within the adult hippocampus. Deletion of KCC2 resulted in compromised GABAergic inhibition and the development of spontaneous, recurrent generalized seizures. Moreover, local ablation of KCC2 activity resulted in hippocampal sclerosis, a key pathological change seen in mTLE. Collectively, our results demonstrate that local deficits in KCC2 activity within the hippocampus are sufficient to precipitate mTLE.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Seizing Control of KCC2: A New Therapeutic Target for Epilepsy

Deficits in GABAergic inhibition result in the abnormal neuronal activation and synchronization that underlies seizures. However, the molecular mechanisms responsible for transforming a normal brain into an epileptic one remain largely unknown. Hyperpolarizing inhibition mediated by type A GABA (GABAA) receptors is dependent on chloride extrusion by the neuron-specific type 2K+-Cl- cotransporter (KCC2). Loss-of-function mutations in KCC2 are a known cause of infantile epilepsy in humans and KCC2 dysfunction is present in patients with both idiopathic and acquired epilepsy. Here we discuss the growing evidence that KCC2 dysfunction has a central role in the development and severity of the epilepsies.

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.

Massively augmented hippocampal dentate granule cell activation accompanies epilepsy development

In a mouse model of temporal lobe epilepsy, multicellular calcium imaging revealed that disease emergence was accompanied by massive amplification in the normally sparse, afferent stimulation-induced activation of hippocampal dentate granule cells. Patch recordings demonstrated reductions in local inhibitory function within the dentate gyrus at time points where sparse activation was compromised. Mimicking changes in inhibitory synaptic function and transmembrane chloride regulation was sufficient to elicit the dentate gyrus circuit collapse evident during epilepsy development. Pharmacological blockade of outward chloride transport had no effect during epilepsy development, and significantly increased granule cell activation in both control and chronically epileptic animals. This apparent occlusion effect implicates reduction in chloride extrusion as a mechanism contributing to granule cell hyperactivation specifically during early epilepsy development. Glutamine plays a significant role in local synthesis of GABA in synapses. In epileptic mice, sparse granule cell activation could be restored by glutamine application, implicating compromised GABA synthesis. Glutamine had no effect on granule cell activation earlier, during epilepsy development. We conclude that compromised feedforward inhibition within the local circuit generates the massive dentate gyrus circuit hyperactivation evident in animals during and following epilepsy development. However, the mechanisms underlying this disinhibition diverge significantly as epilepsy progresses.

Chronic Unpredictable Mild Stress Induces Loss of GABA Inhibition in Corticotrophin-Releasing Hormone-Expressing Neurons through NKCC1 Upregulation

INTRODUCTION

Prolonged and repeated stresses cause hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. The corticotrophin-releasing hormone (CRH)-expressing neurons in the hypothalamic paraventricular nucleus (PVN) are an essential component of the HPA axis.

MATERIALS AND METHODS

Chronic unpredictable mild stress (CUMS) was induced in Sprague-Dawley rats. GABA reversal potentials (EGABA) were determined by using gramicidin-perforated recordings in identified PVN-CRH neurons through expressing enhanced green fluorescent protein driven by the CRH promoter. Plasma corticosterone (CORT) levels were measured in rats implanted with a cannula targeting the lateral ventricles and PVN.

RESULTS

Blocking the GABAA receptor in the PVN with gabazine significantly increased plasma CORT levels in unstressed rats but did not change CORT levels in CUMS rats. CUMS caused a depolarizing shift in EGABA in PVN-CRH neurons compared with EGABA in PVN-CRH neurons in unstressed rats. Furthermore, CUMS induced a long-lasting increase in expression levels of the cation chloride cotransporter Na+-K+-Cl--Cl- (NKCC1) in the PVN but a transient decrease in expression levels of K+-Cl--Cl- in the PVN, which returned to the basal level 5 days after CUMS treatment. The NKCC1 inhibitor bumetanide decreased the basal firing activity of PVN-CRH neurons and normalized EGABA and the gabazine-induced excitatory effect on PVN-CRH neurons in CUMS rats. In addition, central administration of bumetanide decreased basal circulating CORT levels in CUMS rats.

CONCLUSIONS

These data suggest that chronic stress impairs GABAergic inhibition, resulting in HPA axis hyperactivity through upregulation of NKCC1.

Compromised GABAergic inhibition contributes to tumor-associated epilepsy

Glioblastoma Multiforme (GBM) is the most common form of primary brain tumor with 30-50% of patients presenting with epilepsy. These tumor-associated seizures are often resistant to traditional antiepileptic drug treatment and persist after tumor resection. This suggests that changes in the peritumoral tissue underpin epileptogenesis. It is known that glioma cells extrude pathological concentrations of glutamate which is thought to play a role in tumor progression and the development of epilepsy. Given that pathological concentrations of glutamate have been shown to dephosphorylate and downregulate the potassium chloride cotransporter KCC2, we hypothesized that glioma-induced alterations in KCC2 in the peritumoral region may play a role in tumor-associated epilepsy. Consistent with this hypothesis, we observe a decrease in total KCC2 expression and a dephosphorylation of KCC2 at residue Ser940 in a glioma model which exhibits hyperexcitability and the development of spontaneous seizures. To determine whether the reduction of KCC2 could potentially contribute to tumor-associated epilepsy, we generated mice with a focal knockdown of KCC2 by injecting AAV2-Cre-GFP into the cortex of floxed KCC2 mice. The AAV2-Cre-mediated knockdown of KCC2 was sufficient to induce the development of spontaneous seizures. Further, blocking NKCC1 with bumetanide to offset the loss of KCC2 reduced the seizure susceptibility in glioma-implanted mice. These findings support a mechanism of tumor-associated epilepsy involving downregulation of KCC2 in the peritumoral region leading to compromised GABAergic inhibition and suggest that modulating chloride homeostasis may be useful for seizure control.

In search of a periodic table of the neurons: Axonal-dendritic circuitry as the organizing principle: Patterns of axons and dendrites within distinct anatomical parcels provide the blueprint for circuit-based neuronal classification

No one knows yet how to organize, in a simple yet predictive form, the knowledge concerning the anatomical, biophysical, and molecular properties of neurons that are accumulating in thousands of publications every year. The situation is not dissimilar to the state of Chemistry prior to Mendeleev's tabulation of the elements. We propose that the patterns of presence or absence of axons and dendrites within known anatomical parcels may serve as the key principle to define neuron types. Just as the positions of the elements in the periodic table indicate their potential to combine into molecules, axonal and dendritic distributions provide the blueprint for network connectivity. Furthermore, among the features commonly employed to describe neurons, morphology is considerably robust to experimental conditions. At the same time, this core classification scheme is suitable for aggregating biochemical, physiological, and synaptic information.

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.

Ectopic Expression of α6 and δ GABAA Receptor Subunits in Hilar Somatostatin Neurons Increases Tonic Inhibition and Alters Network Activity in the Dentate Gyrus

The role of GABAA receptor (GABAAR)-mediated tonic inhibition in interneurons remains unclear and may vary among subgroups. Somatostatin (SOM) interneurons in the hilus of the dentate gyrus show negligible expression of nonsynaptic GABAAR subunits and very low tonic inhibition. To determine the effects of ectopic expression of tonic GABAAR subtypes in these neurons, Cre-dependent viral vectors were used to express GFP-tagged GABAAR subunits (α6 and δ) selectively in hilar SOM neurons in SOM-Cre mice. In single-transfected animals, immunohistochemistry demonstrated strong expression of either the α6 or δ subunit; in cotransfected animals, both subunits were consistently expressed in the same neurons. Electrophysiology revealed a robust increase of tonic current, with progressively larger increases following transfection of δ, α6, and α6/δ subunits, respectively, indicating formation of functional receptors in all conditions and likely coassembly of the subunits in the same receptor following cotransfection. An in vitro model of repetitive bursting was used to determine the effects of increased tonic inhibition in hilar SOM interneurons on circuit activity in the dentate gyrus. Upon cotransfection, the frequency of GABAAR-mediated bursting in granule cells was reduced, consistent with a reduction in synchronous firing among hilar SOM interneurons. Moreover, in vivo studies of Fos expression demonstrated reduced activation of α6/δ-cotransfected neurons following acute seizure induction by pentylenetetrazole. The findings demonstrate that increasing tonic inhibition in hilar SOM interneurons can alter dentate gyrus circuit activity during strong stimulation and suggest that tonic inhibition of interneurons could play a role in regulating excessive synchrony within the network.

SIGNIFICANCE STATEMENT

In contrast to many hippocampal interneurons, somatostatin (SOM) neurons in the hilus of the dentate gyrus have very low levels of nonsynaptic GABAARs and exhibit very little tonic inhibition. In an effort to increase tonic inhibition selectively in these interneurons, we used Cre-dependent viral vectors in SOM-Cre mice to achieve interneuron-specific expression of the nonsynaptic GABAAR subunits (α6 and δ) in vivo. We show, for the first time, that such recombinant GFP-tagged GABAAR subunits are expressed robustly, assemble to form functional receptors, substantially increase tonic inhibition in SOM interneurons, and alter circuit activity within the dentate gyrus.

Functional Reduction in Cannabinoid-Sensitive Heterotypic Inhibition of Dentate Basket Cells in Epilepsy: Impact on Network Rhythms

Strong perisomatic inhibition by fast-spiking basket cells (FS-BCs) regulates dentate throughput. Homotypic FS-BC interconnections that support gamma oscillations, and heterotypic inputs from diverse groups of interneurons that receive extensive neurochemical regulation, together, shape FS-BC activity patterns. However, whether seizures precipitate functional changes in inhibitory networks and contribute to abnormal network activity in epilepsy is not known. In the first recordings from dentate interneuronal pairs in a model of temporal lobe epilepsy, we demonstrate that status epilepticus (SE) selectively compromises GABA release at synapses from dentate accommodating interneurons (AC-INs) to FS-BCs, while efficacy of homotypic FS-BC synapses is unaltered. The functional decrease in heterotypic cannabinoid receptor type 1 (CB₁R)-sensitive inhibition of FS-BCs resulted from enhanced baseline GABA_B-mediated suppression of synaptic release after SE. The frequency of CB₁R-sensitive inhibitory synaptic events in FS-BCs was depressed early after SE induction and remained reduced in epileptic rats. In biologically based simulations of heterogeneous inhibitory networks and excitatory-inhibitory cell networks, experimentally identified decrease in reliability of AC-IN to FS-BCs synaptic release reduced theta power and theta-gamma coupling and enhanced gamma coherence. Thus, the experimentally identified functional reduction in heterotypic inhibition of FS-BCs can contribute to compromised network oscillations in epilepsy and could precipitate memory and cognitive co-morbidities.

Homeostasis or channelopathy? Acquired cell type-specific ion channel changes in temporal lobe epilepsy and their antiepileptic potential

Neurons continuously adapt the expression and functionality of their ion channels. For example, exposed to chronic excitotoxicity, neurons homeostatically downscale their intrinsic excitability. In contrast, the “acquired channelopathy” hypothesis suggests that proepileptic channel characteristics develop during epilepsy. We review cell type-specific channel alterations under different epileptic conditions and discuss the potential of channels that undergo homeostatic adaptations, as targets for antiepileptic drugs (AEDs). Most of the relevant studies have been performed on temporal lobe epilepsy (TLE), a widespread AED-refractory, focal epilepsy. The TLE patients, who undergo epilepsy surgery, frequently display hippocampal sclerosis (HS), which is associated with degeneration of cornu ammonis subfield 1 pyramidal cells (CA1 PCs). Although the resected human tissue offers insights, controlled data largely stem from animal models simulating different aspects of TLE and other epilepsies. Most of the cell type-specific information is available for CA1 PCs and dentate gyrus granule cells (DG GCs). Between these two cell types, a dichotomy can be observed: while DG GCs acquire properties decreasing the intrinsic excitability (in TLE models and patients with HS), CA1 PCs develop channel characteristics increasing intrinsic excitability (in TLE models without HS only). However, thorough examination of data on these and other cell types reveals the coexistence of protective and permissive intrinsic plasticity within neurons. These mechanisms appear differentially regulated, depending on the cell type and seizure condition. Interestingly, the same channel molecules that are upregulated in DG GCs during HS-related TLE, appear as promising targets for future AEDs and gene therapies. Hence, GCs provide an example of homeostatic ion channel adaptation which can serve as a primer when designing novel anti-epileptic strategies.

Potentiation of tonic GABAergic inhibition by activation of postsynaptic kainate receptors

Presynaptic kainate-type glutamate ionotropic receptors (KARs) that mediate either the depression or the facilitation of GABA release have been intensively studied. Little attention has been given to the modulation of GABAA receptors (GABAARs) by postsynaptic KARs. Recent studies suggest that two GABAAR populations, synaptic (sGABAAR) and extrasynaptic (eGABAAR) GABAARs, mediate phasic and tonic forms of inhibition, respectively. Tonic inhibition plays an important role in the excitability of neuronal circuits and the occurrence of epileptic seizures. For this study, we are the first to report that the activation of postsynaptic KARs by the KAR agonist, Kainic acid (KA, 5 μM), enhanced tonic inhibition by potentiating eGABAARs. KA enhanced THIP-induced eGABAAR currents and prolonged the rise and decay time of muscimol-induced sGABAAR/eGABAAR currents, but also depressed the amplitude of evoked inhibitory postsynaptic currents (IPSCs), unitary IPSCs (uIPSCs), and muscimol-induced sGABAAR/eGABAAR currents. The PKC inhibitor, staurosporine (1 μM), in the patch pipette solution fully blocked the KA-induced potentiation of tonic inhibition, suggesting the involvement of an intracellular PKC pathway. Our study suggests that the activation of postsynaptic KARs potentiates eGABAARs but depresses sGABAARs. By activating postsynaptic KARs, synaptically released glutamate depresses phasic inhibition to facilitate neuronal plasticity, but potentiates tonic inhibition to protect neurons from over-excitation.

Dentate total molecular layer interneurons mediate cannabinoid-sensitive inhibition

Activity of the dentate gyrus, which gates information flow to the hippocampus, is under tight inhibitory regulation by interneurons with distinctive axonal projections, intrinsic and synaptic characteristics and neurochemical identities. Total molecular layer cells (TML-Cs), a class of morphologically distinct GABAergic neurons with axonal projections across the molecular layer, are among the most frequent interneuronal type in the dentate subgranular region. However, little is known about their synaptic and neurochemical properties. We demonstrate that synapses from morphologically identified TML-Cs to dentate interneurons are characterized by low release probability, facilitating short-term dynamics and asynchronous release. TML-Cs consistently show somatic and axonal labeling for the cannabinoid receptor type 1 (CB₁R) yet fail to express cholecystokinin (CCK) indicating their distinctive neurochemical identity. In paired recordings, the release probability at synapses between TML-Cs was increased by the CB₁R antagonist AM251, demonstrating baseline endocannabinoid regulation of TML-C synapses. Apart from defining the synaptic and neurochemical features of TML-Cs, our findings reveal the morphological identity of a class of dentate CB₁R-positive neurons that do not express CCK. Our findings indicate that TML-Cs can mediate cannabinoid sensitive feed-forward and feedback inhibition of dentate perforant path inputs. © 2015 The Authors Hippocampus Published by Wiley Periodicals, Inc.

The importance of metadata to assess information content in digital reconstructions of neuronal morphology

Digital reconstructions of axonal and dendritic arbors provide a powerful representation of neuronal morphology in formats amenable to quantitative analysis, computational modeling, and data mining. Reconstructed files, however, require adequate metadata to identify the appropriate animal species, developmental stage, brain region, and neuron type. Moreover, experimental details about tissue processing, neurite visualization and microscopic imaging are essential to assess the information content of digital morphologies. Typical morphological reconstructions only partially capture the underlying biological reality. Tracings are often limited to certain domains (e.g., dendrites and not axons), may be incomplete due to tissue sectioning, imperfect staining, and limited imaging resolution, or can disregard aspects irrelevant to their specific scientific focus (such as branch thickness or depth). Gauging these factors is critical in subsequent data reuse and comparison. NeuroMorpho.Org is a central repository of reconstructions from many laboratories and experimental conditions. Here, we introduce substantial additions to the existing metadata annotation aimed to describe the completeness of the reconstructed neurons in NeuroMorpho.Org. These expanded metadata form a suitable basis for effective description of neuromorphological data.

Chronic stress shifts the GABA reversal potential in the hippocampus and increases seizure susceptibility

The most commonly reported precipitating factor for seizures is stress. However, the underlying mechanisms whereby stress triggers seizures are not yet fully understood. Here we demonstrate a potential mechanism underlying changes in neuronal excitability in the hippocampus following chronic stress, involving a shift in the reversal potential for GABA (EGABA) associated with a dephosphorylation of the potassium chloride co-transporter, KCC2. Mice subjected to chronic restraint stress (30min/day for 14 consecutive days) exhibit an increase in serum corticosterone levels which is associated with increased susceptibility to seizures induced with kainic acid (20mg/kg). Following chronic stress, but not acute stress, we observe a dephosphorylation of KCC2 residue S940, which regulates KCC2 cell surface expression and function, in the hippocampus. To determine the impact of alterations in KCC2 expression following chronic stress, we performed gramicidin perforated patch recordings to measure changes in EGABA and neuronal excitability of principal hippocampal neurons. We observe a depolarizing shift in EGABA in hippocampal CA1 pyramidal neurons after chronic stress. In addition, there is an increase in the intrinsic excitability of CA1 pyramidal neurons, evident by a shift in the input-output curve which could be reversed with the NKCC1 inhibitor, bumetanide. These data uncover a potential mechanism involving chronic stress-induced plasticity in chloride homeostasis which may contribute to stress-induced seizure susceptibility.

Toll-like receptor 4 enhancement of non-NMDA synaptic currents increases dentate excitability after brain injury

Concussive brain injury results in neuronal degeneration, microglial activation and enhanced excitability in the hippocampal dentate gyrus, increasing the risk for epilepsy and memory dysfunction. Endogenous molecules released during injury can activate innate immune responses including toll-like receptor 4 (TLR4). Recent studies indicate that immune mediators can modulate neuronal excitability. Since non-specific agents that reduce TLR4 signaling can limit post-traumatic neuropathology, we examined whether TLR4 signaling contributes to early changes in dentate excitability after brain injury. Concussive brain injury caused a transient increase in hippocampal TLR4 expression within 4h, which peaked at 24h. Post-injury increase in TLR4 expression in the dentate gyrus was primarily neuronal and persisted for one week. Acute, in vitro treatment with TLR4 ligands caused bidirectional modulation of dentate excitability in control and brain-injured rats, with a reversal in the direction of modulation after brain injury. TLR4 antagonists decreased, and agonist increased, afferent-evoked dentate excitability one week after brain injury. NMDA receptor antagonist did not occlude the ability of LPS-RS, a TLR4 antagonist, to decrease post-traumatic dentate excitability. LPS-RS failed to modulate granule cell NMDA EPSCs but decreased perforant path-evoked non-NMDA EPSC peak amplitude and charge transfer in both granule cells and mossy cells. Our findings indicate an active role for TLR4 signaling in early post-traumatic dentate hyperexcitability. The novel TLR4 modulation of non-NMDA glutamatergic currents, identified herein, could represent a general mechanism by which immune activation influences neuronal excitability in neurological disorders that recruit sterile inflammatory responses.

In vitro gamma oscillations following partial and complete ablation of δ subunit-containing GABAA receptors from parvalbumin interneurons

Perisynaptic and extrasynaptic δ subunit-containing GABAA receptors (δ-GABAARs) mediate tonic conductances in many neurons. On principal cells of the neocortex and hippocampus they comprise α4 subunits, whereas they usually contain α1 on various interneurons. Specific characteristics of δ-GABAARs are their pharmacology and high plasticity. In particular δ-GABAARs are sensitive to low concentrations of neurosteroids (NS) and during times of altered NS production (stress, puberty, ovarian cycle and pregnancy) δ-GABAARs expression varies in many neurons regardless of the α subunits they contain, with direct consequences for neuronal excitability and network synchrony. For example δ-GABAARs plasticity on INs underlies modifications in hippocampal γ oscillations during pregnancy or over the ovarian cycle. Most δ-GABAAR-expressing INs in CA3 stratum pyramidale (SP) are parvalbumin (PV) + INs, whose fundamental role in γ oscillations generation and control has been extensively investigated. In this study we reduced or deleted δ-subunits in PV + INs, with the use of a PV/Cre-Gabrd/floxed genetic system. We find that in vitro CA3 γ oscillations of both PV-Gabrd(+/-)and PV-Gabrd(-/-) mice are characterized by higher frequencies than WT controls. The increased frequencies could be lowered to control levels in PV-Gabrd(+/-) by the NS allopregnanolone (3α,5α-tetrahydroprogesterone, 100 nM) but not the synthetic δ-GABAAR positive allosteric modulator 4-Chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl] benzamide (DS-2, 10 μM). This is consistent with the idea that DS-2, in contrast to ALLO, selectively targets α4/δ-GABAARs but not the α1/δ-GABAARs found on INs. Therefore, development of drugs selective for IN-specific α1/δ-GABAARs may be useful in neurological and psychiatric conditions correlated with altered PV + IN function and aberrant γ oscillations.

Pathophysiological mechanisms underlying increased anxiety after soman exposure: reduced GABAergic inhibition in the basolateral amygdala

The recent sarin attack in Syria killed 1429 people, including 426 children, and left countless more to deal with the health consequences of the exposure. Prior to the Syrian chemical assault, nerve agent attacks in Japan left many victims suffering from neuropsychiatric illnesses, particularly anxiety disorders, more than a decade later. Uncovering the neuro-pathophysiological mechanisms underlying the development of anxiety after nerve agent exposure is necessary for successful treatment. Anxiety is associated with hyperexcitability of the basolateral amygdala (BLA). The present study sought to determine the nature of the nerve agent-induced alterations in the BLA, which could explain the development of anxiety. Rats were exposed to soman, at a dose that induced prolonged status epilepticus. Twenty-four hours and 14-days after exposure, neurons from the BLA were recorded using whole-cell patch-clamp techniques. At both the 24h and 14-day post-exposure time-points, the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) in the BLA were reduced, along with reduction in the frequency but not amplitude of miniature IPSCs. In addition, activation of α7-nicotinic acetylcholine receptors, a cholinergic receptor that participates in the regulation of BLA excitability and is involved in anxiety, increased spontaneous excitatory postsynaptic currents (sEPSCs) in both soman-exposed rats and controls, but was less effective in increasing sIPSCs in soman-exposed rats. Despite the loss of both interneurons and principal cells after soman-induced status epilepticus, the frequency of sEPSCs was increased in the soman-exposed rats. Impaired function and cholinergic modulation of GABAergic inhibition in the BLA may underlie anxiety disorders that develop after nerve agent exposure.

Ovarian cycle-linked plasticity of δ-GABAA receptor subunits in hippocampal interneurons affects γ oscillations in vivo

GABA_A receptors containing δ subunits (δ-GABA_ARs) are GABA-gated ion channels with extra- and perisynaptic localization, strong sensitivity to neurosteroids (NS), and a high degree of plasticity. In selective brain regions they are expressed on specific principal cells and interneurons (INs), and generate a tonic conductance that controls neuronal excitability and oscillations. Plasticity of δ-GABA_ARs in principal cells has been described during states of altered NS synthesis including acute stress, puberty, ovarian cycle, pregnancy and the postpartum period, with direct consequences on neuronal excitability and network dynamics. The defining network events implicated in cognitive function, memory formation and encoding are γ oscillations (30–120 Hz), a well-timed loop of excitation and inhibition between principal cells and PV-expressing INs (PV + INs). The δ-GABA_ARs of INs can modify γ oscillations, and a lower expression of δ-GABA_ARs on INs during pregnancy alters γ frequency recorded in vitro. The ovarian cycle is another physiological event with large fluctuations in NS levels and δ-GABA_ARs. Stages of the cycle are paralleled by swings in memory performance, cognitive function, and mood in both humans and rodents. Here we show δ-GABA_ARs changes during the mouse ovarian cycle in hippocampal cell types, with enhanced expression during diestrus in principal cells and specific INs. The plasticity of δ-GABA_ARs on PV-INs decreases the magnitude of γ oscillations continuously recorded in area CA1 throughout several days in vivo during diestrus and increases it during estrus. Such recurring changes in γ magnitude were not observed in non-cycling wild-type (WT) females, cycling females lacking δ-GABA_ARs only on PV-INs (PV-Gabrd^-/-), and in male mice during a time course equivalent to the ovarian cycle. Our findings may explain the impaired memory and cognitive performance experienced by women with premenstrual syndrome (PMS) or premenstrual dysphoric disorder (PMDD).

WONOEP appraisal: new genetic approaches to study epilepsy

New genetic investigation techniques, including next-generation sequencing, epigenetic profiling, cell lineage mapping, targeted genetic manipulation of specific neuronal cell types, stem cell reprogramming, and optogenetic manipulations within epileptic networks are progressively unraveling the mysteries of epileptogenesis and ictogenesis. These techniques have opened new avenues to discover the molecular basis of epileptogenesis and to study the physiologic effects of mutations in epilepsy-associated genes on a multilayer level, from cells to circuits. This manuscript reviews recently published applications of these new genetic technologies in the study of epilepsy, as well as work presented by the authors at the genetic session of the XII Workshop on the Neurobiology of Epilepsy (WONOEP 2013) in Quebec, Canada. Next-generation sequencing is providing investigators with an unbiased means to assess the molecular causes of sporadic forms of epilepsy and has revealed the complexity and genetic heterogeneity of sporadic epilepsy disorders. To assess the functional impact of mutations in these newly identified genes on specific neuronal cell types during brain development, new modeling strategies in animals, including conditional genetics in mice and in utero knock-down approaches, are enabling functional validation with exquisite cell-type and temporal specificity. In addition, optogenetics, using cell-type-specific Cre recombinase driver lines, is enabling investigators to dissect networks involved in epilepsy. In addition, genetically encoded cell-type labeling is providing new means to assess the role of the nonneuronal components of epileptic networks such as glial cells. Furthermore, beyond its role in revealing coding variants involved in epileptogenesis, next-generation sequencing can be used to assess the epigenetic modifications that lead to sustained network hyperexcitability in epilepsy, including methylation changes in gene promoters and noncoding ribonucleic acid (RNA) involved in modifying gene expression following seizures. In addition, genetically based bioluminescent reporters are providing new opportunities to assess neuronal activity and neurotransmitter levels both in vitro and in vivo in the context of epilepsy. Finally, genetically rederived neurons generated from patient induced pluripotent stem cells and genetically modified zebrafish have become high-throughput means to investigate disease mechanisms and potential new therapies. Genetics has changed the field of epilepsy research considerably, and is paving the way for better diagnosis and therapies for patients with epilepsy.

Seasonal induction of GABAergic excitation in the central mammalian clock

The balance between excitation and inhibition is essential for the proper function of neuronal networks in the brain. The inhibitory neurotransmitter γ-aminobutyric acid (GABA) contributes to the network dynamics within the suprachiasmatic nucleus (SCN), which is involved in seasonal encoding. We investigated GABAergic activity and observed mainly inhibitory action in SCN neurons of mice exposed to a short-day photoperiod. Remarkably, the GABAergic activity in a long-day photoperiod shifts from inhibition toward excitation. The mechanistic basis for this appears to be a change in the equilibrium potential of GABA-evoked current. These results emphasize that environmental conditions can have substantial effects on the function of a key neurotransmitter in the central nervous system.

Distinct effect of impact rise times on immediate and early neuropathology after brain injury in juvenile rats

Traumatic brain injury (TBI) can occur from physical trauma from a wide spectrum of insults ranging from explosions to falls. The biomechanics of the trauma can vary in key features, including the rate and magnitude of the insult. Although the effect of peak injury pressure on neurological outcome has been examined in the fluid percussion injury (FPI) model, it is unknown whether differences in rate of rise of the injury waveform modify cellular and physiological changes after TBI. Using a programmable FPI device, we examined juvenile rats subjected to a constant peak pressure at two rates of injury: a standard FPI rate of rise and a faster rate of rise to the same peak pressure. Immediate postinjury assessment identified fewer seizures and relatively brief loss of consciousness after fast-rise injuries than after standard-rise injuries at similar peak pressures. Compared with rats injured at standard rise, fewer silver-stained injured neuronal profiles and degenerating hilar neurons were observed 4-6 hr after fast-rise FPI. However, 1 week postinjury, both fast- and standard-rise FPI resulted in hilar cell loss and enhanced perforant path-evoked granule cell field excitability compared with sham controls. Notably, the extent of neuronal loss and increase in dentate excitability were not different between rats injured at fast and standard rates of rise to peak pressure. Our data indicate that reduced cellular damage and improved immediate neurological outcome after fast rising primary concussive injuries mask the severity of the subsequent cellular and neurophysiological pathology and may be unreliable as a predictor of prognosis.

The impact of tonic GABAA receptor-mediated inhibition on neuronal excitability varies across brain region and cell type

The diversity of GABA_A receptor (GABA_AR) subunits and the numerous configurations during subunit assembly give rise to a variety of receptors with different functional properties. This heterogeneity results in variations in GABAergic conductances across numerous brain regions and cell types. Phasic inhibition is mediated by synaptically-localized receptors with a low affinity for GABA and results in a transient, rapidly desensitizing GABAergic conductance; whereas, tonic inhibition is mediated by extrasynaptic receptors with a high affinity for GABA and results in a persistent GABAergic conductance. The specific functions of tonic versus phasic GABAergic inhibition in different cell types and the impact on specific neural circuits are only beginning to be unraveled. Here we review the diversity in the magnitude of tonic GABAergic inhibition in various brain regions and cell types, and highlight the impact on neuronal excitability in different neuronal circuits. Further, we discuss the relevance of tonic inhibition in various physiological and pathological contexts as well as the potential of targeting these receptor subtypes for treatment of diseases, such as epilepsy.

Do structural changes in GABA neurons give rise to the epileptic state?

Identifying the role of GABA neurons in the development of an epileptic state has been particularly difficult in acquired epilepsy, in part because of the multiple changes that occur in such conditions. Although once questioned, there is now considerable evidence for loss of GABA neurons in multiple brain regions in models of acquired epilepsy. This loss can affect several cell types, including both somatostatin- and parvalbumin-expressing interneurons, and the cell type that is most severely affected can vary among brain regions and models. Because of the diversity of GABA neurons in the hippocampus and cerebral cortex, resulting functional deficits are unlikely to be compensated fully by remaining GABA neurons of other subtypes. The fundamental importance of GABA neuron loss in epilepsy is supported by findings in genetic mouse models in which GABA neurons appear to be decreased relatively selectively, and increased seizure susceptibility and spontaneous seizures develop. Alterations in remaining GABA neurons also occur in acquired epilepsy. These include alterations in inputs or receptors that could impair function, as well as morphological reorganization of GABAergic axons and their synaptic connections. Such axonal sprouting could be compensatory if normal circuits are reestablished, but the creation of aberrant circuitry could contribute to an epileptic condition. The functional effects of GABA neuron alterations thus may include not only reductions in GABAergic inhibition but also excessive neuronal synchrony and, potentially, depolarizing GABAergic influences. The combination of GABA neuron loss and alterations in remaining GABA neurons provides likely, though still unproven, substrates for the epileptic state.

Interneuronal GABAA receptors inside and outside of synapses

About 20% of the total number of neurons in the brain are interneurons (INs) that utilize GABA as their neurotransmitter. The receptors for GABA have been well studied in principal cells, but INs also express GABA receptors, in particular the GABAA type (GABAARs), which may also be activated in an autocrine manner by the transmitter released by the INs themselves. As more and more neurological and psychiatric disorders are being discovered to be linked to malfunction or deficits of INs, this review will cover how INs communicate with each other through the activation of synaptic and extrasynaptic GABAARs. The properties of GABAARs specific to INs may differ significantly from those found on principal cells to open the prospect of developing IN-specific drugs.

Seizure-induced alterations in fast-spiking basket cell GABA currents modulate frequency and coherence of gamma oscillation in network simulations

Gamma frequency oscillations have been proposed to contribute to memory formation and retrieval. Fast-spiking basket cells (FS-BCs) are known to underlie development of gamma oscillations. Fast, high amplitude GABA synapses and gap junctions have been suggested to contribute to gamma oscillations in FS-BC networks. Recently, we identified that, apart from GABAergic synapses, FS-BCs in the hippocampal dentate gyrus have GABAergic currents mediated by extrasynaptic receptors. Our experimental studies demonstrated two specific changes in FS-BC GABA currents following experimental seizures [Yu et al., J. Neurophysiol. 109, 1746 (2013)]: increase in the magnitude of extrasynaptic (tonic) GABA currents and a depolarizing shift in GABA reversal potential (E(GABA)). Here, we use homogeneous networks of a biophysically based model of FS-BCs to examine how the presence of extrasynaptic GABA conductance (g(GABA-extra)) and experimentally identified, seizure-induced changes in g(GABA-extra) and E(GABA) influence network activity. Networks of FS-BCs interconnected by fast GABAergic synapses developed synchronous firing in the dentate gamma frequency range (40-100 Hz). Systematic investigation revealed that the biologically realistic range of 30 to 40 connections between FS-BCs resulted in greater coherence in the gamma frequency range when networks were activated by Poisson-distributed dendritic synaptic inputs rather than by homogeneous somatic current injections, which were balanced for FS-BC firing frequency in unconnected networks. Distance-dependent conduction delay enhanced coherence in networks with 30-40 FS-BC interconnections while inclusion of gap junctional conductance had a modest effect on coherence. In networks activated by somatic current injections resulting in heterogeneous FS-BC firing, increasing g(GABA-extra) reduced the frequency and coherence of FS-BC firing when E(GABA) was shunting (-74 mV), but failed to alter average FS-BC frequency when E(GABA) was depolarizing (-54 mV). When FS-BCs were activated by biologically based dendritic synaptic inputs, enhancing g(GABA-extra) reduced the frequency and coherence of FS-BC firing when E(GABA) was shunting and increased average FS-BC firing when E(GABA) was depolarizing. Shifting E(GABA) from shunting to depolarizing potentials consistently increased network frequency to and above high gamma frequencies (>80 Hz). Since gamma oscillations may contribute to learning and memory processing [Fell et al., Nat. Neurosci. 4, 1259 (2001); Jutras et al., J. Neurosci. 29, 12521 (2009); Wang, Physiol. Rev. 90, 1195 (2010)], our demonstration that network oscillations are modulated by extrasynaptic inhibition in FS-BCs suggests that neuroactive compounds that act on extrasynaptic GABA receptors could impact memory formation by modulating hippocampal gamma oscillations. The simulation results indicate that the depolarized FS-BC GABA reversal, observed after experimental seizures, together with enhanced spillover extrasynaptic GABA currents are likely to promote generation of focal high frequency activity associated with epileptic networks.

The parvalbumin-positive interneurons in the mouse dentate gyrus express GABAA receptor subunits α1, β2, and δ along their extrasynaptic cell membrane

Neuronal circuitries in the hippocampus are involved in navigation and memory and are controlled by major networks of GABAergic interneurons. Parvalbumin (PV)-expressing interneurons in the dentate gyrus (DG) are identified as fast-spiking cells, playing a crucial role in network oscillation and synchrony. The inhibitory modulation of these interneurons is thought to be mediated mainly through GABAA receptors, the major inhibitory neurotransmitter receptors in the brain. Here we show that all PV-positive interneurons in the granular/subgranular layer (GL/SGL) of the mouse DG express high levels of the GABAA receptor δ subunit. PV-containing interneurons in the hilus and the molecular layer, however, express the δ subunit to a lower extent. Only 8% of the somatostatin-containing interneurons express the δ subunit, whereas calbindin- or calretinin-containing interneurons in the DG seem not to express the GABAA receptor δ subunit at all. Hence, these cells receive a GABAergic control different from that of PV-containing interneurons in the GL/SGL. Experiments investigating a possible co-expression of GABAA receptor α1, α2, α3, α4, α5, β1, β2, β3, or γ2 subunits with PV and δ subunits indicated that α1 and β2 subunits are co-expressed with δ subunits along the extrasynaptic membranes of PV-interneurons. These results suggest a robust tonic GABAergic control of PV-containing interneurons in the GL/SGL of the DG via δ subunit-containing receptors. Our data are important for better understanding of the neuronal circuitries in the DG and the role of specific cell types under pathological conditions.