GluK2 Is a Target for Gene Therapy in Drug-Resistant Temporal Lobe Epilepsy

OBJECTIVE

Temporal lobe epilepsy (TLE) is characterized by recurrent seizures generated in the limbic system, particularly in the hippocampus. In TLE, recurrent mossy fiber sprouting from dentate gyrus granule cells (DGCs) crea an aberrant epileptogenic network between DGCs which operates via ectopically expressed GluK2/GluK5-containing kainate receptors (KARs). TLE patients are often resistant to anti-seizure medications and suffer significant comorbidities; hence, there is an urgent need for novel therapies. Previously, we have shown that GluK2 knockout mice are protected from seizures. This study aims at providing evidence that downregulating KARs in the hippocampus using gene therapy reduces chronic epileptic discharges in TLE.

METHODS

We combined molecular biology and electrophysiology in rodent models of TLE and in hippocampal slices surgically resected from patients with drug-resistant TLE.

RESULTS

Here, we confirmed the translational potential of KAR suppression using a non-selective KAR antagonist that markedly attenuated interictal-like epileptiform discharges (IEDs) in TLE patient-derived hippocampal slices. An adeno-associated virus (AAV) serotype-9 vector expressing anti-grik2 miRNA was engineered to specifically downregulate GluK2 expression. Direct delivery of AAV9-anti grik2 miRNA into the hippocampus of TLE mice led to a marked reduction in seizure activity. Transduction of TLE patient hippocampal slices reduced levels of GluK2 protein and, most importantly, significantly reduced IEDs.

INTERPRETATION

Our gene silencing strategy to knock down aberrant GluK2 expression demonstrates inhibition of chronic seizure in a mouse TLE model and IEDs in cultured slices derived from TLE patients. These results provide proof-of-concept for a gene therapy approach targeting GluK2 KARs for drug-resistant TLE patients. ANN NEUROL 2023;94:745-761.

Maternal hyperhomocysteinemia increases seizures susceptibility of neonatal rats

AIMS

Neonatal seizures are severe pathologies which may result in long-term neurological consequences. High plasma concentrations of homocysteine - hyperhomocysteinemia (hHCy) - are associated with epilepsy. In the present study, we evaluated susceptibility to seizure of neonatal rats with prenatal hHCy.

MAIN METHODS

Prenatal hHCy was induced by feeding females with a high-methionine diet. Experiments were performed on pups during the first three postnatal weeks. Flurothyl-induced epileptic behavior was assessed according to Racine's scale. Epileptiform activity in the hippocampus was recorded using electrophysiological methods. The balance of excitation/inhibition, functional GABAergic inhibition and GABA reversal potential in hippocampal neurons were analyzed.

KEY FINDINGS

Rats with hHCy developed more severe stages of behavioral patterns during flurothyl-induced epilepsy with shorter latency. Electrophysiological recordings demonstrated higher background neuronal activity in rats with hHCy. Seizure-like events triggered by flurothyl (in vivo) or 4-aminopyridine (in vitro) showed shorter latency, higher power and amplitude. An increased glutamate/GABA synaptic ratio was shown in the pyramidal neurons of rats with hHCy and more slices demonstrated excitation by isoguvacine, a selective GABA(A) receptor agonist, during the first and second postnatal weeks. The GABA driving force and the reversal potential of GABA(A) currents were more positive during the second postnatal week for hHCy rats.

SIGNIFICANCE

The higher susceptibility to seizures in rats with prenatal hHCy due to a shift in the balance of excitation/inhibition toward excitation may underlie the clinical evidence about the association of hHCy with an increased risk of epilepsy.

Impaired regulation of KCC2 phosphorylation leads to neuronal network dysfunction and neurodevelopmental pathology

KCC2 is a vital neuronal K+/Cl- cotransporter that is implicated in the etiology of numerous neurological diseases. In normal cells, KCC2 undergoes developmental dephosphorylation at Thr906 and Thr1007 We engineered mice with heterozygous phosphomimetic mutations T906E and T1007E (KCC2E/+ ) to prevent the normal developmental dephosphorylation of these sites. Immature (postnatal day 15) but not juvenile (postnatal day 30) KCC2E/+ mice exhibited altered GABAergic inhibition, an increased glutamate/GABA synaptic ratio, and greater susceptibility to seizure. KCC2E/+ mice also had abnormal ultrasonic vocalizations at postnatal days 10 to 12 and impaired social behavior at postnatal day 60. Postnatal bumetanide treatment restored network activity by postnatal day 15 but failed to restore social behavior by postnatal day 60. Our data indicate that posttranslational KCC2 regulation controls the GABAergic developmental sequence in vivo, indicating that deregulation of KCC2 could be a risk factor for the emergence of neurological pathology.

Bumetanide Prevents Brain Trauma-Induced Depressive-Like Behavior

Brain trauma triggers a cascade of deleterious events leading to enhanced incidence of drug resistant epilepsies, depression, and cognitive dysfunctions. The underlying mechanisms leading to these alterations are poorly understood and treatment that attenuates those sequels are not available. Using controlled-cortical impact as an experimental model of brain trauma in adult mice, we found a strong suppressive effect of the sodium-potassium-chloride importer (NKCC1) specific antagonist bumetanide on the appearance of depressive-like behavior. We demonstrate that this alteration in behavior is associated with an impairment of post-traumatic secondary neurogenesis within the dentate gyrus of the hippocampus. The mechanism mediating the effect of bumetanide involves early transient changes in the expression of chloride regulatory proteins and qualitative changes in GABA(A) mediated transmission from hyperpolarizing to depolarizing after brain trauma. This work opens new perspectives in the early treatment of human post-traumatic induced depression. Our results strongly suggest that bumetanide might constitute an efficient prophylactic treatment to reduce neurological and psychiatric consequences of brain trauma.

Postsynaptic GABA(B) Receptors Contribute to the Termination of Giant Depolarizing Potentials in CA3 Neonatal Rat Hippocampus

During development, hippocampal CA3 network generates recurrent population bursts, so-called Giant Depolarizing Potentials (GDPs). GDPs are characterized by synchronous depolarization and firing of CA3 pyramidal cells followed by afterhyperpolarization (GDP-AHP). Here, we explored the properties of GDP-AHP in CA3 pyramidal cells using gramicidin perforated patch clamp recordings from neonatal rat hippocampal slices. We found that GDP-AHP occurs independently of whether CA3 pyramidal cells fire action potentials (APs) or remain silent during GDPs. However, the amplitude of GDP-AHP increased with the number of APs the cells fired during GDPs. The reversal potential of the GDP-AHP was close to the potassium equilibrium potential. During voltage-clamp recordings, current-voltage relationships of the postsynaptic currents activated during GDP-AHP were characterized by reversal near the potassium equilibrium potential and inward rectification, similar to the responses evoked by the GABA(B) receptor agonists. Finally, the GABA(B) receptor antagonist CGP55845 strongly reduced GDP-AHP and prolonged GDPs, eventually transforming them to the interictal and ictal-like discharges. Together, our findings suggest that the GDP-AHP involves two mechanisms: (i) postsynaptic GABA(B) receptor activated potassium currents, which are activated independently on whether the cell fires or not during GDPs; and (ii) activity-dependent, likely calcium activated potassium currents, whose contribution to the GDP-AHP is dependent on the amount of firing during GDPs. We propose that these two complementary inhibitory postsynaptic mechanisms cooperate in the termination of GDP.

Depolarizing GABA and developmental epilepsies

Early in development, GABA, which is the main inhibitory neurotransmitter in adult brain, depolarizes immature neurons and exerts dual--excitatory and shunting/inhibitory--effects in the developing neuronal networks. The present review discusses some general questions, including the properties of excitation at depolarizing GABAergic synapse and shunting inhibition by depolarizing GABA; technical issues in exploration of depolarizing GABA using various techniques and preparations, including the developmental aspects of traumatic injury and what is known (or rather unknown) on the actions of GABA in vivo; complex roles of depolarizing GABA in developmental epilepsies, including a contribution of depolarizing GABA to enhanced excitability in the immature networks, caused by repetitive seizures accumulation of intracellular chloride concentration that increases excitatory GABA power and its synchronizing proconvulsive effects, and correction of chloride homeostasis as a potential strategy to treat neonatal seizures.

Oxytocin-mediated GABA inhibition during delivery attenuates autism pathogenesis in rodent offspring

We report that the oxytocin-mediated neuroprotective γ-aminobutyric acid (GABA) excitatory-inhibitory shift during delivery is abolished in the valproate and fragile X rodent models of autism. During delivery and subsequently, hippocampal neurons in these models have elevated intracellular chloride levels, increased excitatory GABA, enhanced glutamatergic activity, and elevated gamma oscillations. Maternal pretreatment with bumetanide restored in offspring control electrophysiological and behavioral phenotypes. Conversely, blocking oxytocin signaling in naïve mothers produced offspring having electrophysiological and behavioral autistic-like features. Our results suggest a chronic deficient chloride regulation in these rodent models of autism and stress the importance of oxytocin-mediated GABAergic inhibition during the delivery process. Our data validate the amelioration observed with bumetanide and oxytocin and point to common pathways in a drug-induced and a genetic rodent model of autism.

Tubacin prevents neuronal migration defects and epileptic activity caused by rat Srpx2 silencing in utero

Altered development of the human cerebral cortex can cause severe malformations with often intractable focal epileptic seizures and may participate in common pathologies, notably epilepsy. This raises important conceptual and therapeutic issues. Two missense mutations in the sushi repeat-containing protein SRPX2 had been previously identified in epileptic disorders with or without structural developmental alteration of the speech cortex. In the present study, we aimed to decipher the precise developmental role of SRPX2, to have a better knowledge on the consequences of its mutations, and to start addressing therapeutic issues through the design of an appropriate animal model. Using an in utero Srpx2 silencing approach, we show that SRPX2 influences neuronal migration in the developing rat cerebral cortex. Wild-type, but not the mutant human SRPX2 proteins, rescued the neuronal migration phenotype caused by Srpx2 silencing in utero, and increased alpha-tubulin acetylation. Following in utero Srpx2 silencing, spontaneous epileptiform activity was recorded post-natally. The neuronal migration defects and the post-natal epileptic consequences were prevented early in embryos by maternal administration of tubulin deacetylase inhibitor tubacin. Hence epileptiform manifestations of developmental origin could be prevented in utero, using a transient and drug-based therapeutic protocol.

The GABA excitatory/inhibitory shift in brain maturation and neurological disorders

Ionic currents and the network-driven patterns they generate differ in immature and adult neurons: The developing brain is not a "small adult brain." One of the most investigated examples is the developmentally regulated shift of actions of the transmitter GABA that inhibit adult neurons but excite immature ones because of an initially higher intracellular chloride concentration [Cl(-)](i), leading to depolarizing and often excitatory actions of GABA instead of hyperpolarizing and inhibitory actions. The levels of [Cl(-)](i) are also highly labile, being readily altered transiently or persistently by enhanced episodes of activity in relation to synaptic plasticity or a variety of pathological conditions, including seizures and brain insults. Among the plethora of channels, transporters, and other devices involved in controlling [Cl(-)](i), two have emerged as playing a particularly important role: the chloride importer NKCC1 and the chloride exporter KCC2. Here, the authors stress the importance of determining how [Cl(-)](i) is dynamically regulated and how this affects brain operation in health and disease. In a clinical perspective, agents that control [Cl(-)](i) and reinstate inhibitory actions of GABA open novel therapeutic perspectives in many neurological disorders, including infantile epilepsies, autism spectrum disorders, and other developmental disorders.

Enhanced Synaptic Activity and Epileptiform Events in the Embryonic KCC2 Deficient Hippocampus

The neuronal potassium-chloride co-transporter 2 [indicated thereafter as KCC2 (for protein) and Kcc2 (for gene)] is thought to play an important role in the post natal excitatory to inhibitory switch of GABA actions in the rodent hippocampus. Here, by studying hippocampi of wild-type (Kcc2(+/+)) and Kcc2 deficient (Kcc2(-/-)) mouse embryos, we unexpectedly found increased spontaneous neuronal network activity at E18.5, a developmental stage when KCC2 is thought not to be functional in the hippocampus. Embryonic Kcc2(-/-) hippocampi have also an augmented synapse density and a higher frequency of spontaneous glutamatergic and GABA-ergic postsynaptic currents than naïve age matched neurons. However, intracellular chloride concentration ([Cl(-)](i)) and the reversal potential of GABA-mediated currents (E(GABA)) were similar in embryonic Kcc2(+/+) and Kcc2(-/-) CA3 neurons. In addition, KCC2 immunolabeling was cytoplasmic in the majority of neurons suggesting that the molecule is not functional as a plasma membrane chloride co-transporter. Collectively, our results show that already at an embryonic stage, KCC2 controls the formation of synapses and, when deleted, the hippocampus has a higher density of GABA-ergic and glutamatergic synapses and generates spontaneous and evoked epileptiform activities. These results may be explained either by a small population of orchestrating neurons in which KCC2 operates early as a chloride exporter or by transporter independent actions of KCC2 that are instrumental in synapse formation and networks construction.

Phenobarbital but Not Diazepam Reduces AMPA/kainate Receptor Mediated Currents and Exerts Opposite Actions on Initial Seizures in the Neonatal Rat Hippocampus

Diazepam (DZP) and phenobarbital (PB) are extensively used as first and second line drugs to treat acute seizures in neonates and their actions are thought to be mediated by increasing the actions of GABAergic signals. Yet, their efficacy is variable with occasional failure or even aggravation of recurrent seizures questioning whether other mechanisms are not involved in their actions. We have now compared the effects of DZP and PB on ictal-like events (ILEs) in an in vitro model of mirror focus (MF). Using the three-compartment chamber with the two immature hippocampi and their commissural fibers placed in three different compartments, kainate was applied to one hippocampus and PB or DZP to the contralateral one, either after one ILE, or after many recurrent ILEs that produce an epileptogenic MF. We report that in contrast to PB, DZP aggravated propagating ILEs from the start, and did not prevent the formation of MF. PB reduced and DZP increased the network driven giant depolarizing potentials suggesting that PB may exert additional actions that are not mediated by GABA signaling. In keeping with this, PB but not DZP reduced field potentials recorded in the presence of GABA and NMDA receptor antagonists. These effects are mediated by a direct action on AMPA/kainate receptors since PB: (i) reduced AMPA/kainate receptor mediated currents induced by focal applications of glutamate; (ii) reduced the amplitude and the frequency of AMPA but not NMDA receptor mediated miniature excitatory postsynaptic currents (EPSCs); (iii) augmented the number of AMPA receptor mediated EPSCs failures evoked by minimal stimulation. These effects persisted in MF. Therefore, PB exerts its anticonvulsive actions partly by reducing AMPA/kainate receptors mediated EPSCs in addition to the pro-GABA effects. We suggest that PB may have advantage over DZP in the treatment of initial neonatal seizures since the additional reduction of glutamate receptors mediated signals may reduce the severity of neonatal seizures.

Neuronal chloride accumulation and excitatory GABA underlie aggravation of neonatal epileptiform activities by phenobarbital

Phenobarbital produces its anti-epileptic actions by increasing the inhibitory drive of γ-aminobutyric acid. However, following recurrent seizures, γ-aminobutyric acid excites neurons because of a persistent increase of chloride raising the important issue of whether phenobarbital could aggravate persistent seizures. Here we compared the actions of phenobarbital on initial and established ictal-like events in an in vitro model of mirror focus. Using the in vitro three-compartment chamber preparation with the two hippocampi and their commissural fibres placed in three different chambers, kainate was applied to one hippocampus and phenobarbital contralaterally, either after one ictal-like event or after many recurrent ictal-like events that produce an epileptogenic mirror focus. Field, perforated patch and single-channel recordings were used to determine the effects of γ-aminobutyric acid and their modulation by phenobarbital, and alterations of the chloride cotransporters were investigated using sodium-potassium-chloride cotransporter 1 and potassium chloride cotransporter 2 antagonists, potassium chloride cotransporter 2 immunocytochemistry and sodium-potassium-chloride cotransporter 1 knockouts. Phenobarbital reduced initial ictal-like events and prevented the formation of a mirror focus when applied from the start. In contrast, phenobarbital aggravated epileptiform activities when applied after many ictal-like events by enhancing the excitatory actions of γ-aminobutyric acid due to increased chloride. The accumulation of chloride and the excitatory actions of γ-aminobutyric acid in mirror foci neurons are mediated by the sodium-potassium-chloride cotransporter 1 chloride importer and by downregulation and internalization of the chloride-exporter potassium-chloride cotransporter 2. Finally, concomitant applications of the sodium-potassium-chloride cotransporter 1 antagonist bumetanide and phenobarbital decreased excitatory actions of γ-aminobutyric acid and prevented its paradoxical actions on mirror focus. Therefore, the history of seizures prior to phenobarbital applications determines its effects and rapid treatment of severe potentially epileptogenic-neonatal seizures is recommended to prevent secondary epileptogenesis associated with potassium chloride cotransporter 2 downregulation and acquisition of the excitatory γ-aminobutyric acid phenotype.

Inhibitory actions of the gamma-aminobutyric acid in pediatric Sturge-Weber syndrome

OBJECTIVE

The mechanisms of epileptogenesis in Sturge-Weber syndrome (SWS) are unknown. We explored the properties of neurons from human pediatric SWS cortex in vitro and tested in particular whether gamma-aminobutyric acid (GABA) excites neurons in SWS cortex, as has been suggested for various types of epilepsies.

METHODS

Patch-clamp and field potential recordings and dynamic biphoton imaging were used to analyze cortical tissue samples obtained from four 6- to 14-month-old pediatric SWS patients during surgery.

RESULTS

Neurons in SWS cortex were characterized by a relatively depolarized resting membrane potential, as was estimated from cell-attached recordings of N-methyl-D-aspartate channels. Many cells spontaneously fired action potentials at a rate proportional to the level of neuronal depolarization. The reversal potential for GABA-activated currents, assessed by cell-attached single channel recordings, was close to the resting membrane potential. All spontaneously firing neurons recorded in cell-attached mode or imaged with biphoton microscopy were inhibited by GABA. Spontaneous epileptiform activity in the form of recurrent population bursts was suppressed by glutamate receptor antagonists, the GABA(A) receptor agonist isoguvacine, and the positive allosteric GABA(A) modulator diazepam. Blockade of GABA(A) receptors aggravated spontaneous epileptiform activity. The NKCC1 antagonist bumetanide had little effect on epileptiform activity.

INTERPRETATION

SWS cortical neurons have a relatively depolarized resting membrane potential and spontaneously fire action potentials that may contribute to increased network excitability. In contrast to previous data depicting excitatory and proconvulsive actions of GABA in certain pediatric and adult epilepsies, GABA plays mainly an inhibitory and anticonvulsive role in SWS pediatric cortex.

Bumetanide, an NKCC1 antagonist, does not prevent formation of epileptogenic focus but blocks epileptic focus seizures in immature rat hippocampus

Excitatory GABA action induced by high [Cl(-)](i) is thought to contribute to seizure generation in neonatal neurons although the mechanism of this effect remains unclear. We report that bumetanide, a NKCC1 antagonist, reduces driving force of GABA-mediated currents (DF(GABA)) in neonatal hippocampal neurons and blocks the giant depolarizing potentials (GDPs), a spontaneous pattern of network activity. In the preparation composed of two intact interconnected hippocampi, bumetanide did not prevent generation of kainate-induced seizures, their propagation to the contralateral hippocampus, and formation of an epileptogenic mirror focus. However, in the isolated mirror focus, bumetanide effectively blocked spontaneous epileptiform activity transforming it to the GDP-like activity pattern. Bumetanide partially reduced DF(GABA) and therefore the excitatory action of GABA in epileptic neurons. Therefore bumetanide is a potent anticonvulsive agent although it cannot prevent formation of the epileptogenic mirror focus. We suggest that an additional mechanism other than NKCC1-mediated contributes to the persistent increase of DF(GABA) in epileptic neurons.

Maternal oxytocin triggers a transient inhibitory switch in GABA signaling in the fetal brain during delivery

We report a signaling mechanism in rats between mother and fetus aimed at preparing fetal neurons for delivery. In immature neurons, gamma-aminobutyric acid (GABA) is the primary excitatory neurotransmitter. We found that, shortly before delivery, there is a transient reduction in the intracellular chloride concentration and an excitatory-to-inhibitory switch of GABA actions. These events were triggered by oxytocin, an essential maternal hormone for labor. In vivo administration of an oxytocin receptor antagonist before delivery prevented the switch of GABA actions in fetal neurons and aggravated the severity of anoxic episodes. Thus, maternal oxytocin inhibits fetal neurons and increases their resistance to insults during delivery.

The dark side of high-frequency oscillations in the developing brain

Adult brain networks generate a wide range of oscillations. Some of these are behaviourally relevant, whereas others occur during seizures and other pathological conditions. This raises the question of how physiological oscillations differ from pathogenic ones. In this review, this issue is discussed from a developmental standpoint. Indeed, both epileptic and physiological high-frequency oscillations (HFOs) appear progressively during maturation, and it is therefore possible to determine how this program corresponds to maturation of the neuronal populations that generate these oscillations. We review here important differences in the development of neuronal populations that might contribute to their different oscillatory properties. In particular, at an early stage, the density of glutamatergic synapses is too low for physiological HFOs but an additional drive can be provided by excitatory GABA, triggering epileptic HFOs and the cascades involved in long-lasting epileptogenic transformations. This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).

Epileptogenic Actions of GABA and Fast Oscillations in the Developing Hippocampus

GABA excites immature neurons and inhibits adult ones, but whether this contributes to seizures in the developing brain is not known. We now report that in the developing, but not the adult, hippocampus, seizures beget seizures only if GABAergic synapses are functional. In the immature hippocampus, seizures generated with functional GABAergic synapses include fast oscillations that are required to transform a naive network to an epileptic one: blocking GABA receptors prevents the long-lasting sequels of seizures. In contrast, in adult neurons, full blockade of GABA(A) receptors generates epileptogenic high-frequency seizures. Therefore, purely glutamatergic seizures are not epileptogenic in the developing hippocampus. We suggest that the density of glutamatergic synapses is not sufficient for epileptogenesis in immature neurons; excitatory GABAergic synapses are required for that purpose. We suggest that the synergistic actions of GABA and NMDA receptors trigger the cascades involved in epileptogenesis in the developing hippocampus.

Interneurons set the tune of developing networks

Despite a rather long migratory journey, interneurons are functional before vertically migrating pyramidal neurons and they constitute the source and target of the first functional synapses in the developing hippocampus. Interneuron-driven network patterns are already present in utero while principal cells are mostly quiescent. At that early stage, GABAergic synapses--which are formed before glutamatergic ones--are excitatory, suggesting that GABA is a pioneer, much like the neurons from which it is released. This review discusses this sequence of events, its functional significance and the role that interneurons might play in the construction of cortical networks.

Developmental changes in GABAergic actions and seizure susceptibility in the rat hippocampus

The immature brain is prone to seizures but the underlying mechanisms are poorly understood. We explored the hypothesis that increased seizure susceptibility during early development is due to the excitatory action of GABA. Using noninvasive extracellular field potential and cell-attached recordings in CA3 of Sprague-Dawley rat hippocampal slices, we compared the developmental alterations in three parameters: excitatory actions of GABA, presence of the immature pattern of giant depolarizing potentials (GDPs) and severity of epileptiform activity generated by high potassium. The GABA(A) receptor agonist isoguvacine increased firing of CA3 pyramidal cells in neonatal slices while inhibiting activity in adults. A switch in the GABA(A) signalling from excitation to inhibition occurred at postnatal day (P) 13.5 +/- 0.4. Field GDPs were present in the form of spontaneous population bursts until P12.7 +/- 0.3. High potassium (8.5 mm) induced seizure-like events (SLEs) in 35% of slices at P7-16 (peak at P11.3 +/- 0.4), but only interictal activity before and after that age. The GABA(A) receptor antagonist bicuculline reduced the frequency or completely blocked SLEs and induced interictal clonic-like activity accompanied by a reduction in the frequency but an increase in the amplitude of the population spikes. In slices with interictal activity, bicuculline typically caused a large amplitude interictal clonic-like activity at all ages; in slices from P5-16 rats it was often preceded by one SLE at the beginning of bicuculline application. These results suggest that, in the immature hippocampus, GABA exerts dual (both excitatory and inhibitory) actions and that the excitatory component in the action of GABA may contribute to increased excitability during early development.

In vitro formation of a secondary epileptogenic mirror focus by interhippocampal propagation of seizures

We have determined whether seizures generate an epileptogenic focus in distal structures using an in vitro preparation composed of three independent chambers that accommodate two intact hippocampi and their connecting commissures. This enabled us to apply a convulsive agent to one hippocampus, allow the propagation of a given number of seizures to the other side and block the connections reversibly by applying tetrodotoxin (TTX) to the commissural chamber. The propagation of seizures from the kainate-treated side to the naive side transformed the latter into an independent epileptogenic focus that was capable of generating spontaneous and evoked seizures. The induction mechanism required activation of NMDA receptors and the epileptogenic transformation was associated with long-term alterations in GABAergic synapses, which became excitatory because of a shift in the chloride reversal potential, E(Cl). These data indicate that the excitatory actions of GABA may be a fundamental property of epileptogenic structures.

Paracrine intercellular communication by a Ca2+- and SNARE-independent release of GABA and glutamate prior to synapse formation

GABA and glutamate receptors are expressed in immature "silent" CA1 pyramidal neurons prior to synapse formation, but their function is unknown. We now report the presence of tonic, spontaneous, and evoked currents in embryonic and neonatal CA1 neurons mediated primarily by the activation of GABA(A) receptors. These currents are mediated by a nonconventional release of transmitters, as they persist in the presence of calcium channel blockers or botulinium toxin and are observed in Munc18-1-deficient mice in which vesicular release is abolished. This paracrine communication is modulated by glutamate but not GABA transporters, which do not operate during this period of life. Thus, a Ca(2+)- and SNARE-independent release of transmitters underlies a paracrine mode of communication before synapse formation.

Early sequential formation of functional GABA(A) and glutamatergic synapses on CA1 interneurons of the rat foetal hippocampus

During postnatal development of CA1 pyramidal neurons, GABAergic synapses are excitatory and established prior to glutamatergic synapses. As interneurons are generated before pyramidal cells, we have tested the hypothesis that the GABAergic interneuronal network is operative before glutamate pyramidal neurons and provides the initial patterns of activity. We patch-clamp recorded interneurons in foetal (69 neurons) and neonatal P0 (162 neurons) hippocampal slices and performed a morphofunctional analysis of biocytin-filled neurons. At P0, three types of interneurons were found: (i) non-innervated "silent" interneurons (5%) with no spontaneous or evoked synaptic currents; (ii) G interneurons (17%) with GABA(A) synapses only; and (iii) GG interneurons with GABA and glutamatergic synapses (78%). Relying on the neuronal capacitance, cell body size and arborization of dendrites and axons, the three types of interneurons correspond to three stages of development with non-innervated neurons and interneurons with GABA(A) and glutamatergic synapses being, respectively, the least and the most developed. Recordings from both pyramidal neurons and interneurons in foetuses (E18-20) revealed that the majority of interneurons (65%) had functional synapses whereas nearly 90% of pyramidal neurons were quiescent. Therefore, interneurons follow the same GABA-glutamate sequence of synapse formation but earlier than the principal cells. Interneurons are the source and the target of the first synapses formed in the hippocampus and are thus in a position to modulate the development of the hippocampus in the foetal stage.

Paradoxical anti-epileptic effects of a GluR5 agonist of kainate receptors

Kainate generates in adult hippocampal neurons a seizure but also a massive excitation of interneurons and a dramatic increase of the inhibitory drive that impinges on principal cells. This "overinhibition" is largely mediated by GluR5-containing kainate receptors that are enriched on interneurons. Here, using the neonatal intact hippocampus in vitro and the triple chamber, we first show that this mechanism is fully operative in neonatal neurons. We then report that application to one hippocampus of (RS)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid-a relatively selective agonist of GluR5 containing kainate receptors-depresses the propagation of seizure generated in the opposite hippocampus by a convulsive agent. We conclude that the selective excitation of interneurons by GluR5-containing kainate receptor agonists opens a new therapeutic approach for the epilepsies.

Correlative fluorescence and electron microscopy of biocytin-filled neurons with a preservation of the postsynaptic ultrastructure

Several techniques enable to inject intracellularly neurons with dyes and to use light and electron microscopy to correlate the physiological data with the morphological properties of the neuron. However, the ultrastructure of the neuron is usually obscured by the injected dye thus notably precluding the analysis of the postsynaptic specialisation and that of the other organelles. To overcome this problem, we have developed a technique based on fluorophore- and ultra small gold-conjugated streptavidins. We report, that this method facilitates the identification of intracellular organelles of the biocytin-filled neuron and of postsynaptic densities. This method is valid for the study of early postnatal neurons that are particularly refractory to this type of analysis. The procedure introduced here consists of the following steps: (1) injection of biocytin into the neuron by a patch-clamp pipette, (2) aldehyde fixation, (3) reaction with a fluorophore-conjugated streptavidin, (4) analysis with a fluorescence microscope, (5) formation of avidin-biotin complexes (ABC), (6) reaction with an ultra small gold-conjugated streptavidin, (7) silver enhancement of gold, (8) postfixation with osmium tetroxide and embedding in resin, (9) ultrathin sectioning and analysis with an electron microscope. Using this method, we show that in early postnatal hippocampal neurons, that have been injected with biocytine, it is possible to determine the morphology of the dendritic and axonal trees (including very thin details such as spines and filopodia) and to identify the localisation of the symmetric and asymmetric synapses on dendrites of the injected neuron.

Early development of neuronal activity in the primate hippocampus in utero

Morphological studies suggest that the primate hippocampus develops extensively before birth, but little is known about its functional development. Patch-clamp recordings of hippocampal neurons and reconstruction of biocytin-filled pyramidal cells were performed in slices of macaque cynomolgus fetuses delivered by cesarean section. We found that during the second half of gestation, axons and dendrites of pyramidal cells grow intensively by hundreds of micrometers per day to attain a high level of maturity near term. Synaptic currents appear around midgestation and are correlated with the level of morphological differentiation of pyramidal cells: the first synapses are GABAergic, and their emergence correlates with the growth of apical dendrite into stratum radiatum. A later occurrence of glutamatergic synaptic currents correlates with a further differentiation of the axodendritic tree and the appearance of spines. Relying on the number of dendritic spines, we estimated that hundreds of new glutamatergic synapses are established every day on a pyramidal neuron during the last third of gestation. Most of the synaptic activity is synchronized in spontaneous slow ( approximately 0.1 Hz) network oscillations reminiscent of the giant depolarizing potentials in neonatal rodents. Epileptiform discharges can be evoked by the GABA(A) receptor antagonist bicuculline by the last third of gestation, and postsynaptic GABA(B) receptors contribute to the termination of epileptiform discharges. Comparing the results obtained in primates and rodents, we conclude that the template of early hippocampal network development is conserved across the mammalian evolution but that it is shifted toward fetal life in primate.

Neuronal mechanisms of the anoxia-induced network oscillations in the rat hippocampus in vitro

1. A spindle of fast network oscillations precedes the ischaemia-induced rapid depolarisation in the rat hippocampus in vivo. However, this oscillatory pattern could not be reproduced in slices and the underlying mechanisms remain poorly understood. We have found that anoxia-induced network oscillations (ANOs, 20-40 Hz, lasting for 1-2 min) can be reproduced in the intact hippocampi of postnatal day P7-10 rats in vitro, and we have examined the underlying mechanisms using whole-cell and extracellular field potential recordings in a CA3 pyramidal layer. 2. ANOs were generated at the beginning of the anoxic depolarisation, when pyramidal cells depolarised to subthreshold values. Maximal power of the ANOs was attained when pyramidal cells depolarised to -56 mV; depolarisation above -47 mV resulted in a depolarisation block of pyramidal cells and a waning of ANOs. 3. A multiple unit activity in extracellular field recordings was phase locked to the negative and ascending phases of ANOs. Pyramidal cells recorded in current-clamp mode generated action potentials with an average probability of about 0.05 per cycle. The AMPA receptor-mediated EPSCs and the GABA receptor-mediated IPSCs in CA3 pyramidal cells were also phase locked with ANOs. 4. ANOs were prevented by tetrodotoxin and glutamate receptor antagonists CNQX and APV, and were slowed down by the allosteric GABA(A) receptor modulator diazepam. In the presence of the GABA(A) receptor antagonist bicuculline, ANOs were transformed to epileptiform discharges. 5. In the presence of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the anoxia induced an epileptiform activity and no ANOs were observed. 6. In normoxic conditions, a rise of extracellular potassium to 10 mM induced an epileptiform activity. Increasing extracellular potassium in conjunction with a bath application of the adenosine A1 receptor agonist cyclopentyladenosine induced oscillations similar to ANOs. 7. Multisite recordings along the septo-temporal hippocampal axis revealed that ANOs and anoxic depolarisation originate in the temporal part, and propagate towards the septal pole at a speed of 1.9 mm x min(-1). 8. ANOs were observed starting from P7, i.e. at a developmental stage when the effects of GABA change from depolarisation to hyperpolarisation. 9. These results suggest that the synchronisation of anoxia-induced oscillations relies on synaptic mechanisms; that the inhibition by GABA and adenosine sets the tune for a generation of oscillations and prevents an epileptiform activity; and that a synchronous GABAergic inhibition is instrumental in a phase locking neuronal activity similarly to other types of oscillatory activities in the gamma frequency range.

Morphology of CA3 non-pyramidal cells in the developing rat hippocampus

Although several investigations have shown that the local GABAergic circuit in the rat hippocampus is functional very early in development, this result has not been yet completed by the investigation of the full dendritic and axonal arborization of the neonatal interneurones. In the present study, intracellular injection of biocytin was used to assess the branching pattern of interneurones in the hippocampal CA3 region of rat between 2 and 6 days of age. Based on their dendritic morphology, the biocytin-filled interneurones were divided into four classes: bipolar, stellate, pyramidal-like and fusiform interneurones. About half of the biocytin-filled neonatal interneurones exhibited dendritic or somatic filopodial processes. The axonal arbors of the filled-interneurones were widely spread into the CA3 region, and in four out of nine cases extended beyond the CA3 region to branch into the CA1 region. These results show that, despite immature features, the filopodial processes, the hippocampal interneurones are well developed early in development at a time when their target cells, the pyramidal neurones, are still developing. These observations are consistent with a trophic role that GABA may play early in development.

Dual role of GABA in the neonatal rat hippocampus

The effects of modulators of GABA-A receptors on neuronal network activity were studied in the neonatal (postnatal days 0-5) rat hippocampus in vitro. Under control conditions, the physiological pattern of activity of the neonatal hippocampal network was characterized by spontaneous network-driven giant depolarizing potentials (GDPs). The GABA-A receptor agonist isoguvacine (1-2 microM) and the allosteric modulator diazepam (2 microM) induced biphasic responses: initially the frequency of GDPs increased 3 to 4 fold followed by blockade of GDPs and desynchronization of the network activity. The GABA-A receptor antagonists bicuculline (10 microM) and picrotoxin (100 microM) blocked GDPs and induced glutamate (AMPA and NMDA)-receptor-mediated interictal- and ictal-like activities in the hippocampal slices and the intact hippocampus. These data suggest that at early postnatal ages GABA can exert a dual - both excitatory and inhibitory - action on the network activity.

GABA is the principal fast-acting excitatory transmitter in the neonatal brain

gamma-aminobutyric acid (GABA) is the principal neurotransmitter of inhibition in the adult mammalian brain. However, at early stages of development, including the embryonic period and first week of postnatal life, GABA plays the role of main neurotransmitter of excitation. The paradoxical excitatory effect of GABA is caused by an inverted chloride gradient and, therefore, a depolarizing direction of GABA type A (GABAA) receptor mediated responses. In addition, another type of GABAergic inhibition mediated by postsynaptic GABA type B (GABAB) receptors is not functional at early stage of life. In the neonatal rat hippocampus, GABA, acting via GABAA receptors, activates voltage-gated sodium and calcium channels and potentiates the activity of N-methyl-D-aspartate (NMDA) receptors by reducing their voltage-dependent Mg2+ block. The temporal window when GABA exerts excitatory actions coincides with a particular pattern of activity of hippocampal neuronal network that is characterized by periodical giant depolarizing potentials (GDPs) reminiscent of interictal-like epileptiform discharges. Recent studies have shown that GDPs result from the synchronous discharge of GABAergic interneurons and principal glutamatergic pyramidal cells, and they are mediated by the synergistic excitatory actions of GABAA and glutamate receptors. GDPs provide synchronous intracellular Ca2+ oscillations and may, therefore, be implicated in hebbian modulation of developing synapses and activity-dependent formation of the hippocampal network.

Maturation of kainate-induced epileptiform activities in interconnected intact neonatal limbic structures in vitro

In vivo studies suggest that ontogenesis of limbic seizures is determined by the development of the limbic circuit. We have now used the newly-developed in vitro intact interconnected neonatal rat limbic structures preparation to determine the developmental profile of kainate-induced epileptiform activity in the hippocampus and its propagation to other limbic structures. We report gradual alterations in the effects of kainate during the first postnatal week on an almost daily basis; from no epileptiform activity at birth, through interictal seizures around postnatal day (P) 2 and ictal seizures by the end of the first week. The developmental profile of kainate-induced hippocampal seizures is paralleled by the expression of postsynaptic kainate receptor-mediated currents in CA3 pyramidal cells. Intralimbic propagation of the hippocampal seizures is also age-dependent: whereas seizures readily propagate to the septum and to the contralateral hippocampus via the commissures on P2, propagation to the entorhinal cortex only takes place from P4 onwards. Finally, repeated brief applications of kainate to the hippocampus induce recurrent spontaneous glutamatergic ictal and interictal discharges which persist for several hours after the kainate is washed away and which replace the physiological pattern of network activity. Paroxysmal activities are thus generated by kainate in the hippocampus at an early developmental stage and are initially restricted to this structure. Before the end of the first week of postnatal life, kainate generates the epileptiform activities that may perturb activity-dependent mechanisms that modulate neuronal development. Although at this stage neurons are relatively resistant to the pathological effects of kainate, the epileptiform activities that it generates will perturb activity-dependent mechanisms that modulate neuronal development.

Three-independent-compartment chamber to study in vitro commissural synapses

We describe a novel chamber in which the two intact neonatal rat hippocampi and the commissural fibers are placed in three independent compartments separated by latex membranes and perfused selectively with different solutions. A set of control tests showed that the compartments are well isolated: 1) methylene blue or eosin applied to one compartment did not diffuse to other compartments when verified via the microscope, and spectrophotometry revealed that <1/10.000th of the dye diffuses to other compartments; 2) tetrodotoxin (1 microM) applied to the commissural compartment blocked the synaptic responses evoked contralaterally without affecting those evoked on the ipsilateral side. This chamber enables a wide range of experiments that cannot be performed in conventional chambers, e.g., to study the maturation and plasticity of the commissural connections, bilateral synchronization of the rhythmic activities in the limbic system, commissural propagation of the epileptiform activities, etc.

Giant depolarizing potentials: the septal pole of the hippocampus paces the activity of the developing intact septohippocampal complex in vitro

In neonatal hippocampal slices, recurrent spontaneous giant depolarizing potentials (GDPs) provide neuronal synchronized firing and Ca2+ oscillations. To investigate the possible role of GDPs in the synchronization of neuronal activity in intact neonatal limbic structures, we used multiple simultaneous electrophysiological recordings in the recently described preparation of intact neonatal septohippocampal complex in vitro. Combined whole-cell (in single or pairs of cells) and extracellular field recordings (one to five simultaneous recording sites) from the CA3 hippocampal region and various parts of the septum indicated that spontaneous GDPs, which can be initiated anywhere along the longitudinal hippocampal axis, are most often initiated in the septal poles of hippocampus and propagate to medial septum and temporal poles of both hippocampi simultaneously. GDPs were abolished in the medial septum but not in the hippocampus after surgical separation of both structures, suggesting hippocampal origin of GDPs. The preferential septotemporal orientation of GDP propagation observed in the intact hippocampus was associated with a corresponding gradient of GDP frequency in isolated portions of hippocampus. Accordingly, most GDPs propagated in the septotemporal direction in both septal and temporal hippocampal isolated halves, and whereas GDP frequency remained similar in the septal part of hippocampus after its surgical isolation, it progressively decreased in more temporally isolated portions of the hippocampus. Because GDPs provide most of the synaptic drive of neonatal neurons, they may modulate the development of neuronal connections in the immature limbic system.

A novel in vitro preparation: the intact hippocampal formation

The intact hippocampal formation (IHF) of neonatal or young rats can be kept alive for an extended period in a fully submerged chamber with excellent morphological preservation. Field or patch-clamp recordings, intracellular Ca2+ measurements, and 3-D reconstruction of biocytin-filled neurons can be performed routinely. The generation and propagation of network-driven activities can be studied within the IHF or between connected intact structures such as the septum and the hippocampus or two hippocampi, and the use of a dual chamber enables the application of drugs separately to each structure. This preparation will be useful to study intact neuronal networks in the developing hippocampus in vitro.

Synchronization of GABAergic interneuronal network in CA3 subfield of neonatal rat hippocampal slices

1. Cell-attached and whole-cell recordings from interneurons localized in the stratum radiatum of the CA3 subfield (SR-CA3) of neonatal (postnatal days 2-5) rat hippocampal slices were performed to study their activity during the generation of GABAergic giant depolarizing potentials (GDPs) in CA3 pyramidal cells. 2. Dual recordings revealed that during the generation of GDPs in CA3 pyramidal cells, the interneurons fire bursts of spikes, on average 4.5 +/- 1.4 spikes per burst (cell-attached mode). There bursts were induced by periodical large inward currents (interneuronal GDPs) recorded in whole-cell mode. 3. Interneuronal GDPs revealed typical features of polysynaptic neuronal network-driven events: they were blocked by TTX and by high divalent cation medium and they could be evoked in an all-or-none manner by electrical stimulation in different regions of the hippocampus. The network elements required for the generation of GDPs are present in local CA3 circuits since spontaneous GDPs were present in the isolated CA3 subfield of the hippocampal slice. 4. Interneuronal GDPs were mediated by GABAA and glutamate receptors, since: (i) their reversal potential strongly depended on [Cl-]i; (ii) at the reversal potential of GABAA postsynaptic currents an inward component of GDPs was composed of events with the same kinetics as alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-mediated EPSCs; and (iii) once GABAA receptors were blocked intracellularly by dialysis with F(-)-MgATP-free solution, the remaining component of interneuronal GDPs reversed near 0 mV and rectified at membrane potentials more negative than -20 mV, suggesting an important contribution of NMDA receptors in addition to AMPA receptors. 5. In cell-attached recordings from interneurons, electrical stimulation in the stratum radiatum evoked a burst of spikes that corresponded to evoked GDPs. Pharmacological study of this response revealed that excitation of SR-CA3 interneurons during GDPs is determined by the co-operative depolarizing actions mediated by GABAA and glutamate (AMPA and NMDA) receptors. Interestingly, after blockade of AMPA receptors, GABAA receptor-mediated depolarization enabled the activation of NMDA receptors presumably via attenuation of their voltage-dependent magnesium block. 6. It is concluded that synchronous activation of SR-CA3 interneurons during generation of GDPs is mediated synaptically and is determined by the co-operation of (i) excitatory GABAergic connections between interneurons and (ii) glutamatergic connections to interneurons originating presumably from the pyramidal cells.

Ca2+ oscillations mediated by the synergistic excitatory actions of GABA(A) and NMDA receptors in the neonatal hippocampus

We asked whether GABA(A) and NMDA receptors may act in synergy in neonatal hippocampal slices, at a time when GABA exerts a depolarizing action. The GABA(A) receptor agonist isoguvacine reduced the voltage-dependent Mg2+ block of single NMDA channels recorded in cell-attached configuration from P(2-5) CA3 pyramidal neurons and potentiated the Ca2+ influx through NMDA channels. The synaptic response evoked by electrical stimulation of stratum radiatum was mediated by a synergistic interaction between GABA(A) and NMDA receptors. Network-driven Giant Depolarizing Potentials, which are a typical feature of the neonatal hippocampal network, provided coactivation of GABA(A) and NMDA receptors and were associated with spontaneous and synchronous Ca2+ increases in CA3 pyramidal neurons. Thus, at the early stages of development, GABA is a major excitatory transmitter that acts in synergy with NMDA receptors. This provides in neonatal neurons a hebbian stimulation that may be involved in neuronal plasticity and network formation in the developing hippocampus.

Bicuculline induces ictal seizures in the intact hippocampus recorded in vitro

The effects of the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline on rat hippocampal neurons recorded in slices and in the intact hippocampi kept in vitro were studied using whole-cell patch-clamp recordings. Bicuculline (10 microM) evoked ictal discharges in the intact hippocampus but only interictal discharges in conventional slices. Recording from the intact hippocampus in vitro is an alternative preparation to study the organization of the hippocampal neuronal network.