GABAergic inhibition suppresses paroxysmal network activity in the neonatal rodent hippocampus and neocortex

How Staying Negative Is Good for the (Adult) Brain: Maintaining Chloride Homeostasis and the GABA-Shift in Neurological Disorders

Excitatory-inhibitory (E-I) imbalance has been shown to contribute to the pathogenesis of a wide range of neurodevelopmental disorders including autism spectrum disorders, epilepsy, and schizophrenia. GABA neurotransmission, the principal inhibitory signal in the mature brain, is critically coupled to proper regulation of chloride homeostasis. During brain maturation, changes in the transport of chloride ions across neuronal cell membranes act to gradually change the majority of GABA signaling from excitatory to inhibitory for neuronal activation, and dysregulation of this GABA-shift likely contributes to multiple neurodevelopmental abnormalities that are associated with circuit dysfunction. Whilst traditionally viewed as a phenomenon which occurs during brain development, recent evidence suggests that this GABA-shift may also be involved in neuropsychiatric disorders due to the “dematuration” of affected neurons. In this review, we will discuss the cell signaling and regulatory mechanisms underlying the GABA-shift phenomenon in the context of the latest findings in the field, in particular the role of chloride cotransporters NKCC1 and KCC2, and furthermore how these regulatory processes are altered in neurodevelopmental and neuropsychiatric disorders. We will also explore the interactions between GABAergic interneurons and other cell types in the developing brain that may influence the GABA-shift. Finally, with a greater understanding of how the GABA-shift is altered in pathological conditions, we will briefly outline recent progress on targeting NKCC1 and KCC2 as a therapeutic strategy against neurodevelopmental and neuropsychiatric disorders associated with improper chloride homeostasis and GABA-shift abnormalities.

NKCC1 Deficiency in Forming Hippocampal Circuits Triggers Neurodevelopmental Disorder: Role of BDNF-TrkB Signalling

The time-sensitive GABA shift from excitatory to inhibitory is critical in early neural circuits development and depends upon developmentally regulated expression of cation-chloride cotransporters NKCC1 and KCC2. NKCC1, encoded by the SLC12A2 gene, regulates neuronal Cl⁻ homeostasis by chloride import working opposite KCC2. The high NKCC1/KCC2 expression ratio decreases in early neural development contributing to GABA shift. Human SLC12A2 loss-of-function mutations were recently associated with a multisystem disorder affecting neural development. However, the multisystem phenotype of rodent Nkcc1 knockout models makes neurodevelopment challenging to study. Brain-Derived Neurotrophic Factor (BDNF)-NTRK2/TrkB signalling controls KCC2 expression during neural development, but its impact on NKCC1 is still controversial. Here, we discuss recent evidence supporting BDNF-TrkB signalling controlling Nkcc1 expression and the GABA shift during hippocampal circuit formation. Namely, specific deletion of Ntrk2/Trkb from immature mouse hippocampal dentate granule cells (DGCs) affects their integration and maturation in the hippocampal circuitry and reduces Nkcc1 expression in their target region, the CA3 principal cells, leading to premature GABA shift, ultimately influencing the establishment of functional hippocampal circuitry and animal behaviour in adulthood. Thus, immature DGCs emerge as a potential therapeutic target as GABAergic transmission is vital for specific neural progenitors generating dentate neurogenesis in early development and the mature brain.

When Are Depolarizing GABAergic Responses Excitatory?

The membrane responses upon activation of GABA(A) receptors critically depend on the intracellular Cl⁻ concentration ([Cl⁻]_i), which is maintained by a set of transmembrane transporters for Cl⁻. During neuronal development, but also under several pathophysiological conditions, the prevailing expression of the Cl⁻ loader NKCC1 and the low expression of the Cl⁻ extruder KCC2 causes elevated [Cl⁻]_i, which result in depolarizing GABAergic membrane responses. However, depolarizing GABAergic responses are not necessarily excitatory, as GABA(A) receptors also reduces the input resistance of neurons and thereby shunt excitatory inputs. To summarize our knowledge on the effect of depolarizing GABA responses on neuronal excitability, this review discusses theoretical considerations and experimental studies illustrating the relation between GABA conductances, GABA reversal potential and neuronal excitability. In addition, evidences for the complex spatiotemporal interaction between depolarizing GABAergic and glutamatergic inputs are described. Moreover, mechanisms that influence [Cl⁻]_i beyond the expression of Cl⁻ transporters are presented. And finally, several in vitro and in vivo studies that directly investigated whether GABA mediates excitation or inhibition during early developmental stages are summarized. In summary, these theoretical considerations and experimental evidences suggest that GABA can act as inhibitory neurotransmitter even under conditions that maintain substantial depolarizing membrane responses.

Modelling the spatial and temporal constrains of the GABAergic influence on neuronal excitability

GABA (γ-amino butyric acid) is an inhibitory neurotransmitter in the adult brain that can mediate depolarizing responses during development or after neuropathological insults. Under which conditions GABAergic membrane depolarizations are sufficient to impose excitatory effects is hard to predict, as shunting inhibition and GABAergic effects on spatiotemporal filtering of excitatory inputs must be considered. To evaluate at which reversal potential a net excitatory effect was imposed by GABA (E_GABA^Thr), we performed a detailed in-silico study using simple neuronal topologies and distinct spatiotemporal relations between GABAergic and glutamatergic inputs.

These simulations revealed for GABAergic synapses located at the soma an E_GABA^Thr close to action potential threshold (E_AP^Thr), while with increasing dendritic distance E_GABA^Thr shifted to positive values. The impact of GABA on AMPA-mediated inputs revealed a complex temporal and spatial dependency. E_GABA^Thr depends on the temporal relation between GABA and AMPA inputs, with a striking negative shift in E_GABA^Thr for AMPA inputs appearing after the GABA input. The spatial dependency between GABA and AMPA inputs revealed a complex profile, with E_GABA^Thr being shifted to values negative to E_AP^Thr for AMPA synapses located proximally to the GABA input, while for distally located AMPA synapses the dendritic distance had only a minor effect on E_GABA^Thr. For tonic GABAergic conductances E_GABA^Thr was negative to E_AP^Thr over a wide range of g_GABA^tonic values. In summary, these results demonstrate that for several physiologically relevant situations E_GABA^Thr is negative to E_AP^Thr, suggesting that depolarizing GABAergic responses can mediate excitatory effects even if E_GABA did not reach E_AP^Thr.

The neurotransmitter GABA mediates an inhibitory action in the mature brain, while it was found that GABA provokes depolarizations in the immature brain or after neurological insults. It is, however, not clear to which extend these GABAergic depolarizations can contribute to an excitatory effect. In the present manuscript we approached this question with a computational model of a simplified neurons to determine what amount of a GABAergic depolarizing effect, which we quantified by the so called GABA reversal potential (E_GABA), was required to turn GABAergic inhibition to excitation. The results of our simulations revealed that if GABA was applied alone a GABAergic excitation was induced when E_GABA was around the action potential threshold. When GABA was applied together with additional excitatory inputs, which is the physiological situation in the brain, only for spatially and temporally correlated inputs E_GABA was close to the action potential threshold. For situations in which the additional excitatory inputs appear after the GABA input or are distant to the GABA input, an excitatory effect of GABA could be observed already at E_GABA substantially negative to the action potential threshold. This results indicate that even slightly depolarizing GABA responses, which may be induced during or after neurological insults, can potentially turn GABAergic inhibition into GABAergic excitation.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

Depolarizing GABA Transmission Restrains Activity-Dependent Glutamatergic Synapse Formation in the Developing Hippocampal Circuit

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mature brain but has the paradoxical property of depolarizing neurons during early development. Depolarization provided by GABA_A transmission during this early phase regulates neural stem cell proliferation, neural migration, neurite outgrowth, synapse formation, and circuit refinement, making GABA a key factor in neural circuit development. Importantly, depending on the context, depolarizing GABA_A transmission can either drive neural activity or inhibit it through shunting inhibition. The varying roles of depolarizing GABA_A transmission during development, and its ability to both drive and inhibit neural activity, makes it a difficult developmental cue to study. This is particularly true in the later stages of development when the majority of synapses form and GABA_A transmission switches from depolarizing to hyperpolarizing. Here, we addressed the importance of depolarizing but inhibitory (or shunting) GABA_A transmission in glutamatergic synapse formation in hippocampal CA1 pyramidal neurons. We first showed that the developmental depolarizing-to-hyperpolarizing switch in GABA_A transmission is recapitulated in organotypic hippocampal slice cultures. Based on the expression profile of K⁺−Cl⁻ co-transporter 2 (KCC2) and changes in the GABA reversal potential, we pinpointed the timing of the switch from depolarizing to hyperpolarizing GABA_A transmission in CA1 neurons. We found that blocking depolarizing but shunting GABA_A transmission increased excitatory synapse number and strength, indicating that depolarizing GABA_A transmission can restrain glutamatergic synapse formation. The increase in glutamatergic synapses was activity-dependent but independent of BDNF signaling. Importantly, the elevated number of synapses was stable for more than a week after GABA_A inhibitors were washed out. Together these findings point to the ability of immature GABAergic transmission to restrain glutamatergic synapse formation and suggest an unexpected role for depolarizing GABA_A transmission in shaping excitatory connectivity during neural circuit development.

Taurine potentiates the anticonvulsive effect of the GABAA agonist muscimol and pentobarbital in the immature mouse hippocampus

OBJECTIVE

The high incidence of epileptic seizures in neonates and their frequent refractoriness to pharmacologic therapies require identification of new therapeutical options. Therefore, we investigated whether the modulatory effect of taurine on γ-aminobutyric acid (GABA)_A receptors can enhance the anticonvulsive potential of the GABA_A receptor agonist muscimol and of the barbiturate pentobarbital.

METHODS

We performed field potential recordings in in toto hippocampus preparations of immature (postnatal days 4-7) C57Bl/6 mouse pups. Spontaneous epileptiform activity was induced by the continuous presence of the potassium channel blocker 4-aminopyridine and the glycinergic antagonist strychnine in Mg²⁺ -free solutions.

RESULTS

Bath application of 0.1 μmol/L muscimol increases the occurrence of recurrent epileptiform discharges, whereas they are significantly attenuated in a dose-dependent manner by muscimol in concentrations between 0.5 and 5 μmol/L. Taurine at concentrations between 0.1 and 0.5 mmol/L induces a proconvulsive effect, but upon coapplication, it significantly augments the anticonvulsive effect of moderate muscimol doses (0.5-1 μmol/L). In addition, the anticonvulsive effect of 100 and 200 μmol/L pentobarbital is increased significantly in the presence of 0.5 μmol/L taurine.

SIGNIFICANCE

These observations demonstrate that taurine can indeed enhance the anticonvulsive effects of muscimol and pentobarbital, suggesting that taurine may act as a positive modulator on GABA_A receptors. Thus, interfering with the modulatory taurine binding site of GABA_A receptors or the interstitial taurine concentration may provide new therapeutical options for anticonvulsive therapies in neonates.

Ethanol and the Developing Brain: Inhibition of Neuronal Activity and Neuroapoptosis

Ethanol induces massive neuroapoptosis in the developing brain. One of the main hypotheses that has been put forward to explain the deleterious actions of ethanol in the immature brain involves an inhibition of neuronal activity. Here, we review recent evidence for this hypothesis obtained in the somatosensory cortex and hippocampus of neonatal rodents. In both structures, ethanol strongly inhibits brain activity. At the doses inducing massive neuroapoptosis, ethanol completely suppresses the early activity patterns of spindle-bursts and gamma oscillations in the neocortex and the early sharp-waves in the hippocampus. The inhibitory effects of ethanol decrease with age and in adult animals, ethanol only mildly depresses neuronal firing and induces delta-wave activity. Suppression of cortical activity in neonatal animals likely involves inhibition of the myoclonic twitches, an important physiological trigger for the early activity bursts, and inhibition of the thalamocortical and intracortical circuits through a potentiation of GABAergic transmission and an inhibition of N-methyl-d-aspartate (NMDA) receptors, that is in keeping with the neuroapoptotic effects of other agents acting on GABA and NMDA receptors. These findings provide support for the hypothesis that the ethanol-induced inhibition of cortical activity is an important pathophysiological mechanism underlying massive neuroapoptosis induced by ethanol in the developing brain.

Network burst activity in hippocampal neuronal cultures: the role of synaptic and intrinsic currents

The goal of this work was to define the contributions of intrinsic and synaptic mechanisms toward spontaneous network-wide bursting activity, observed in dissociated rat hippocampal cell cultures. This network behavior is typically characterized by short-duration bursts, separated by order of magnitude longer interburst intervals. We hypothesize that while short-timescale synaptic processes modulate spectro-temporal intraburst properties and network-wide burst propagation, much longer timescales of intrinsic membrane properties such as persistent sodium (Nap) currents govern burst onset during interburst intervals. To test this, we used synaptic receptor antagonists picrotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP) to selectively block GABAA, AMPA, and NMDA receptors and riluzole to selectively block Nap channels. We systematically compared intracellular activity (recorded with patch clamp) and network activity (recorded with multielectrode arrays) in eight different synaptic connectivity conditions: GABAA + NMDA + AMPA, NMDA + AMPA, GABAA + AMPA, GABAA + NMDA, AMPA, NMDA, GABAA, and all receptors blocked. Furthermore, we used mixed-effects modeling to quantify the aforementioned independent and interactive synaptic receptor contributions toward spectro-temporal burst properties including intraburst spike rate, burst activity index, burst duration, power in the local field potential, network connectivity, and transmission delays. We found that blocking intrinsic Nap currents completely abolished bursting activity, demonstrating their critical role in burst onset within the network. On the other hand, blocking different combinations of synaptic receptors revealed that spectro-temporal burst properties are uniquely associated with synaptic functionality and that excitatory connectivity is necessary for the presence of network-wide bursting. In addition to confirming the critical contribution of direct excitatory effects, mixed-effects modeling also revealed distinct combined (nonlinear) contributions of excitatory and inhibitory synaptic activity to network bursting properties.

Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning

Sharp wave ripples (SPW-Rs) represent the most synchronous population pattern in the mammalian brain. Their excitatory output affects a wide area of the cortex and several subcortical nuclei. SPW-Rs occur during "off-line" states of the brain, associated with consummatory behaviors and non-REM sleep, and are influenced by numerous neurotransmitters and neuromodulators. They arise from the excitatory recurrent system of the CA3 region and the SPW-induced excitation brings about a fast network oscillation (ripple) in CA1. The spike content of SPW-Rs is temporally and spatially coordinated by a consortium of interneurons to replay fragments of waking neuronal sequences in a compressed format. SPW-Rs assist in transferring this compressed hippocampal representation to distributed circuits to support memory consolidation; selective disruption of SPW-Rs interferes with memory. Recently acquired and pre-existing information are combined during SPW-R replay to influence decisions, plan actions and, potentially, allow for creative thoughts. In addition to the widely studied contribution to memory, SPW-Rs may also affect endocrine function via activation of hypothalamic circuits. Alteration of the physiological mechanisms supporting SPW-Rs leads to their pathological conversion, "p-ripples," which are a marker of epileptogenic tissue and can be observed in rodent models of schizophrenia and Alzheimer's Disease. Mechanisms for SPW-R genesis and function are discussed in this review.

Activation of glycine receptors modulates spontaneous epileptiform activity in the immature rat hippocampus

While the expression of glycine receptors in the immature hippocampus has been shown, no information about the role of glycine receptors in controlling the excitability in the immature CNS is available. Therefore, we examined the effect of glycinergic agonists and antagonists in the CA3 region of an intact corticohippocampal preparation of the immature (postnatal days 4-7) rat using field potential recordings. Bath application of 100 μM taurine or 10 μM glycine enhanced the occurrence of recurrent epileptiform activity induced by 20 μM 4-aminopyridine in low Mg(2+) solution. This proconvulsive effect was prevented by 3 μM strychnine or after incubation with the loop diuretic bumetanide (10 μM), suggesting that it required glycine receptors and an active NKCC1-dependent Cl(-) accumulation. Application of higher doses of taurine (≥ 1 mM) or glycine (100 μM) attenuated recurrent epileptiform discharges. The anticonvulsive effect of taurine was also observed in the presence of the GABAA receptor antagonist gabazine and was attenuated by strychnine, suggesting that it was partially mediated by glycine receptors. Bath application of the glycinergic antagonist strychnine (0.3 μM) induced epileptiform discharges. We conclude from these results that in the immature hippocampus, activation of glycine receptors can mediate both pro- and anticonvulsive effects, but that a persistent activation of glycine receptors is required to suppress epileptiform activity. In summary, our study elucidated the important role of glycine receptors in the control of neuronal excitability in the immature hippocampus.

Inhibition of different GABA transporter systems is required to attenuate epileptiform activity in the CA3 region of the immature rat hippocampus

GABA transporters (GATs) are an essential element of the GABAergic system, which regulate excitability in the central nervous system and are thus used as targets for anticonvulsive therapy. However, in the immature nervous system the functions of the GABAergic system and the expression profile of GATs are distinct from the adult situation, obscuring to predict how different GAT isoforms influence epileptiform activity. Therefore we analyzed the effects of subtype specific GAT inhibitors on repetitive epileptiform discharges using field potential and whole-cell patch-clamp recordings in the CA3 region of hippocampal slices of immature (postnatal days 4-7) rats. These experiments revealed that inhibition of GAT-1 with either tiagabine (30 μM) or NO-711 (10 μM) exhibited only a minor anticonvulsive effect on repetitive epileptiform discharges. Blockade of GAT-2/3 with SNAP-5114 (40 μM) had no anticonvulsive effect, but significantly prolonged the decay of spontaneous GABAergic postsynaptic currents. In contrast, the combined application of 10 μM NO-711 and 40 μM SNAP-5114 blocked epileptiform activity in 33% of all slices and reduced the occurrence of epileptiform discharges by 54% in the remaining slices. In addition, the input resistance decreased by 10.5 ± 1.0% under this condition. These results indicate that both GAT-1 and GAT-2/3 are functional in the immature hippocampus and that only the combined inhibition of GAT 1-3 is sufficient to promote a considerable anticonvulsive effect. We conclude from these results that both GAT-1 and GAT-2/3 act synergistically to regulate the excitability in the immature hippocampus.

Cation-chloride cotransporters NKCC1 and KCC2 as potential targets for novel antiepileptic and antiepileptogenic treatments

In cortical and hippocampal neurons, cation-chloride cotransporters (CCCs) control the reversal potential (EGABA) of GABAA receptor-mediated current and voltage responses and, consequently, they modulate the efficacy of GABAergic inhibition. Two members of the CCC family, KCC2 (the major neuron-specific K-Cl cotransporter; KCC isoform 2) and NKCC1 (the Na-K-2Cl cotransporter isoform 1 which is expressed in both neurons and glial cells) have attracted much interest in studies on GABAergic signaling under both normal and pathophysiological conditions, such as epilepsy. There is tentative evidence that loop diuretic compounds such as furosemide and bumetanide may have clinically relevant antiepileptic actions, especially when administered in combination with conventional GABA-mimetic drugs such as phenobarbital. Furosemide is a non-selective inhibitor of CCCs while at low concentrations bumetanide is selective for NKCCs. Search for novel antiepileptic drugs (AEDs) is highly motivated especially for the treatment of neonatal seizures which are often resistant to, or even aggravated by conventional AEDs. This review shows that the antiepileptic effects of loop diuretics described in the pertinent literature are based on widely heterogeneous mechanisms ranging from actions on both neuronal NKCC1 and KCC2 to modulation of the brain extracellular volume fraction. A promising strategy for the development of novel CCC-blocking AEDs is based on prodrugs that are activated following their passage across the blood-brain barrier. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Asphyxia induced by umbilical cord occlusion alters glutamatergic and GABAergic synaptic transmission in neurons of the superior colliculus in fetal rats

Using optical recordings, we studied the effects of asphyxia on intracellular Cl(-) and Ca(2+) concentrations ([Cl(-)]i; [Ca(2+)]i) in the superior colliculus of fetal rats, which were connected via the umbilical cord to the dam. Acute asphyxia was induced by umbilical cord occlusion. The number of fetal superior colliculus neurons showing GABA-mediated increases in [Cl(-)]i (leading to hyperpolarization) following local synaptic electrical stimulation had decreased by 3 h post-asphyxiation, while the number showing GABA-mediated decreases in [Cl(-)]i (leading to depolarization) increased. [Ca(2+)]i rise, which occurred after acute asphyxiation, was antagonized by both non-NMDA and NMDA receptor antagonists. The increase in [Ca(2+)]i following focal superior colliculus stimulation was markedly attenuated at 3 h post-asphyxiation. These findings suggest that asphyxia induced by umbilical occlusion induces changes in glutamatergic and GABAergic synaptic transmission in the fetal brain.

Development of coherent neuronal activity patterns in mammalian cortical networks: common principles and local hetereogeneity

Many mammals are born in a very immature state and develop their rich repertoire of behavioral and cognitive functions postnatally. This development goes in parallel with changes in the anatomical and functional organization of cortical structures which are involved in most complex activities. The emerging spatiotemporal activity patterns in multi-neuronal cortical networks may indeed form a direct neuronal correlate of systemic functions like perception, sensorimotor integration, decision making or memory formation. During recent years, several studies--mostly in rodents--have shed light on the ontogenesis of such highly organized patterns of network activity. While each local network has its own peculiar properties, some general rules can be derived. We therefore review and compare data from the developing hippocampus, neocortex and--as an intermediate region--entorhinal cortex. All cortices seem to follow a characteristic sequence starting with uncorrelated activity in uncoupled single neurons where transient activity seems to have mostly trophic effects. In rodents, before and shortly after birth, cortical networks develop weakly coordinated multineuronal discharges which have been termed synchronous plateau assemblies (SPAs). While these patterns rely mostly on electrical coupling by gap junctions, the subsequent increase in number and maturation of chemical synapses leads to the generation of large-scale coherent discharges. These patterns have been termed giant depolarizing potentials (GDPs) for predominantly GABA-induced events or early network oscillations (ENOs) for mostly glutamatergic bursts, respectively. During the third to fourth postnatal week, cortical areas reach their final activity patterns with distinct network oscillations and highly specific neuronal discharge sequences which support adult behavior. While some of the mechanisms underlying maturation of network activity have been elucidated much work remains to be done in order to fully understand the rules governing transition from immature to mature patterns of network activity.

Phasic GABAA -receptor activation is required to suppress epileptiform activity in the CA3 region of the immature rat hippocampus

PURPOSE

  Despite the consistent observation that γ-aminobutyric acid A (GABA(A) ) receptors mediate excitatory responses at perinatal stages, the role of the GABAergic system in the generation of neonatal epileptiform activity remains controversial. Therefore, we analyzed whether tonic and phasic GABAergic transmission had differential effects on neuronal excitability during early development.

METHODS

  We performed whole cell patch-clamp and field potential recordings in the CA3 region of hippocampal slices from immature (postnatal day 4-7) rats to analyze the effect of specific antagonists and modulators of tonic and phasic GABAergic components on neuronal excitability.

KEY FINDINGS

  The GABAergic antagonists gabazine (3 μm) and picrotoxin (100 μm) induced epileptiform discharges, whereas activation of GABA(A) receptors attenuated epileptiform discharges. Under low-Mg(2+) conditions, 100 nm gabazine and 1 μm picrotoxin were sufficient to provoke epileptiform activity in 63.2% (n = 19) and 53.8% (n = 26) of the slices, respectively. Whole-cell patch-clamp experiments revealed that these concentrations significantly reduced the amplitude of phasic GABAergic postsynaptic currents but had no effect on tonic currents. In contrast, 1-μm 4,5,6,7-tetrahydroisoxaz-olo[5,4-c]-pyridin-3-ol (THIP) induced a tonic current of -12 ± 2.5 pA (n = 6) and provoked epileptiform discharges in 57% (n = 21) of the slices.

SIGNIFICANCE

  We conclude from these results that in the early postnatal rat hippocampus a constant phasic synaptic activity is required to control excitability and prevent epileptiform activity, whereas tonic GABAergic currents can mediate excitatory responses. Pharmacologic intervention at comparable human developmental stages should consider these ambivalent GABAergic actions.

Postnatal differentiation of cortical interneuron signalling

Most GABAergic interneurons in the cortex are born at embryonic stages in the ganglionic eminences and migrate tangentially to their final destination. They continue, however, to differentiate and functionally integrate in the circuitry until later postnatal stages of the rodent brain. Recent investigations show that interneurons undergo marked changes in their morphological, intrinsic and synaptic properties as they mature. Action potential shape and its propagation, the period of transmitter release and the time course of the postsynaptic GABA(A) receptor-mediated conductance become faster during the first three to four postnatal weeks, resulting in a developmental switch of interneurons from slow to fast signalling units. At the same time, the nature of GABAergic signalling is classically considered to shift from depolarizing to hyperpolarizing. However, recent studies oppose this view as interneuron synapses can be shunting, excitatory or hyperpolarizing in the mature cortex, demonstrating the coexistence of diverse developmental rules for the emerging effects of GABAergic synapses. Thus, mature interneuron signalling comes in many forms and is apparently optimized to the network in which the neurons are embedded.

Developmental mechanisms for the generation of telencephalic interneurons

Interneurons, which release the neurotransmitter γ-aminobutyric acid (GABA), are the major inhibitory cells of the central nervous system (CNS). Despite comprising only 20-30% of the cerebral cortical neuronal population, these cells play an essential and powerful role in modulating the electrical activity of the excitatory pyramidal cells onto which they synapse. Although interneurons are present in all regions of the mature telencephalon, during embryogenesis these cells are generated in specific compartments of the ventral (subpallial) telencephalon known as ganglionic eminences. To reach their final destinations in the mature brain, immature interneurons migrate from the ganglionic eminences to developing telencephalic structures that are both near and far from their site of origin. The specification and migration of these cells is a complex but precisely orchestrated process that is regulated by a combination of intrinsic and extrinsic signals. The final outcome of which is the wiring together of excitatory and inhibitory neurons that were born in separate regions of the developing telencephalon. Disruption of any aspect of this sequence of events during development, either from an environmental insult or due to genetic mutations, can have devastating consequences on normal brain function.

GABAergic depolarization during early cortical development and implications for anticonvulsive therapy in neonates

Epileptic seizures rank among the most frequent neurologic symptoms during the neonatal period. Accumulating data from experimental animal studies and clinical trials in humans suggest that neonatal seizures could adversely affect normal brain development and result in long-term neurologic sequelae. Unfortunately, currently used anticonvulsive drugs are often ineffective in the neonatal period. One particularity of the immature neuronal network during neonatal development is that the neurotransmitter γ-aminobutyric acid (GABA) is mainly depolarizing, rather than hyperpolarizing as commonly observed in adults. This might, in part, explain not only the higher seizure propensity of the immature neuronal network, but also the limited anticonvulsive efficacy of GABA-enhancing drugs during early postnatal life. Accordingly, pharmacologic attenuation of GABAergic depolarization has been proposed as a strategy for neonatal seizure control. However, the underlying conjecture of a depolarizing mode of GABA action has been seriously challenged recently. In the present review, we will summarize the state of knowledge regarding GABAergic depolarization in early life and discuss how these data might impact a currently tested anticonvulsive strategy.

Kainate receptor-induced ectopic spiking of CA3 pyramidal neurons initiates network bursts in neonatal hippocampus

Kainate receptors (KARs) are expressed at high levels in the brain during early development and may be critical for the proper development of neuronal networks. Here we elucidated a physiological role of high-affinity KARs in developing hippocampal network by studying the effects of 25-100 nM kainate (KA) on intrinsic network activity in slice preparations. Whereas 100 nM KA resulted in hyperexcitability of the network and the disruption of natural activity patterns, ≤ 50 nM KA concentrations enhanced the initiation of network bursts yet preserved the characteristic patterns of endogenous activity. This was not dependent on changes in GABAergic transmission or on activation of GluK1 subunit containing KARs. However, the activation of high-affinity KARs increased glutamatergic drive by promoting spontaneous firing of CA3 pyramidal neurons without affecting action potential independent glutamate release. This was not because of changes in the intrinsic somatic properties of pyramidal neurons but seemed to reside in an electrically remote site, most probably in an axonal compartment. Although application of KAR agonists has mainly been used to study pathological type of network activities, this study provides a novel mechanism by which endogenous activity of KARs can modulate intrinsic activities of the emerging neuronal network in a physiologically relevant manner. The results support recent studies that KARs play a central role in the activity-dependent maturation of synaptic circuitries.

Inhibitory neurons modulate spontaneous signaling in cultured cortical neurons: density-dependent regulation of excitatory neuronal signaling

Cortical neuronal activity depends on a balance between excitatory and inhibitory influences. Culturing of neurons on multi-electrode arrays (MEAs) has provided insight into the development and maintenance of neuronal networks. Herein, we seeded MEAs with murine embryonic cortical/hippocampal neurons at different densities (<150 or >1000 cells mm(-2)) and monitored resultant spontaneous signaling. Sparsely seeded cultures displayed a large number of bipolar, rapid, high-amplitude individual signals with no apparent temporal regularity. By contrast, densely seeded cultures instead displayed clusters of signals at regular intervals. These patterns were observed even within thinner and thicker areas of the same culture. GABAergic neurons (25% of total neurons in our cultures) mediated the differential signal patterns observed above, since addition of the inhibitory antagonist bicuculline to dense cultures and hippocampal slice cultures induced the signal pattern characteristic of sparse cultures. Sparsely seeded cultures likely lacked sufficient inhibitory neurons to modulate excitatory activity. Differential seeding of MEAs can provide a unique model for analyses of perturbation in the interaction between excitatory and inhibitory function during aging and neuropathological conditions where dysregulation of GABAergic neurons is a significant component.

Intrinsic activation of GABA(A) receptors suppresses epileptiform activity in the cerebral cortex of immature mice

PURPOSE

Activation of ionotropic γ-aminobutyric acid type A (GABA(A) ) receptors induces in immature neocortical neurons a membrane depolarization that may contribute to the higher epilepsy susceptibility in newborns. To elucidate whether depolarizing GABAergic responses enhance or attenuate epileptiform activity in the immature neocortex, we investigated the effect of agonists, antagonists, and positive modulators of GABA(A) receptors on epileptiform activity.

METHODS

We performed in vitro field potential recordings on isolated whole neocortex preparations and whole cell recordings of identified pyramidal neurons in 400-μm slices of immature (postnatal day 1-7) mice. Epileptiform activity was induced by low Mg²(+) solutions with or without 50-100 μm 4-aminopyridine.

RESULTS

  Bath application of GABA (3-100 μm, in the presence of tiagabine) attenuated epileptiform activity. The GABA transporter isoform 1 (GAT-1) inhibitor tiagabine (30 μm) and the GAT-2/3 specific inhibitor SNAP 5114 (40 μm) reduced the frequency of epileptiform activity. The benzodiazepines midazolam (0.2 μm) and zolpidem (0.5 μm) as well as the barbiturate phenobarbital (30 μm) slightly attenuated epileptiform activity. Continuous bath application of the GABAergic antagonist gabazine (SR-95531, 2-3 μm) or picrotoxin (15 μm) induced epileptiform discharges.

DISCUSSION

These results demonstrate, that (1) the activation or positive modulation of GABA(A) receptors attenuates epileptiform activity, (2) GABA(A) antagonists mediate a disinhibition, and (3) GABA uptake contributes to the regulation of extracellular GABA in immature neocortex. We conclude from these findings that a constant inhibition via GABA(A) receptors is required to suppress epileptiform activity already in the immature neocortex.

Recurrent neonatal seizures result in long-term increases in neuronal network excitability in the rat neocortex

Neonatal seizures are associated with a high likelihood of adverse neurological outcomes, including mental retardation, behavioral disorders, and epilepsy. Early seizures typically involve the neocortex, and post-neonatal epilepsy is often of neocortical origin. However, our understanding of the consequences of neonatal seizures for neocortical function is limited. In the present study, we show that neonatal seizures induced by flurothyl result in markedly enhanced susceptibility of the neocortex to seizure-like activity. This change occurs in young rats studied weeks after the last induced seizure and in adult rats studied months after the initial seizures. Neonatal seizures resulted in reductions in the amplitude of spontaneous inhibitory postsynaptic currents and the frequency of miniature inhibitory postsynaptic currents, and significant increases in the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in pyramidal cells of layer 2/3 of the somatosensory cortex. The selective N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate eliminated the differences in amplitude and frequency of sEPSCs and mEPSCs in the control and flurothyl groups, suggesting that NMDA receptors contribute significantly to the enhanced excitability seen in slices from rats that experienced recurrent neonatal seizures. Taken together, our results suggest that recurrent seizures in infancy result in a persistent enhancement of neocortical excitability.

Differences in cortical versus subcortical GABAergic signaling: a candidate mechanism of electroclinical uncoupling of neonatal seizures

Electroclinical uncoupling of neonatal seizures refers to electrographic seizure activity that is not clinically manifest. Uncoupling increases after treatment with Phenobarbital, which enhances the GABA(A) receptor (GABA(A)R) conductance. The effects of GABA(A)R activation depend on the intracellular Cl(-) concentration ([Cl(-)](i)) that is determined by the inward Cl(-) transporter NKCC1 and the outward Cl(-) transporter KCC2. Differential maturation of Cl(-) transport observed in cortical versus subcortical regions should alter the efficacy of GABA-mediated inhibition. In perinatal rat pups, most thalamic neurons maintained low [Cl(-)](i) and were inhibited by GABA. Phenobarbital suppressed thalamic seizure activity. Most neocortical neurons maintained higher [Cl(-)](i), and were excited by GABA(A)R activation. Phenobarbital had insignificant anticonvulsant responses in the neocortex until NKCC1 was blocked. Regional differences in the ontogeny of Cl(-) transport may thus explain why seizure activity in the cortex is not suppressed by anticonvulsants that block the transmission of seizure activity through subcortical networks.

Impaired GABAergic transmission disrupts normal homeostatic plasticity in rat cortical networks

In the cortex, homeostatic plasticity appears to be a key process for maintaining neuronal network activity in a functional range. This phenomenon depends on close interactions between excitatory and inhibitory circuits. We previously showed that application of a high frequency of stimulation (HFS) protocol in layer 2/3 induces parallel potentiation of excitatory and inhibitory inputs on layer 5 pyramidal neurons, leading to an unchanged excitation/inhibition (E/I) balance. These coordinated long-term potentiations of excitation and inhibition correspond to homeostatic plasticity of the neuronal networks. We showed here, on the rat visual cortex, that blockade (with gabazine) or overactivation (with 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) of GABA(A) receptors enhanced the E/I balance and prevented the potentiation of excitatory and inhibitory inputs after an HFS protocol. These impairements of the GABAergic transmission led to a long-term depression-like effect after an HFS protocol. We also observed that the blockade of inhibition reduced excitation (by 60%), and conversely, the blockade of excitation decreased inhibition (by 90%). These results support the idea that inhibitory interneurons are critical for recurrent interactions underlying homeostatic plasticity in cortical networks.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

Timing of the developmental switch in GABA(A) mediated signaling from excitation to inhibition in CA3 rat hippocampus using gramicidin perforated patch and extracellular recordings

The timing of the developmental switch in the GABA(A) mediated responses from excitatory to inhibitory was studied in Wistar rat CA3 hippocampal pyramidal cells using gramicidin perforated patch-clamp and extracellular recordings. Gramicidin perforated patch recordings revealed a gradual developmental shift in the reversal potential of synaptic and isoguvacine-induced GABA(A) mediated responses from -55 +/- 4 mV at postnatal days P0-2 to -74 +/- 3 mV at P13-15 with a midpoint of disappearance of the excitatory effects of GABA at around P8. Extracellular recordings in CA3 pyramidal cell layer revealed that the effect of isoguvacine on multiple unit activity (MUA) switched from an increase to a decrease at around P10. The effect of synaptic GABA(A) mediated responses on MUA switched from an increase to a decrease at around P8. It is concluded that the developmental switch in the action of GABA via GABA(A) receptors from excitatory to inhibitory occurs in Wistar rat CA3 pyramidal cells at around P8-10, an age that coincides with the transition from immature to mature hippocampal rhythms. We propose that excitatory GABA contributes to enhanced excitability and ictogenesis in the neonatal rat hippocampus.

Model-specific effects of bumetanide on epileptiform activity in the in-vitro intact hippocampus of the newborn mouse

The immature brain has a higher susceptibility to develop seizures, which often respond poorly to classical pharmacological treatment. It has been recently suggested that bumetanide, which blocks Na(+)-dependent K(+)-Cl(-)-cotransporter isoform 1 (NKCC1) and thus attenuates depolarizing GABAergic responses, could soothe epileptiform activity in immature nervous systems. To evaluate whether bumetanide consistently attenuates epileptiform activity, we investigated the effect of 10 microM bumetanide in five different in-vitro epilepsy models using field potential recordings in the CA3 region of intact mouse hippocampal preparations at postnatal day 4-7. Bumetanide reduced amplitude and frequency of ictal-like events (ILE) induced by 8.5 mM K(+), but it increased the frequency of ILE induced by 1 microM kainate. Inhibition of ligand-gated Cl(-) channels by 10 microM gabazine and 30 microM strychnine induced interictal activity (IA) that was only marginally affected by bumetanide. Removal of extracellular Mg(2+) induced both ILE and IA. Bumetanide had no effect on these ILE but enhanced the IA. Low-Mg(2+) solution containing 20 microM 4-AP induced late-recurrent discharges, which were slightly attenuated by bumetanide. In summary, our results demonstrate that bumetanide exerts diverse effects in different in-vitro epilepsy models.

Status epilepticus alters zolpidem sensitivity of [3H]flunitrazepam binding in the developing rat brain

GABA, the main inhibitory neurotransmitter in the adult brain, exerts its effects through multiple GABA(A) receptor subtypes with different pharmacological profiles, the alpha subunit variant mainly determining the binding properties of benzodiazepine site on the receptor protein. In adult experimental epileptic animals and in humans with epilepsy, increased excitation, i.e. seizures, alters GABA(A) receptor subunit expression leading to changes in the receptor structure, function, and pharmacology. Whether this also occurs in the developing brain, in which GABA has a trophic, excitatory effect, is not known. We have now applied autoradiography to study properties of GABA(A)/benzodiazepine receptors in 9-day-old rats acutely (6 h) and sub-acutely (7 days) after kainic acid-induced status epilepticus by analyzing displacement of [(3)H]flunitrazepam binding by zolpidem, a ligand selective for the alpha1beta2gamma2 receptor subtype. Regional changes in the binding properties were further corroborated at the cellular level by immunocytochemistry. The results revealed that status epilepticus significantly decreased displacement of [(3)H]flunitrazepam binding by zolpidem 6 h after the kainic acid-treatment in the dentate gyrus of the hippocampus, parietal cortex, and thalamus, and in the hippocampal CA3 and CA1 cell layers 1 week after the treatment. Our results suggest that status epilepticus modifies region-specifically the pharmacological properties of GABA(A) receptors, and may thus disturb the normal, strictly developmentally-regulated maturation of zolpidem-sensitive GABA(A) receptors in the immature rat brain. A part of these changes could be due to alterations in the cell surface expression of receptor subtypes.

Shunting and hyperpolarizing GABAergic inhibition in the high-potassium model of ictogenesis in the developing rat hippocampus

Ontogenesis of GABAergic signaling may play an important role in developmental changes in seizure susceptibility in the high-potassium model of ictogenesis in vitro. The age-dependent effects of [K(+)](o) on the reversal potential of the GABA(A)-mediated responses and membrane potential in hippocampal slices in vitro were compared with the effect of GABA(A)-receptors antagonists and GABA(A) modulators on high-potassium induced seizures in the CA3 pyramidal layer of rat hippocampus in vivo. GABA(A) responses were depolarizing at P8-12 and hyperpolarizing at P17-21. In P8-12 rats, GABA(A) responses switch their polarity from depolarizing to hyperpolarizing upon elevation of extracellular potassium. At approximately 10 mM [K(+)](o), activation of GABA(A) receptors produced an isoelectric, purely shunting response characterized by no changes in the membrane potential but an increase in the membrane conductance. In P17-21 rats, the hyperpolarizing GABA(A) driving force progressively increased with elevation of [K(+)](o). In P8-12 rats in vivo, GABA(A)-receptor antagonists did not affect the occurrence of ictal discharges induced by intrahippocampal injection of 10 mM [K(+)](o), but significantly increased seizure duration. Diazepam and isoguvacine completely prevented seizures induced by 10 mM [K(+)](o). In P17-21 rats, GABA(A)-receptor antagonists strongly increased the occurrence of ictal activity induced both by 10 mM [K(+)](o). Taken together, these results suggest that anticonvulsive effects of GABA are because of the combination of shunting and hyperpolarizing actions of GABA. Although shunting GABA is already efficient in the young age group, a developmental increase in the hyperpolarizing GABA(A) driving force likely contributes to the increase in the GABAergic control of seizures upon maturation.

Coordinated developmental recruitment of latent fast spiking interneurons in layer IV barrel cortex

Feedforward inhibitory GABAergic transmission is critical for mature cortical circuit function; in the neonate, however, GABA is depolarizing and believed to have a different role. Here we show that the GABAA receptor-mediated conductance is depolarizing in excitatory (stellate) cells in neonatal (postnatal day [P]3-5) layer IV barrel cortex, but GABAergic transmission at this age is not engaged by thalamocortical input in the feedforward circuit and has no detectable circuit function. However, recruitment occurs at P6-7 as a result of coordinated increases in thalamic drive to fast-spiking interneurons, fast-spiking interneuron-stellate cell connectivity and hyperpolarization of the GABAA receptor-mediated response. Thus, GABAergic circuits are not engaged by thalamocortical input in the neonate, but are poised for a remarkably coordinated development of feedforward inhibition at the end of the first postnatal week, which has profound effects on circuit function at this critical time in development.

Network mechanisms of spindle-burst oscillations in the neonatal rat barrel cortex in vivo

Early in development, cortical networks generate particular patterns of activity that participate in cortical development. The dominant pattern of electrical activity in the neonatal rat neocortex in vivo is a spatially confined spindle-burst. Here, we studied network mechanisms of generation of spindle-bursts in the barrel cortex of neonatal rats using a superfused cortex preparation in vivo. Both spontaneous and sensory-evoked spindle-bursts were present in the superfused barrel cortex. Pharmacological analysis revealed that spindle-bursts are driven by glutamatergic synapses with a major contribution of AMPA/kainate receptors, but slight participation of NMDA receptors and gap junctions. Although GABAergic synapses contributed minimally to the pacing the rhythm of spindle-burst oscillations, surround GABAergic inhibition appeared to be crucial for their compartmentalization. We propose that local spindle-burst oscillations, driven by glutamatergic synapses and spatially confined by GABAergic synapses, contribute to the development of barrel cortex during the critical period of developmental plasticity.

Postnatal development of intrinsic GABAergic rhythms in mouse hippocampus

The local circuitry of the mammalian limbic cortices, including the hippocampus, is capable of generating spontaneous rhythmic activities of 0.5-4 Hz when isolated in vitro. These rhythmic activities are mediated by synchronous inhibitory postsynaptic potentials in pyramidal neurons as the result of repeated discharges of inhibitory interneurons. As such, they are thought to represent an intrinsic inhibitory rhythm. It is unknown at present whether such a rhythm occurs in the immature rodent hippocampus and, if so, the postnatal time window in which it develops. We explored these issues using our recently developed whole mouse hippocampal isolate preparation in vitro. We found that spontaneous rhythmic field potentials started to emerge in mouse hippocampal isolates around postnatal day 10, stabilized after postnatal day 15 and persisted into adulthood. In postnatal days 11-14 mouse hippocampi, the properties of these rhythmic potentials were in keeping with a CA3-driven, IPSP-based intrinsic network activity. The lack of spontaneous field rhythm in neonatal (postnatal days 2-7) hippocampi cannot be attributed to the excitatory activities mediated by gamma-aminobutyric acid type A (GABA-A) receptors, as chloride-dependent hyperpolarizing inhibitory postsynaptic potentials were detectable in neonatal pyramidal neurons at voltages near resting potentials and pharmacological antagonisms of GABA-A receptors produced robust epileptiform discharges in neonatal hippocampi. High frequency afferent stimulation or applications of 4-aminopyridine at low micromolar concentrations failed to induce persistent field rhythm in neonatal hippocampi, suggesting that an overall weak glutamatergic drive is not the sole causing factor. We suggest that the inhibitory postsynaptic potential-based spontaneous rhythmic field potentials develop in a discrete time window during the second postnatal week in the mouse hippocampus due to a fine-tuning in the structure and function of CA3 recurrent circuitry and associated GABAergic inhibitory interneurons.

Simultaneous glutamate and GABA(A) receptor agonist administration increases calbindin levels and prevents hippocampal damage induced by either agent alone in a model of perinatal brain injury

Perinatal brain injury is associated with the release of amino acids, principally glutamate and GABA, resulting in massive increases in intracellular calcium and eventual cell death. We have previously demonstrated that independent administration of kainic acid (KA), an AMPA/kainate receptor agonist, or muscimol, a GABA(A) receptor agonist, to newborn rats results in hippocampal damage [Hilton, G.D., Ndubuizu, A., and McCarthy, M.M., 2004. Neuroprotective effects of estradiol in newborn female rat hippocampus. Dev. Brain Res. 150, 191-198; Hilton, G. D., Nunez, J.L. and McCarthy, M.M., 2003. Sex differences in response to kainic acid and estradiol in the hippocampus of newborn rats. Neuroscience. 116, 383-391; Nunez, J.L. and McCarthy, M.M., 2003. Estradiol exacerbates hippocampal damage in a model of preterm infant brain injury. Endocrinology. 144, 2350-2359; Nunez, J.L., Alt, J.J. and McCarthy, M.M., 2003. A new model for prenatal brain damage. I. GABA(A) receptor activation induces cell death in developing rat hippocampus. Exp. Neurol. 181, 258-269]. We now report that KA or muscimol alone administered to immature hippocampal neurons in culture induces significant cell death as evidenced by TUNEL assay. Surprisingly, simultaneous administration of equimolar quantities of these two agonists blocks the effect of either one alone. Moreover, treatment of newborn pups results in less damage compared to either muscimol or KA alone. We further observed that immunoreactivity for the calcium-binding protein, calbindin D(28K), is increased in the brains of pups simultaneously administered KA and muscimol as compared to either alone.

Anticonvulsant action of GABA in the high potassium-low magnesium model of ictogenesis in the neonatal rat hippocampus in vivo and in vitro

Previous developmental studies in vitro suggested that the inhibitory neurotransmitter GABA exerts depolarizing and excitatory actions on the immature neurons and that depolarizing GABA is causally linked to ictal activity during the first weeks of postnatal life. However, remarkably little is known on the role of GABA in the generation of neonatal seizures in vivo. Here, using extracellular recordings from CA3 hippocampus, we studied the effects of GABA(A)-acting drugs on electrographic seizures induced by local intrahippocampal injection of the epileptogenic agents (high K(+)/low Mg(2+)) in the nonanesthetized rats in vivo and in the hippocampal slices in vitro during the second postnatal week (postnatal days P8-12). We found that in vivo, the induction of ictal-like events was facilitated by co-infusion of high-K(+)/low Mg(2+) together with the GABA(A) antagonist bicuculline or gabazine. Moreover, the infusion of bicuculline alone caused ictal-like activity in approximately 30% of cases. Co-infusion of the GABA(A) receptor agonist isoguvacine or the GABA(A)-positive allosteric modulator diazepam completely prevented high-K(+)/low Mg(2+)-induced seizures. In in vitro studies using hippocampal slices, we also found that high-K(+)/low Mg(2+) produced ictal activity that was exacerbated by bicuculline and gabazine and reduced by isoguvacine. Thus in the model of high-K(+)/low Mg(2+)-induced seizures both in in vivo and in vitro conditions, GABA, acting via GABA(A) receptors, has an anticonvulsant effect during the critical developmental period of enhanced excitability.

Expression of alpha5 nicotinic acetylcholine receptor subunit mRNA during hippocampal and cortical development

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated pentameric cation channels of alpha and beta subunits. The alpha5 subunit, when added to heteromeric complexes, alters pharmacological and physiological properties of nAChRs, which may be important during development. Here we have evaluated the pre- and postnatal expression of alpha5 subunit mRNA in rat cortex and hippocampus using highly sensitive in situ hybridization. In the cortex, alpha5 mRNA was detected in the subplate, claustrum, and endopiriform nucleus at embryonic day 18 (E18), areas with sustained expression into adulthood. Transient cortical expression was detected in numerous cells, scattered mainly in layers V and II/III, during the first 2 postnatal weeks. In the hippocampus, alpha5 transcripts first appeared in the CA1 region at E18. During postnatal development, increased hybridization signal was detected in CA1 and CA3 pyramidal neurons and granule cells of the dentate gyrus, which strongly contrasted with the low levels in the adult. Interneurons in CA1/CA3 stratum radiatum and moleculare, and in the hilus region of the dentate gyrus, strongly expressed alpha5 mRNA in postnatal and adult animals. With double in situ hybridization, co-expression of alpha5 and alpha7 mRNAs was detected in a subpopulation of hippocampal interneurons. In contrast, in the subiculum, numerous cells exhibited strong hybridization signal for alpha5 and alpha7 mRNAs, but co-expression was rarely detected. In conclusion, similar to other nAChR subunits, there is transient expression of alpha5 mRNA during cortical and hippocampal development. This expression pattern places nicotinic receptors containing the alpha5 subunit in a position to regulate glutamatergic and GABAergic transmission during the postnatal period.

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.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.

GABA receptors containing Rdl subunits mediate fast inhibitory synaptic transmission in Drosophila neurons

GABAergic inhibition in Drosophila, as in other insects and mammals, is important for regulation of activity in the CNS. However, the functional properties of synaptic GABA receptors in Drosophila have not been described. Here, we report that spontaneous GABAergic postsynaptic currents (sPSCs) in cultured embryonic Drosophila neurons are mediated by picrotoxin-sensitive chloride-conducting receptors. A rapid increase in spontaneous firing in response to bath application of picrotoxin demonstrates that these GABA receptors mediate inhibition in the neuronal networks formed in culture. Many of the spontaneous GABAergic synaptic currents are sodium action potential independent [miniature IPSCs (mIPSCs)] but are regulated by external calcium levels. The large variation in mIPSC frequency, amplitude, and kinetics properties between neurons suggests heterogeneity in GABA receptor number, location, and/or subtype. A decrease in the mean mIPSC decay time constant between 2 and 5 d, in the absence of a correlated change in rise time, demonstrates that the functional properties of the synaptic GABA receptors are regulated during maturation in vitro. Finally, neurons from the GABA receptor subunit mutant Rdl exhibit reduced sensitivity to picrotoxin blockade of the mIPSCs and resistance to picrotoxin-induced increases in spontaneous firing frequency. This demonstrates that Rdl containing GABA receptors play a direct role in mediating synaptic inhibition in Drosophila neural circuits formed in culture.

The role of the hyperpolarization-activated cationic current I(h) in the timing of interictal bursts in the neonatal hippocampus

Under both pathological and experimental conditions, area CA3 of the adult or juvenile hippocampus generates periodic population discharges known as interictal bursts. Whereas the ionic and synaptic basis of individual bursts has been comprehensively studied experimentally and computationally, the pacemaker mechanisms underlying interictal rhythmicity remain conjectural. We showed previously that rhythmic population discharges resembling interictal bursts can be induced in hippocampal slices from first postnatal week mice, in Mg2+-free solution with GABA(A) receptor-mediated inhibition blocked. Here we show that these neonatal bursts occurred with high temporal precision and that their frequency and regularity were greatly reduced by the bradycardic agent ZD-7288 when applied at concentrations and durations that selectively block the hyperpolarization-activated, cationic current I(h). Augmenting I(h) by elevating intracellular cAMP dramatically increased burst frequency in a protein kinase A-independent manner. Burst amplitudes were strongly correlated with the preceding, but not the following, interburst intervals. The experimentally observed distribution of interburst intervals was modeled by assuming that a burst was triggered whenever the instantaneous rate of spontaneous EPSPs (sEPSPs) exceeded a threshold and that the mean sEPSP rate was minimal immediately after a burst and then relaxed exponentially to a steady-state level. The effect of blocking I(h) in any given slice could be modeled by decreasing only the steady-state sEPSP rate, suggesting that the instantaneous rate of sEPSPs is governed by the level of I(h) activation and raising the novel possibility that interburst intervals reflected the slow activation kinetics of I(h) in the neonatal CA3.

Gap junctions and their implications for neurogenesis and maturation of synaptic circuitry in the developing neocortex

More and more data accumulate which provide evidence for an important role of gap junctions for the development and function of the mammalian brain. In the neocortex, gap junctions are already present at very early stages of development and they seem to be involved in neurogenesis and neuronal migration. At postnatal stages of development, gap junctions obviously participate in the differentiation of neurons and formation of synapses. Recently, it has been shown that they are responsible for the synchronization of inhibitory network activity, even within the adult mammalian neocortex. Gap junction-mediated interneuronal communication seems to be complementary to the signal transfer created by chemical synapses and, in some cases, these two systems interact or act synergistically. There are, however, many open questions which need to be answered before we can achieve a comprehensive understanding of the function of gap junctions and electrical synapses for the development and function of the neocortex.

Excitatory actions of endogenously released GABA contribute to initiation of ictal epileptiform activity in the developing hippocampus

In the developing rat hippocampus, ictal epileptiform activity can be elicited easily in vitro during the first three postnatal weeks. Changes in neuronal ion transport during this time cause the effects of GABA(A) receptor (GABA(A)-R) activation to shift gradually from strongly depolarizing to hyperpolarizing. It is not known whether the depolarizing effects of GABA and the propensity for ictal activity are causally linked. A key question is whether the GABA-mediated depolarization is excitatory, which we defined operationally as being sufficient to trigger action potentials. We assessed the effect of endogenous GABA on ictal activity and neuronal firing rate in hippocampal slices from postnatal day 1 (P1) to P30. In extracellular recordings, there was a strong correlation between the postnatal age at which GABA(A)-R antagonists decreased action potential frequency (P23) and the age at which ictal activity could be induced by elevated potassium (P23). In addition, there was a strong correlation between the fraction of slices in which ictal activity was induced by elevated potassium concentrations and the fractional decrease in action potential firing when GABA(A)-Rs were blocked in the presence of ionotropic glutamate receptor antagonists. Finally, ictal activity induced by elevated potassium was blocked by the GABA(A)-R antagonists bicuculline and SR-95531 (gabazine) and increased in frequency and duration by GABA(A)-R agonists isoguvacine and muscimol. Thus, the propensity of the developing hippocampus for ictal activity is highly correlated with the effect of GABA on action potential probability and reversed by GABA(A) antagonists, indicating that GABA-mediated excitation is causally linked to ictal activity in this developmental window.