Activity-dependent scaling of GABAergic excitation by dynamic Cl- changes in Cajal-Retzius cells

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.

Coincident glutamatergic depolarizations enhance GABAA receptor-dependent Cl- influx in mature and suppress Cl- efflux in immature neurons

The impact of GABAergic transmission on neuronal excitability depends on the Cl^--gradient across membranes. However, the Cl^--fluxes through GABA_A receptors alter the intracellular Cl^- concentration ([Cl^-]_i) and in turn attenuate GABAergic responses, a process termed ionic plasticity. Recently it has been shown that coincident glutamatergic inputs significantly affect ionic plasticity. Yet how the [Cl^-]_i changes depend on the properties of glutamatergic inputs and their spatiotemporal relation to GABAergic stimuli is unknown. To investigate this issue, we used compartmental biophysical models of Cl^- dynamics simulating either a simple ball-and-stick topology or a reconstructed CA3 neuron. These computational experiments demonstrated that glutamatergic co-stimulation enhances GABA receptor-mediated Cl^- influx at low and attenuates or reverses the Cl^- efflux at high initial [Cl^-]_i. The size of glutamatergic influence on GABAergic Cl^--fluxes depends on the conductance, decay kinetics, and localization of glutamatergic inputs. Surprisingly, the glutamatergic shift in GABAergic Cl^--fluxes is invariant to latencies between GABAergic and glutamatergic inputs over a substantial interval. In agreement with experimental data, simulations in a reconstructed CA3 pyramidal neuron with physiological patterns of correlated activity revealed that coincident glutamatergic synaptic inputs contribute significantly to the activity-dependent [Cl^-]_i changes. Whereas the influence of spatial correlation between distributed glutamatergic and GABAergic inputs was negligible, their temporal correlation played a significant role. In summary, our results demonstrate that glutamatergic co-stimulation had a substantial impact on ionic plasticity of GABAergic responses, enhancing the attenuation of GABAergic inhibition in the mature nervous systems, but suppressing GABAergic [Cl^-]_i changes in the immature brain. Therefore, glutamatergic shift in GABAergic Cl^--fluxes should be considered as a relevant factor of short-term plasticity.

Information processing in the brain requires that excitation and inhibition are balanced. The main inhibitory neurotransmitter in the brain is gamma-amino-butyric acid (GABA). GABA actions depend on the Cl^--gradient, but activation of ionotropic GABA receptors causes Cl^--fluxes and thus reduces GABAergic inhibition. Here, we investigated how a coincident membrane depolarization by excitatory glutamatergic synapses influences GABA-induced Cl^--fluxes using a biophysical compartmental model of Cl^- dynamics, simulating either simple or realistic neuron topologies. We demonstrate that glutamatergic co-stimulation directly affects GABA-induced Cl^--fluxes, with the size of glutamatergic effects depending on the conductance, the decay kinetics, and localization of glutamatergic inputs. We also show that the glutamatergic shift in GABAergic Cl^--fluxes is surprisingly stable over a substantial range of latencies between glutamatergic and GABAergic inputs. We conclude from these results that glutamatergic co-stimulation alters GABAergic Cl^--fluxes and in turn affects the strength of GABAergic inhibition. These coincidence-dependent ionic changes should be considered as a relevant factor of short-term plasticity in the CNS.

NKCC-1 mediated Cl- uptake in immature CA3 pyramidal neurons is sufficient to compensate phasic GABAergic inputs

Activation of GABA_A receptors causes in immature neurons a functionally relevant decrease in the intracellular Cl^- concentration ([Cl^-]_i), a process termed ionic plasticity. Amount and duration of ionic plasticity depends on kinetic properties of [Cl^-]_i homeostasis. In order to characterize the capacity of Cl^- accumulation and to quantify the effect of persistent GABAergic activity on [Cl^-]_i, we performed gramicidin-perforated patch-clamp recordings from CA3 pyramidal neurons of immature (postnatal day 4-7) rat hippocampal slices. These experiments revealed that inhibition of NKCC1 decreased [Cl^-]_i toward passive distribution with a time constant of 381 s. In contrast, active Cl^- accumulation occurred with a time constant of 155 s, corresponding to a rate of 15.4 µM/s. Inhibition of phasic GABAergic activity had no significant effect on steady state [Cl^-]_i. Inhibition of tonic GABAergic currents induced a significant [Cl^-]_i increase by 1.6 mM, while activation of tonic extrasynaptic GABA_A receptors with THIP significantly reduced [Cl^-]_i.. Simulations of neuronal [Cl^-]_i homeostasis supported the observation, that basal levels of synaptic GABAergic activation do not affect [Cl^-]_i. In summary, these results indicate that active Cl^--uptake in immature hippocampal neurons is sufficient to maintain stable [Cl^-]_i at basal levels of phasic and to some extent also to compensate tonic GABAergic activity.

Coincident Activation of Glutamate Receptors Enhances GABAA Receptor-Induced Ionic Plasticity of the Intracellular Cl--Concentration in Dissociated Neuronal Cultures

Massive activation of γ-amino butyric acid A (GABA_A) receptors during pathophysiological activity induces an increase in the intracellular Cl^--concentration ([Cl^-]_i), which is sufficient to render GABAergic responses excitatory. However, to what extent physiological levels of GABAergic activity can influence [Cl^-]_i is not known. Aim of the present study is to reveal whether moderate activation of GABA_A receptors mediates functionally relevant [Cl^-]_i changes and whether these changes can be augmented by coincident glutamatergic activity. To address these questions, we used whole-cell patch-clamp recordings from cultured cortical neurons [at days in vitro (DIV) 6-22] to determine changes in the GABA reversal potential (E_GABA) induced by short bursts of GABAergic and/or synchronized glutamatergic stimulation. These experiments revealed that pressure-application of 10 short muscimol pulses at 10 Hz induced voltage-dependent [Cl^-]_i changes. Under current-clamp conditions this muscimol burst induced a [Cl^-]_i increase of 3.1 ± 0.4 mM (n = 27), which was significantly enhanced to 4.6 ± 0.5 mM (n = 27) when glutamate was applied synchronously with the muscimol pulses. The muscimol-induced [Cl^-]_i increase significantly attenuated the inhibitory effect of GABA, as determined by the GABAergic rheobase shift. The synchronous coapplication of glutamate pulses had no additional effect on the attenuation of GABAergic inhibition, despite the larger [Cl^-]_i transients under these conditions. In summary, these results indicate that moderate GABAergic activity can induce functionally relevant [Cl^-]_i transients, which were enhanced by coincident glutamate pulses. This ionic plasticity of [Cl^-]_i may contribute to short-term plasticity of the GABAergic system.

Glutamate versus GABA in neuron-oligodendroglia communication

In the central nervous system (CNS), myelin sheaths around axons are formed by glial cells named oligodendrocytes (OLs). In turn, OLs are generated by oligodendrocyte precursor cells (OPCs) during postnatal development and in adults, according to a process that depends on the proliferation and differentiation of these progenitors. The maturation of OL lineage cells as well as myelination by OLs are complex and highly regulated processes in the CNS. OPCs and OLs express an array of receptors for neurotransmitters, in particular for the two main CNS neurotransmitters glutamate and GABA, and are therefore endowed with the capacity to respond to neuronal activity. Initial studies in cell cultures demonstrated that both glutamate and GABA signaling mechanisms play important roles in OL lineage cell development and function. However, much remains to be learned about the communication of glutamatergic and GABAergic neurons with oligodendroglia in vivo. This review focuses on recent major advances in our understanding of the neuron-oligodendroglia communication mediated by glutamate and GABA in the CNS, and highlights the present controversies in the field. We discuss the expression, activation modes and potential roles of synaptic and extrasynaptic receptors along OL lineage progression. We review the properties of OPC synaptic connectivity with presynaptic glutamatergic and GABAergic neurons in the brain and consider the implication of glutamate and GABA signaling in activity-driven adaptive myelination.

Interactions between Membrane Resistance, GABA-A Receptor Properties, Bicarbonate Dynamics and Cl--Transport Shape Activity-Dependent Changes of Intracellular Cl- Concentration

The effects of ionotropic γ-aminobutyric acid receptor (GABA-A, GABA_A) activation depends critically on the Cl⁻-gradient across neuronal membranes. Previous studies demonstrated that the intracellular Cl⁻-concentration ([Cl⁻]_i) is not stable but shows a considerable amount of activity-dependent plasticity. To characterize how membrane properties and different molecules that are directly or indirectly involved in GABAergic synaptic transmission affect GABA-induced [Cl⁻]_i changes, we performed compartmental modeling in the NEURON environment. These simulations demonstrate that GABA-induced [Cl⁻]_i changes decrease at higher membrane resistance, revealing a sigmoidal dependency between both parameters. Increase in GABAergic conductivity enhances [Cl⁻]_i with a logarithmic dependency, while increasing the decay time of GABA_A receptors leads to a nearly linear enhancement of the [Cl⁻]_i changes. Implementing physiological levels of HCO₃⁻-conductivity to GABA_A receptors enhances the [Cl⁻]_i changes over a wide range of [Cl⁻]_i, but this effect depends on the stability of the HCO₃⁻ gradient and the intracellular pH. Finally, these simulations show that pure diffusional Cl⁻-elimination from dendrites is slow and that a high activity of Cl⁻-transport is required to improve the spatiotemporal restriction of GABA-induced [Cl⁻]_i changes. In summary, these simulations revealed a complex interplay between several key factors that influence GABA-induced [Cl]_i changes. The results suggest that some of these factors, including high resting [Cl⁻]_i, high input resistance, slow decay time of GABA_A receptors and dynamic HCO₃⁻ gradient, are specifically adapted in early postnatal neurons to facilitate limited activity-dependent [Cl⁻]_i decreases.

Giant Depolarizing Potentials Trigger Transient Changes in the Intracellular Cl- Concentration in CA3 Pyramidal Neurons of the Immature Mouse Hippocampus

Giant depolarizing potentials (GDPs) represent a typical spontaneous activity pattern in the immature hippocampus. GDPs are mediated by GABAergic and glutamatergic synaptic inputs and their initiation requires an excitatory GABAergic action, which is typical for immature neurons due to their elevated intracellular Cl^- concentration ([Cl^-]_i). Because GABA_A receptors are ligand-gated Cl^- channels, activation of these receptors can potentially influence [Cl^-]_i. However, whether the GABAergic activity during GDPs influences [Cl^-]_i is unclear. To address this question we performed whole-cell and gramicidin-perforated patch-clamp recordings from visually identified CA3 pyramidal neurons in immature hippocampal slices of mice at postnatal days 4–7. These experiments revealed that the [Cl^-]_i of CA3 neurons displays a considerable heterogeneity, ranging from 13 to 70 mM (average 38.1 ± 3.2 mM, n = 36). In accordance with this diverse [Cl^-]_i, GDPs induced either Cl^--effluxes or Cl^--influxes. In high [Cl^-]_i neurons with a negative Cl^--driving force (DF_Cl) the [Cl^-]_i decreased after a GDP by 12.4 ± 3.4 mM (n = 10), while in low [Cl^-]_i neurons with a positive DF_Cl [Cl^-]_i increased by 4.4 ± 0.9 mM (n = 6). Inhibition of GDP activity by application of the AMPA receptor antagonist CNQX led to a [Cl^-]_i decrease to 24.7 ± 2.9 mM (n = 8). We conclude from these results, that Cl^--fluxes via GABA_A receptors during GDPs induced substantial [Cl^-]_i changes and that this activity-dependent ionic plasticity in neuronal [Cl^-]_i contributes to the functional consequences of GABAergic responses, emphasizing the concept that [Cl^-]_i is a state- and compartment-dependent parameter of individual cells.

Cajal-Retzius cells and GABAergic interneurons of the developing hippocampus: Close electrophysiological encounters of the third kind

In contrast to the large number of studies investigating the electrophysiological properties and synaptic connectivity of hippocampal pyramidal neurons, granule cells, and GABAergic interneurons, much less is known about Cajal-Retzius cells. In this review article, we discuss the possible reasons underlying this difference, and review experimental work performed on this cell type in the hippocampus, comparing it with results obtained in the neocortex. Our main emphasis is on data obtained with in vitro electrophysiology. In particular, we address the bidirectional connectivity between Cajal-Retzius cells and GABAergic interneurons, examine their synaptic properties and propose specific functions of Cajal-Retzius cell/GABAergic interneuron microcircuits. Lastly, we discuss the potential involvement of these microcircuits in critical physiological hippocampal functions such as postnatal neurogenesis or pathological scenarios such as temporal lobe epilepsy.

GABAergic Transmission during Brain Development: Multiple Effects at Multiple Stages

In recent years, considerable progress has been achieved in deciphering the cellular and network functions of GABAergic transmission in the intact developing brain. First, in vivo studies in non-mammalian and mammalian species confirmed the long-held assumption that GABA acts as a mainly depolarizing neurotransmitter at early developmental stages. At the same time, GABAergic transmission was shown to spatiotemporally constrain spontaneous cortical activity, whereas firm evidence for GABAergic excitation in vivo is currently missing. Second, there is a growing body of evidence indicating that depolarizing GABA may contribute to the activity-dependent refinement of neural circuits. Third, alterations in GABA actions have been causally linked to developmental brain disorders and identified as potential targets of timed prophylactic interventions. In this article, we review these major recent findings and argue that both depolarizing and inhibitory GABA actions may be crucial for physiological brain maturation.

GABA depolarizes immature neurons and inhibits network activity in the neonatal neocortex in vivo

A large body of evidence from in vitro studies suggests that GABA is depolarizing during early postnatal development. However, the mode of GABA action in the intact developing brain is unknown. Here we examine the in vivo effects of GABA in cells of the upper cortical plate using a combination of electrophysiological and Ca(2+)-imaging techniques. We report that at postnatal days (P) 3-4, GABA depolarizes the majority of immature neurons in the occipital cortex of anaesthetized mice. At the same time, GABA does not efficiently activate voltage-gated Ca(2+) channels and fails to induce action potential firing. Blocking GABA(A) receptors disinhibits spontaneous network activity, whereas allosteric activation of GABA(A) receptors has the opposite effect. In summary, our data provide evidence that in vivo GABA acts as a depolarizing neurotransmitter imposing an inhibitory control on network activity in the neonatal (P3-4) neocortex.

Cajal-Retzius cells: update on structural and functional properties of these mystic neurons that bridged the 20th century

Cajal-Retzius cells (CRc) represent a mostly transient neuronal cell type localized in the uppermost layer of the developing neocortex. The observation that CRc are a major source of the extracellular matrix protein reelin, which is essential for the laminar development of the cerebral cortex, attracted the interest in this unique cell type. In this review we will (i) describe the morphological and molecular properties of neocortical CRc, with a special emphasize on the question which markers can be used to identify CRc, (ii) summarize reports that identified the different developmental origins of CRc, (iii) discuss the fate of CRc, including recent evidence for apoptotic cell death and a possible persistence of some CRc, (iv) provide a detailed description of the electrical membrane properties and transmitter receptors of CRc, and (v) address the role of CRc in early neuronal circuits and cortical development. Finally, we speculate whether CRc may provide a link between early network activity and the structural maturation of neocortical circuits.

Activity-dependent endogenous taurine release facilitates excitatory neurotransmission in the neocortical marginal zone of neonatal rats

In the developing cerebral cortex, the marginal zone (MZ), consisting of early-generated neurons such as Cajal-Retzius cells, plays an important role in cell migration and lamination. There is accumulating evidence of widespread excitatory neurotransmission mediated by γ-aminobutyric acid (GABA) in the MZ. Cajal-Retzius cells express not only GABA_A receptors but also α2/β subunits of glycine receptors, and exhibit glycine receptor-mediated depolarization due to high [Cl⁻]_i. However, the physiological roles of glycine receptors and their endogenous agonists during neurotransmission in the MZ are yet to be elucidated. To address this question, we performed optical imaging from the MZ using the voltage-sensitive dye JPW1114 on tangential neocortical slices of neonatal rats. A single electrical stimulus evoked an action-potential-dependent optical signal that spread radially over the MZ. The amplitude of the signal was not affected by glutamate receptor blockers, but was suppressed by either GABA_A or glycine receptor antagonists. Combined application of both antagonists nearly abolished the signal. Inhibition of Na⁺, K⁺-2Cl⁻ cotransporter by 20 µM bumetanide reduced the signal, indicating that this transporter contributes to excitation. Analysis of the interstitial fluid obtained by microdialysis from tangential neocortical slices with high-performance liquid chromatography revealed that GABA and taurine, but not glycine or glutamate, were released in the MZ in response to the electrical stimulation. The ambient release of taurine was reduced by the addition of a voltage-sensitive Na⁺ channel blocker. Immunohistochemistry and immunoelectron microscopy indicated that taurine was stored both in Cajal-Retzius and non-Cajal-Retzius cells in the MZ, but was not localized in presynaptic structures. Our results suggest that activity-dependent non-synaptic release of endogenous taurine facilitates excitatory neurotransmission through activation of glycine receptors in the MZ.

Taurine activates GABAergic networks in the neocortex of immature mice

Although it has been suggested that taurine is the main endogenous neurotransmitter acting on glycine receptors, the implications of glycine receptor-mediated taurine actions on immature neocortical networks have not been addressed yet. To investigate the influence of taurine on the excitability of neuronal networks in the immature neocortex, we performed whole-cell patch-clamp recordings from visually identified pyramidal neurons and interneurons in coronal slices from C57Bl/6 and GAD67-green fluorescent protein (GFP) transgenic mice (postnatal days 2–4). In 46% of the pyramidal neurons bath-application of taurine at concentrations ≥ 300 μM significantly enhanced the frequency of postsynaptic currents (PSCs) by 744.3 ± 93.8% (n = 120 cells). This taurine-induced increase of PSC frequency was abolished by 0.2 μM tetrodotoxin (TTX), 1 μM strychnine or 3 μM gabazine, but was unaffected by the glutamatergic antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and (±) R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP), suggesting that taurine specifically activates GABAergic network activity projecting to pyramidal neurons. Cell-attached recordings revealed that taurine enhanced the frequency of action potentials (APs) in pyramidal neurons, indicating an excitatory action of the GABAergic PSCs. In order to identify the presynaptic targets of taurine we demonstrate that bath application of taurine induced in GAD67-GFP labeled interneurons an inward current that is mainly mediated by glycine receptors and can generate APs in these cells. We conclude from these results that taurine can enhance network excitability in the immature neocortex by selectively activating GABAergic interneurons via interactions with glycine receptors.

Early GABAergic circuitry in the cerebral cortex

In the cerebral cortex GABAergic signaling plays an important role in regulating early developmental processes, for example, neurogenesis, migration and differentiation. Transient cell populations, namely Cajal-Retzius in the marginal zone and thalamic input receiving subplate neurons, are integrated as active elements in transitory GABAergic circuits. Although immature pyramidal neurons receive GABAergic synaptic inputs already at fetal stages, they are integrated into functional GABAergic circuits only several days later. In consequence, GABAergic synaptic transmission has only a minor influence on spontaneous network activity during early corticogenesis. Concurrent with the gradual developmental shift of GABA action from excitatory to inhibitory and the maturation of cortical synaptic connections, GABA becomes more important in synchronizing neuronal network activity.

Malformations of cortical development and neocortical focus

Developmental neocortical malformations resulting from abnormal neurogenesis, disturbances in programmed cell death, or neuronal migration disorders may cause a long-term hyperexcitability. Early generated Cajal-Retzius and subplate neurons play important roles in transient cortical circuits, and structural/functional disorders in early cortical development may induce persistent network disturbances and epileptic disorders. In particular, depolarizing GABAergic responses are important for the regulation of neurodevelopmental events, like neurogenesis or migration, while pathophysiological alterations in chloride homeostasis may cause epileptic activity. Although modern imaging techniques may provide an estimate of the structural lesion, the site and extent of the cortical malformation may not correlate with the epileptogenic zone. The neocortical focus may be surrounded by widespread molecular, structural, and functional disturbances, which are difficult to recognize with imaging technologies. However, modern imaging and electrophysiological techniques enable focused hypotheses of the neocortical epileptogenic zone, thus allowing more specific epilepsy surgery. Focal cortical malformation can be successfully removed with minimal rim, close to or even within eloquent cortex with a promising risk-benefit ratio.

Excitatory actions of GABA in the intact neonatal rodent hippocampus in vitro

The excitatory action of gamma-aminobutyric acid (GABA) is considered to be a hallmark of the developing nervous system. However, in immature brain slices, excitatory GABA actions may be secondary to neuronal injury during slice preparation. Here, we explored GABA actions in the rodent intact hippocampal preparations and at different depths of hippocampal slices during the early post-natal period [post-natal days (P) 1–7]. We found that in the intact hippocampus at P1–3: (i) GABA exerts depolarizing action as seen in cell-attached single GABA(A) channel recordings; (ii) GABA(A) receptor (GABA(A)-R) agonist isoguvacine and synaptic activation of the GABA(A)-Rs increase the frequency of multiple unit activity and the frequency of the network-driven giant depolarizing potentials (GDPs); and that (iii) Na⁺–K⁺–2Cl^- cotransporter (NKCC1) antagonist bumetanide suppresses GDPs and the excitatory actions of isoguvacine. In the hippocampal slices at P2–5, isoguvacine and synaptic activation of GABA(A)-Rs-evoked excitatory responses at all slice depths, including surface and core. Thus, GABA exerts excitatory actions in the intact hippocampus (P1–3) and at all depths of hippocampal slices (P2–5). Therefore, the excitatory actions of GABA in hippocampal slices during the first post-natal days are not due to neuronal injury during slice preparation, and the trauma-related excitatory GABA actions at the slice surface are a fundamentally different phenomenon observed during the second post-natal week.

Short-term ionic plasticity at GABAergic synapses

Fast synaptic inhibition in the brain is mediated by the pre-synaptic release of the neurotransmitter γ-Aminobutyric acid (GABA)and the post-synaptic activation of GABA-sensitive ionotropic receptors. As with excitatory synapses, it is being increasinly appreciated that a variety of plastic processes occur at inhibitory synapses, which operate over a range of timescales. Here we examine a form of activity-dependent plasticity that is somewhat unique to GABAergic transmission. This involves short-lasting changes to the ionic driving force for the post-synaptic receptors, a process referred to as short-term ionic plasticity. These changes are directly related to the history of activity at inhibitory synapses and are influenced by a variety of factors including the location of the synapse and the post-synaptic cell's ion regulation mechanisms. We explore the processes underlying this form of plasticity, when and where it can occur, and how it is likely to impact network activity.

Refuting the challenges of the developmental shift of polarity of GABA actions: GABA more exciting than ever!

During brain development, there is a progressive reduction of intracellular chloride associated with a shift in GABA polarity: GABA depolarizes and occasionally excites immature neurons, subsequently hyperpolarizing them at later stages of development. This sequence, which has been observed in a wide range of animal species, brain structures and preparations, is thought to play an important role in activity-dependent formation and modulation of functional circuits. This sequence has also been considerably reinforced recently with new data pointing to an evolutionary preserved rule. In a recent “Hypothesis and Theory Article,” the excitatory action of GABA in early brain development is suggested to be “an experimental artefact” (Bregestovski and Bernard, 2012). The authors suggest that the excitatory action of GABA is due to an inadequate/insufficient energy supply in glucose-perfused slices and/or to the damage produced by the slicing procedure. However, these observations have been repeatedly contradicted by many groups and are inconsistent with a large body of evidence including the fact that the developmental shift is neither restricted to slices nor to rodents. We summarize the overwhelming evidence in support of both excitatory GABA during development, and the implications this has in developmental neurobiology.

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.

Effect of depolarizing GABA(A)-mediated membrane responses on excitability of Cajal-Retzius cells in the immature rat neocortex

In immature neurons activation of ionotropic GABA receptors induces depolarizing membrane responses due to a high intracellular Cl(-) concentration ([Cl(-)](i)). However, it is difficult to draw conclusions about the functional consequences of subthreshold GABAergic depolarizations, since GABAergic membrane shunting and additional effects on voltage-dependent ion channels or action potential threshold must be considered. To systematically investigate factors that determine the GABAergic effect on neuronal excitability we performed whole cell patch-clamp recordings from Cajal-Retzius cells in immature rat neocortex, using [Cl(-)](i) between 10 and 50 mM. The effect of focal GABA application was quantified by measuring various parameters of GABAergic responses including the shift in minimal threshold current (rheobase). The rheobase shift was correlated with other parameters of the GABAergic responses by multiple linear regression analyses with a set of simple mathematical models. Our experiments demonstrate that focal GABA application induces heterogeneous rheobase shifts in Cajal-Retzius cells that could not be predicted reliably from [Cl(-)](i) or the GABAergic membrane depolarization. Implementation of a linear mathematical model, which takes the GABAergic membrane conductance and the difference between action potential threshold and GABA reversal potential into account, resulted in a close correlation between calculated and experimentally obtained rheobase shifts. Addition of a linear term proportional to the GABAergic membrane depolarization improved the accuracy of correlation. The main advantage of using multiple linear regression with simple models is that direction and strength of GABAergic excitability shifts can be analyzed by using only measured parameters of GABAergic responses and with minimal a priori information about cellular parameters.