GAT-1 acts to limit a tonic GABAA current in rat CA3 pyramidal neurons at birth

Caffeine Improves GABA Transport in the Striatum of Spontaneously Hypertensive Rats (SHR)

The spontaneously hypertensive rat (SHR) is an excellent animal model that mimics the behavioral and neurochemical phenotype of attention-deficit/hyperactivity disorder (ADHD). Here, we characterized the striatal GABA transport of SHR and investigated whether caffeine, a non-selective antagonist of adenosine receptors, could influence GABAergic circuitry. For this purpose, ex vivo striatal slices of SHR and Wistar (control strain) on the 35th postnatal day were dissected and incubated with [3H]-GABA to quantify the basal levels of uptake and release. SHR exhibited a reduced [3H]-GABA uptake and release, suggesting a defective striatal GABAergic transport system. GAT-1 appears to be the primary transporter for [3H]-GABA uptake in SHR striatum, as GAT-1 selective blocker, NO-711, completely abolished it. We also verified that acute exposure of striatal slices to caffeine improved [3H]-GABA uptake and release in SHR, whereas Wistar rats were not affected. GABA-uptake increase and cAMP accumulation promoted by caffeine was reverted by A1R activation with N6-cyclohexyl adenosine (CHA). As expected, the pharmacological blockade of cAMP-PKA signaling by H-89 also prevented caffeine-mediated [3H]-GABA uptake increment. Interestingly, a single caffeine exposure did not affect GAT-1 or A1R protein density in SHR, which was not different from Wistar protein levels, suggesting that the GAT-1-dependent transport in SHR has a defective functional activity rather than lower protein expression. The current data support that caffeine regulates GAT-1 function and improves striatal GABA transport via A1R-cAMP-PKA signaling, specifically in SHR. These results reinforce that caffeine may have therapeutic use in disorders where the GABA transport system is impaired.

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.

GABA transporters regulate tonic and synaptic GABAA receptor-mediated currents in the suprachiasmatic nucleus neurons

GABA is a principal neurotransmitter in the hypothalamic suprachiasmatic nucleus (SCN) that contributes to intercellular communication between individual circadian oscillators within the SCN network and the stability and precision of the circadian rhythms. GABA transporters (GAT) regulate the extracellular GABA concentration and modulate GABAA receptor (GABAAR)-mediated currents. GABA transport inhibitors were applied to study how GABAAR-mediated currents depend on the expression and function of GAT. Nipecotic acid inhibits GABA transport and induced an inward tonic current in concentration-dependent manner during whole cell patch-clamp recordings from SCN neurons. Application of either the selective GABA transporter 1 (GAT1) inhibitors NNC-711 or SKF-89976A, or the GABA transporter 3 (GAT3) inhibitor SNAP-5114, produced only small changes of the baseline current. Coapplication of GAT1 and GAT3 inhibitors induced a significant GABAAR-mediated tonic current that was blocked by gabazine. GAT inhibitors decreased the amplitude and decay time constant and increased the rise time of spontaneous GABAAR-mediated postsynaptic currents. However, inhibition of GAT did not alter the expression of either GAT1 or GAT3 in the hypothalamus. Thus GAT1 and GAT3 functionally complement each other to regulate the extracellular GABA concentration and GABAAR-mediated synaptic and tonic currents in the SCN. Coapplication of SKF-89976A and SNAP-5114 (50 µM each) significantly reduced the circadian period of Per1 expression in the SCN by 1.4 h. Our studies demonstrate that GAT are important regulators of GABAAR-mediated currents and the circadian clock in the SCN.NEW & NOTEWORTHY In the suprachiasmatic nucleus (SCN), the GABA transporters GAT1 and GAT3 are expressed in astrocytes. Inhibition of these GABA transporters increased a tonic GABA current and reduced the circadian period of Per1 expression in SCN neurons. GAT1 and GAT3 showed functional cooperativity: inhibition of one GAT increased the activity but not the expression of the other. Our data demonstrate that GABA transporters are important regulators of GABAA receptor-mediated currents and the circadian clock.

Early postnatal switch in GABAA receptor α-subunits in the reticular thalamic nucleus

The GABAergic neurons of the thalamic reticular nucleus (nRt) provide the primary source of inhibition within the thalamus. Using physiology, pharmacology, and immunohistochemistry in mice, we characterized postsynaptic developmental changes in these inhibitory projection neurons. First, at postnatal days 3-5 (P3-5), inhibitory postsynaptic currents (IPSCs) decayed very slowly, followed by a biphasic developmental progression, becoming faster at P6-8 and then slower again at P9-11 before stabilizing in a mature form around P12. Second, the pharmacological profile of GABA(A) receptor (GABA(A)R)-mediated IPSCs differed between neonatal and mature nRt neurons, and this was accompanied by reciprocal changes in α3 (late) and α5 (early) subunit expression in nRt. Zolpidem, selective for α1- and α3-containing GABA(A)Rs, augmented only mature IPSCs, whereas clonazepam enhanced IPSCs at all stages. This effect was blocked by the α5-specific inverse agonist L-655,708, but only in immature neurons. In α3(H126R) mice, in which α3-subunits were mutated to become benzodiazepine insensitive, IPSCs were enhanced compared with those in wild-type animals in early development. Third, tonic GABA(A)R activation in nRt is age dependent and more prominent in immature neurons, which correlates with early expression of α5-containing GABA(A)Rs. Thus neonatal nRt neurons show relatively high expression of α5-subunits, which contributes to both slow synaptic and tonic extrasynaptic inhibition. The postnatal switch in GABA(A)R subunits from α5 to α3 could facilitate spontaneous network activity in nRt that occurs at this developmental time point and which is proposed to play a role in early circuit development.

Glycine transporter-1 controls nonsynaptic inhibitory actions of glycine receptors in the neonatal rat hippocampus

Although functional glycinergic synapses have not been identified in the hippocampus, neurons in this area express Cl(-) permeable extrasynaptic glycine receptors (GlyRs). In experiments on CA3 pyramidal neurons on postnatal day 0-6 rat hippocampal slices, we detected robust GlyR activity as a tonic current and as single-channel events. Glycine release was independent of neuronal activity or extracellular Ca(2+). The endogenous GlyR activity was strongly enhanced by inhibition of the glycine-transporter-1 (GlyT1). Blockade of GlyT1 also caused a profound increase in the baseline current induced by exogenous glycine. Inhibition of GlyT1 reduced the frequency of spontaneous network events known as field giant depolarizing potentials (fGDPs) and of the unit activity in the absence of synaptic transmission. This inhibitory action on fGDPs was mimicked by applying 2 μm glycine or 0.1 μm isoguvacine, a GABAA-receptor agonist. Furthermore, 2 μm glycine suppressed unit spiking in the absence of synaptic transmission. Hence, despite the well known depolarizing Cl(-) equilibrium potential of neonatal hippocampal neurons, physiologically relevant extracellular glycine concentrations can exert an inhibitory action. The present data show that, akin to GABA uptake, GlyT1 exerts a powerful modulatory action on network events in the newborn 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.

Role of tonic GABAergic currents during pre- and early postnatal rodent development

In the last three decades it became evident that the GABAergic system plays an essential role for the development of the central nervous system, by influencing the proliferation of neuronal precursors, neuronal migration and differentiation, as well as by controlling early activity patterns and thus formation of neuronal networks. GABA controls neuronal development via depolarizing membrane responses upon activation of ionotropic GABA receptors. However, many of these effects occur before the onset of synaptic GABAergic activity and thus require the presence of extrasynaptic tonic currents in neuronal precursors and immature neurons. This review summarizes our current knowledge about the role of tonic GABAergic currents during early brain development. In this review we compare the temporal sequence of the expression and functional relevance of different GABA receptor subunits, GABA synthesizing enzymes and GABA transporters. We also refer to other possible endogenous agonists of GABAA receptors. In addition, we describe functional consequences mediated by the GABAergic system during early developmental periods and discuss current models about the origin of extrasynaptic GABA and/or other endogenous GABAergic agonists during early developmental states. Finally, we present evidence that tonic GABAergic activity is also critically involved in the generation of physiological as well as pathophysiological activity patterns before and after the establishment of functional GABAergic synaptic connections.

Functional role of ambient GABA in refining neuronal circuits early in postnatal development

Early in development, γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the mature brain, depolarizes and excites targeted neurons by an outwardly directed flux of chloride, resulting from the peculiar balance between the cation-chloride importer NKCC1 and the extruder KCC2. The low expression of KCC2 at birth leads to accumulation of chloride inside the cell and to the equilibrium potential for chloride positive respect to the resting membrane potential. GABA exerts its action via synaptic and extrasynaptic GABAA receptors mediating phasic and tonic inhibition, respectively. Here, recent data on the contribution of "ambient" GABA to the refinement of neuronal circuits in the immature brain have been reviewed. In particular, we focus on the hippocampus, where, prior to the formation of conventional synapses, GABA released from growth cones and astrocytes in a calcium- and SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor)-independent way, diffuses away to activate in a paracrine fashion extrasynaptic receptors localized on distal neurons. The transient increase in intracellular calcium following the depolarizing action of GABA leads to inhibition of DNA synthesis and cell proliferation. Tonic GABA exerts also a chemotropic action on cell migration. Later on, when synapses are formed, GABA spilled out from neighboring synapses, acting mainly on extrasynaptic α5, β2, β3, and γ containing GABAA receptor subunits, provides the membrane depolarization necessary for principal cells to reach the window where intrinsic bursts are generated. These are instrumental in triggering calcium transients associated with network-driven giant depolarizing potentials which act as coincident detector signals to enhance synaptic efficacy at emerging GABAergic and glutamatergic synapses.

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.

Localization and Function of GABA Transporters GAT-1 and GAT-3 in the Basal Ganglia

GABA transporter type 1 and 3 (GAT-1 and GAT-3, respectively) are the two main subtypes of GATs responsible for the regulation of extracellular GABA levels in the central nervous system. These transporters are widely expressed in neuronal (mainly GAT-1) and glial (mainly GAT-3) elements throughout the brain, but most data obtained so far relate to their role in the regulation of GABA(A) receptor-mediated postsynaptic tonic and phasic inhibition in the hippocampus, cerebral cortex and cerebellum. Taking into consideration the key role of GABAergic transmission within basal ganglia networks, and the importance for these systems to be properly balanced to mediate normal basal ganglia function, we analyzed in detail the localization and function of GAT-1 and GAT-3 in the globus pallidus of normal and Parkinsonian animals, in order to further understand the substrate and possible mechanisms by which GABA transporters may regulate basal ganglia outflow, and may become relevant targets for new therapeutic approaches for the treatment of basal ganglia-related disorders. In this review, we describe the general features of GATs in the basal ganglia, and give a detailed account of recent evidence that GAT-1 and GAT-3 regulation can have a major impact on the firing rate and pattern of basal ganglia neurons through pre- and post-synaptic GABA(A)- and GABA(B)-receptor-mediated effects.

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.

Non-cell-autonomous factor induces the transition from excitatory to inhibitory GABA signaling in retina independent of activity

During development, the effect of activating GABA(A) receptors switches from depolarizing to hyperpolarizing. Several environmental factors have been implicated in the timing of this GABA switch, including neural activity, although these observations remain controversial. By using acutely isolated retinas from KO mice and pharmacological manipulations in retinal explants, we demonstrate that the timing of the GABA switch in retinal ganglion cells (RGCs) is unaffected by blockade of specific neurotransmitter receptors or global activity. In contrast to RGCs in the intact retina, purified RGCs remain depolarized by GABA, indicating that the GABA switch is not cell-autonomous. Indeed, purified RGCs cocultured with dissociated cells from the superior colliculus or cultured in media conditioned by superior collicular cells undergo a normal switch. Thus, a diffusible signal that acts independent of local circuit activity regulates the maturation of GABAergic inhibition in mouse RGCs.

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.

GABA Actions in Hippocampal Area CA3 During Postnatal Development: Differential Shift From Depolarizing to Hyperpolarizing in Somatic and Dendritic Compartments

γ-Aminobutyric acid type A receptor (GABAA-R) activation leads to depolarization of pyramidal cells during the first postnatal week and produces hyperpolarization from the second week. However, immunohistochemical evidence has suggested that during the second and third postnatal weeks the NKCC1 cotransporter relocates from the soma to the dendrites of CA3 pyramidal cells. We hypothesized that this leads to depolarizing responses in apical dendrites. Here we show that the activation of GABAA-R in the distal dendrites of CA3 pyramidal cells at P15 by restricted application of muscimol or synaptic activation by stimulation of interneurons in stratum radiatum (SR) causes depolarizing postsynaptic potentials (PSPs), which are blocked by NKCC1 cotransporter antagonists. By contrast, activation of proximal GABAA-R by muscimol application or by stimulation of interneurons in s. oriens (SO) leads to hyperpolarizing PSPs. Activation of the dentate gyrus (DG) in the presence of glutamatergic blockers evokes hyperpolarizing responses during the second postnatal week; however, the reversal potential of the DG-evoked inhibitory (I)PSPs is more depolarized than that of IPSPs evoked by activation of SO interneurons. Despite the shift of GABA action from depolarizing to hyperpolarizing, DG-evoked field potentials (f-PSPs) recorded in s. lucidum/radiatum (SL/R) do not change in polarity until the third week. Current source density analysis yielded results consistent with depolarizing actions of GABA in the dendritic compartment. Our data suggest that GABAergic input to apical dendrites of pyramidal cells of CA3 evokes depolarizing PSPs long after synaptic inhibition has become hyperpolarizing in the somata, in the axon initial segments and in basal dendrites.

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.