Intracellular chloride and calcium transients evoked by gamma-aminobutyric acid and glycine in neurons of the rat inferior colliculus

BIG1, a brefeldin A-inhibited guanine nucleotide-exchange factor, is required for GABA-gated Cl⁻ influx through regulation of GABAA receptor trafficking

GABAA receptors (GABAARs) mediate the majority of fast synaptic inhibition. Trafficking regulation and protein-protein interactions that maintain the appropriate number of GABAARs at the cell surface are considered to be important mechanisms for controlling the strength of synaptic inhibition. Here, we report that BIG1, a brefeldin A (BFA)-inhibited guanine nucleotide-exchange factor (GEF) which has a known role in vesicle trafficking, is a new binding partner of GABAARs. Treatment of neurons with BFA, an uncompetitive inhibitor of BIG1 GEF activity, or depletion of BIG1 by small RNA interference (siRNA) significantly decreased GABAARs at the neuronal surface and suppressed GABA-gated influx of chloride ions. Over-expression of HA-tagged BIG1-E793K, a dominant-negative mutant, also significantly decreased GABAARs at the neuronal surface, but had no effect on the total amount of GABAARs. Inhibition of GABAAR endocytosis by muscimol increased both GABAARs and BIG1 at the neuronal surface in a time-dependent fashion, and this increase could be abolished by bicuculline. Finally, depletion of BIG1 by siRNA inhibited the muscimol-stimulated increase of GABAARs. Those data suggest an important function of BIG1 in trafficking of GABAARs to the cell surface through its GEF activity. Thus, we identify an important role of BIG1 in modulating GABA-gated Cl(-) influx through the regulation of cell surface expression of GABAARs.

Chloride cotransporters, chloride homeostasis, and synaptic inhibition in the developing auditory system

The role of glycine and GABA as inhibitory neurotransmitters in the adult vertebrate nervous system has been well characterized in a variety of model systems, including the auditory, which is particularly well suited for analyzing inhibitory neurotransmission. However, a full understanding of glycinergic and GABAergic transmission requires profound knowledge of how the precise organization of such synapses emerges. Likewise, the role of glycinergic and GABAergic signaling during development, including the dynamic changes in regulation of cytosolic chloride via chloride cotransporters, needs to be thoroughly understood. Recent literature has elucidated the developmental expression of many of the molecular components that comprise the inhibitory synaptic phenotype. An equally important focus of research has revealed the critical role of glycinergic and GABAergic signaling in sculpting different developmental aspects in the auditory system. This review examines the current literature detailing the expression patterns and function (chapter 1), as well as the regulation and pharmacology of chloride cotransporters (chapter 2). Of particular importance is the ontogeny of glycinergic and GABAergic transmission (chapter 3). The review also surveys the recent work on the signaling role of these two major inhibitory neurotransmitters in the developing auditory system (chapter 4) and concludes with an overview of areas for further research (chapter 5).

Genetically encoded Cl-Sensor as a tool for monitoring of Cl-dependent processes in small neuronal compartments

Chloride (Cl) participates in a variety of physiological functions. To study processes connected with Cl homeostasis we need effective and quantitative probes allowing measurements of intracellular Cl concentration ([Cl(-)](i)) in different cell types, particularly in specialized small cellular compartments such as dendrites and dendritic spines. Of the different tools proposed for monitoring [Cl(-)](i), the genetically encoded Cl-sensitive indicators are the most promising. Recently, a ratiometric CFP-YFP based construct, termed "Cl-Sensor", with a relatively high sensitivity to Cl has been proposed (Markova et al., 2008). In the present study, we have developed conditions for the efficient expression of Cl-Sensor in tiny neuronal compartments including distal dendrites and spines. We also propose a new approach for the calibration of intracellularly expressed probes using a natural triterpenoid saponin, β-escin. We have mapped [Cl(-)](i) distribution in different neuronal compartments of cultured hippocampal and spinal cord neurons. The maximum Cl concentration was observed in the soma and it had a tendency to decrease gradually along dendritic branches, reaching minimum values in thin distal dendrites. We have also monitored transient increases in intracellular Cl in dendritic spines caused by glutamate application. These results demonstrate that Cl-Sensor enables non-invasive monitoring of the [Cl(-)](i) distribution in different types of neurons with variable morphology. This probe represents an effective tool for the quantitative estimation of [Cl(-)](i) in various cellular compartments including dendritic spines.

Activity-dependent intracellular chloride accumulation and diffusion controls GABA(A) receptor-mediated synaptic transmission

In the CNS, prolonged activation of GABA(A) receptors (GABA(A)Rs) has been shown to evoke biphasic postsynaptic responses, consisting of an initial hyperpolarization followed by a depolarization. A potential mechanism underlying the depolarization is an acute chloride (Cl(-)) accumulation resulting in a shift of the GABA(A) reversal potential (E(GABA)). The amount of GABA-evoked Cl(-) accumulation and accompanying depolarization depends on presynaptic and postsynaptic properties of GABAergic transmission, as well as on cellular morphology and regulation of Cl(-) intracellular concentration ([Cl(-)](i)). To analyze the influence of these factors on the Cl(-) and voltage behavior, we studied spatiotemporal dynamics of activity-dependent [Cl(-)](i) changes in multicompartmental models of hippocampal cells based on realistic morphological data. Simulated Cl(-) influx through GABA(A) Rs was able to exceed physiological Cl(-) extrusion rates thereby evoking HCO(3)(-) -dependent E(GABA) shift and depolarizing responses. Depolarizations were observed in spite of GABA(A) receptor desensitization. The amplitude of the depolarization was frequency-dependent and determined by intracellular Cl(-) accumulation. Changes in the dendritic diameter and in the speed of GABA clearance in the synaptic cleft were significant sources of depolarization variability. In morphologically reconstructed granule cells subjected to an intense GABAergic background activity, dendritic inhibition was more affected by accumulation of intracellular Cl(-) than somatic inhibition. Interestingly, E(GABA) changes induced by activation of a single dendritic synapse propagated beyond the site of Cl(-) influx and affected neighboring synapses. The simulations suggest that E(GABA) may differ even along a single dendrite supporting the idea that it is necessary to assign E(GABA) to a given GABAergic input and not to a given neuron.

Genetically encoded optical sensors for monitoring of intracellular chloride and chloride-selective channel activity

This review briefly discusses the main approaches for monitoring chloride (Cl⁻), the most abundant physiological anion. Noninvasive monitoring of intracellular Cl⁻ ([Cl⁻]i) is a challenging task owing to two main difficulties: (i) the low transmembrane ratio for Cl⁻, approximately 10:1; and (ii) the small driving force for Cl⁻, as the Cl⁻ reversal potential (E_Cl) is usually close to the resting potential of the cells. Thus, for reliable monitoring of intracellular Cl⁻, one has to use highly sensitive probes. From several methods for intracellular Cl⁻ analysis, genetically encoded chloride indicators represent the most promising tools. Recent achievements in the development of genetically encoded chloride probes are based on the fact that yellow fluorescent protein (YFP) exhibits Cl⁻-sensitivity. YFP-based probes have been successfully used for quantitative analysis of Cl⁻ transport in different cells and for high-throughput screening of modulators of Cl⁻-selective channels. Development of a ratiometric genetically encoded probe, Clomeleon, has provided a tool for noninvasive estimation of intracellular Cl⁻ concentrations. While the sensitivity of this protein to Cl⁻ is low (EC₅₀ about 160 mM), it has been successfully used for monitoring intracellular Cl⁻ in different cell types. Recently a CFP–YFP-based probe with a relatively high sensitivity to Cl⁻ (EC₅₀ about 30 mM) has been developed. This construct, termed Cl-Sensor, allows ratiometric monitoring using the fluorescence excitation ratio. Of particular interest are genetically encoded probes for monitoring of ion channel distribution and activity. A new molecular probe has been constructed by introducing into the cytoplasmic domain of the Cl⁻-selective glycine receptor (GlyR) channel the CFP–YFP-based Cl-Sensor. This construct, termed BioSensor-GlyR, has been successfully expressed in cell lines. The new genetically encoded chloride probes offer means of screening pharmacological agents, analysis of Cl⁻ homeostasis and functions of Cl⁻-selective channels under different physiological and pathological conditions.

Control of a depolarizing GABAergic input in an auditory coincidence detection circuit

Neurons in the chicken nucleus laminaris (NL), the third-order auditory neurons that detect the interaural time differences that enable animals to localize sounds in the horizontal plane, receive glutamatergic excitation from the cochlear nucleus magnocellularis (NM) and GABAergic inhibition from the ipsilateral superior olivary nucleus. Here, we study metabotropic glutamate receptor (mGluR)- and GABAB receptor (GABABR)-mediated modulation of synaptic transmission in NL neurons. Gramicidin-perforated recordings from acute brain stem slice preparations showed that the reversal potential of the GABAergic responses in NL neurons was more depolarized than the spike threshold. Activation of the GABAergic input produced a mix of inhibitory and excitatory actions in NL neurons. The inhibitory action is known to be critical in improving the acuity of temporal processing of sounds. The excitatory action, however, would reduce the phase locking fidelity of NL neurons in response to their excitatory inputs from the NM. We show that activation of presynaptic mGluRs or GABABRs by either exogenous agonists or synaptically released neurotransmitters reduced the GABAergic responses, preventing the excitatory action of GABA while leaving the inhibitory action intact. Unlike most CNS synapses, the glutamatergic transmission in the NL was not modulated by either mGluRs or GABABRs, indicating that fixed (nonmodulatory) excitatory inputs to the NL may be optimal for coincidence detection. This study contributes to our understanding of how selective neuromodulation is achieved to suit a particular function of neuronal circuits in the brain.

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.

Progestin facilitation of lordosis in rodents involves adenylyl cyclase activity in the ventral tegmental area

Increasing cAMP, or activating dopamine type 1 (D(1)) or GABA(A)/benzodiazepine receptor complexes (GBRs), in the ventral tegmental area (VTA) enhances lordosis of rodents. Whether D(1)- and/or GBR-mediated increases in progestin-facilitated lordosis involve the cAMP-synthesizing enzyme, adenylyl cyclase, in the VTA, was investigated. In Experiment 1, ovariectomized estradiol (E(2); 10 microg at h 0)+progesterone (P; 250 microg at h 45)-primed hamsters first received bilateral infusions of the adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine (DDA; 12 microM/side), or vehicle, and then were infused with the D(1) agonist, SKF38393 (100 ng/side), the GBR agonist, muscimol (100 ng/side), or vehicle, to the VTA. Lordosis was evaluated before and 30 min after each infusion. In Experiment 2, ovariectomized, E(2)-primed (10 microg at h 0) rats received VTA infusions of DDA (12 microM/side) or vehicle; SKF38393 (100 ng/side), muscimol (100 ng/side), or vehicle; and the neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP; 100 or 200 ng/side), or beta-cyclodextrin vehicle. Lordosis was assessed before the series of infusions, immediately after drug infusions and 10 or 60 min after 3alpha,5alpha-THP infusions. Progestin- or progestin plus SKF38393-or muscimol-mediated increases in lordosis were blocked by DDA pretreatment. Thus, in the VTA, progestins' membrane action may involve adenylyl cyclase.

Bilirubin potentiates inhibitory synaptic transmission in lateral superior olive neurons of the rat

Bilirubin is a well-known neurotoxin that can result in multiple neurologic deficits. Previous studies have suggested that bilirubin affects aspects of synaptic transmission; however the acute effects of bilirubin on synaptic transmission have not been examined in real-time. In this study, using whole-cell voltage-clamp recordings, we observed the effect of bilirubin on inhibitory postsynaptic currents (IPSC) in postnatal 13-15-day-old neurons dissociated from lateral superior olive nuclei (LSO), one of the brainstem auditory nucleus that are highly vulnerable to bilirubin. The results showed that 10(-5)M bilirubin increased the frequency of spontaneous IPSC without causing change in their amplitudes or in the response to bath applied glycine, suggesting a presynaptic locus for the action. In the presence of tetrodotoxin, the frequency of miniature IPSC was also potentiated by 10(-5)M bilirubin. The facilitation by bilirubin was concentration dependent and increased with an increase in exposure time. Bicuculline only partially reduced the action of bilirubin. The action of bilirubin was observed in extracellular Ca(2+)-free ([Ca(2+)](o) free) solution but was fully occluded by pretreatment with BAPTA-AM in [Ca(2+)](o) free solution. Thus, in LSO neurons, bilirubin facilitates inhibitory synaptic transmission, in a manner independent of voltage-activated Na(+) and Ca(2+) channels but dependent on presynaptic [Ca(2+)](i). The increase of inhibitory synaptic transmission in response to acute bilirubin is a novel effect of bilirubin on the central nervous system and may have implications for neurotoxicity and the impairment of auditory transduction seen in hyperbilirubinemia.

Synaptically activated Cl- accumulation responsible for depolarizing GABAergic responses in mature hippocampal neurons

It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABAA receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABAA receptor-permeable anions, chloride (Cl-) and bicarbonate (HCO3-). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl- accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl- imaging technique with a Cl- indicator 6-methoxy-N-ethylquinolinium iodide (MEQ). The synaptically activated Cl- accumulation was mediated dominantly through GABAA receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl- accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABAA-mediated Cl- accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl- accumulation during synaptic activation.

Coincident pre- and postsynaptic activity modifies GABAergic synapses by postsynaptic changes in Cl- transporter activity

Coincident pre- and postsynaptic activation is known to induce long-term modification of glutamatergic synapses. We report here that, in both hippocampal cultures and acute hippocampal slices, repetitive postsynaptic spiking within 20 ms before and after the activation of GABAergic synapses also led to a persistent change in synaptic strength. This synaptic modification required Ca2+ influx through postsynaptic L-type Ca2+ channels and was due to a local decrease in K+-Cl- cotransport activity, effectively reducing the strength of inhibition. Thus, GABAergic synapses can detect and be modified by coincident pre- and postsynaptic spiking, allowing the level of inhibition to be modulated in accordance to the temporal pattern of postsynaptic excitation.

Glycinergic and GABAergic calcium responses in the developing lateral superior olive

The lateral superior olive (LSO), a binaural nucleus involved in sound localization, receives tonotopically organized inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). During development, the tonotopic organization of this glycinergic/GABAergic MNTB-LSO pathway is established by activity-dependent axonal reorganization. However, the underlying mechanisms by which this reorganization takes place have remained largely unknown. As cytosolic calcium is one of the most important second messengers responsible for inducing synaptic plasticity and reorganization, we examined whether and how activity in the MNTB-LSO pathway changes the intracellular calcium concentration ([Ca2+]i) in developing LSO neurons. By applying calcium imaging techniques to Fura-2-labelled slices from neonatal rats and mice, we found that glycine and GABA (gamma-aminobutyric acid) affect [Ca2+]i in LSO neurons in an age-dependent manner; during the first postnatal week, the period at which glycine and GABA are depolarizing in the LSO, glycine and GABA always increased [Ca2+]i. However, in 2-week-old animals, the time around hearing onset when glycine and GABA are hyperpolarizing, glycine and GABA slightly decreased [Ca2+]i. Calcium responses could also be elicited by stimulation of afferent fibres from the MNTB, and these synaptic responses were mediated by glycine and GABA(A) receptors. Furthermore, GABA, which is a neurotransmitter only in the immature MNTB-LSO pathway, played a major role in generating MNTB-elicited Ca2+ responses. The direct link of glycinergic/GABAergic synaptic activity to intracellular calcium signalling during the period of inhibitory synaptic plasticity could be one of the mechanisms by which tonotopic MNTB-LSO connections become established.

Characterization of the spontaneous synaptic activity of amacrine cells in the mouse retina

Amacrine cells are a heterogeneous class of interneurons that modulate the transfer of the light signals through the retina. In addition to ionotropic glutamate receptors, amacrine cells express two types of inhibitory receptors, GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs). To characterize the functional contribution of these different receptors, spontaneous postsynaptic currents (sPSCs) were recorded with the whole cell configuration of the patch-clamp technique in acutely isolated slices of the adult mouse retina. All amacrine cells investigated (n = 47) showed spontaneous synaptic activity. In six amacrine cells, spontaneous excitatory postsynaptic currents could be identified by their sensitivity to kynurenic acid. They were characterized by small amplitudes [mean: -13.7 +/- 1.5 (SE) pA] and rapid decay kinetics (mean tau: 1.35 +/- 0.16 ms). In contrast, the reversal potential of sPSCs characterized by slow decay kinetics (amplitude-weighted time constant, tau(w), >4 ms) was dependent on the intracellular Cl(-) concentration (n = 7), indicating that they were spontaneous inhibitory postsynaptic currents (sIPSCs). In 14 of 34 amacrine cells sIPSCs were blocked by bicuculline (10 microM), indicating that they were mediated by GABA(A)Rs. Only four amacrine cells showed glycinergic sIPSCs that were inhibited by strychnine (1 microM). In one amacrine cell, sIPSCs mediated by GABA(A)Rs and GlyRs were found simultaneously. GABAergic sIPSCs could be subdivided into one group best fit by a monoexponential decay function and another biexponentially decaying group. The mean amplitude of GABAergic sIPSCs (-42.1 +/- 5.8 pA) was not significantly different from that of glycinergic sIPSCs (-28.0 +/- 8.5 pA). However, GlyRs (mean T10/90: 2.4 +/- 0.08 ms) activated significantly slower than GABA(A)Rs (mean T10/90: 1.2 +/- 0.03 ms). In addition, the decay kinetics of monoexponentially decaying GABA(A)Rs (mean tau(w): 20.3 +/- 0.50), biexponentially decaying GABA(A)Rs (mean tau(w): 30.7 +/- 0.95), and GlyRs (mean tau(w) = 25.3 +/- 1.94) were significantly different. These differences in the activation and decay kinetics of sIPSCs indicate that amacrine cells of the mouse retina express at least three types of functionally different inhibitory receptors: GlyRs and possibly two subtypes of GABA(A)Rs.

Negative cross talk between anionic GABAA and cationic P2X ionotropic receptors of rat dorsal root ganglion neurons

Using whole-cell patch-clamp recording and intracellular Ca(2+) imaging of rat cultured DRG neurons, we studied the cross talk between GABA(A) and P2X receptors. A rapidly fading current was the main response to ATP, whereas GABA elicited slowly desensitizing inward currents. Coapplication of these agonists produced a total current much smaller than the linear summation of individual responses (68 +/- 5% with 10 microm ATP plus 100 microm GABA). Occlusion was observed regardless of ATP response type. Neurons without functional P2X receptors manifested no effect of ATP on GABA currents (and vice versa). Occlusion was also absent in the presence of the P2X blocker trinitrophenyl-ATP (TNP-ATP) or of the GABA blocker picrotoxin, indicating a lack of involvement by metabotropic ATP or GABA receptors. Less occlusion was obtained when ATP was applied 2 sec after GABA than when GABA was applied after ATP. Changing the polarity of GABA currents by using intracellular SO(4)2- instead of Cl(-) significantly reduced the occlusion of ATP currents by GABA, suggesting an important role for Cl(-) efflux in this phenomenon. Occlusion was enhanced whenever intracellular Ca(2+) ([Ca(2+)](i)) was not buffered, indicating the cross talk-facilitating role of this divalent cation. Ca(2+) imaging showed that ATP (but not GABA) increased [Ca(2+)](i) in voltage-clamped or intact neurons. Our data demonstrated a novel Cl(-) and Ca(2+)-dependent interaction between cationic P2X and anionic GABA(A) receptors of DRG neurons. Such negative cross talk might represent a model for a new mechanism to inhibit afferent excitation to the spinal cord as GABA and ATP are coreleased within the dorsal horn.

Modulation of glycine responses by dihydropyridines and verapamil in rat spinal neurons

Although glycine receptors (GlyRs) are responsible for the main spinal inhibitory responses in adult vertebrates, in the embryo they have been reported to mediate depolarizing responses, which can sometimes activate dihydropyridine-sensitive L-type calcium channels. However, these channels are not the only targets of dihydropyridines (DHPs), and we questioned whether GlyRs might be directly modulated by DHPs. By whole-cell recording of cultured spinal neurons, we investigated modulation of glycine responses by the calcium channel antagonists, nifedipine, nitrendipine, nicardipine and (R)-Bay K 8644, and by the calcium channel, agonist (S)-Bay K 8644. At concentrations between 1 and 10 microM, all these DHPs could block glycine responses, even in the absence of extracellular Ca2+. The block was stronger at higher glycine concentrations, and increased with time during each glycine application. Nicardipine blocked GABAA responses from the same neurons in a similar manner. In addition to their blocking effects, nitrendipine and nicardipine potentiated the peak responses to low glycine concentrations. Both effects of extracellular nitrendipine on glycine responses persisted when the drug was present in the intracellular solution. Thus, these modulations are related neither to calcium channel modulation nor to possible intracellular effects of DHPs. Another type of calcium antagonist, verapamil (10-50 microM), also blocked glycine responses. Our results suggest that some of the effects of calcium antagonists, including the neuroprotective and anticonvulsant effects of DHPs, might result partly from their interactions with ligand-gated chloride channels.

Role of bicarbonate and chloride in GABA- and glycine-induced depolarization and [Ca2+]i rise in fetal rat motoneurons in situ

Ca(2+) imaging and (perforated) patch recording were used to analyze the mechanism of GABA- and glycine-induced depolarizations in lumbar motoneurons of spinal cord slices from fetal rats. In fura-2 ester-loaded cells, the agonist-induced depolarizations increased [Ca(2+)](i) by up to 100 nm. The GABA- and glycine-evoked [Ca(2+)](i) transients were suppressed by bicuculline and strychnine, respectively. Their magnitude decreased by approximately 50% between embryonic days 15.5 and 19.5. The [Ca(2+)](i) increases were abolished by Ca(2+)-free superfusate and attenuated by approximately 65% by nifedipine, showing that the responses were mediated by voltage-activated Ca(2+) channels. The [Ca(2+)](i) rises were potentiated by >300% immediately after removal of Cl(-) from the superfusate but recovered to values of 50-200% of control during repeated agonist administration in Cl(-)-free saline. Bumetanide gradually suppressed the [Ca(2+)](i) increases by >75%. Subsequent removal of Cl(-) reconstituted the responses and increased, upon repeated agonist application, the peak [Ca(2+)](i) rises to values above control. Removal of HCO(3)(-) from the Cl(-)-free (bumetanide-containing) superfusate reversibly abolished both the agonist-induced [Ca(2+)](i) rises and depolarizations that were reestablished by formate anions. In Cl(-)-containing superfusate, removal of HCO(3)(-) decreased both the peak and duration of the agonist-evoked membrane depolarization and [Ca(2+)](i) response. Our findings show that HCO(3)(-) efflux has a major contribution to depolarizations mediated by GABA(A) and glycine receptor-coupled anion channels in prenatal neurons. We hypothesize that the HCO(3)(-)-dependent depolarizing component, which is likely to produce an intracellular acidosis, might play an important role during the early postnatal period when the Cl(-)-dependent component gradually shifts to hyperpolarization.

A genetically encoded ratiometric indicator for chloride: capturing chloride transients in cultured hippocampal neurons

We constructed a novel optical indicator for chloride ions by fusing the chloride-sensitive yellow fluorescent protein with the chloride-insensitive cyan fluorescent protein. The ratio of FRET-dependent emission of these fluorophores varied in proportion to the concentration of Cl and was used to measure intracellular chloride concentration ([Cl-]i) in cultured hippocampal neurons. [Cl-]i decreased during neuronal development, consistent with the shift from excitation to inhibition during maturation of GABAergic synapses. Focal activation of GABAA receptors caused large changes in [Cl-]i that could underlie use-dependent depression of GABA-dependent synaptic transmission. GABA-induced changes in somatic [Cl-]i spread into dendrites, suggesting that [Cl-]i can signal the location of synaptic activity. This genetically encoded indicator will permit new approaches ranging from high-throughput drug screening to direct recordings of synaptic Cl- signals in vivo.