Glycine-activated currents are changed by coincident membrane depolarization in developing rat auditory brainstem neurones

Inhibitory Synaptic Influences on Developmental Motor Disorders

During development, GABA and glycine play major trophic and synaptic roles in the establishment of the neuromotor system. In this review, we summarise the formation, function and maturation of GABAergic and glycinergic synapses within neuromotor circuits during development. We take special care to discuss the differences in limb and respiratory neuromotor control. We then investigate the influences that GABAergic and glycinergic neurotransmission has on two major developmental neuromotor disorders: Rett syndrome and spastic cerebral palsy. We present these two syndromes in order to contrast the approaches to disease mechanism and therapy. While both conditions have motor dysfunctions at their core, one condition Rett syndrome, despite having myriad symptoms, has scientists focused on the breathing abnormalities and their alleviation-to great clinical advances. By contrast, cerebral palsy remains a scientific quagmire or poor definitions, no widely adopted model and a lack of therapeutic focus. We conclude that the sheer abundance of diversity of inhibitory neurotransmitter targets should provide hope for intractable conditions, particularly those that exhibit broad spectra of dysfunction-such as spastic cerebral palsy and Rett syndrome.

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.

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.

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.

Spines slow down dendritic chloride diffusion and affect short-term ionic plasticity of GABAergic inhibition

Cl⁻ plays a crucial role in neuronal function and synaptic inhibition. However, the impact of neuronal morphology on the diffusion and redistribution of intracellular Cl⁻ is not well understood. The role of spines in Cl⁻ diffusion along dendritic trees has not been addressed so far. Because measuring fast and spatially restricted Cl⁻ changes within dendrites is not yet technically possible, we used computational approaches to predict the effects of spines on Cl⁻ dynamics in morphologically complex dendrites. In all morphologies tested, including dendrites imaged by super-resolution STED microscopy in live brain tissue, spines slowed down longitudinal Cl⁻ diffusion along dendrites. This effect was robust and could be observed in both deterministic as well as stochastic simulations. Cl⁻ extrusion altered Cl⁻ diffusion to a much lesser extent than the presence of spines. The spine-dependent slowing of Cl⁻ diffusion affected the amount and spatial spread of changes in the GABA reversal potential thereby altering homosynaptic as well as heterosynaptic short-term ionic plasticity at GABAergic synapses in dendrites. Altogether, our results suggest a fundamental role of dendritic spines in shaping Cl⁻ diffusion, which could be of relevance in the context of pathological conditions where spine densities and neural excitability are perturbed.

Glycine receptors and brain development

Glycine receptors (GlyRs) are ligand-gated chloride ion channels that mediate fast inhibitory neurotransmission in the spinal cord and the brainstem. There, they are mainly involved in motor control and pain perception in the adult. However, these receptors are also expressed in upper regions of the central nervous system, where they participate in different processes including synaptic neurotransmission. Moreover, GlyRs are present since early stages of brain development and might influence this process. Here, we discuss the current state of the art regarding GlyRs during embryonic and postnatal brain development in light of recent findings about the cellular and molecular mechanisms that control brain development.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Transient increase in neuronal chloride concentration by neuroactive aminoacids released from glioma cells

Neuronal chloride concentration ([Cl⁻]_i) is known to be dynamically modulated and alterations in Cl⁻ homeostasis may occur in the brain at physiological and pathological conditions, being also likely involved in glioma-related seizures. However, the mechanism leading to changes in neuronal [Cl⁻]_i during glioma invasion are still unclear. To characterize the potential effect of glioma released soluble factors on neuronal [Cl⁻]_i, we used genetically encoded CFP/YFP-based ratiometric Cl-(apical) Sensor transiently expressed in cultured hippocampal neurons. Exposition of neurons to glioma conditioned medium (GCM) caused rapid and transient elevation of [Cl⁻]_i, resulting in the increase of fluorescence ratio, which was strongly reduced by blockers of ionotropic glutamate receptors APV and NBQX. Furthermore, in HEK cells expressing GluR1-AMPA receptors, GCM activated ionic currents with efficacy similar to those caused by glutamate, supporting the notion that GCM contains glutamate or glutamatergic agonists, which cause neuronal depolarization, activation of NMDA and AMPA/KA receptors leading to elevation of [Cl⁻]_i. Chromatographic analysis of the GCM showed that it contained several aminoacids, including glutamate, whose release from glioma cells did not occur via the most common glial mechanisms of transport, or in response to hypoosmotic stress. GCM also contained glycine, whose action contrasted the glutamate effect. Indeed, strychnine application significantly increased GCM-induced depolarization and [Cl⁻]_i rise. GCM-evoked [Cl⁻]_i elevation was not inhibited by antagonists of Cl⁻ transporters and significantly reduced in the presence of anion channels blocker NPPB, suggesting that Cl⁻ selective channels are a major route for GCM-induced Cl⁻ influx. Altogether, these data show that glioma released aminoacids may dynamically alter Cl⁻ equilibrium in surrounding neurons, deeply interfering with their inhibitory balance, likely leading to physiological and pathological consequences.

Glycine receptors support excitatory neurotransmitter release in developing mouse visual cortex

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(-) gradient maintained by the Na(+)-K(+)-2Cl(-) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex.

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).

Decreased Immunoreactivities and Functions of the Chloride Transporters, KCC2 and NKCC1, in the Lateral Superior Olive Neurons of Circling Mice

Objectives

We tested the possibility of differential expression and function of the potassium-chloride (KCC2) and sodium-potassium-2 chloride (NKCC1) co-transporters in the lateral superior olive (LSO) of heterozygous (+/cir) or homozygous (cir/cir) mice.

Methods

Mice pups aged from postnatal (P) day 9 to 16 were used. Tails from mice were cut for DNA typing. For Immunohistochemical analysis, rabbit polyclonal anti-KCC2 or rabbit polyclonal anti-NKCC1 was used and the density of immunolabelings was evaluated using the NIH image program. For functional analysis, whole cell voltage clamp technique was used in brain stem slices and the changes of reversal potentials were evaluated at various membrane potentials.

Results

Immunohistochemical analysis revealed both KCC2 and NKCC1 immunoreactivities were more prominent in heterozygous (+/cir) than homozygous (cir/cir) mice on P day 16. In P9-P12 heterozygous (+/cir) mice, the reversal potential (E_gly) of glycine-induced currents was shifted to a more negative potential by 50 µM bumetanide, a known NKCC1 blocker, and the negatively shifted E_gly was restored by additional application of 1 mM furosemide, a KCC2 blocker (-58.9±2.6 mV to -66.0±1.5 mV [bumetanide], -66.0±1.5 mV to -59.8±2.8 mV [furosemide+bumetanide], n=11). However, only bumetanide was weakly, but significantly effective (-60.1±2.9 mV to -62.7±2.6 mV [bumetanide], -62.7±2.6 mV to -62.1±2.5 mV [furosemide+bumetanide], n=7) in P9-P12 homozygous (cir/cir) mice.

Conclusion

The less prominent immunoreactivities and weak or absent responses to bumetanide or furosemide suggest impaired function or delayed development of both transporters in homozygous (cir/cir) mice.

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.

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.

Sustained depolarizing shift of the GABA reversal potential by glutamate receptor activation in hippocampal neurons

The inhibitory action of GABA is a consequence of a relatively hyperpolarized Cl(-) reversal potential (E(Cl)), which results from the activity of K(+)-Cl(-) cotransporter (KCC2). In this study we investigated the effects of glutamate and glutamatergic synaptic activity on E(Cl). In dissociated culture of mature hippocampal neurons, the application of glutamate caused positive E(Cl) shifts with two distinct temporal components. Following a large transient depolarizing state, the sustained depolarizing state (E(Cl)-sustained) lasted more than 30 min. The E(Cl)-sustained disappeared in the absence of external Ca(2+) during glutamate application and was blocked by both AP5 and MK801, but not by nifedipine. The E(Cl)-sustained was also induced by NMDA. The E(Cl)-sustained was blocked by furosemide, a blocker of both KCC2 and NKCC1, but not bumetanide, a blocker of NKCC1. On the other hand, in immature neurons having less expression of KCC2, NMDA failed to induce the sustained depolarizing E(Cl) shift. In organotypic slice cultured neurons, repetitive activation of glutamatergic afferents also generated a sustained depolarizing E(Cl) shift. These results suggest that Ca(2+) influx through NMDA receptors causes the down-regulation of KCC2 and gives rise to long lasting positive E(Cl) shifts, which might contribute to hyperexcitability, LTP, and epileptiform discharges.

Regulation of excitation by glycine receptors

Our knowledge of glycine receptor (GlyR) regulation of excitation has advanced significantly in recent years. GlyRs are widespread in the CNS, are heterogeneous, and undergo developmental changes. Activation of GlyRs of immature neurons induces outflow of Cl( - ), membrane depolarization, neuronal excitation, calcium influx, and transmitter release, in contrast to the inhibitory effects these receptors have in mature neurons. Thus, GlyRs are important for neuronal excitability in both the developing and the mature CNS. This chapter is an overview of selective studies on the newly discovered roles of GlyRs in regulating neuronal excitation, and inhibition, particularly in the upper brain areas.

Taurine acts as a glycine receptor agonist in slices of rat inferior colliculus

Taurine is an important endogenous amino acid for neural development and for many physiological functions, but little is known about its functional role in the central auditory system. We investigated in young rats (P10-P14) the effects of taurine on the neuronal responses and synaptic transmissions in the central nucleus of the inferior colliculus (ICC) with a brain slice preparation and with whole-cell patch-clamp recordings. Perfusion of taurine at 1mM reliably evoked a current across the membrane and decreased the input resistance in neurons of the ICC. Taurine also depressed the spontaneous and current-evoked firing of ICC neurons. All these effects were reversible after washout and could be blocked by 3 microM strychnine, an antagonist of glycine receptors, but not by 10 microM bicuculline, an antagonist of GABA(A) receptors. When the inhibitory receptors were not pharmacologically blocked, taurine reversibly reduced the postsynaptic currents/potentials evoked by electrically stimulating the commissure of the inferior colliculus or the ipsilateral lateral lemniscus. The results demonstrate that taurine reduces the neuronal excitability and depresses the synaptic transmission in the ICC by activating glycine-gated chloride channels. Our findings suggest that taurine acts as a ligand of glycine receptors in the ICC and can be involved in the information processing of the central auditory system similarly like the neurotransmitter glycine.

Glycine receptors in CNS neurons as a target for nonretrograde action of cannabinoids

At many central synapses, endocannabinoids released by postsynaptic cells act retrogradely on presynaptic G-protein-coupled cannabinoid receptors to inhibit neurotransmitter release. Here, we demonstrate that cannabinoids may directly affect the functioning of inhibitory glycine receptor (GlyR) channels. In isolated hippocampal pyramidal and Purkinje cerebellar neurons, endogenous cannabinoids anandamide and 2-arachidonylglycerol, applied at physiological concentrations, inhibited the amplitude and altered the kinetics of rise time, desensitization, and deactivation of the glycine-activated current (I(Gly)) in a concentration-dependent manner. These effects of cannabinoids were observed in the presence of cannabinoid CB1/CB3, vanilloid receptor 1 antagonists, and the G-protein inhibitor GDPbetaS, suggesting a direct action of cannabinoids on GlyRs. The effect of cannabinoids on I(Gly) desensitization was strongly voltage dependent. We also demonstrate that, in the presence of a GABA(A) receptor antagonist, GlyRs may contribute to the generation of seizure-like activity induced by short bursts (seven stimuli) of high-frequency stimulation of inputs to hippocampal CA1 region, because this activity was diminished by selective GlyR antagonists (strychnine and ginkgolides B and J). The GlyR-mediated rhythmic activity was also reduced by cannabinoids (anandamide) in the presence of a CB1 receptor antagonist. These results suggest that the direct inhibition of GlyRs by endocannabinoids can modulate the hippocampal network activity.

Taurine activates strychnine-sensitive glycine receptors in neurons of the rat inferior colliculus

Taurine (Tau) is one of the most abundant free amino acids in the mammalian central nervous system. Whether the neurotransmission of the central auditory system is regulated or modulated by Tau is not clear. In the present study, we investigated the electrophysiological and pharmacological properties of Tau-activated currents in acutely dissociated neurons of the rat inferior colliculus (IC) using whole cell patch clamp recordings. At a holding potential of -60 mV and under a condition of chloride equilibrium potential near 0 mV, Tau activated an inward current and its half-maximal activation concentration was equal to 0.37 mM. The measured reversal potential of Tau-activated currents was close to theoretical chloride equilibrium potential. The currents evoked by Tau at both low (1 mM) and high (10 mM) concentrations were almost completely inhibited by strychnine, a glycine receptor antagonist. The Tau-activated current, however, was not affected by bicuculline, a GABA(A) receptor antagonist. Tau at increased concentrations progressively reduced the current response to subsequent glycine application. At saturated concentrations, Tau-activated current and glycine-activated current were mutually cross-desensitized by each other. These findings indicate that Tau activates glycine receptors in neurons of the rat IC and thus may have a functional role in regulating or modulating the neurotransmission of the central auditory system in mammals.

Changes in Na(+)-K(+)-Cl(-) cotransporter immunoreactivity in the gerbil hippocampus following transient ischemia

We examined alterations in Na(+)-K(+)-Cl(-) cotransporter 1 (NKCC1) immunoreactivity following ischemia. Twelve hours after ischemia, NKCC1 immunoreactivity in the CA1 region and in the hilar region was significantly diminished. Twenty-four hours after ischemia, NKCC1 immunoreactivity was intensified in these hippocampal regions as well as CA2-3. Two days after ischemia, NKCC1 immunoreactivity in the CA1 and the hilar neurons had disappeared, although in the CA2-3 and the granule cell layer NKCC1 immunoreactivities had recovered to the sham level. This finding suggests that NKCC1 may play an important role in the ischemic neuronal injury induced by excitotoxicity as well as neuronal edema.

Dopamine transporter-mediated conductances increase excitability of midbrain dopamine neurons

Uptake by Na(+)/Cl(-)-dependent neurotransmitter transporters is the principal mechanism by which extracellular biogenic amine concentrations are regulated. In addition to uptake, the cloned transporter proteins also elicit ion channel-like currents, but the physiological consequences of these currents are unknown. Here, whole-cell patch clamp and perforated-patch recordings show that substrates of the dopamine transporter (DAT), such as dopamine (DA) and amphetamine, increase the firing activity of rat DA neurons in culture. We found that these substrates elicit inward currents that are Na(+)-dependent and blocked by cocaine. These currents are primarily comprised of anions and result in an excitatory response in DA neurons at lower DA concentrations than are required for D2 autoreceptor activation. Thus, in addition to clearing extracellular DA, our results suggest that the currents associated with DAT modulate excitability and may regulate release of neurotransmitter from midbrain DA neurons.

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.

Glycinergic/GABAergic synapses in the lateral superior olive are excitatory in neonatal C57Bl/6J mice

The lateral superior olive (LSO), a nucleus involved in sound localization, receives tonotopically organized, inhibitory input from the medial nucleus of the trapezoid body (MNTB). To better understand the development of this glycinergic/GABAergic pathway, we used Gramicidin-perforated patch clamp recordings to characterize MNTB-evoked postsynaptic potentials in LSO neurons of neonatal C57Bl/6J mice. We found that during the first postnatal week, MNTB-evoked responses change from being depolarizing to being hyperpolarizing. Most interestingly, depolarizing glycinergic/GABAergic synaptic potentials were able to trigger action potentials, demonstrating that the MNTB-LSO pathway can act as a true excitatory pathway. This transient excitatory action of immature MNTB-LSO synapses might play an important role in activity-dependent sharpening of the tonotopic organization of inhibitory connections in the LSO.

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.

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.

Developmental changes in physiological properties in the rat's dorsal nucleus of the lateral lemniscus

To better understand the development of the dorsal nucleus of the lateral lemniscus (DNLL), intrinsic membrane properties and synaptic responses of DNLL neurons in brain slice preparations were examined. Intracellular recordings were taken from DNLL neurons of rat pups at postnatal days 4-8 (early group), 10-12 (intermediate group) and 16-18 (late group). In response to positive current injection, neurons in the early group displayed firing with lower frequency and a longer action potential duration in comparison to the intermediate and late groups. The action potential amplitude of DNLL neurons increased during development. Postsynaptic potentials (PSPs), with excitatory predominance, were elicited by electrical stimulation of the lateral lemniscus and commissure of Probst throughout the three age groups. Neurons showed a longer latency and rise time of the PSPs in the early group in comparison with those in the intermediate and late groups. These results suggest that the early DNLL neurons display physiological characteristics associated with immature neurons, while the other two groups tend to illustrate mature-like neuronal properties. Furthermore, it seems that the neurons at day 10-12 are in a transitional period of development, which coincides with the onset of hearing.

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.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Afferent regulation of inhibitory synaptic transmission in the developing auditory midbrain

To determine whether afferent innervation regulates the strength of inhibitory connections in the gerbil auditory midbrain, both cochleas were surgically removed in postnatal day 7 animals, before sound-driven activity is first observed. Inhibitory synaptic currents were measured in a brain slice preparation 1-7 d after the ablations. Whole-cell and gramicidin-perforated patch recordings were obtained from inferior colliculus neurons, and IPSCs were evoked by stimulation of the commissure of the inferior colliculus (CIC) or the ipsilateral lateral lemniscus (LL) in the presence of kynurenic acid. Deafferentation led to a 24 mV depolarizing shift in the IPSC equilibrium potential within 1 d of deafferentation. As a consequence, there was a large reduction of IPSC amplitude at a holding potential of -20 mV in neurons from bilaterally ablated animals. Furthermore, both afferent pathways displayed a 50% reduction of the inhibitory synaptic conductance after deafferentation, indicating that driving force was not solely responsible for the decline in IPSC amplitude. When paired pulses were delivered to the LL or CIC pathway in control neurons, the evoked IPSCs exhibited facilitation. However, paired pulse facilitation was nearly eliminated after deafferentation. Thus, normal innervation affects inhibitory synaptic strength by regulating postsynaptic chloride homeostasis and presynaptic transmitter release properties.

Afferent regulation of inhibitory synaptic transmission in the developing auditory midbrain

To determine whether afferent innervation regulates the strength of inhibitory connections in the gerbil auditory midbrain, both cochleas were surgically removed in postnatal day 7 animals, before sound-driven activity is first observed. Inhibitory synaptic currents were measured in a brain slice preparation 1-7 d after the ablations. Whole-cell and gramicidin-perforated patch recordings were obtained from inferior colliculus neurons, and IPSCs were evoked by stimulation of the commissure of the inferior colliculus (CIC) or the ipsilateral lateral lemniscus (LL) in the presence of kynurenic acid. Deafferentation led to a 24 mV depolarizing shift in the IPSC equilibrium potential within 1 d of deafferentation. As a consequence, there was a large reduction of IPSC amplitude at a holding potential of -20 mV in neurons from bilaterally ablated animals. Furthermore, both afferent pathways displayed a 50% reduction of the inhibitory synaptic conductance after deafferentation, indicating that driving force was not solely responsible for the decline in IPSC amplitude. When paired pulses were delivered to the LL or CIC pathway in control neurons, the evoked IPSCs exhibited facilitation. However, paired pulse facilitation was nearly eliminated after deafferentation. Thus, normal innervation affects inhibitory synaptic strength by regulating postsynaptic chloride homeostasis and presynaptic transmitter release properties.

Shift from depolarizing to hyperpolarizing glycine action in rat auditory neurones is due to age-dependent Cl- regulation

1. The inhibitory neurotransmitter glycine can elicit depolarizing responses in immature neurones. We investigated the changes in glycine responses and their ionic mechanism in developing neurones of the rat lateral superior olive (LSO), an auditory brainstem nucleus involved in sound localization. 2. Whole-cell and gramicidin perforated-patch recordings were performed from visually identified LSO neurones in brain slices and glycine was pressure applied for 3-100 ms to the soma. Glycine-evoked currents were reversibly blocked by strychnine. They were mostly monophasic, but biphasic responses occurred in approximately 30 % of P8-11 neurones in perforated-patch recordings. 3. In whole-cell recordings from P2-11 neurones, the reversal potential of glycine-evoked currents (EGly) was determined by the transmembranous Cl- gradient and corresponded closely to the Nernst potential for Cl-, regardless of age. This indicates that Cl- is the principle ion permeating glycine receptors, but is also consistent with a low relative (10-20 %) permeability for HCO3-. The Cl- gradient also determined the polarity and amplitude of glycine-evoked membrane potential changes. 4. Leaving the native intracellular [Cl-] undisturbed with gramicidin perforated-patch recordings, we found a highly significant, age-dependent change of EGly from -46.8 +/- 1.8 mV (P1-4, n = 28) to -67.6 +/- 3.3 mV (P5-8, n = 10) to -82.2 +/- 4.1 mV (P9-11, n = 18). The majority of P1-4 neurones were depolarized by glycine ( approximately 80 %) and spikes were evoked in approximately 30 %. In contrast, P9-11 neurones were hyperpolarized. 5. In perforated-patch recordings, EGly was influenced by the voltage protocol and the glycine application interval; it could be shifted in the positive and negative direction. For a given application interval, these shifts were always larger in P1-4 than in P8-11 neurones, pointing to less effective Cl- regulation mechanisms in younger neurones. 6. Furosemide (frusemide), a blocker of cation-Cl- cotransporters, reversibly shifted EGly in the negative direction in P2-4 neurones, yet in the positive direction in P8-10 neurones, suggesting the blockade of net inward and net outward Cl- transporters, respectively. 7. Taken together, age-dependent changes in active Cl- regulation are likely to cause the developmental shift from depolarizing to hyperpolarizing glycine responses. A high intracellular [Cl-] is generated in neonatal LSO neurones which decreases during maturation.

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

Microfluorometric recordings showed that the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated transient increases in the intracellular Cl- concentration in neurons of the inferior colliculus (IC) from acutely isolated slices of the rat auditory midbrain. Current recordings in gramicidin-perforated patch mode disclosed that GABA and glycine mainly evoked inward or biphasic currents. These currents were dependent on HCO3- and characterized by a continuous shift of their reversal potential (E(GABA/gly)) in the positive direction. In HCO3- -buffered saline, GABA and glycine could also evoke an increase in the intracellular Ca2+ concentration. Ca2+ transients occurred only with large depolarizations and were blocked by Cd2+, suggesting an activation of voltage-gated Ca2+ channels. However, in the absence of HCO3-, only a small rise, if any, in the intracellular Ca2+ concentration could be evoked by GABA or glycine. We suggest that the activation of GABAA or glycine receptors results in an acute accumulation of Cl- that is enhanced by the depolarization owing to HCO3- efflux, thus shifting E(GABA/gly) to more positive values. A subsequent activation of these receptors would result in a strenghtened depolarization and an enlarged Ca2+ influx that might play a role in the stabilization of inhibitory synapses in the auditory pathway.

Regulation of intracellular chloride by cotransporters in developing lateral superior olive neurons

The regulatory mechanisms of intracellular Cl- concentration ([Cl-]i) were investigated in the lateral superior olive (LSO) neurons of various developmental stages by taking advantage of gramicidin perforated patch recording mode, which enables neuronal [Cl-]i measurement. Responses to glycine changed from depolarization to hyperpolarization during the second week after birth, resulting from [Cl-]i decrease. Furosemide equally altered the [Cl-]i of both immature and mature LSO neurons, indicating substantial contributions of furosemide-sensitive intracellular Cl- regulators; i.e., K+-Cl- cotransporter (KCC) and Na+-K+-Cl- cotransporter (NKCC), throughout this early development. Increase of extracellular K+ concentration and replacement of intracellular K+ with Cs+ resulted in [Cl-]i elevation at postnatal days 13-15 (P13-P15), but not at P0-P2, indicating that the mechanism of neuronal Cl- extrusion is sensitive to both furosemide and K+-gradient and poorly developed in immature LSO neurons. In addition, removal of extracellular Na+ decreased [Cl-]i at P0-P2, suggesting the existence of extracellular Na+-dependent and furosemide-sensitive Cl- accumulation in immature LSO neurons. These data show clearly that developmental changes of Cl- cotransporters alter [Cl-]i and are responsible for the switch from the neonatal Cl- efflux to the mature Cl- influx in LSO neurons. Such maturational changes in Cl- cotransporters might have the important functional roles for glycinergic and GABAergic synaptic transmission and the broader implications for LSO and auditory development.

Regulation of intracellular chloride by cotransporters in developing lateral superior olive neurons

The regulatory mechanisms of intracellular Cl- concentration ([Cl-]i) were investigated in the lateral superior olive (LSO) neurons of various developmental stages by taking advantage of gramicidin perforated patch recording mode, which enables neuronal [Cl-]i measurement. Responses to glycine changed from depolarization to hyperpolarization during the second week after birth, resulting from [Cl-]i decrease. Furosemide equally altered the [Cl-]i of both immature and mature LSO neurons, indicating substantial contributions of furosemide-sensitive intracellular Cl- regulators; i.e., K+-Cl- cotransporter (KCC) and Na+-K+-Cl- cotransporter (NKCC), throughout this early development. Increase of extracellular K+ concentration and replacement of intracellular K+ with Cs+ resulted in [Cl-]i elevation at postnatal days 13-15 (P13-P15), but not at P0-P2, indicating that the mechanism of neuronal Cl- extrusion is sensitive to both furosemide and K+-gradient and poorly developed in immature LSO neurons. In addition, removal of extracellular Na+ decreased [Cl-]i at P0-P2, suggesting the existence of extracellular Na+-dependent and furosemide-sensitive Cl- accumulation in immature LSO neurons. These data show clearly that developmental changes of Cl- cotransporters alter [Cl-]i and are responsible for the switch from the neonatal Cl- efflux to the mature Cl- influx in LSO neurons. Such maturational changes in Cl- cotransporters might have the important functional roles for glycinergic and GABAergic synaptic transmission and the broader implications for LSO and auditory development.

The superior olivary nucleus and its influence on nucleus laminaris: a source of inhibitory feedback for coincidence detection in the avian auditory brainstem

Located in the ventrolateral region of the avian brainstem, the superior olivary nucleus (SON) receives inputs from nucleus angularis (NA) and nucleus laminaris (NL) and projects back to NA, NL, and nucleus magnocellularis (NM). The reciprocal connections between the SON and NL are of particular interest because they constitute a feedback circuit for coincidence detection. In the present study, the chick SON was investigated. In vivo tracing studies show that the SON projects predominantly to the ipsilateral NM, NL, and NA. In vitro whole-cell recording reveals single-cell morphology, firing properties, and postsynaptic responses. SON neurons are morphologically and physiologically suited for temporal integration; their firing patterns do not reflect the temporal structure of their excitatory inputs. Of most interest, direct stimulation of the SON evokes long-lasting inhibition in NL neurons. The inhibition blocks both intrinsic spike generation and orthodromically evoked activity in NL neurons and can be eliminated by bicuculline methiodide, a potent antagonist for GABAA receptor-mediated neurotransmission. These results strongly suggest that the SON provides GABAergic inhibitory feedback to laminaris neurons. We discuss a mechanism whereby SON-evoked GABAergic inhibition can influence the coding of interaural time differences for sound localization in the avian auditory brainstem.

The superior olivary nucleus and its influence on nucleus laminaris: a source of inhibitory feedback for coincidence detection in the avian auditory brainstem

Located in the ventrolateral region of the avian brainstem, the superior olivary nucleus (SON) receives inputs from nucleus angularis (NA) and nucleus laminaris (NL) and projects back to NA, NL, and nucleus magnocellularis (NM). The reciprocal connections between the SON and NL are of particular interest because they constitute a feedback circuit for coincidence detection. In the present study, the chick SON was investigated. In vivo tracing studies show that the SON projects predominantly to the ipsilateral NM, NL, and NA. In vitro whole-cell recording reveals single-cell morphology, firing properties, and postsynaptic responses. SON neurons are morphologically and physiologically suited for temporal integration; their firing patterns do not reflect the temporal structure of their excitatory inputs. Of most interest, direct stimulation of the SON evokes long-lasting inhibition in NL neurons. The inhibition blocks both intrinsic spike generation and orthodromically evoked activity in NL neurons and can be eliminated by bicuculline methiodide, a potent antagonist for GABAA receptor-mediated neurotransmission. These results strongly suggest that the SON provides GABAergic inhibitory feedback to laminaris neurons. We discuss a mechanism whereby SON-evoked GABAergic inhibition can influence the coding of interaural time differences for sound localization in the avian auditory brainstem.

Synaptic inhibition in the isolated respiratory network of neonatal rats

Gramicidin-perforated patch-clamp recording revealed phasic Cl(-)-mediated hyperpolarizations in respiratory neurons of the brainstem-spinal cord preparation from newborn rats. The in vitro respiratory rhythm persisted after block of gamma-aminobutyric acid (GABA), i.e. GABAA, receptor-mediated inhibitory postsynaptic potentials (IPSPs) with bicuculline and/or glycinergic IPSPs with strychnine. In one class of expiratory neurons, bicuculline unmasked inspiration-related excitatory postsynaptic potentials (EPSPs), leading to spike discharge. Bicuculline also blocked hyperpolarizations and respiratory arrest due to bath-applied muscimol, whereas strychnine antagonized similar responses to glycine. The reversal potential of respiration-related IPSPs and responses to GABA, muscimol or glycine was not affected by CO2/HCO3(-)-free solutions, but shifted from about -65 mV to values more positive than -20 mV upon dialysis of the cells with 144 instead of 4 mM Cl-. Impairment of GABA uptake with nipecotic acid or glycine uptake with sarcosine evoked a bicuculline- or strychnine-sensitive decrease of respiratory frequency which could lead to respiratory arrest. Also, the GABAB receptor agonist baclofen led to reversible suppression of respiratory rhythm. This in vitro apnoea was accompanied by a K+ channel-mediated hyperpolarization (reversal potential -88 mV) of tonic cells, whereas membrane potential of neighbouring respiratory neurons remained almost unaffected. Both baclofen-induced hyperpolarization and respiratory depression were antagonised by 2-OH-saclofen, which did not affect respiration-related IPSPs per se. The results show that synaptic inhibition is not essential for rhythmogenesis in the isolated neonatal respiratory network, although (endogenous) GABA and glycine have a strong modulatory action. Hyperpolarizing IPSPs mediated by GABAA and glycine receptors provide a characteristic pattern of membrane potential oscillations in respiratory neurons, whereas GABAB receptors rather appear to be a feature of non-respiratory neurons, possibly providing excitatory drive to the network.

Development of glycinergic synaptic transmission to rat brain stem motoneurons

Using an in vitro rat brain stem slice preparation, we examined the postnatal changes in glycinergic inhibitory postsynaptic currents (IPSCs) and passive membrane properties that underlie a developmental change in inhibitory postsynaptic potentials (IPSPs) recorded in hypoglossal motoneurons (HMs). Motoneurons were placed in three age groups: neonate (P0-3), intermediate (P5-8), and juvenile (P10-18). During the first two postnatal weeks, the decay time course of both unitary evoked IPSCs [mean decay time constant, taudecay = 17.0 +/- 1.6 (SE) ms in neonates and 5.5 +/- 0.4 ms in juveniles] and spontaneous miniature IPSCs (taudecay = 14.2 +/- 2.4 ms in neonates and 6.3 +/- 0.7 ms in juveniles) became faster. As glycine uptake does not influence IPSC time course at any postnatal age, this change most likely results from a developmental alteration in glycine receptor (GlyR) subunit composition. We found that expression of fetal (alpha2) GlyR subunit mRNA decreased, whereas expression of adult (alpha1) GlyR subunit mRNA increased postnatally. Single GlyR-channels recorded in outside-out patches excised from neonate motoneurons had longer mean burst durations than those from juveniles (18.3 vs. 11.1 ms). Concurrently, HM input resistance (RN) and membrane time constant (taum) decreased (RN from 153 +/- 12 MOmega to 63 +/- 7 MOmega and taum from 21.5 +/- 2.7 ms to 9.1 +/- 1.0 ms, neonates and juveniles, respectively), and the time course of unitary evoked IPSPs also became faster (taudecay = 22.4 +/- 1.8 and 7.7 +/- 0.9 ms, neonates vs. juveniles, respectively). Simulated synaptic currents were used to probe more closely the interaction between IPSC time course and taum, and these simulations demonstrated that IPSP duration was reduced as a consequence of postnatal changes in both the kinetics of the underlying GlyR channel and the membrane properties that transform the IPSC into a postsynaptic potential. Additionally, gramicidin perforated-patch recordings of glycine-evoked currents reveal a postnatal change in reversal potential, which is shifted from -37 to -73 mV during this same period. Glycinergic PSPs are therefore depolarizing and prolonged in neonate HMs and become faster and hyperpolarizing during the first two postnatal weeks.