Signaling pathway downstream of GABAA receptor in the growth cone

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

GABAergic system in the endocrine pancreas: a new target for diabetes treatment

Excessive loss of functional pancreatic β-cell mass, mainly due to apoptosis, is a major factor in the development of hyperglycemia in both type 1 and type 2 diabetes (T1D and T2D). In T1D, β-cells are destroyed by immunological mechanisms. In T2D, while metabolic factors are known to contribute to β-cell failure and subsequent apoptosis, mounting evidence suggests that islet inflammation also plays an important role in the loss of β-cell mass. Therefore, it is of great importance for clinical intervention to develop new therapies. γ-Aminobutyric acid (GABA), a major neurotransmitter, is also produced by islet β-cells, where it functions as an important intraislet transmitter in regulating islet-cell secretion and function. Importantly, recent studies performed in rodents, including in vivo studies of xenotransplanted human islets, reveal that GABA exerts β-cell regenerative effects. Moreover, it protects β-cells against apoptosis induced by cytokines, drugs, and other stresses, and has anti-inflammatory and immunoregulatory activities. It ameliorates the manifestations of diabetes in preclinical models, suggesting potential applications for the treatment of diabetic patients. This review outlines the actions of GABA relevant to β-cell regeneration, including its signaling mechanisms and potential interactions with other mediators. These studies increase our understanding of the regenerative processes of pancreatic β-cells, and help pave the way for the development of regenerative medicine for diabetes.

The interdependence of membrane shape and cellular signal processing

Cellular membranes are constantly reshaped by vesicular fission and fusion as well as by interactions with the dynamic cytoskeleton. Signaling activity at membranes depends on their geometric parameters, such as surface area and curvature; these affect local concentration and thereby regulate the potency of molecular reactions. A membrane's shape is thus inextricably tied to information processing. Here, we review how a trinity of signaling, cytoskeletal dynamics, and membrane shape interact within a closed-loop causality that gives rise to an energy-consuming, self-organized system that changes shape to sense the extracellular environment.

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.

Detection of structural and metabolic changes in traumatically injured hippocampus by quantitative differential proteomics

Traumatic brain injury (TBI) is a complex and common problem resulting in the loss of cognitive function. In order to build a comprehensive knowledge base of the proteins that underlie these cognitive deficits, we employed unbiased quantitative mass spectrometry, proteomics, and bioinformatics to identify and quantify dysregulated proteins in the CA3 subregion of the hippocampus in the fluid percussion model of TBI in rats. Using stable isotope 18O-water differential labeling and multidimensional tandem liquid chromatography (LC)-MS/MS with high stringency statistical analyses and filtering, we identified and quantified 1002 common proteins, with 124 increased and 76 decreased. The ingenuity pathway analysis (IPA) bioinformatics tool identified that TBI had profound effects on downregulating global energy metabolism, including glycolysis, the Krebs cycle, and oxidative phosphorylation, as well as cellular structure and function. Widespread upregulation of actin-related cytoskeletal dynamics was also found. IPA indicated a common integrative signaling node, calcineurin B1 (CANB1, CaNBα, or PPP3R1), which was downregulated by TBI. Western blotting confirmed that the calcineurin regulatory subunit, CANB1, and its catalytic binding partner PP2BA, were decreased without changes in other calcineurin subunits. CANB1 plays a critical role in downregulated networks of calcium signaling and homeostasis through calmodulin and calmodulin-dependent kinase II to highly interconnected structural networks dominated by tubulins. This large-scale knowledge base lays the foundation for the identification of novel therapeutic targets for cognitive rescue in TBI.

Mechanistic involvement of the calpain-calpastatin system in Alzheimer neuropathology

The mechanism by which amyloid-β peptide (Aβ) accumulation causes neurodegeneration in Alzheimer's disease (AD) remains unresolved. Given that Aβ perturbs calcium homeostasis in neurons, we investigated the possible involvement of calpain, a calcium-activated neutral protease. We first demonstrated close postsynaptic association of calpain activation with Aβ plaque formation in brains from both patients with AD and transgenic (Tg) mice overexpressing amyloid precursor protein (APP). Using a viral vector-based tracer, we then showed that axonal termini were dynamically misdirected to calpain activation-positive Aβ plaques. Consistently, cerebrospinal fluid from patients with AD contained a higher level of calpain-cleaved spectrin than that of controls. Genetic deficiency of calpastatin (CS), a calpain-specific inhibitor protein, augmented Aβ amyloidosis, tau phosphorylation, microgliosis, and somatodendritic dystrophy, and increased mortality in APP-Tg mice. In contrast, brain-specific CS overexpression had the opposite effect. These findings implicate that calpain activation plays a pivotal role in the Aβ-triggered pathological cascade, highlighting a target for pharmacological intervention in the treatment of AD.

A mechanism for the polarity formation of chemoreceptors at the growth cone membrane for gradient amplification during directional sensing

Accurate response to external directional signals is essential for many physiological functions such as chemotaxis or axonal guidance. It relies on the detection and amplification of gradients of chemical cues, which, in eukaryotic cells, involves the asymmetric relocalization of signaling molecules. How molecular events coordinate to induce a polarity at the cell level remains however poorly understood, particularly for nerve chemotaxis. Here, we propose a model, inspired by single-molecule experiments, for the membrane dynamics of GABA chemoreceptors in nerve growth cones (GCs) during directional sensing. In our model, transient interactions between the receptors and the microtubules, coupled to GABA-induced signaling, provide a positive-feedback loop that leads to redistribution of the receptors towards the gradient source. Using numerical simulations with parameters derived from experiments, we find that the kinetics of polarization and the steady-state polarized distribution of GABA receptors are in remarkable agreement with experimental observations. Furthermore, we make predictions on the properties of the GC seen as a sensing, amplification and filtering module. In particular, the growth cone acts as a low-pass filter with a time constant ∼10 minutes determined by the Brownian diffusion of chemoreceptors in the membrane. This filtering makes the gradient amplification resistent to rapid fluctuations of the external signals, a beneficial feature to enhance the accuracy of neuronal wiring. Since the model is based on minimal assumptions on the receptor/cytoskeleton interactions, its validity extends to polarity formation beyond the case of GABA gradient sensing. Altogether, it constitutes an original positive-feedback mechanism by which cells can dynamically adapt their internal organization to external signals.

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.

Asymmetric redistribution of GABA receptors during GABA gradient sensing by nerve growth cones analyzed by single quantum dot imaging

During development of the nervous system, the tip of a growing axon, the growth cone (GC), must respond accurately to stimuli that direct its growth. This axonal navigation depends on extracellular concentration gradients of numerous guidance cues, including GABA. GCs can detect even weak directional signals, yet the mechanisms underlying this sensitivity remain unclear. Past studies in other eukaryotic chemotactic systems have pointed to the role of the spatial reorganization of the transduction pathway in their sensitive response. Here we have developed a single-molecule assay to observe individual GABA(A) receptors (GABA(A)Rs) in the plasma membrane of nerve GCs subjected to directional stimuli. We report that in the presence of an external GABA gradient GABA(A)Rs redistribute asymmetrically across the GC toward the gradient source. Single-particle tracking of GABA(A)Rs shows that the redistribution results from transient interactions between the receptors and the microtubules. Moreover, the relocalization is accompanied by an enhancement in the asymmetry of intracellular calcium concentration. Altogether, our results reveal a microtubule-dependent polarized reorganization of chemoreceptors at the cell surface and suggest that this polarization serves as an amplification step in GABA gradient sensing by nerve GCs.

Transient directed motions of GABA(A) receptors in growth cones detected by a speed correlation index

Single-molecule tracking of membrane proteins has become an important tool for investigating dynamic processes in live cells, such as cell signaling, membrane compartmentation or trafficking. The extraction of relevant parameters, such as interaction times between molecular partners or confinement-zone sizes, from the trajectories of single molecules requires appropriate statistical methods. Here we report a new tool, the speed correlation index, designed to detect transient periods of directed motion within trajectories of diffusing molecules. The ability to detect such events in a wide range of biologically relevant parameter values (speed, diffusion coefficient, and durations of the directed period) was first established on simulated data. The method was next applied to analyze the trajectories of quantum-dot-labeled GABA(A) receptors in nerve growth cones. The use of the speed correlation index revealed that the receptors had a "conveyor belt" type of motion due to temporary interactions ( approximately 4.0 s) between the receptors and the microtubules, leading to an average directed motion (velocity approximately 0.3 mum s(-1)) in the growth-cone membrane. Our observations point to the possibility of a cytoskeleton-dependent redistribution of the sensing molecules in the membrane, which could play a role in the modulation of the cell response to external signals.

Insights into GABA receptor signalling in TM3 Leydig cells

Gamma-aminobutyric acid (GABA) is an emerging signalling molecule in endocrine organs, since it is produced by endocrine cells and acts via GABA(A) receptors in a paracrine/autocrine fashion. Testicular Leydig cells are producers and targets for GABA. These cells express GABA(A) receptor subunits and in the murine Leydig cell line TM3 pharmacological activation leads to increased proliferation. The signalling pathway of GABA in these cells is not known in this study. We therefore attempted to elucidate details of GABA(A) signalling in TM3 and adult mouse Leydig cells using several experimental approaches. TM3 cells not only express GABA(A )receptor subunits, but also bind the GABA agonist [(3)H]muscimol with a binding affinity in the range reported for other endocrine cells (K(d) = 2.740 +/- 0.721 nM). However, they exhibit a low B(max) value of 28.08 fmol/mg protein. Typical GABA(A) receptor-associated events, including Cl(-) currents, changes in resting membrane potential, intracellular Ca(2+) or cAMP, were not measurable with the methods employed in TM3 cells, or, as studied in part, in primary mouse Leydig cells. GABA or GABA(A) agonist isoguvacine treatment resulted in increased or decreased levels of several mRNAs, including transcription factors (c-fos, hsf-1, egr-1) and cell cycle-associated genes (Cdk2, cyclin D1). In an attempt to verify the cDNA array results and because egr-1 was recently implied in Leydig cell development, we further studied this factor. RT-PCR and Western blotting confirmed a time-dependent regulation of egr-1 in TM3. In the postnatal testis egr-1 was seen in cytoplasmic and nuclear locations of developing Leydig cells, which bear GABA(A) receptors and correspond well to TM3 cells. Thus, GABA acts via an atypical novel signalling pathway in TM3 cells. Further details of this pathway remain to be elucidated.

GABAergic modulation of motor-driven behaviors in juvenileDrosophila and evidence for a nonbehavioral role for GABA transport

We have identified specific GABAergic-modulated behaviors in the juvenile stage of the fruit fly, Drosophila melanogaster via systemic treatment of second instar larvae with the potent GABA transport inhibitor DL-2,4-diaminobutyric acid (DABA). DABA significantly inhibited motor-controlled body wall and mouth hook contractions and impaired rollover activity and contractile responses to touch stimulation. The perturbations in locomotion and rollover activity were reminiscent of corresponding DABA-induced deficits in locomotion and the righting reflex observed in adult flies. The effects were specific to these motor-controlled behaviors, because DABA-treated larvae responded normally in olfaction and phototaxis assays. Recovery of these behaviors was achieved by cotreatment with the vertebrate GABA(A) receptor antagonist picrotoxin. Pharmacological studies performed in vitro with plasma membrane vesicles isolated from second instar larval tissues verified the presence of high-affinity, saturable GABA uptake mechanisms. GABA uptake was also detected in plasma membrane vesicles isolated from behaviorally quiescent stages. Competitive inhibition studies of [3H]-GABA uptake into plasma membrane vesicles from larval and pupal tissues with either unlabeled GABA or the transport inhibitors DABA, nipecotic acid, or valproic acid, revealed differences in affinities. GABAergic-modulation of motor behaviors is thus conserved between the larval and adult stages of Drosophila, as well as in mammals and other vertebrate species. The pharmacological studies reveal shared conservation of GABA transport mechanisms between Drosophila and mammals, and implicate the involvement of GABA and GABA transporters in regulating physiological processes distinct from neurotransmission during behaviorally quiescent stages of development.

Role of sex hormones in the sexually dimorphic expression of KCC2 in rat substantia nigra

KCC2 is a neuronal-specific potassium chloride cotransporter. The level of KCC2 expression is a factor determining whether GABA(A) receptor agonists depolarize or hyperpolarize neurons. Substantia nigra reticulata (SNR) neurons of male postnatal day 15 (PN15) rats have low KCC2 mRNA expression and respond to GABA(A) receptor activation with depolarization and activation of calcium-regulated gene expression. Female PN15 SNR neurons have high KCC2 mRNA expression and GABA(A) receptor agonists cannot activate calcium-dependent signaling processes. We investigate whether sex hormones regulate KCC2 mRNA expression in PN15 rat SNR. Using in situ hybridization, we studied the effects of acute (4 h) or prolonged (52 h) subcutaneous (s.c.) administration of testosterone (100 microg), dihydrotestosterone (180 microg) or 17beta-estradiol benzoate (5 microg) on KCC2 mRNA expression in male and female PN15 rat SNR. Different doses of estradiol (1 and 10 microg s.c., 4 h) were also acutely administered in female PN15 rats. Controls received oil injections. Separate groups of PN15 male rats were pretreated with antagonists of L-type voltage-sensitive calcium channels (L-VSCCs) [nifedipine, 100 mg/kg s.c.] or GABA(A) receptors [bicuculline, 2 mg/kg intraperitoneally (i.p.)] or their vehicles, 30 min before estradiol (5 microg s.c., 4 h). Testosterone and dihydrotestosterone upregulated KCC2 mRNA in both sexes. Estradiol downregulated KCC2 mRNA in males but not in females. Both acute and prolonged hormonal administration had similar effects. In male PN15 SNR, nifedipine and bicuculline decreased KCC2 mRNA acutely and prevented further downregulation of KCC2 mRNA by estradiol. Estradiol therefore downregulates KCC2 mRNA in male PN15 SNR, by interacting with the GABA(A) receptor and L-VSCC signaling pathway.

In developing Drosophila neurones the production of gamma-amino butyric acid is tightly regulated downstream of glutamate decarboxylase translation and can be influenced by calcium

The presented work pioneers the embryonic Drosophila CNS for studies of the developmental regulation and function of gamma-amino butyric acid (GABA). We describe for the first time the developmental pattern of GABA in Drosophila and address underlying regulatory mechanisms. Surprisingly, and in contrast to vertebrates, detectable levels of GABA occur late during Drosophila neurogenesis, after essential neuronal proliferation and growth have taken place and synaptogenesis has been initiated. This timeline is almost unchanged when the GABA synthetase glutamate decarboxylase (GAD) is strongly misexpressed throughout the nervous system suggesting a tight post-translational regulation of GABA expression. We confirmed such GABA control mechanisms in an independent model system, i.e. primary Drosophila cell cultures raised in elevated [K+]. The data suggest that, in both systems, GABA suppression occurs via control of GAD activity. Using developing embryos and cell cultures as parallel assay systems for pharmacological and genetic studies we show that the negative regulation of GAD can be overridden by drugs known to elevate intracellular free [Ca2+]. Our results provide the basis for investigations of genetic mechanisms underlying the observed phenomenon, and we discuss the potential implications of this work for Drosophila neurogenesis but also for a general understanding of GAD regulation.

GABA expression dominates neuronal lineage progression in the embryonic rat neocortex and facilitates neurite outgrowth via GABA(A) autoreceptor/Cl- channels

GABA emerges as a trophic signal during rat neocortical development in which it modulates proliferation of neuronal progenitors in the ventricular/subventricular zone (VZ/SVZ) and mediates radial migration of neurons from the VZ/SVZ to the cortical plate/subplate (CP/SP) region. In this study we investigated the role of GABA in the earliest phases of neuronal differentiation in the CP/SP. GABAergic-signaling components emerging during neuronal lineage progression were comprehensively characterized using flow cytometry and immunophenotyping together with physiological indicator dyes. During migration from the VZ/SVZ to the CP/SP, differentiating cortical neurons became predominantly GABAergic, and their dominant GABA(A) receptor subunit expression pattern changed from alpha4beta1gamma1 to alpha3beta3gamma2gamma3 coincident with an increasing potency of GABA on GABA(A) receptor-mediated depolarization. GABA(A) autoreceptor/Cl(-) channel activity in cultured CP/SP neurons dominated their baseline potential and indirectly their cytosolic Ca(2+) (Ca(2+)c) levels via Ca(2+) entry through L-type Ca(2+) channels. Block of this autocrine circuit at the level of GABA synthesis, GABA(A) receptor activation, intracellular Cl(-) ion homeostasis, or L-type Ca(2+) channels attenuated neurite outgrowth in most GABAergic CP/SP neurons. In the absence of autocrine GABAergic signaling, neuritogenesis could be preserved by depolarizing cells and elevating Ca(2+)c. These results reveal a morphogenic role for GABA during embryonic neocortical neuron development that involves GABA(A) autoreceptors and L-type Ca(2+) channels.

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

In the adult cerebral cortex, the neurotransmitter GABA is strongly inhibitory, as it profoundly decreases neuronal excitability and suppresses the network propensity for synchronous activity. When fast, GABA(A) receptor (GABA(A)R)-mediated neurotransmission is blocked in the mature cortex, neuronal firing is synchronized via recurrent excitatory (glutamatergic) synaptic connections, generating population discharges manifested extracellularly as spontaneous paroxysmal field potentials (sPFPs). This epileptogenic effect of GABA(A)R antagonists has rarely been observed in the neonatal cortex, and indeed, GABA in the neonate has been proposed to have an excitatory, rather than inhibitory, action. In contrast, we show here that when fast GABAergic neurotransmission was blocked in slices of neonatal mouse and rat hippocampus and neocortex, sPFPs occurred in nearly half the slices from postnatal day 4 (P4) to P7 neocortex and in most slices from P2 to P7 hippocampus. In Mg(2+)-free solution, GABA(A)R antagonists elicited sPFPs in nearly all slices of P2 and older neocortex and P0 and older hippocampus. Mg(2+)-free solution alone induced spontaneous events in the majority of P2 and older slices from both regions; addition of GABA(A)R antagonists caused a dramatic increase in the mean amplitude, but not frequency, of these events in the hippocampus and in their mean frequency, but not amplitude, in the neocortex. In the hippocampus, GABA(A)R agonists suppressed amplitudes, but not frequency, of sPFPs, whereas glutamate antagonists suppressed frequency but not amplitudes. We conclude that neonatal rodent cerebral cortex possesses glutamatergic circuits capable of generating synchronous network activity and that, as in the adult, tonic GABA(A)R-mediated inhibition prevents this activity from becoming paroxysmal.

bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.

Nitrous oxide, but not xenon, affects the signaling in the neuronal growth cone

1. Xenon (Xe) is an inert gas with the anesthetic property. To investigate whether Xe affects the neural network formation, the authors examined the biochemical characteristics of growth cones prepared from rat forebrains at different perinatal periods, in comparison with inhalation of N2O. 2. Fetal or neonatal rats were exposed to an atmosphere containing inhalational anesthetics (70% Xe or N2O) or the control atmosphere (30% O2 and 70% N2) for 6 h. After the exposure, isolated growth cone particles (IGCs) were prepared from their forebrains using a subcellular fractionation method. Protein composition, Ca2+-dependent protein phosphorylation, protein kinase C (PKC) activity, and degradation of PKC in the IGCs were compared among three groups. 3. No apparent change of protein composition in IGCs was observed by electrophoresis. Ca2+dependent phosphorylation of GAP-43 and MARCKS protein, and PKC activity in IGCs significantly decreased after exposure to N20. The degradation of PKC increased significantly after inhalation of N2O. 4. The authors concluded that Xe dose not change the above biochemical characteristic of the growth cones, suggesting that Xe is free from the teratogenic effect on the neuronal network formation and that Xe is a safe anesthetics for the perinatal neuronal development.

The mechanism of the neurotransmitter release in growth cones

The growth cone is considered the precursor of the presynaptic terminal. To elucidate the minimal molecular machinery required for exocytosis, we examined the characteristics of alpha-latrotoxin-induced exocytosis in growth cones. In isolated growth cones (IGC), neurotransmitters were released in a SNARE-dependent manner, but rab3A cycling was blocked. By supplying rabphilin, a rab3A acceptor found in low levels in IGC, the IGC obtained as high an exocytotic efficiency as adult synaptosomes, and the complete GDP-GTP conversion of rab3A occurred on growth cone vesicles (GCV). GCVs bound SNAREs but not NSF or alpha-SNAP; whereas in the rabphilin-supplied IGC, GCVs recruited both NSF and alpha-SNAP, to form the SNARE-NSF-SNAP complex. These results suggest that rab3A cycling is dependent upon the accumulation of rabphilin and is completed later than the SNARE mechanism, and that rabphilin is involved in determining the efficiency of exocytosis by modifying the SNARE mechanism.

GABA release from mouse axonal growth cones

1. Using developing hypothalamic neurons from transgenic mice that express high levels of green fluorescent protein in growing axons, and an outside-out patch from mature neuronal membranes that contain neurotransmitter receptors as a sensitive detector, we found that GABA is released by a vesicular mechanism from the growth cones of developing axons prior to synapse formation. 2. A low level of GABA release occurs spontaneously from the growth cone, and this is substantially increased by evoked action potentials. 3. Neurotransmitters such as acetylcholine can enhance protein kinase C (PKC) activity even prior to synapse formation; PKC activation caused a substantial increase in spontaneous GABA release from the growth cone, probably acting at the axon terminal. 4. These data indicate that GABA is secreted from axons during a stage of neuronal development when GABA is excitatory, and that neuromodulators could alter GABA release from the growing axon, potentially enabling other developing neurons of different transmitter phenotype to modulate the early actions of GABA.

Functional NMDA and GABAA receptors in pioneer neurons of the cortical marginal zone

Transient pioneer neurons in the neocortical marginal zone generate an early corticofugal axonal projection at E12-E16 (Meyer et al. 1998). We have analysed the functional activity of glutamate and GABA receptors in such cells by measuring changes in intracellular calcium concentrations ([Ca2+]i). The activation of GABAA receptors with muscimol, as well as bath application of glutamate, lead to increases in [Ca2+]i in pioneer neurons. The stimulatory action of glutamate is mostly produced through the NMDA-type of ionotropic receptors. Metabotropic glutamate receptor activation has no effect on [Ca2+]i. Consistent with such results, immunocytochemical studies showed a prominent expression of GABAA and NMDA receptors in pioneer neurons. The activation of such receptors may be implicated in the remodelling of pioneer neurons during development.

Neurosteroid modulation of the GABAA receptor in the developing guinea pig cerebral cortex

Developmental changes in 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone; 5alpha-3alpha-P) potentiation of muscimol and benzodiazepine binding to the GABAA receptor were studied in the guinea pig cerebral cortex at three prenatal ages (gestational day (GD) 40, GD 50, GD 62), and three postnatal ages (postnatal day (PD) 11, PD 21, PD 61) (term, about GD 68). The number and affinity of [3H]flunitrazepam binding sites, and 5alpha-3alpha-P potentiation of [3H]muscimol and [3H]flunitrazepam binding to the GABAA receptor were determined at each age. There was no age effect on the affinity (Kd) for [3H]flunitrazepam. However, the number (Bmax) of [3H]flunitrazepam binding sites doubled between GD 40 and GD 62, and then declined slightly to reach adult levels by PD 11. 5alpha-3alpha-P produced a concentration-dependent potentiation of [3H]muscimol and [3H]flunitrazepam binding at each developmental age examined. The potency (high-affinity) for 5alpha-3alpha-P potentiation of both [3H]muscimol and [3H]flunitrazepam binding was lowest at GD 40, and increased to adult levels by GD 62. In contrast, the efficacy for 5alpha-3alpha-P potentiation of both [3H]muscimol and [3H]flunitrazepam binding was greatest at GD 40, and decreased to adult levels between GD 50 and GD 62. The percentage of high-affinity zolpidem binding sites increased in an age-dependent manner from 34.2+/-2.2% at GD 40, to reach adult levels by GD 62 (59. 4+/-2.5%). These data suggest that 5alpha-3alpha-P can modulate GABAA receptors in the immature cerebral cortex, and that changes in 5alpha-3alpha-P action are temporally related to changes in GABAA receptor benzodiazepine pharmacology late in gestation in the guinea pig.

Neurite outgrowth stimulated by neural cell adhesion molecules requires growth-associated protein-43 (GAP-43) function and is associated with GAP-43 phosphorylation in growth cones

The mechanisms whereby cell adhesion molecules (CAMs) promote axonal growth and synaptic plasticity are poorly understood. Here we show that the neurite outgrowth stimulated by NCAM-mediated fibroblast growth factor (FGF) receptor activation in cerebellar granule cells is associated with increased GAP-43 phosphorylation on serine-41. In contrast, neither NCAM nor FGF was able to stimulate neurite outgrowth in similar neurons from mice in which the GAP-43 gene had been deleted by homologous recombination. Integrin-mediated neurite outgrowth was unaffected by GAP-43 deletion. Both neurite outgrowth and rapid phosphorylation of GAP-43 in isolated growth cones required the first three Ig domains of a NCAM-Fc chimera and were stimulated maximally at 5 micrograms/ml (approximately 50 nM). Likewise, GAP-43 phosphorylation in isolated growth cones also was stimulated by an L1-Fc chimera. Both neurite outgrowth and NCAM-stimulated GAP-43 phosphorylation were inhibited by antibodies to the FGF receptor and a diacylglycerol lipase inhibitor (RHC80267) that blocks the production of arachidonic acid in response to activation of the FGF receptor. Direct activation of the FGF receptor and the arachidonic acid cascade with either basic FGF or melittin also resulted in increased GAP-43 phosphorylation. These data suggest that the stimulation of neurite outgrowth by NCAM requires GAP-43 function and that GAP-43 phosphorylation in isolated growth cones occurs via an FGF receptor-dependent increase in arachidonic acid.

GABAB receptor-mediated inhibition of GABAA receptor calcium elevations in developing hypothalamic neurons

In the CNS, gamma-aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABAA receptor channel and the G protein-coupled GABAB receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABAA receptor increases neural excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 +/- 2 days in vitro, we found that cytosolic Ca2+ rises triggered by synaptically activated GABAA receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABAB receptor agonist baclofen (100 nM-100 microM). Coadministration of the GABAB receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABAB antagonist alone elicited a reproducible Ca2+ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABAA receptor-mediated Ca2+ rises via GABAB receptor activation. In the presence of tetrodotoxin the GABAA receptor agonist muscimol elicited robust postsynaptic Ca2+ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca2+ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABAB receptors are functionally active at an early stage of neuronal development. Ca2+ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABAB receptor activation depresses GABAA receptor-mediated Ca2+ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB receptors play an important inhibitory role regulating Ca2+ rises elicited by GABAA receptor activation. Changes in cytosolic Ca2+ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

GABA activity mediating cytosolic Ca2+ rises in developing neurons is modulated by cAMP-dependent signal transduction

In the majority of developing neurons, GABA can exert depolarizing actions, thereby raising neuronal Ca2+. Ca2+ elevations can have broad consequences during development, inducing gene expression, altering neurite outgrowth and growth cone turning, activating enzyme pathways, and influencing neuronal survival. We used fura-2 and fluo-3 Ca2+ digital imaging to assess the effects of inhibiting or activating the cAMP signal transduction pathway on GABA activity mediating Ca2+ rises during the early stages of in vitro hypothalamic neural development. Our experiments stemmed from the finding that stimulation of transmitter receptors shown to either activate or inhibit adenylyl cyclase activity caused a rapid decrease in Ca2+ rises mediated by synaptically released GABA. Both the adenylyl cyclase activator forskolin and the inhibitor SQ-22,536 reduced the Ca2+ rise elicited by the synaptic release of GABA. Bath application of the membrane-permeable cAMP analogs 8-bromo-cAMP (8-Br-cAMP) or 8-(4-chlorophenylthio)-cAMP (0.2-5 mM) produced a rapid, reversible, dose-dependent inhibition of Ca2+ rises triggered by synaptic GABA release. Potentiation of GABAergic activity mediating Ca2+ rises was observed in some neurons at relatively low concentrations of the membrane-permeable cAMP analogs (20-50 microM). In the presence of tetrodotoxin (TTX), postsynaptic Ca2+ rises triggered by the bath application of GABA were only moderately depressed (13%) by 8-Br-cAMP (1 mM), suggesting that the inhibitory effects of 8-Br-cAMP were largely the result of a presynaptic mechanism. The protein kinase A (PKA) inhibitors H89 and Rp-3', 5'-cyclic monophosphothioate triethylamine also caused a large reduction (>70%) in Ca2+ rises triggered by synaptic GABA release. Unlike the short-term depression elicited by activation of the cAMP signal transduction pathway, Ca2+ depression elicited by PKA inhibition persisted for an extended period (>30 min) after PKA inhibitor washout. Postsynaptic depression of GABA-evoked Ca2+ rises triggered by H89 (in the presence of TTX) recovered rapidly, suggesting that the extended depression observed during synaptic GABA release was largely through a presynaptic mechanism. Long-term Ca2+ modulation by cAMP-regulating hypothalamic peptides may be mediated through a parallel mechanism. Together, these results suggest that GABAergic activity mediating Ca2+ rises is dependent on ongoing PKA activity that is maintained within a narrow zone for GABA to elicit a maximal Ca2+ elevation. Thus, neuromodulator-mediated changes in the cAMP-dependent signal transduction pathway (activation or inhibition) could lead to a substantial decrease in GABA-mediated Ca2+ rises during early development.

GABA immunoreactivity in hypothalamic neurons and growth cones in early development in vitro before synapse formation

GABA (gamma-aminobutyrate) is the most prevalent inhibitory transmitter in the mature hypothalamus. In contrast, in the developing hypothalamus, GABA may exert depolarizing actions leading to neuronal excitation. To determine whether GABA is present in hypothalamic neurons early in development, and whether there is a preferential expression in axonal growth cones, immunogold and peroxidase studies were used with light and whole mount transmission electron microscopy. At embryonic day 15, a stage of development at the beginning of hypothalamic neurogenesis, histological sections showed GABA immunoreactivity in fibers and weakly stained perikarya. Hypothalamic neurons (13%) cultured at embryonic day 15 were immunoreactive after 1 day in vitro. The percentage of neurons stained, and the intensity of staining increased during the next few days to 39% at 4 days in vitro. Neuritic growth cones, including lamellipodia and long filopodia, showed strong immunoreactivity before synaptogenesis. By using neuronal whole mounts studied with transmission electron microscopy and GABA silver-enhanced immunogold staining, a quantitative comparison of growth cones after a day and a half in culture revealed that the growth cone of the longest process, the putative axon, had a greater level of immunogold labeling than that of the shorter processes, the putative dendrites. This finding is one of the earliest biochemical differences between putative axons and dendrites. Astrocytes in the same cultures showed no immunolabeling. These results indicate that GABA is present very early in the development of hypothalamic neurons and is in a position to be released.