Indirect effects of amino-acids on sympathetic ganglion cells mediated through the release of gamma-aminobutyric acid from glial cells

Disrupted Association of Sensory Neurons With Enveloping Satellite Glial Cells in Fragile X Mouse Model

Among most prevalent deficits in individuals with Fragile X syndrome (FXS) is hypersensitivity to sensory stimuli and somatosensory alterations. Whether dysfunction in peripheral sensory system contributes to these deficits remains poorly understood. Satellite glial cells (SGCs), which envelop sensory neuron soma, play critical roles in regulating neuronal function and excitability. The potential contributions of SGCs to sensory deficits in FXS remain unexplored. Here we found major structural defects in sensory neuron-SGC association in the dorsal root ganglia (DRG), manifested by aberrant covering of the neuron and gaps between SGCs and the neuron along their contact surface. Single-cell RNAseq analyses demonstrated transcriptional changes in both neurons and SGCs, indicative of defects in neuronal maturation and altered SGC vesicular secretion. We validated these changes using fluorescence microscopy, qPCR, and high-resolution transmission electron microscopy (TEM) in combination with computational analyses using deep learning networks. These results revealed a disrupted neuron-glia association at the structural and functional levels. Given the well-established role for SGCs in regulating sensory neuron function, altered neuron-glia association may contribute to sensory deficits in FXS.

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

Crossing the Chloride Channel: The Current and Potential Therapeutic Value of the Neuronal K+-Cl- Cotransporter KCC2

Chloride (Cl-) homeostasis is an essential process involved in neuronal signalling and cell survival. Inadequate regulation of intracellular Cl- interferes with synaptic signalling and is implicated in several neurological diseases. The main inhibitory neurotransmitter of the central nervous system is γ-aminobutyric acid (GABA). GABA hyperpolarises the membrane potential by activating Cl- permeable GABAA receptor channels (GABAAR). This process is reliant on Cl- extruder K+-Cl- cotransporter 2 (KCC2), which generates the neuron's inward, hyperpolarising Cl- gradient. KCC2 is encoded by the fifth member of the solute carrier 12 family (SLC12A5) and has remained a poorly understood component in the development and severity of many neurological diseases for many years. Recent advancements in next-generation sequencing and specific gene targeting, however, have indicated that loss of KCC2 activity is involved in a number of diseases including epilepsy and schizophrenia. It has also been implicated in neuropathic pain following spinal cord injury. Any variant of SLC12A5 that negatively regulates the transporter's expression may, therefore, be implicated in neurological disease. A recent whole exome study has discovered several causative mutations in patients with epilepsy. Here, we discuss the implications of KCC2 in neurological disease and consider the evolving evidence for KCC2's potential as a therapeutic target.

Ammonia modifies enteric neuromuscular transmission through glial γ-aminobutyric acid signaling

Impaired gut motility may contribute, at least in part, to the development of systemic hyperammonemia and systemic neurological disorders in inherited metabolic disorders, or in severe liver and renal disease. It is not known whether enteric neurotransmission regulates intestinal luminal and hence systemic ammonia levels by induced changes in motility. Here, we propose and test the hypothesis that ammonia acts through specific enteric circuits to influence gut motility. We tested our hypothesis by recording the effects of ammonia on neuromuscular transmission in tissue samples from mice, pigs, and humans and investigated specific mechanisms using novel mutant mice, selective drugs, cellular imaging, and enzyme-linked immunosorbent assays. Exogenous ammonia increased neurogenic contractions and decreased neurogenic relaxations in segments of mouse, pig, and human intestine. Enteric glial cells responded to ammonia with intracellular Ca²⁺ responses. Inhibition of glutamine synthetase and the deletion of glial connexin-43 channels in hGFAP::CreER^T2+/-/connexin43^f/f mice potentiated the effects of ammonia on neuromuscular transmission. The effects of ammonia on neuromuscular transmission were blocked by GABA_A receptor antagonists, and ammonia drove substantive GABA release as did the selective pharmacological activation of enteric glia in GFAP::hM3Dq transgenic mice. We propose a novel mechanism whereby local ammonia is operational through GABAergic glial signaling to influence enteric neuromuscular circuits that regulate intestinal motility. Therapeutic manipulation of these mechanisms may benefit a number of neurological, hepatic, and renal disorders manifesting hyperammonemia.NEW & NOTEWORTHY We propose that local circuits in the enteric nervous system sense and regulate intestinal ammonia. We show that ammonia modifies enteric neuromuscular transmission to increase motility in human, pig, and mouse intestine model systems. The mechanisms underlying the effects of ammonia on enteric neurotransmission include GABAergic pathways that are regulated by enteric glial cells. Our new data suggest that myenteric glial cells sense local ammonia and directly modify neurotransmission by releasing GABA.

Norman Bowery's discoveries about extrasynaptic and asynaptic GABA systems and their significance

Before discovering the GABA-B receptor, Norman Bowery completed a series of studies on an extrasynaptic or asynaptic "GABA system" in the rat superior cervical sympathetic ganglion. First, he discovered an uptake system for GABA in neuroglial cells in the ganglia and in peripheral nerves, with a different substrate specificity than that in neurons. Second, he showed that accumulated GABA in sympathetic glial cells was metabolized to succinate by a transaminase enzyme. Third, he provided detailed structure-activity information about compounds activating an extrasynaptic GABA-A receptor on neurons in the rat sympathetic ganglion. Fourth, he showed that some amino acid substrates for the neuroglial transporter could indirectly stimulate neurons by releasing GABA from adjacent glial cells, and that GABA could also be released from neuroglial cells by membrane depolarization. In this review, these discoveries are briefly described and updated and some of their implications assessed. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Tonic GABAA conductance decreases membrane time constant and increases EPSP-spike precision in hippocampal pyramidal neurons

Because of a complex dendritic structure, pyramidal neurons have a large membrane surface relative to other cells and so a large electrical capacitance and a large membrane time constant (τ_m). This results in slow depolarizations in response to excitatory synaptic inputs, and consequently increased and variable action potential latencies, which may be computationally undesirable. Tonic activation of GABA_A receptors increases membrane conductance and thus regulates neuronal excitability by shunting inhibition. In addition, tonic increases in membrane conductance decrease the membrane time constant (τ_m), and improve the temporal fidelity of neuronal firing. Here we performed whole-cell current clamp recordings from hippocampal CA1 pyramidal neurons and found that bath application of 10μM GABA indeed decreases τ_m in these cells. GABA also decreased first spike latency and jitter (standard deviation of the latency) produced by current injection of 2 rheobases (500 ms). However, when larger current injections (3–6 rheobases) were used, GABA produced no significant effect on spike jitter, which was low. Using mathematical modeling we demonstrate that the tonic GABA_A conductance decreases rise time, decay time and half-width of EPSPs in pyramidal neurons. A similar effect was observed on EPSP/IPSP pairs produced by stimulation of Schaffer collaterals: the EPSP part of the response became shorter after application of GABA. Consistent with the current injection data, a significant decrease in spike latency and jitter was obtained in cell attached recordings only at near-threshold stimulation (50% success rate, S₅₀). When stimulation was increased to 2- or 3- times S₅₀, GABA significantly affected neither spike latency nor spike jitter. Our results suggest that a decrease in τ_m associated with elevations in ambient GABA can improve EPSP-spike precision at near-threshold synaptic inputs.

Tonic GABA(A) receptor-mediated signalling in temporal lobe epilepsy

The tonic activation of extrasynaptic GABAA receptors by extracellular GABA provides a powerful means of regulating neuronal excitability. A consistent finding from studies that have used various models of temporal lobe epilepsy is that tonic GABAA receptor-mediated conductances are largely preserved in epileptic brain (in contrast to synaptic inhibition which is often reduced). Tonic inhibition is therefore an attractive target for antiepileptic drugs. However, the network consequences of a commonly used approach to augment tonic GABAA receptor-mediated conductances by global manipulation of extracellular GABA are difficult to predict without understanding how epileptogenesis alters the pharmacology and GABA sensitivity of tonic inhibition, and how manipulation of tonic conductances modulates the output of individual neurons. Here we review the current literature on epilepsy-associated changes in tonic GABAA receptor-mediated signalling, and speculate about possible effects they have at the network level. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Neurotransmitters and integration in neuronal-astroglial networks

Two major neural cell types, glia, astrocytes in particular, and neurones can release chemical transmitters that act as soluble signalling compounds for intercellular communication. Exocytosis, a process which depends on an increase in cytosolic Ca(2+) levels, represents a common denominator for release of neurotransmitters, stored in secretory vesicles, from these neural cells. While neurones rely predominately on the immediate entry of Ca(2+) from the extracellular space to the cytosol in this process, astrocytes support their cytosolic Ca(2+) increases by appropriating this ion from the intracellular endoplasmic reticulum store and extracellular space. Additionally, astrocytes can release neurotransmitters using a variety of non-vesicular pathways which are mediated by an assortment of plasmalemmal channels and transporters. Once a neuronal and/or astrocytic neurotransmitter is released into the extracellular space, it can activate plasma membrane neurotransmitter receptors on neural cells, causing autocrine and/or paracrine signalling. Moreover, chemical transmission is essential not only for homocellular, but also for heterocellular bi-directional communication in the brain. Further detailed understanding of chemical transmission will aid our comprehension of the brain (dys)function in heath and disease.

Tonic excitation or inhibition is set by GABA(A) conductance in hippocampal interneurons

Inhibition is a physiological process that decreases the probability of a neuron generating an action potential. The two main mechanisms that have been proposed for inhibition are hyperpolarization and shunting. Shunting results from increased membrane conductance, and it reduces the neuron-firing probability. Here we show that ambient GABA, the main inhibitory neurotransmitter in the brain, can excite adult hippocampal interneurons. In these cells, the GABA(A) current reversal potential is depolarizing, making baseline tonic GABA(A) conductance excitatory. Increasing the tonic conductance enhances shunting-mediated inhibition, which eventually overpowers the excitation. Such a biphasic change in interneuron firing leads to corresponding changes in the GABA(A)-mediated synaptic signalling. The described phenomenon suggests that the excitatory or inhibitory actions of the current are set not only by the reversal potential, but also by the conductance.

Recent advances in (patho)physiology of astroglia

Our view of astrocytes in the operation of the brain is changing dramatically over the last 3 decades. Astroglial calcium excitability controls the release of gliotransmitters, which can occur at the tripartite synapse. Astrocytes not only modulate synaptic transmission by releasing and taking up transmitters, but also receiving neuronal signals that act upon astrocytic plasma membrane receptors. This process represents the bidirectional neurone-glia communication. Additionally, astrocytes play role in the regulation of blood flow as well as ion and water homeostasis. Many of the brain dysfunctions are primary astropathies, including hepatic encephalopathy and Alexander disease, while other brain malfunctions, such as epilepsy and Alzheimer disease, may have substantial astrocytic contribution. Thus, these star-shaped cells by their roles in (patho)physiology of the brain seem to live up to the expectation one can have from their given name - astrocyte.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

GABA increases electrical excitability in a subset of human unmyelinated peripheral axons

Background

A proportion of small diameter primary sensory neurones innervating human skin are chemosensitive. They respond in a receptor dependent manner to chemical mediators of inflammation as well as naturally occurring algogens, thermogens and pruritogens. The neurotransmitter GABA is interesting in this respect because in animal models of neuropathic pain GABA pre-synaptically regulates nociceptive input to the spinal cord. However, the effect of GABA on human peripheral unmyelinated axons has not been established.

Methodology/Principal Findings

Electrical stimulation was used to assess the effect of GABA on the electrical excitability of unmyelinated axons in isolated fascicles of human sural nerve. GABA (0.1–100 µM) increased electrical excitability in a subset (ca. 40%) of C-fibres in human sural nerve fascicles suggesting that axonal GABA sensitivity is selectively restricted to a sub-population of human unmyelinated axons. The effects of GABA were mediated by GABA_A receptors, being mimicked by bath application of the GABA_A agonist muscimol (0.1–30 µM) while the GABA_B agonist baclofen (10–30 µM) was without effect. Increases in excitability produced by GABA (10–30 µM) were blocked by the GABA_A antagonists gabazine (10–20 µM), bicuculline (10–20 µM) and picrotoxin (10–20 µM).

Conclusions/Significance

Functional GABA_A receptors are present on a subset of unmyelinated primary afferents in humans and their activation depolarizes these axons, an effect likely due to an elevated intra-axonal chloride concentration. GABA_A receptor modulation may therefore regulate segmental and peripheral components of nociception.

Gliotransmission: Exocytotic release from astrocytes

Gliotransmitters are chemicals released from glial cells fulfilling a following set of criteria: (i) they are synthesized by and/or stored in glia; (ii) their regulated release is triggered by physiological and/or pathological stimuli; (iii) they activate rapid (milliseconds to seconds) responses in neighboring cells; and (iv) they play a role in (patho)physiological processes. Astrocytes can release a variety of gliotransmitters into the extracellular space using several different mechanisms. In this review, we focus on exocytotic mechanism(s) underlying the release of three classes of gliotransmitters: (i) amino acids, such as, glutamate and d-serine; (ii) nucleotides, like adenosine 5'-triphosphate; and (iii) peptides, such as, atrial natriuretic peptide and brain-derived neurotrophic factor. It is becoming clear that astrocytes are endowed with elements that qualify them as cells communicating with neurons and other cells within the central nervous system by employing regulated exocytosis.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

Electrophysiological effects of GABA on cat pancreatic neurons

In mammalian peripheral sympathetic ganglia GABA acts presynaptically to facilitate cholinergic transmission and postsynaptically to depolarize membrane potential. The GABA effect on parasympathetic pancreatic ganglia is unknown. We aimed to determine the effect of locally applied GABA on cat pancreatic ganglion neurons. Ganglia with attached nerve trunks were isolated from cat pancreata. Conventional intracellular recording techniques were used to record electrical responses from ganglion neurons. GABA pressure microejection depolarized membrane potential with an amplitude of 17.4 +/- 0.7 mV. Electrically evoked fast excitatory postsynaptic potentials were significantly inhibited (5.4 +/- 0.3 to 2.9 +/- 0.2 mV) after GABA application. GABA-evoked depolarizations were mimicked by the GABA(A) receptor agonist muscimol and abolished by the GABA(A) receptor antagonist bicuculline and the Cl(-) channel blocker picrotoxin. GABA was taken up and stored in ganglia during preincubation with 1 mM GABA; beta-aminobutyric acid application after GABA loading significantly (P < 0.05) increased depolarizing response to GABA (15.6 +/- 1.0 vs. 7.8 +/- 0.8 mV without GABA preincubation). Immunolabeling with antibodies to GABA, glial cell fibrillary acidic protein, protein gene product 9.5, and glutamic acid decarboxylase (GAD) immunoreactivity showed that GABA was present in glial cells, but not in neurons, and that glial cells did not contain GAD, whereas islet cells did. The data suggest that endogenous GABA released from ganglionic glial cells acts on pancreatic ganglion neurons through GABA(A) receptors.

Persistent activation of GABA(A) receptor/Cl(-) channels by astrocyte-derived GABA in cultured embryonic rat hippocampal neurons

Whole cell patch-clamp recordings using Cl(-)-filled pipettes revealed more negative levels of baseline current and associated current variance in embryonic rat hippocampal neurons co-cultured on a monolayer of astrocytes than those cultured on poly-D-lysine. These effects were mimicked by culturing neurons on poly-D-lysine in astrocyte-conditioned medium (ACM). The baseline current and variance decreased immediately in all cells after either local perfusion with saline or exposure to bicuculline, an antagonist of GABA at GABA(A) receptor/Cl(-) channels. Baseline current and variance in all cells reached a nadir at approximately 0 mV, the calculated equilibrium potential for Cl(-). Perfusion of ACM rapidly induced a sustained current in neurons, which also reversed polarity at approximately 0 mV. Bicuculline attenuated or eliminated the ACM-induced current at a concentration that completely blocked micromolar GABA-induced current. Quantitative analyses of spontaneously occurring fluctuations superimposed on the ACM-induced current revealed estimated unitary properties of the underlying channel activity similar to those calculated for GABA's activation of GABA(A) receptor/Cl(-) channels. Bicuculline-sensitive synaptic-like transients, which reversed at approximately 0 mV, were also detected in neurons cultured in ACM, and these were immediately eliminated along with the negative baseline current and superimposed current fluctuations by perfusion. Furthermore bicuculline-sensitive synaptic-like transients were rapidly and reversibly triggered when ACM was acutely applied. ACM induced an increase in cytoplasmic Ca(2+) in cultured embryonic hippocampal neurons that was completely blocked by bicuculline and strychnine. We conclude that astrocytes release diffusible substances, most likely GABA, that persistently activate GABA(A) receptor/Cl(-) channels in co-cultured neurons.

In vitro techniques for the assessment of neurotoxicity

Risk assessment is a process often divided into the following steps: a) hazard identification, b) dose-response assessment, c) exposure assessment, and d) risk characterization. Regulatory toxicity studies usually are aimed at providing data for the first two steps. Human case reports, environmental research, and in vitro studies may also be used to identify or to further characterize a toxic hazard. In this report the strengths and limitations of in vitro techniques are discussed in light of their usefulness to identify neurotoxic hazards, as well as for the subsequent dose-response assessment. Because of the complexity of the nervous system, multiple functions of individual cells, and our limited knowledge of biochemical processes involved in neurotoxicity, it is not known how well any in vitro system would recapitulate the in vivo system. Thus, it would be difficult to design an in vitro test battery to replace in vivo test systems. In vitro systems are well suited to the study of biological processes in a more isolated context and have been most successfully used to elucidate mechanisms of toxicity, identify target cells of neurotoxicity, and delineate the development and intricate cellular changes induced by neurotoxicants. Both biochemical and morphological end points can be used, but many of the end points used can be altered by pharmacological actions as well as toxicity. Therefore, for many of these end points it is difficult or impossible to set a criterion that allows one to differentiate between a pharmacological and a neurotoxic effect. For the process of risk assessment such a discrimination is central. Therefore, end points used to determine potential neurotoxicity of a compound have to be carefully selected and evaluated with respect to their potential to discriminate between an adverse neurotoxic effect and a pharmacologic effect. It is obvious that for in vitro neurotoxicity studies the primary end points that can be used are those affected through specific mechanisms of neurotoxicity. For example, in vitro systems may be useful for certain structurally defined compounds and mechanisms of toxicity, such as organophosphorus compounds and delayed neuropathy, for which target cells and the biochemical processes involved in the neurotoxicity are well known. For other compounds and the different types of neurotoxicity, a mechanism of toxicity needs to be identified first. Once identified, by either in vivo or in vitro methods, a system can be developed to detect and to evaluate predictive ability for the type of in vivo neurotoxicity produced. Therefore, in vitro tests have their greatest potential in providing information on basic mechanistic processes in order to refine specific experimental questions to be addressed in the whole animal.

Synthesis and release of neuroactive substances by glial cells

Glia contain, synthesize, or release more than 20 neuroactive compounds including neuropeptides, amino acid transmitters, eicosanoids, steroids, and growth factors. The stimuli that elicit release differ among compounds but include neuropeptides, neurotransmitters, receptor agonists, and elevated external [K+]. The mechanisms of release are poorly understood in most cases. Many of the neuroactive compounds are localized in discrete subpopulations of glia. Thus, glia are equipped to send as well as receive chemical messages and appear to be present as classes of cells with differing abilities to communicate chemically. It is possible that glia are as diverse as neurons in their functional characteristics.

Localization of L-glutamate decarboxylase and GABA transaminase immunoreactivity in the sympathetic ganglia of the rat

The location of L-glutamate decarboxylase and gamma-aminobutyrate (GABA)-transaminase immunoreactivity in the superior cervical ganglion and in the coeliac-superior mesenteric ganglion complex of the rat was studied by an indirect immunofluorescence method and by immunoelectron microscopy, with specific antisera raised in rabbits against the corresponding enzymes. In light microscopy, several glutamate decarboxylase- or GABA-transaminase-immunoreactive principal nerve cells were detected in the superior cervical ganglion and coeliac-superior mesenteric ganglion complex. In addition, numerous small cells in both the superior cervical ganglion and coeliac-superior mesenteric ganglion complex showed intense immunoreactivity to glutamate decarboxylase or GABA-transaminase. The small cells were 10-20 micron in diameter and resembled in size and morphology the small intensely fluorescent cells. In consecutive sections, the small glutamate decarboxylase-immunoreactive cell clusters also showed immunoreactivity to tyrosine hydroxylase, suggesting that these cells contain the enzymes for both GABA and catecholamine synthesis. In the superior cervical ganglion and in the coeliac-superior mesenteric ganglion complex, GABA-transaminase immunoreactivity was also localized in fibre-like processes around and between the principal nerve cells, in nerve trunks traversing the ganglia, and around or in close contact with ganglionic blood vessels. Furthermore, GABA-transaminase immunoreactivity was observed in fibre-like structures close to the capsule of the ganglia. Division of the preganglionic nerve trunk of the superior cervical ganglion caused no detectable change in GABA-transaminase immunoreactivity in the ganglion. In immunoelectron microscopy of the superior cervical ganglion, GABA-transaminase immunoreactivity was localized in nerve fibres in association with neurotubules. A large number of GABA-transaminase labelled principal nerve cells were detected, containing immunoreactivity evenly distributed in their cytoplasm. GABA-transaminase immunoreactivity was also observed in satellite cells and their processes in the superior cervical ganglion. The present immunocytochemical results provide evidence that the rat sympathetic ganglia contain an intrinsic neuronal system showing histochemical markers for GABA synthesis and inactivation, but its functional role in the modulation of ganglionic neurotransmission remains to be established.

Saturable binding of 35S-t-butylbicyclophosphorothionate to the sites linked to the GABA receptor and the interaction with GABAergic agents

35S-t-Butylbicyclophosphorothionate (35S-TBPS) binds in a concentration-saturable manner to specific sites on membranes from rat cerebral cortex. Using a filtration assay at 25 degrees C, in 250 mM NaCl, specific binding of 35S-TBPS constitutes about 84 to 94 percent of total binding, depending on radioligand concentrations. 35S-TBPS binding is optimal in the presence of NaCl or NaBr and substantially less in the presence of NaI or NaF. It is sensitive to the treatment with 0.05 percent Triton X-100 but not to repeated freezing and thawing, procedures which increase 3H-GABA binding. Pharmacological studies show that 35S-TBPS binding is strongly inhibited by GABA-A receptor agonists (e.g., GABA and muscimol) and by the noncompetitive antagonist, picrotoxin, but not the competitive antagonist, bicuculline. Compounds which enhance binding of radioactive GABA and benzodiazepines, such as the pyrazolopyridines, cartazolate and tracazolate, and a diaryltriazine, LY81067, are also potent inhibitors of 35S-TBPS binding, with LY81067 being the most effective. The effects of GABA, picrotoxin and LY81067 on the saturable binding of 35S-TBPS in cortical membranes are compared. The present findings are consistent with the interpretation that 35S-TBPS binds at or near the picrotoxin-sensitive anion recognition sites of the GABA/benzodiazepine/picrotoxin receptor complex.

RO5-4864 inhibits the binding of [35S]t-butylbicyclophosphorothionate to rat brain membranes

RO5-4864, a 1,4-benzodiazepine, has recently been shown to possess anticonvulsant, convulsant and anxiogenic properties and to inhibit Ca++-calmodulin-stimulated membrane phosphorylation. RO5-4864 inhibited the binding of [35S]t-butylbicyclophosphorothionate (TBPT) to cerebral cortex, cerebellar and hippocampus membranes, with an IC50 value of approximately 20 microM. TBPT binds apparently to the picrotoxinin site of the benzodiazepine-GABA receptor-ionophore complex and appears to be a site of action for several classes of convulsant, depressant and anxiolytic drugs that modulate GABAergic transmission. RO5-4864 inhibited [35S] TBPT binding in cerebral cortex, apparently competitively. Antagonists of GABA and central benzodiazepine sites did not interfere with the ability of RO5-4864 to inhibit [35S] TBPT binding. The properties of RO5-4864 to inhibit TBPT binding are similar to other convulsants and GABA antagonists (except bicuculline) which inhibit TBPT binding. These results suggest that RO5-4864 interacts with the TBPT binding sites of the oligomeric GABA receptor complex.

Ketamine potentiates the responses of the rat superior cervical ganglion to GABA

The responses of the rat superior cervical ganglion to gamma-aminobutyric acid (GABA) were recorded in vitro using extracellular electrodes. Ketamine was found to increase the amplitude of these responses by up to 100%. This potentiation was seen at ketamine concentrations as low as 18 microM, reached maximum at 180 microM and then declined as the concentration was raised further. Ketamine (90 microM and over) depressed the nicotinic responses of the ganglia. The log concentration-effect curve to GABA was shifted to the left, and the maximum response increased, by ketamine. Inhibition of glial uptake of GABA did not prevent the effect of ketamine. The effects of 3-aminopropane sulphonic acid, which has a very low affinity for this GABA uptake mechanism, were also increased by ketamine. We conclude that ketamine, at concentrations which are found during general anaesthesia, potentiates the responses of the ganglion to GABA, by a mechanism other than inhibition of uptake, and suggest this may be due to action at the GABA receptor site.

Comparison of the distribution of convulsant/barbiturate and benzodiazepine receptors using light microscopic autoradiography

Some convulsant drugs elicit CNS excitation by blocking neuronal activity at GABAergic synapses whereas depressant compounds may result in the enhancement of GABAergic transmission. These effects are thought to involve drug actions at a multireceptor complex involving a benzodiazepine receptor, GABA receptor, picrotoxin receptor and a chloride ionophore. A radiolabeled convulsant, [35S]t-butylbicyclophosphorothionate [( 35S]-TBT) has been developed and used to characterize the binding to the "picrotoxin" or convulsant/barbiturate site. The microscopic distribution of the convulsant/barbiturate sites are reported in this communication, as demonstrated by receptor autoradiography after labeling tissue sections with [35S]-TBT. Comparison of the distribution of these sites with those of the benzodiazepine receptors show a close regional correlation in many areas. The convulsant/barbiturate sites and the benzodiazepine receptors, however, are unevenly distributed in the rat cerebellum and exist in separate lamina.

Substrate specificity and developmental aspects of a presynaptic GABA receptor regulating glutamate release in the rat cerebellum

In order to better characterize the presynaptic GABA receptors regulating glutamate release in the cerebellum [Levi and Gallo, 1981], a number of GABA agonists and GABA transport inhibitors were tested for their ability to potentiate the depolarization-induced release of the glutamate analog D-[3H]aspartate from superfused cerebellar synaptosomes. Of all the compounds tested, only those which are known to interact specifically with GABA receptors were effective when tested on synaptosomal preparations. The order of effectiveness found was the following: muscimol congruent to 3-APS greater than or equal to P4S greater than isoguvacine greater than THIP. GABA uptake inhibitors were unable to enhance D-[3H]aspartate evoked release from synaptosomes, but were effective when tested in cerebellar slices; in the latter case, the activation of the GABA receptors may be achieved indirectly, through an increase of the extracellular GABA concentrations. The substrate specificity of the presynaptic GABA receptors regulating cerebellar acidic amino acid release appears to be similar to that reported for GABA receptors in radioligand binding studies and for GABA autoreceptors. Studies on synaptosomes from immature cerebella suggested that the presence of the potentiating effect on the acidic amino acid release by GABA agonists is correlated with the development of the parallel fiber terminals, which are believed to be the main site from which glutamate is released in the adult cerebellum.

Phosphatidylserine enhancement of [3H]gamma-aminobutyric acid uptake by rat whole brain synaptosomes

[3H]gamma-Aminobutyric acid ([3H]GABA) binding to purified lipids was examined in an organic solvent-aqueous partition system. In addition, the [3H]GABA binding capacity in the partition system was compared with the capacity of lipids to alter sodium-dependent [3H]GABA uptake into synaptosomes isolated from rat whole brains. [3H]GABA was found to bind to all of the lipids studied in the organic solvent-aqueous partition system [phosphatidic acid (PA), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), gangliosides, and sulfatide], although PS exhibited the greatest binding capacity. [3H]GABA uptake into synaptosomes was enhanced by PS (48.0%) but was not altered by any other lipid. PS enhancement of [3H]GABA uptake required the presence of sodium and was blocked by nipecotic acid (10 microM). These results suggest that PS may play a role in the sodium-dependent GABA reuptake process in the presynaptic nerve end.

Neurochemical studies of the mesolimbic dopaminergic pathway: glycinergic mechanisms and glycinergic-dopaminergic interactions in the rat ventral tegmentum

The rat ventral tegmentum (containing somata and dendrites of mesolimbic dopaminergic neurones) contained 1.3 mumol/g wet weight of glycine. Slices of ventral tegmentum accumulated exogenous [3H]glycine by an energy-, temperature- and sodium-dependent mechanism. The uptake was medicated by two different transport systems; one system with relatively low affinity for glycine (Km approximately 400 microM) and the other a higher affinity for glycine (Km approximately 10 microM). Small amino acid analogues of glycine inhibited the uptake process, the most potent being taurine and beta-alanine (47% and 44% inhibition, respectively, at 1 mM). Release of exogenous [3H]glycine by elevated potassium and by protoveratrine A was calcium-dependent and tetrodotoxin-sensitive. Glycine (500 microM--2mM) potentiated the protoveratrine A-induced release of exogenous [3H]dopamine from slices of ventral tegmentum; this potentiation was blocked by atrychnine (10 microM). A convulsant dose of strychnine elevated the concentration of 3,4-dihydroxyphenylacetic acid in the ventral tegmentum. Glycine is likely to be a transmitter in the ventral tegmentum and to have a role regulating the activity of somatodendritic regions of mesolimbic dopaminergic neurones.

Does glial uptake affect GABA responses? AN intracellular study on rat dorsal root ganglion neurones in vitro

1. Using single barrel pipettes, intracellular records were obtained from surface neurones of isolated rat dorsal root ganglia (DRG) impaled under microscopic vision.2. Responses to gamma-aminobutyric acid (GABA) were elicited either by ionophoresis or by placing drops of concentrated GABA solutions directly into the flow of superfusing Ringer. Using this latter method it was estimated that the GABA concentration eliciting threshold ( approximately 1 mV) responses was 3-20 muM.3. Short (</= 1 sec) ionophoretic or drop administrations of GABA elicited depolarizing responses associated with an increased membrane conductance. With longer applications the initial depolarization was not sustained but decayed to a lower plateau level (desensitization) associated with a minimal conductance change.4. Low chloride superfusions did not affect subsequent responses to GABA unless GABA was also administered during the low chloride superfusion, in which case responses declined markedly. This suggests that GABA caused appreciable chloride fluxes when it was administered regularly (e.g. for 1 sec every minute).5. Glial GABA uptake was inhibited by adding 1 mM-beta-alanine or 0.25 mM-chlorpromazine to the bicarbonate-Ringer superfusate or by substituting lithium for sodium in a Tris-Ringer superfusate. Uptake inhibition had no consistent effect on any of the parameters studied, namely membrane potential, input resistance, amplitude and time course of responses to GABA, and GABA desensitization.6. Muscimol and isoguvacine, which are probably not substrates for the glial GABA carrier, elicited responses with time course and desensitization characteristics indistinguishable from those of responses to GABA.7. GABA superfused at concentrations as low as 1 muM could reduce responses to ionophoretic GABA, i.e. cause a desensitization of GABA receptors.8. It is concluded firstly that in DRG, glial uptake does not affect the amplitude or time course of responses to GABA when the neurone under study is close to the source of GABA; and secondly that desensitization can occur independently of GABA uptake.9. The findings are discussed in relation to their possible relevance to GABA systems in the central nervous system.

Influence of cellular transport on the interaction of amino acids with gamma-aminobutyric acid (GABA)-receptors in the isolated olfactory cortex of the guinea-pig

1 Freshly cut guniea-pig olfactory cortex slices contained 2.2 mmol gamma-aminobutyric acid (GABA)/kg tissue weight. This declined during in vitro incubation at 25 degrees C in the absence of exogenous GABA, but increased to 6.95 mmol/kg after 1.5 h incubation in 1 mM GABA. 2 Uptake of [3H]-GABA (1 microM) was inhibited by 1 mM (+/-)-nipecotic acid (-83%), beta-amino-n-butyric acid (BABA) (-59%), L-2,4-diaminobutyric acid (DABA) (-63%), (+/-)cis-3-aminocyclohexane carboxylic acid (ACHC) (-53%), and 3-aminopropanesulphonic acid (3-APS) (-26%), but was increased by beta-alanine (BALA) (+23%). 3 Autoradiographs showed steep concentration gradients of radioactivity across slices incubated for short periods in [3H]-GABA. 4 Efflux of [3H]-GABA from pre-loaded slices was accelerated strongly by nipecotic acid, BABA, DABA and ACHC but weakly or not all by BALA or 3-APS. 5 Nipecotic acid (1 mM) potentiated the surface-depolarization of the slice produced by GABA but not that produced by 3-APS. 6 The depolarizing actions of DABA, BABA, nipecotic acid and ACHC, but not that of 3-APS or BALA, were potentiated when the endogenous GABA content of slices was raised. 7 It is concluded that: (a) the depolarizing action of exogenous GABA is limited by cellular uptake; (b) surface-depolarizations produced by nipecotic acid, DABA, BABA and ACHC may be mediated by the release of GABA; and (c) neuronal, rather than glial, transport systems are responsible for these effects.

gamma-Aminobutyric acid efflux from sympathetic glial cells: effect of 'depolarizing' agents

1. Isolated desheathed rat superior cervical ganglia were incubated in [3H]2,3,-gamma-aminobutyric acid ([3H]GABA) solution (1--10 microM for 2--3 hr) in the presence of 10 microM-amino-oxyacetic acid (AOAA). The subsequent efflux of tritium into a stream of superfused non-radioactive GABA-free Krebs solution at 25 degrees C was measured. 2. In the presence of 10 micrometer-AOAA the mean basal efflux rate coefficient (k0) for exit of tritium into the superfusion fluid was 0.7 x 10(-3) min-1. More than 98% of effluent tritium comprised unchanged [3H]GABA. The rate coefficient showed no correlation with the amount of [3H]GABA previously accumulated by the ganglion. 3. Elevation of [K+]o to greater than 50 mM increased the rate coefficient for [3H]GABA release by up to four times. Changes in efflux rate were not correlated with osmotic changes, and persisted after re-accumulation of effluent [3H]GABA by the inward carrier was inhibited. The effect of alkali metal cations diminished in the order Rb+ greater than K+ greater than Cs+Li+. Effects of K+ solutions were not reduced by omitting Ca2+ ions, with or without the addition of Mg2+. 4. Application of electrical pulses (0.1--1 msec duration, 1--10 Hz, 4 min trains) to the ganglion soma or to the preganglionic nerve trunk also raised k0. This effect declined with repeated stimulus trains, without an accompanying diminution in the response to K+. Responses to electrical stimulation were not reduced by amethocaine (300 microM), tetrodotoxin (3 microM) or raised [Mg2+i1 (0 mM-[Ca2+]/30 mM-[Mg3+]). Separate local superfusion of the pre- and post-ganglionic nerve trunks and of the ganglion soma showed that the response to electrical stimulation was localized to the vicinity of the stimulus and was not propagated along the nerve trunks or across the synapses. 5. Electrical recording from impaled 'inexcitable' cells (presumed to be neuroglial cells (Appendix)) indicated that the quantities of K+ ion accumulating during repetitive nerve stimulation are insufficient to stimulate the release of GABA from the glial cells. No physiological role for the release process in modulating neuronal excitability could be adduced.

[3H]gamma-Aminobutyric acid uptake into neuroglial cells of rat superior cervical sympathetic ganglia

1. The influx of [3H]gamma-aminobutyric acid ([3H]GABA) into isolated rat superior cervical ganglia has been measured by radioassay, supplemented by autoradiography. Ganglia were incubated in oxygenated Krebs solution at 25 degrees C, containing 10 microM-amino-oxyacetic acid. Under these conditions more than 95% of accumulated tritium was unmetabolized [3H]GABA. 2. Ganglionic radioactivity increased linearly with incubation time, to yield an intracellular fluid/extracellular fluid concentration ratio (Ci/Co) of about 200 after 6 hr in 0.5 microM-external [3H]GABA. 3. Uptake showed saturation with an apparent transport constant (KT) of 6.8 microM and maximum influx velocity (Jmaxi) of 7 mumole 1. cell fluid-1- min-1. 4. The influx rate at Co = 0.5 microM was unaltered by raising intracellular GABA from 0.2 to 1 mM. 5. Influx velocity increased with temperature (5--35 degrees C) in a monotonic manner with an apparent activation energy of 14 kcal mole-1. 6. Concentrative uptake was depressed by reducing external [Na+] with ouabain, by raising [K+]o above 20 mM, or by removing external Cl-. Uptake was not particularly sensitive to Ca2+ or Mg2+ ions. 7. Utake of [3H]GABA (0.5 microM) was inhibited by beta-guanidinopropionic acid (apparent KI, 28 microM), beta-alanine (KI, 55 microM), gamma-amino-beta-hydroxybutyric acid (KI, 220 microM), beta-amino-n-butyric acid (KI, 708 microM), 3-aminopropanesulphonic acid (KI, 832 microM) and taurine (KI greater than 1 mM). Uptake was not depressed by 1 mM-glycine, alpha-alanine, leucine, serine, methionine or alpha-amino-iso-butyric acid. 8. Radioactively labelled methionine, leucine, glycine, serine, beta-alanine and taurine (concentrations less than or equal to 5 microM) were also taken up by ganglia. Of these, only uptake of beta-alanine and taurine were significantly depressed by 1 mM-GABA. 9. Autoradiographs confirmed that [3H]GABA and [3H] beta-alanine were taken up predominantly into extraneuronal sites (presumed to be neuroglial cells). Methionine, leucine, glycine and serine showed preferential accumulation in neurones. Neuronal uptake of leucine was not prevented by inhibiting protein synthesis. 10. Calculations of net fluxes from unidirectional tracer fluxes suggest that the sympathetic glial cells are capable of promoting net uptake of GABA at external concentrations above 1 microM.

Potentiation of the effects of GABA by pentobarbitone

Pentobarbitone (50 microM) has been shown to potentiate GABA-induced responses in rat isolated superior cervical ganglia and in dorsal or ventral root fibres of immature rat isolated spinal cords. Measurements of dose ratios for the antagonism of GABA-induced responses of dorsal root fibres by bicuculline showed that pentobarbitone (50 microM) did not significantly alter this antagonism. This result indicates that the antagonism of GABA by bucuculline at dorsal root fibres is not reversed by pentobarbitone. Ventral root responses to GABA (but not glycine) were also potentiated by pentobarbitone (+/-)-Nipecotic acid (300 microM) potentiated responses to GABA much more than those to 3-aminopropane-sulphonic acid.

Neuron-glia interactions: indirect effect of GABA on cultured glial cells

A study was made of the action of GABA on the membrane potentail and resistance of satellite glial (SG) cells in cultures of rat dorsal root ganglia. GABA (10(-4) M) depolarized all SG cells tested without producing significant changes in membrane resistance. Similar results were obtained from astrocytes of cultured rat spinal cord and brain stem, although only half of the cells tested were depolarized by GABA. Bicuculline (10(-5) and 10(-6) M) which blocked the GABA-depolarization on cultured dorsal root ganglion (DRG) neurons, also markedly reduced or blocked the action of GABA on SG cells. When GABA was tested in sodium-free bathing solution, the amino acid caused a depolarization of similar shape and amplitude as in normal (137 mM Na+) bathing fluid, indicating that uptake processes are probably not involved in producing the depolarization by GABA. It is suggested that the depolarizing action of GABA on glial cells is an indirect effect due to the release of potassium from adjacent neurons during the action of the amino acid.

Isoguvacine, isonipecotic acid, muscimol and N-methyl isoguvacine on the GABA receptor in rat sympathetic ganglia

The GABA-mimetic activities of 4 analogues muscimol, isonipecotic acid, isoguvacine and N-methyl isoguvacine have been examined at the GABA receptor in the rat isolated superior cervical ganglion. The depolarizing action of all 4 analogues could be selectively antagonized by bicuculline methochloride and isopropyl bicyclophosphate. Muscimol was the only analogue more potent than GABA (molar potency ratio = 5.08 +/- 0.707). The potency of isoguvacine was 0.23 +/- 0.026 and isonipecotic acid 0.011 +/- 0.0028. N-methyl isoguvacine was less than 0.001 GABA.

A study of the interactions between glutamate and aspartate at the lobster neuromuscular junction

The depolarization produced by bath-applied or iontophoretically applied glutamate and aspartate were recorded from lobster muscle fibres by means of intracellular microelectrodes. 2 Bath-applied glutamate or aspartate evoked reversible, membrane depolarizations; however, responses to repeated applications of aspartate decreased progressively in amplitude until a plateau level was attained. Repeated applications of glutamate, kainate, domoate or quisqualate did not produce a similar effect. 3 After a dose of glutamate, responses to bath-applied aspartate were enhanced. Responses to other depolarizing agonists were little affected by previous administration of glutamate. Aspartate dose-depolarization curves were therefore constructed after initial aspartate responses had stabilized. The log-log transforms of the aspartate and glutamate curves had limiting slopes of 0.8 and 2.1 respectively. 4 Iontophoretic application of aspartate to single glutamate-sensitive sites produced small depolarizations with slow time course, compared with the glutamate potentials. When aspartate and glutamate were pulsed simultaneously from a twin-barrelled pipette, the resultant glutamate potential was enhanced. It is suggested that this potentiation was due to summation of agonist concentrations in the receptor region interacting with a second-order dose-response relationship. 5 Bath-applied aspartate increased the amplitude and prolonged the half-decay time of the glutamate potential. This effect was particularly noticeable when the glutamate potential was of slow time course. 6 It is proposed that bath-applied aspartate has an agonist effect whose magnitude is possibly exaggerated by concomitant release of glutamate and/or inhibition by glutamate of aspartate uptake. This agonist action of aspartate is thought to be exerted mainly on extrajunctional areas of the glutamate-sensitive sites.