Bicuculline-insensitive GABA receptors on peripheral autonomic nerve terminals

Somatostatin-expressing interneurons modulate neocortical network through GABAb receptors in a synapse-specific manner

The firing activity of somatostatin-expressing inhibitory neurons (SST-INs) can suppress network activity via both GABAa and GABAb receptors (Rs). Although SST-INs do not receive GABAaR input from other SST-INs, it is possible that SST-IN-released GABA could suppress the activity of SST-INs themselves via GABAbRs, providing a negative feedback loop. Here we characterized the influence of GABAbR modulation on SST-IN activity in layer 2/3 of the somatosensory cortex in mice. We compared this to the effects of GABAbR activation on parvalbumin-expressing interneurons (PV-INs). Using in vitro whole-cell patch clamp recordings, pharmacological and optogenetic manipulations, we found that the firing activity of SST-INs suppresses excitatory drive to themselves via presynaptic GABAbRs. Postsynaptic GABAbRs did not influence SST-IN spontaneous activity or intrinsic excitability. Although GABAbRs at pre- and postsynaptic inputs to PV-INs are modestly activated during cortical network activity in vitro, the spontaneous firing of SST-INs was not the source of GABA driving this GABAbR activation. Thus, SST-IN firing regulates excitatory synaptic strength through presynaptic GABAbRs at connections between pyramidal neurons (Pyr-Pyr) and synapses between pyramidal neurons and SST-INs (Pyr-SST), but not Pyr-PV and PV-Pyr synapses. Our study indicates that two main types of neocortical inhibitory interneurons are differentially modulated by SST-IN-mediated GABA release.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

Antihypertensive effect of giant embryo brown rice and pre-germinated giant embryo brown rice on spontaneously hypertensive rats

"Shangshida NO.5" is a giant embryo mutant resulting from giant embryo gene (GE) dysfunction in "Chao2-10" rice. Here, we compared the antihypertensive effects of "Chao2-10" brown rice (C2-10), "Shangshida NO.5" brown rice (GER), and pre-germinated "Shangshida NO.5" brown rice (PGER) in spontaneously hypertensive rats (SHR). Male SHR at 6 weeks of age were divided into four groups and were fed with (a) a control diet (control), (b) a 40% C2-10-supplemented diet (C2-10), (c) a 40% GER-supplemented diet (GER), or (d) a 40% PGER-supplemented diet (PGER) for 8 weeks, and their physiological and biochemical parameters were measured. The results showed that the C2-10-, GER-, and PGER-supplemented diets significantly decreased systolic blood pressure (SBP) and diastolic blood pressure (DBP) during the experiment. At the end of the experimental period, the SBP and DBP of the C2-10, GER, and PGER groups were 7.6, 23.3, and 31.1 mmHg and 9.8, 21.1, and 29 mmHg lower than those in the control group, respectively, suggesting the GER and PGER diets were better able to inhibit blood pressure elevation than the C2-10 diet. The serum creatinine levels in the C2-10, GER, and PGER groups and the blood urea nitrogen content in the PGER group were significantly lower than those of the control group, indicating that C2-10-, GER-, and especially PGER-supplemented diets improved renal function. In addition, the antioxidant activity of the C2-10 group and especially of the GER and PGER groups also improved. The above results suggest that "Shangshida NO.5" rice, particularly pre-germinated rice, is a good dietary supplement for preventing the development of hypertension.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Bypassing Glutamic Acid Decarboxylase 1 (Gad1) Induced Craniofacial Defects with a Photoactivatable Translation Blocker Morpholino

γ-Amino butyric acid (GABA) mediated signaling is critical in the central and enteric nervous systems, pancreas, lungs, and other tissues. It is associated with many neurological disorders and craniofacial development. Glutamic acid decarboxylase (GAD) synthesizes GABA from glutamate, and knockdown of the gad1 gene results in craniofacial defects that are lethal in zebrafish. To bypass this and enable observation of the neurological defects resulting from knocking down gad1 expression, a photoactivatable morpholino oligonucleotide (MO) against gad1 was prepared by cyclization with a photocleavable linker rendering the MO inactive. The cyclized MO was stable in the dark and toward degradative enzymes and was completely linearized upon brief exposure to 405 nm light. In the course of investigating the function of the ccMOs in zebrafish, we discovered that zebrafish possess paralogous gad1 genes, gad1a and gad1b. A gad1b MO injected at the 1–4 cell stage caused severe morphological defects in head development, which could be bypassed, enabling the fish to develop normally, if the fish were injected with a photoactivatable, cyclized gad1b MO and grown in the dark. At 1 day post fertilization (dpf), light activation of the gad1b MO followed by observation at 3 and 7 dpf led to increased and abnormal electrophysiological brain activity compared to wild type animals. The photocleavable linker can be used to cyclize and inactivate any MO, and represents a general strategy to parse the function of developmentally important genes in a spatiotemporal manner.

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

The Gamma-Aminobutyric Acid B Receptor in Depression and Reward

The metabotropic gamma-aminobutyric acid B (GABA_B) receptor was the first described obligate G protein-coupled receptor heterodimer and continues to set the stage for discoveries in G protein-coupled receptor signaling complexity. In this review, dedicated to the life and work of Athina Markou, we explore the role of GABA_B receptors in depression, reward, and the convergence of these domains in anhedonia, a shared symptom of major depressive disorder and withdrawal from drugs of abuse. GABA_B receptor expression and function are enhanced by antidepressants and reduced in animal models of depression. Generally, GABA_B receptor antagonists are antidepressant-like and agonists are pro-depressive. Exceptions to this rule likely reflect the differential influence of GABA_B1 isoforms in depression-related behavior and neurobiology, including the anhedonic effects of social stress. A wealth of data implicate GABA_B receptors in the rewarding effects of drugs of abuse. We focus on nicotine as an example. GABA_B receptor activation attenuates, and deactivation enhances, nicotine reward and associated neurobiological changes. In nicotine withdrawal, however, GABA_B receptor agonists, antagonists, and positive allosteric modulators enhance anhedonia, perhaps owing to differential effects of GABA_B1 isoforms on the dopaminergic system. Nicotine cue-induced reinstatement is more reliably attenuated by GABA_B receptor activation. Separation of desirable and undesirable side effects of agonists is achievable with positive allosteric modulators, which are poised to enter clinical studies for drug abuse. GABA_B1 isoforms are key to understanding the neurobiology of anhedonia, whereas allosteric modulators may offer a mechanism for targeting specific brain regions and processes associated with reward and depression.

The influence of midazolam on heart rate arises from cardiac autonomic tones alterations in Burmese pythons, Python molurus

The GABA_A receptor agonist midazolam is a compound widely used as a tranquilizer and sedative in mammals and reptiles. It is already known that this benzodiazepine produces small to intermediate heart rate (HR) alterations in mammals, however, its influence on reptiles' HR remains unexplored. Thus, the present study sought to verify the effects of midazolam on HR and cardiac modulation in the snake Python molurus. To do so, the snakes' HR, cardiac autonomic tones, and HR variability were evaluated during four different experimental stages. The first stage consisted on the data acquisition of animals under untreated conditions, in which were then administered atropine (2.5mgkg^-1; intraperitoneal), followed later by propranolol (3.5mgkg^-1; intraperitoneal) (cardiac double autonomic blockade). The second stage focused on the data acquisition of animals under midazolam effect (1.0mgkg^-1; intramuscular), which passed through the same autonomic blockade protocol of the first stage. The third and fourth stages consisted of the same protocol of stages one and two, respectively, with the exception that atropine and propranolol injections were reversed. By comparing the HR of animals that received midazolam (second and fourth stages) with those that did not (first and third stages), it could be observed that this benzodiazepine reduced the snakes' HR by ~60%. The calculated autonomic tones showed that such cardiac depression was elicited by an ~80% decrease in cardiac adrenergic tone and an ~620% increase in cardiac cholinergic tone - a finding that was further supported by the results of HR variability analysis.

Differential surface density and modulatory effects of presynaptic GABAB receptors in hippocampal cholecystokinin and parvalbumin basket cells

The perisomatic domain of cortical neurons is under the control of two major GABAergic inhibitory interneuron types: regular-spiking cholecystokinin (CCK) basket cells (BCs) and fast-spiking parvalbumin (PV) BCs. CCK and PV BCs are different not only in their intrinsic physiological, anatomical and molecular characteristics, but also in their presynaptic modulation of their synaptic output. Most GABAergic terminals are known to contain GABA_B receptors (GABA_BR), but their role in presynaptic inhibition and surface expression have not been comparatively characterized in the two BC types. To address this, we performed whole-cell recordings from CCK and PV BCs and postsynaptic pyramidal cells (PCs), as well as freeze-fracture replica-based quantitative immunogold electron microscopy of their synapses in the rat hippocampal CA1 area. Our results demonstrate that while both CCK and PV BCs contain functional presynaptic GABA_BRs, their modulatory effects and relative abundance are markedly different at these two synapses: GABA release is dramatically inhibited by the agonist baclofen at CCK BC synapses, whereas a moderate reduction in inhibitory transmission is observed at PV BC synapses. Furthermore, GABA_BR activation has divergent effects on synaptic dynamics: paired-pulse depression (PPD) is enhanced at CCK BC synapses, but abolished at PV BC synapses. Consistent with the quantitative differences in presynaptic inhibition, virtually all CCK BC terminals were found to contain GABA_BRs at high densities, but only 40% of PV BC axon terminals contain GABA_BRs at detectable levels. These findings add to an increasing list of differences between these two interneuron types, with implications for their network functions.

γ-Aminobutyric acid (GABA) signalling in plants

The role of γ-aminobutyric acid (GABA) as a signal in animals has been documented for over 60 years. In contrast, evidence that GABA is a signal in plants has only emerged in the last 15 years, and it was not until last year that a mechanism by which this could occur was identified-a plant 'GABA receptor' that inhibits anion passage through the aluminium-activated malate transporter family of proteins (ALMTs). ALMTs are multigenic, expressed in different organs and present on different membranes. We propose GABA regulation of ALMT activity could function as a signal that modulates plant growth, development, and stress response. In this review, we compare and contrast the plant 'GABA receptor' with mammalian GABAA receptors in terms of their molecular identity, predicted topology, mode of action, and signalling roles. We also explore the implications of the discovery that GABA modulates anion flux in plants, its role in signal transduction for the regulation of plant physiology, and predict the possibility that there are other GABA interaction sites in the N termini of ALMT proteins through in silico evolutionary coupling analysis; we also explore the potential interactions between GABA and other signalling molecules.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Reflections on more than 30 years association with Hanns

I first met Hanns in 1977 and soon learnt of his extraordinary ability as a researcher. He became a friend as well as a mentor providing enthusiasm for my own research. I watched closely over the years how his research uncovered details of the association of the benzodiazepines and GABA and delineated the structural composition of the GABAA receptor associated with the action of individual drugs such as antianxiety and antiepileptic agents. His work produced many important contributions to medicine notable of which was the discovery of the first benzodiazepine antagonists, which are now routinely used in clinical practice. But for me his most important contribution was the discovery of the benzodiazepine receptor. During this time, my group uncovered a novel receptor for GABA and my progress in this work was encouraged and enhanced by discussions with Hanns.

Nerve regenerative effects of GABA-B ligands in a model of neuropathic pain

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the peripheral somatosensory system. It may be associated with allodynia and increased pain sensitivity. Few studies correlated neuropathic pain with nerve morphology and myelin proteins expression. Our aim was to test if neuropathic pain is related to nerve degeneration, speculating whether the modulation of peripheral GABA-B receptors may promote nerve regeneration and decrease neuropathic pain. We used the partial sciatic ligation- (PSL-) induced neuropathic model. The biochemical, morphological, and behavioural outcomes of sciatic nerve were analysed following GABA-B ligands treatments. Simultaneous 7-days coadministration of baclofen (10 mg/kg) and CGP56433 (3 mg/kg) alters tactile hypersensitivity. Concomitantly, specific changes of peripheral nerve morphology, nerve structure, and myelin proteins (P0 and PMP22) expression were observed. Nerve macrophage recruitment decreased and step coordination was improved. The PSL-induced changes in nociception correlate with altered nerve morphology and myelin protein expression. Peripheral synergic effects, via GABA-B receptor activation, promote nerve regeneration and likely ameliorate neuropathic pain.

Sympathetic single axonal discharge after spinal cord injury in humans: activity at rest and after bladder stimulation

STUDY DESIGN

Clinical experimental mechanistic study.

OBJECTIVES

(1) To determine in three spinal cord-injured patients whether individual muscle sympathetic nerve fibres below the level of the spinal lesion display spontaneous activity. (2) To determine in these patients if individual sympathetic vasoconstrictor fibres show a prolonged discharge following a bladder stimulus.

SETTING

University hospital in Gothenburg, Sweden.

METHODS

Microneurographic recordings of action potentials from individual muscle nerve sympathetic fibres in a peroneal nerve. Recordings of skin blood flow and electrodermal responses in a foot.

RESULTS

In all patients, there was sparse ongoing spontaneous impulse traffic in individual sympathetic fibres. Brisk mechanical pressure over the urinary bladder evoked a varying number of action potentials in individual fibres, but the activity was brief and did not continue after the end of the evoked multiunit burst.

CONCLUSION

Prolonged discharges in individual sympathetic fibres are unlikely to contribute to a long duration of blood pressure increases induced by brief bladder stimuli.

GABA heteroreceptors modulate noradrenaline release in human dental pulp

γ-aminobutyric-acid-containing neurons and GABA(B) receptors have been identified in human dental pulp; however, their significance in pulpal physiology is unclear. The purpose of this study was to determine whether pre-synaptic GABAergic heteroreceptors influence the release of noradrenaline (NA). Segments of vital pulp were incubated in [(3)H]NA (0.6 μM) and superfused with Krebs solution. GABA, a GABA(B) receptor agonist (baclofen), GABA(A and B) receptor antagonists [bicuculline and (+)-(S)-5, 5-dimethylmorpholinyl-2-acetic acid (Sch 50911), respectively], and a GABA(A) receptor-mediated Cl(-) channel inhibitor (picrotoxin) were added to the superfusion medium at least 10 min prior to the second period of stimulation (S2). Sympathetic nerves were stimulated electrically after 70 (S1) and 115 (S2) min. We determined the effects of agonists/antagonists by comparing the overflow of [(3)H]NA at S2 with that at S1 in the presence and absence of the compound. Baclofen (3 µM) inhibited the release of [(3)H]NA (IC50 = 2 µM), an action reversed by Sch 50911 (10 µM). GABA (100 µM) inhibited the release of [(3)H]NA (IC50 = 75 µM), an effect reversed by Sch 50911 (10 µM) but not by bicuculline (10 µM). However, picrotoxin (100 µM) prevented the inhibitory action of GABA. GABA(B) and GABA(A) heteroceptors mediate the release of NA from sympathetic nerves in human dental pulp in vitro.

Functional coupling of cerebral ?-aminobutyric acid (GABA)(B) receptor with adenylate cyclase system: effect of phaclofen

Using synaptic membrane from bovine cerebral cortex, effects of ?-aminobutyric acid (GABA), (?)-baclofen, and phaclofen on the cyclic AMP formation mediated by adenylate cyclase were studied. In addition, the binding affinity of phaclofen, a GABA(B) antagonist, to synaptic membrane was compared with those of GABA and (?)-baclofen. GABA and (?)-baclofen, GABA(B) receptor agonists, induced significant inhibitions on the basal and forskolin-stimulated adenylate cyclase activities. Treatment of synaptic membrane with the islet-activating protein, pertussis toxin, completely eliminated the inhibitory effects of GABA and (?)-baclofen on the forskolin-stimulated adenylate cyclase activity. In solubilized fraction of synaptic membrane, the forskolin-stimulated adenylate cyclase was no longer affected by the additions of GABA and (?)-baclofen. Phaclofen displaced 50% of the bound [(3)H](?)-baclofen from synaptic membrane at the concentration of 10(?3) M, and also completely abolished inhibitory effects of GABA and (?)-baclofen on the forskolin-stimulated adenylate cyclase activity. These results suggest that GABA(B) receptor in synaptic membrane of the bovine cerebral cortex may be functionally coupled with adenylate cyclase system via Ni and/or No proteins. The present results also suggest that phaclofen may have selective affinity to the same binding sites as those of GABA(B) receptor agonists such as (?)-baclofen, and induce a suppressive effect on GABA(B) receptor mediated inhibition of adenylate cyclase.

Schwann-like adult stem cells derived from bone marrow and adipose tissue express γ-aminobutyric acid type B receptors

γ-Aminobutyric acid type B receptors (GABA-B) are expressed in glial cells of the central and peripheral nervous systems, and recent evidence has shown their importance in modulating physiological parameters of Schwann cell (SC). SC play essential roles in peripheral nerve regeneration, but several drawbacks prevent their use for nerve repair. Adult stem cells from adipose tissue (ASC) or bone marrow (BM-MSC) can be differentiated into an SC-like phenotype and used as SC replacements. The aim of this study was to investigate GABA-B receptor functional expression in differentiated stem cells by assessing the similarity to SC. By means of RT-PCR and Western blot methodologies, BM-MSC and ASC were found to express both GABA-B1 and GABA-B2 receptor subunits. The expression levels of GABA-B1b and GABA-B2 receptors were influenced by SC-like differentiation, as shown by Western blot studies. GABA-B receptor stimulation with baclofen reduced the proliferation rate of SC and differentiated ASC (dASC) but not that of dBM-MSC. In conclusion, both of the subunits that assemble into a functional GABA-B receptor are present in differentiated stem cells. Furthermore, GABA-B receptors in dASC are functionally active, regulating a key process such as proliferation. The presence of functional GABA-B receptors on differentiated stem cells opens new opportunities for a possible pharmacological modulation of their physiology and phenotype.

GABA and neuroactive steroid interactions in glia: new roles for old players?

In recent years it has becoming clear that glial cells of the central and peripheral nervous system play a crucial role from the earliest stages of development throughout adult life. Glial cells are important for neuronal plasticity, axonal conduction and synaptic transmission. In this respect, glial cells are able to produce, uptake and metabolize many factors that are essential for neuronal physiology, including classic neurotransmitters and neuroactive steroids. In particular, neuroactive steroids, which are mainly synthesized by glial cells, are able to modulate some neurotransmitter receptors affecting both glia and neurons. Among the signaling systems that are specialized for neuron-glial communication, we can include neurotransmitter GABA.The main focus of this review is to illustrate the cross-talk between neurons and glial cells in terms of GABA neurotransmission and actions of neuroactive steroids. To this purpose, we will review the presence of the different GABA receptors in the glial cells of the central and peripheral nervous system. Then, we will discuss their modulation by some neuroactive steroids.

Antihypertensive effect of a gamma-aminobutyric acid rich tomato cultivar 'DG03-9' in spontaneously hypertensive rats

This study aimed to investigate the effects of a gamma-aminobutyric acid (GABA) rich tomato (Solanum lycopersicum L.) cultivar 'DG03-9' in comparison with 'Momotaro', a commonly consumed tomato cultivar in Japan, on systolic blood pressure (SBP) in spontaneously hypertensive rats (SHR). In a single administration study, treatment with the GABA-rich cultivar elicited a significant decrease in SBP compared to the control group. In a chronic administration study, SHR were fed diets containing one of the tomato cultivars for 4 weeks. Both cultivars significantly reduced the increase in SBP compared to the control. The antihypertensive effect of the GABA-rich cultivar was higher than that of the commonly consumed cultivar in both the single- and chronic-administration studies. Treatment with a comparable amount of GABA elicited a similar response to treatment with the GABA-rich cultivar. These results suggest that the GABA-rich cultivar 'DG03-9' is a potent antihypertensive food and may be useful for treating hypertension effectively.

Involvement of chloride channel coupled GABA(C) receptors in the peripheral antinociceptive effect induced by GABA(C) receptor agonist cis-4-aminocrotonic acid

We investigated the effect of chloride and potassium channel blockers on the antinociception induced by GABA(C) receptor agonist CACA (cis-4-aminocrotonic acid) using the paw pressure test, in which pain sensitivity was increased by an intraplantar injection (2 microg) of prostaglandin E(2) (PGE(2)). CACA administered locally into the right hindpaw (25, 50 and 100 microg/paw) elicited a dose-dependent antinociceptive effect which was demonstrated to be local, since only higher doses produced an effect when injected in the contralateral paw. The GABA(C) receptor antagonist (1,2,5,6 tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA; 5, 10 and 20 microg/paw) antagonized, in a dose-dependent manner, the peripheral antinociception induced by CACA (100 microg), suggesting a specific effect. This effect was reversed by the chloride channel coupled receptor blocker picrotoxin (0.8 microg/paw). Glibenclamide (160 microg) and tolbutamide (320 microg), blockers of ATP-sensitive potassium channels, charybdotoxin (2 microg), a large-conductance potassium channel blocker, dequalinium (50 microg), a small-conductance potassium channel blocker, and cesium (500 microg), a non-specific potassium channel blocker did not modify the peripheral antinociception induced by CACA. This study provides evidence that activation of GABA(C) receptors in the periphery induces antinociception, that this effect results from the activation of chloride channel coupled GABA(C) receptors and that potassium channels appear not to be involved.

GABA(B) receptors: structure and function

In the basal ganglia the effects of gamma-aminobutyrate (GABA) are mediated by both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. Although the existence and widespread distribution in the CNS of the GABA(B) receptor had been established in the 1980s the field of GABA(B) research was revolutionized with the discovery that two related G-protein-coupled receptors (GPCRs) needed to dimerize to form the functional GABA(B) receptor at the cell surface. This finding lead to a number of studies of oligomerization in GPCRs and detailed pharmacological studies of the cloned receptors and their splice variants. Particular interest has focused on the proteins interacting with the receptor which may be important in mediating the longer term signalling effects of the receptor and modifying its cellular localization or physiology. The cloning of the GABA(B) receptors also lead to the identification of the first compounds interacting in an allosteric fashion with the receptor some of which may have therapeutic value. Most recently "knockouts" of both the GABA(B) subunits have been produced where in general as expected there is a loss of the majority of the inhibitory effects of the GABA(B) receptor.

Baclofen, an agonist at peripheral GABAB receptors, induces antinociception via activation of TEA-sensitive potassium channels

BACKGROUND AND PURPOSE

Central anti-nociceptive actions of baclofen involve activation of K+ channels. Here we assessed what types of K+ channel might participate in the peripheral anti-nociception induced by baclofen.

EXPERIMENTAL APPROACH

Nociceptive thresholds to mechanical stimulation in rat paws treated with intraplantar prostaglandin E2.(PGE2) to induce hyperalgesia were measured 3 h after PGE2 injection. Other agents were also given by intraplantar injection.

KEY RESULTS

Baclofen elicited a dose-dependent (15 - 240 microg per paw) anti-nociceptive effect. An intermediate dose of baclofen (60 microg) did not produce antinociception in the contralateral paw, showing its peripheral site of action. The GABAB receptor antagonist saclofen (12.5 - 100 microg per paw) antagonized, in a dose-dependent manner, peripheral antinociception induced by baclofen (60 microg), suggesting a specific effect. This antinociceptive action of baclofen was unaffected by bicuculline, GABAA receptor antagonist (80 microg per paw), or by (1,2,5,6 tetrahydropyridin-4-yl) methylphosphinic acid, GABAC receptor antagonist (20 microg per paw). The peripheral antinociception induced by baclofen (60 microg) was reversed, in a dose-dependent manner, by the voltage-dependent K+ channel blockers tetraethylammonium (7.5 - 30 microg per paw) and 4-aminopyridine (2.5 - 10 microg per paw). The blockers of other K+ channels, glibenclamide (160 microg), tolbutamide (320 microg), charybdotoxin (2 microg), dequalinium (50 microg) and caesium (500 microg) had no effect.

CONCLUSIONS AND IMPLICATIONS

This study provides evidence that the peripheral antinociceptive effect of the GABAB receptor agonist baclofen results from the activation of tetraethylammonium-sensitive K+ channels. Other K+ channels appear not to be involved.

GABA receptor-mediated effects in the peripheral nervous system: A cross-interaction with neuroactive steroids

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult mammalian central nervous system (CNS), exerts its action via an interaction with specific receptors (e.g., GABAA and GABAB). These receptors are expressed not only in neurons but also on glial cells of the CNS, which might represent a target for the allosteric action of neuroactive steroids. Herein, we have demonstrated first that in the peripheral nervous system (PNS), the sciatic nerve and myelin-producing Schwann cells express both GABAA and GABAB receptors. Specific ligands, muscimol and baclofen, respectively, control Schwann-cell proliferation and expression of some specific myelin proteins (i.e., glycoprotein P0 and peripheral myelin protein 22 [PMP22]). Moreover, the progesterone (P) metabolite allopregnanolone, acting via the GABAA receptor, can influence PMP22 synthesis. In addition, we demonstrate that P, dihydroprogesterone, and allopregnanolone influence the expression of GABAB subunits in Schwann cells. The results suggest, at least in the myelinating cells of the PNS, a cross-interaction within the GABAergic receptor system, via GABAA and GABAB receptors and neuroactive steroids.

Synthesis and Biological Activity of Allosteric Modulators of GABAB Receptors, Part 2. 3-(2,6-Bis-tert-butyl-4-hydroxyphenyl)propanols

A series of 13 2,2-disubstituted 3-(3,5-di-t-butyl-4-hydroxyphenyl)propan-1-ol derivatives have been prepared for evaluation as allosteric modulators of GABAB receptors. The activity (EC50, 4–7 μM) was greatest for the cyclohexyl and cyclopentyl analogues.

Synthesis and Biological Activity of Allosteric Modulators of GABAB Receptors, Part 1. N-(Phenylpropyl)-1-arylethylamines

A series of 15 analogues of fendiline, and 34 derivatives of N-(3-phenylpropyl)-1-arylethylamine have been prepared for evaluation as positive allosteric modulators of GABAB receptors. The most active (EC50, 10 nM) was N-(3,3-diphenylpropyl)-1-(3-chloro-4-methoxyphenyl)ethylamine 6g.

GABAB receptors on central terminals of C-afferents mediate intersegmental Adelta-afferent evoked hypoalgesia

The current study tested the hypothesis that repetitive activation of sciatic Adelta-afferents evokes a saphenous C-afferent hypoalgesia mediated by pre-synaptic GABA(B) receptors. Tonic activation of sciatic Adelta-afferents was produced by cutaneous application of dimethyl sulfoxide (DMSO) followed by repetitive thermal activation of Adelta-afferents on the dorsolateral hind paw. The tonic activation of sciatic Adelta-afferents produced hypoalgesia in saphenous C-afferents. Intrathecal administration of the GABA(B) receptor antagonist, saclofen, attenuated saphenous hypoalgesia demonstrating at least partial mediation by central GABA(B) receptors. To determine if this central GABA(B) receptor activation occurs at pre-synaptic primary afferent terminals or postsynaptic spinal cord neurons, the dorsal hind paws of mice were infected with a recombinant herpes simplex virus type 1 (HSV-1) designed to selectively knock down expression of the GABA(B1a) receptor subunit (PAGB1a) in primary afferents or a control virus encoding the E. coli lacZ gene (PZ). Four weeks after infection, GABA(B) receptor immunoreactivity in the superficial dorsal horns ipsilateral to PAGB1a application was reduced and hypoalgesia in saphenous C-afferents was attenuated when compared to PZ-infected mice. These findings indicate an intersegmental, sciatic Adelta-afferent-evoked hypoalgesic effect on saphenous C-afferent responses that is mediated by pre-synaptic GABA(B) receptors on the terminals of those C-afferents.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

GABAB receptor agonists and antagonists: pharmacological properties and therapeutic possibilities

Gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, is widely distributed throughout the brain and spinal cord. Two major families of GABA receptors have been identified, GABAA and GABAB. While much is known about the pharmacological and molecular properties of GABAA receptors, it is only within the last few years that potent and selective antagonists have been developed for the GABAB site, and only within the past few months that this receptor has been cloned. Thus, tools are now available to define more fully the GABAB receptor in terms of its biology and the therapeutic potential of manipulating this site. Data suggest that, in addition to their established use as muscle relaxants, GABAB receptor agonists possess analgesic and antitussive properties, and may be useful for treating bladder dysfunction. While there is less clinical data on GABAB receptor antagonists, preclinical results indicate that they may be of value in treating absence epilepsy, cognitive dysfunction and, possibly, pulmonary and intestinal disorders. However, for this potential to be fully exploited, it is necessary to identify and characterise molecularly and pharmacologically distinct GABAB receptor subtypes.

GABA(B) receptor alterations as indicators of physiological and pharmacological function

Given the widespread distribution of GABA(B) receptors throughout the central nervous system, and within certain peripheral organs, it is likely their selective pharmacological manipulation could be of benefit in the treatment of a variety of disorders. Studies aimed at defining the clinical potential of GABA(B) receptor agonists and antagonists have included gene deletion experiments, examination of changes in receptor binding, subunit expression and function in diseased tissue, as well as after the chronic administration of drugs. The results indicate that a functional GABA(B) receptor requires the combination of GABA(B(1)) and GABA(B(2)) subunits, that receptor function does not always correlate with subunit expression and receptor binding, and that GABA(B) receptor modifications may be associated with the clinical response to antidepressants, mood stabilizers, and GABA(B) receptor agonists and antagonists. Moreover, changes in GABA(B) binding or expression suggest this receptor may be involved in mediating symptoms associated with chronic pain, epilepsy and schizophrenia. This, together with results from other types of studies, indicates the potential therapeutic value of developing drugs capable of selectively activating, inhibiting, or modulating GABA(B) receptor function.

Characterization of GABA(B) receptors involved in inhibition of motility associated with acetylcholine release in the dog small intestine: possible existence of a heterodimer of GABA(B1) and GABA(B2) subunits

Characterization of the gamma-aminobutyric acid (GABA)(B) receptor involved in the motility of dog small intestine was analyzed by application of the microdialysis method to the small intestine of the whole body of the dog. The reverse transcription-polymerase chain reaction (RT-PCR) was used. Intraarterial administration of muscimol induced acceleration of motility associated with acetylcholine (ACh) release, these responses being antagonized by bicuculline. Intraarterial administration of baclofen induced inhibition of motility associated with ACh release, these responses being antagonized by CGP62349. GABA induced inhibition of motility associated with decrease in ACh release. CGP62349 alone induced acceleration of motility associated with increase in ACh release. RT-PCR revealed the presence of mRNAs for both subunits of GABA(B) receptor, GABA(B1) and GABA(B2), in the dog small intestine, although GABA(B1) subunits were 6 isoforms of GABA(B1) (GABA(B1(a)) - GABA(B1(g))), except GABA(B1(d)). Thus, the GABA(B) receptor located at cholinergic neurons as a heterodimer with subunits of GABA(B1) and GABA(B2) in the dog small intestine operates predominantly relative to the GABA(A) receptor in physiological motility.

GABAB receptors in Schwann cells influence proliferation and myelin protein expression

The location and the role of gamma-aminobutyric acid type B (GABA(B)) receptors in the central nervous system have recently received considerable attention, whilst relatively little is known regarding the peripheral nervous system. In this regard, here we demonstrate for the first time that GABA(B) receptor isoforms [i.e. GABA(B(1)) and GABA(B(2))] are specifically localized in the rat Schwann cell population of the sciatic nerve. Using the selective GABA(B) agonist [i.e. (-)-baclofen] and the antagonists (i.e. CGP 62349, CGP 56999 A, CGP 55845 A), such receptors are shown to be functionally active and negatively coupled to the adenylate cyclase system. Furthermore, exposure of cultured Schwann cells to (-)-baclofen inhibits their proliferation and reduces the synthesis of specific myelin proteins (i.e. glycoprotein Po, peripheral myelin protein 22, myelin-associated glycoprotein, connexin 32), providing evidence for a physiological role of GABA(B) receptors in the glial cells of the peripheral nervous system.

GABA(A) receptors in the basolateral amygdala are involved in mediating morphine reward

In the present study, the effects of intra-basolateral amygdala (BLA) injection of GABA(A) receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) in male Wistar rats have been investigated. Subcutaneous (s.c.) administration of different doses of morphine sulfate (1-9 mg/kg) produced a dose-dependent CPP. Using a 3-day schedule of conditioning, it was found that the GABA(A) receptor agonist, muscimol (0.125, 0.25 and 0.5 microg/rat) or the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 microg/rat), did not produce a significant place preference or place aversion. Intra-BLA administration of muscimol (0.25 and 0.5 microg/rat) decreased the acquisition of CPP induced by morphine (6 mg/kg). On the other hand, intra-BLA injection of bicuculline (0.25 and 0.5 microg/rat) in combination with an ineffective dose of morphine (1 mg/kg) elicited a significant CPP. The response of different doses of muscimol was attenuated by bicuculline (0.125 and 0.25 microg/rat). Furthermore, intra-BLA administration of bicuculline but not muscimol before testing significantly decreased the expression of morphine (6 mg/kg)-induced place preference. The administration of the higher doses of bicuculline (0.25 and 0.5 microg/rat) during acquisition and the higher dose of muscimol (2 microg/rat) on the test day decreased the locomotor activity of the animals on the testing phase. It can be concluded that GABA(A) receptors in the amygdala are involved in morphine reward.

Involvement of GABA(B) receptors in presynaptic inhibition of fibers of the descending projections of the spinal cord in the frog Rana ridibunda

Isolated spinal cord preparations from Rana ridibunda frogs were used for studies of the effects of the GABA(B) receptor agonists (-)-baclofen (50 and 100 microM) and GABA (4-8 mM) and the specific GABA(B) receptor antagonist 2-hydroxysaclofen (100 microM) on the transmission of signals from fibers of the ventral columns monosynaptically connected with motoneurons in segments 9 and 10. These experiments showed that (-)-baclofen (50 and 100 microM) produced significant and dose-dependent suppression of excitatory postsynaptic potentials (EPSP) in motoneurons and ventral root potentials evoked by stimulation of fibers of the ipsi- and contralateral ventral columns. The inhibitory effect of (-)-baclofen (100 microM) on descending EPSP was 35-50% blocked by the GABA(B) receptor antagonist 2-hydroxysaclofen (0.2 mM). The inhibitory effect of GABA (4-8 mM) on descending EPSP was 60% blocked by the GABA(A) receptor antagonist picrotoxin (0.05 mM). (-)-Baclofen (50 microM) and GABA (4 and 6 mM) were found to have inhibitory effects on ventral root potentials evoked by stimulation of the ipsi- and contralateral ventral columns. The data obtained here, as well as data obtained by pharmacological analysis and conditioning by stimulation of the ipsi- and contralateral ventral columns, are regarded as a significant argument supporting the existence of GABA(B) receptor-mediated presynaptic inhibition of descending fibers connected monosynaptically to spinal cord motoneurons in the frog Rana ridibunda.

Baclofen analgesia: involvement of the GABAergic system

The effect of baclofen, a GABA(B) agonist, has been studied in the hot plate test in mice, to analyze the possible involvement of the GABAergic system in baclofen analgesia. Baclofen (1-3 mg kg(-1) intraperitoneal (i.p.)) was found to elicit a dose-dependent antinociceptive effect. The antinociceptive effect of baclofen cannot be due to motor incoordination or sedation as the doses of baclofen which produce analgesia did not induce these effects during the rota-rod test. The antinociceptive effect of baclofen was reversed by 2-hydroxysaclofen, a GABA(B) antagonist by both systemic (3 mg kg(-1)) and intra cisterna magna (intracisternal (i.c.)) (0.3 mg kg(-1)) administration. The antagonist dose administered via i.c. produced a complete blockade and was 10-fold lower than the dose employed in i.p. administration. The data suggest that the antinociceptive effect of baclofen is GABA(B) receptor-mediated and reveal a central location of the analgesic effect of baclofen.

Involvement of gamma-aminobutyric acid (GABA) B receptors in the hypotensive effect of systemically administered GABA in spontaneously hypertensive rats

We investigated the effects of intraduodenally (i.d.) administered gamma-aminobutyric acid (GABA) on blood pressure (BP) in anesthetized spontaneously hypertensive rats (SHR) and the mechanism underlying this effect, especially the type of GABA receptor involved in the depressive effect of this amino acid. GABA (0.3 to 300 mg/kg, i.d.) caused a dose-related decrease in the BP of 9.20 +/- 3.96 to 35.0 +/- 5.34 mmHg (mean +/- S.E.M.) that lasted for 30 to 50 min. The minimum effective i.d. dose of GABA was 0.3 to 1.0 mg/kg. Results pertaining to the mechanism underlying the GABA-induced effects on BP were as follows: a) GABA did not alter the BP-related effects of exogenous noradrenaline and acetylcholine; b) pretreatment with hexamethonium decreased the GABA-induced fall in BP, and GABA tended to reduce the pressor response associated with injection of dimethyl phenylpiperazinium; and c) pretreatment with 2-hydroxysaclofen markedly reduced the GABA-induced drop in BP, whereas pretreatment with bicuculline did not. In conclusion, in SHR, low-dose (0.3 to 1.0 mg/kg, i.d.) GABA had a hypotensive effect, which may result from attenuation of sympathetic transmission through the activation of GABA(B) receptors at presynaptic or ganglionic sites.

GABA(A) but not GABA(B) receptor stimulation induces antianxiety profile in rats

The effect of GABA receptor agonists and antagonists on anxiety behavior in rats in the elevated-plus-maze has been investigated. The increase in two parameters of %open arm entries (%OAE) and %time spent in the open arms (%OAT) and decrease in the %time spent in closed arm (%CAT) was considered as antianxiety effects. Intracerebroventricular (i.c.v.) injection of different doses of the GABA(A) receptor agonist muscimol (0.25, 0.5, and 1 microg/rat) increased %OAE and %OAT and decreased %CAT in rats dose-dependently. The higher response was obtained with 1 microg/rat of the drug. Neither icv (0.05, 0.1, and 0.2 microg/rat) nor intraperitoneal (i.p.) (1, 2, and 4 mg/kg) injection of the GABA(B) receptor agonist baclofen altered %OAE, %OAT, and %CAT. However, the GABA(B) receptor antagonist CGP35348 (5, 10, and 30 microg/rat i.c.v.) increased %OAE and %OAT and decreased %CAT in the animals. The response induced by injection of muscimol (0.5 microg/rat i.c.v.) or administration of CGP35348 (10 microg/rat i.c.v.) was reduced by i.c.v. (1, 2, and 4 microg/rat) or i.p. (0.25, 0.5, and 0.75 mg/kg) injection of the GABA(A) receptor antagonist bicuculline, except the effect of CGP35348 on %CAT which was not significantly altered by i.p. administration of bicuculline. Ip but not i.c.v. administration of bicuculline by itself reduced both %OAE and %OAT but did not alter %CAT. None of the drugs altered the locomotor activity of the animals. The current findings support our hypothesis that the anxiolytic effects of GABA(B) antagonist are mediated by autoreceptor blockade-induced release of endogenous GABA, which in turn activates postsynaptic GABA(A) receptors.