Addition Of Gamma-aminobutyric Acid Research Articles

Mechanism of neuronal activity and synaptic transmission in rostral ventrolateral medulla

Rostral ventrolateral medulla (RVLM) plays an essential role in blood pressure homeostasis. This study was aimed to investigate the mechanism of neuronal activity and synaptic transmission in the RVLM. Medulla oblongata slices were carefully obtained from brainstem of rats. With continues perfusion of artificial cerebrospinal fluid (ACSF), the spontaneous firing of slices and amplitudes were assayed by conventional whole cell patch-clamp recording after addition of gamma-aminobutyric acid (GABA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl d-aspartate (NMDA). Furthermore, the effects of agonist or antagonist targeted Type-A GABA (GABAA) or glutamate receptors on spontaneous excitatory postsynaptic potential (sEPSP) and spontaneous inhibitory postsynaptic potential (sIPSP) of neurons in RVLM were determined. The spontaneous firing of neurons in RVLM were inhibited by GABA (P<0.001) while were promoted by NMDA or AMPA (P<0.01 or P<0.001). In terms of sEPSP and sIPSP, the numbers of firing neurons in RVLM were both improved by GABAA receptor antagonist (P<0.01 or P<0.001) while were both decreased by GABAA receptor agonist or glutamate receptor antagonist (P<0.05, P<0.01 or P<0.001). The corresponding effects of agonist and antagonist on amplitudes were the same as the effects on number of firing neurons in RVLM. The spontaneous firing, sEPSP and sIPSP of neurons in RVLM were all activated by GABAA receptor antagonist while were all suppressed by GABAA receptor agonist or glutamate receptor antagonist.

Effect of Propofol on Carotid Body Chemosensitivity and Cholinergic Chemotransduction

Propofol decreases the acute hypoxic ventilatory response in humans and depresses in vivo carotid body chemosensitivity. The mechanisms behind this impaired oxygen sensing and signaling are not understood. Cholinergic transmission is involved in oxygen signaling, and because general anesthetics such as propofol have affinity to neuronal nicotinic acetylcholine receptors, the authors hypothesized that propofol depresses carotid body chemosensitivity and cholinergic signaling. An isolated rabbit carotid body preparation was used. Chemoreceptor activity was recorded from the whole carotid sinus nerve. The effect of propofol on carotid body chemosensitivity was tested at three different degrees of PO2 reduction. Nicotine-induced chemoreceptor response was evaluated using bolus doses of nicotine given before and after propofol 10-500 microM. The contribution of the gamma-aminobutyric acid A receptor complex was tested by addition of gamma-aminobutyric acid A receptor antagonists. Propofol reduced carotid body chemosensitivity; the magnitude of depression was dependent on the reduction in PO2. Furthermore, propofol caused a concentration-dependent (10-500 microM) depression of nicotine-induced chemoreceptor response, with a 50% inhibitory concentration (propofol) of 40 microM. Bicuculline in combination with propofol did not have any additional effect, whereas addition of picrotoxin gave a slightly more pronounced inhibition. It is concluded that propofol impairs carotid body chemosensitivity, the magnitude of depression being dependent on the severity of PO2 reduction, and that propofol causes a concentration-dependent block of cholinergic chemotransduction via the carotid sinus nerve, whereas it seems unlikely that an activation of the gamma-aminobutyric acid A receptor complex is involved in this interaction.

Cytotoxic action of lindane in neocortical GABAergic neurons is primarily mediated by interaction with flunitrazepam-sensitive GABAA receptors

The cytotoxic action of the gamma-isomer of hexachlorocyclohexane (y-HCH; lindane) was studied in cultured mouse neocortical neurons by measurements of the reduction in mitochondrial function using the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) test. The cells were exposed to 30-300 microM lindane in the culture medium for different periods of time and lindane cytotoxicity was found to be time- and concentration-dependent. Lindane cytotoxicity could be ameliorated by addition of gamma aminobutyric acid (GABA) in a concentration-dependent manner but this effect of GABA was not blocked by bicuculline or picrotoxinin (PTX). Lindane induced cytotoxicity was also reduced by the GABA(A) receptor agonists muscimol and THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol). This effect was enhanced by the simultaneous presence of flunitrazepam but only at the highest lindane concentrations studied (200 and 300 microM). Flunitrazepam by itself had no effect on lindane-induced cytotoxicity. The protective effect of GABA plus flunitrazepam was blocked by the benzodiazepine receptor antagonist flumazenil and by the GABA(A) antagonist bicuculline, suggesting the involvement of central benzodiazepine receptors allosterically coupled to the GABA recognition site at the GABA(A) receptor. When 100 microM PTX was used to suppress the protective effect of GABA and flunitrazepam, a significant effect of PTX was observed only at 300 microM lindane. The GABA(B) receptor agonist, baclophen, only marginally reduced the cytotoxic effect induced by the highest lindane concentrations. It is concluded that the cytotoxic action of lindane in neocortical neurons in culture is mediated primarily through an interaction with allosterically coupled GABA-benzodiazepine recognition sites at the GABA(A) receptor.

Cholinergic Mediation of γ-Aminobutyric Acid-Induced Gastrin and Somatostatin Release From Rat Antrum

Addition of gamma-aminobutyric acid (GABA) to antral mucosal fragments in short-term incubation results in dose-dependent and bicuculline-sensitive stimulation of gastrin release and inhibition of somatostatin release, respectively. These effects of GABA on antral gastrin and somatostatin release closely resembled the actions of cholinergic agonists on G- and D-cell function. The present study examines the possibility that the effects of GABA on antral peptide release may be mediated, in part, through stimulation of antral cholinergic neurons. Inclusion of either atropine or pirenzepine in incubation medium prevented GABA-induced stimulation of gastrin release and inhibition of somatostatin release. Addition of the acetylcholinesterase inhibitor, physostigmine, caused a leftward shift in the GABA dose-response curve and increased by 10-fold the sensitivity of the antral preparation to GABA stimulation. Studies with tetrodotoxin suggest that GABA-stimulated gastrin release is mediated through activation of neurons contained within the antral mucosal/submucosal fragments. Hexamethonium, the ganglionic nicotinic receptor antagonist, did not affect GABA-induced gastrin release. These results indicate that GABA affects antral gastrin and somatostatin release through stimulation of antral postganglionic cholinergic neurons.

Excitatory transmitter release induced by high concentrations of gamma-aminobutyric acid (GABA) in crayfish neuromuscular junctions.

At the neuromuscular junction of very small crayfish (0.4-2 g) addition of gamma-aminobutyric acid (GABA) to the superfusing solution at concentrations exceeding 100 mmol/l elicited high frequency release of excitatory transmitter quanta. In seven experiments single application of 500 mmol/l GABA gave rise to instantaneous release of 70,000 to 130,000 quanta. These stores of transmitter were released by GABA in a first order process with time constants, tau q, of between 9 s and 20 s, the maximum rate of release, ñ0, reaching 10,000 quanta/s in some cases. After release had ceased in the presence of GABA, the preparation was allowed to recover for five minutes in normal solution. Subsequently, a second trial evoked about 50% of the release induced during the first application of GABA. Pretreatment of the preparation with 2 mumol/l serotonin (5-HT) facilitated GABA-induced transmitter release resulting in larger rates of release and consequently in a larger output of transmitter by a factor of about 3. The largest amount of transmitter released on a single application of GABA in the presence of serotonin comprised about 220,000 quanta with a maximum rate of release ñ0 approximately equal to 25,000 quanta/s. The release evoked by high GABA-concentrations did not depend markedly on extracellular Ca2+ or Mg2+, but required extracellular Na+. The effects induced by high concentrations of GABA on release of excitatory transmitter quanta were quantitatively similar to the effects of high glycine-concentrations on release of quanta from the inhibitory terminals (Finger 1983a, b).

Photoaffinity labelling of the benzodiazepine receptor compromises the recognition site but not its effector mechanism.

When subjected to ultraviolet light flunitrazepam could be irreversibly attached to brain membrane preparations such that the affinity of the benzodiazepine receptor for certain ligands, such as diazepam, becomes markedly reduced. However, the apparent affinity for diazepam is increased by the addition of gamma-aminobutyric acid and sodium chloride, to the same extent in this membrane preparation as in membranes not so pretreated. This observation suggests that photoaffinity labelling of the benzodiazepine receptor with flunitrazepam modifies the recognition characteristics of the receptor but not its effector mechanism.

Electrophysiological investigation of GABA-mediated inhibition at the hermit crab neuromuscular junction.

1. The inhibitory neuromuscular junction of the abductor muscle of the large claw of the hermit crab (Eupagurus bernhardus) was investigated using electrophysiological intracellular techniques in order to elucidate further the relative contributions of the pre- and post-synaptic mechanisms of action of GABA and of neural inhibition.2. The electrical constants of the post-synaptic membrane, calculated using the equations for a ;short cable' model, were characteristic of a poorly developed electrical excitability; the specific membrane resistance was usually < 1000 Omega cm(2) and the specific membrane capacitance was > 40 muF/cm(2).3. Stimulation of the excitatory axon to the abductor muscle of the large claw at a frequency of 20 Hz evoked highly facilitating excitatory junction potentials (e.j.p.s); stimulation of the inhibitory axon (60-220 Hz) during the excitatory train elicited inhibition which was manifest as an attenuation of the e.j.p.s.4. The addition of gamma-aminobutyric acid (GABA) to the bathing solution produced a dose-dependent reduction of e.j.p. amplitude and membrane resistance. The inhibitory effect of concentrations (5 x 10(-5) and 1 x 10(-4)M) which caused a 40-75% e.j.p. attenuation could largely be accounted for by a post-synaptic action on membrane conductance.5. Experiments with picrotoxin suggest that presynaptic inhibitory mechanisms have an important role in neurally evoked inhibition.6. Picrotoxin (1-5 x 10(-5)M) effectively blocked neural inhibition and the actions of GABA in this preparation, whereas bicuculline proved to be considerably less potent and therefore less useful as a physiological tool for studying GABA-mediated inhibition in crustacea.

Anion interaction at the inhibitory post-synaptic membrane of the crayfish neuromuscular junction.

1. The membrane potential and the membrane conductance of the crayfish muscle fibre in solutions containing various anions were measured with intracellular micro-electrodes.2. When Cl(-) in the solution was replaced by Br(-), NO(3) (-), I(-) or CNS(-), the addition of gamma-amino butyric acid (GABA) produced a transient hyperpolarization of the membrane.3. The reversal potential of the inhibitory junctional potentials (i.j.p.s) was at a slightly depolarized level relative to the resting potential in the normal Cl(-) solution. When Cl(-) in the bathing solution was replaced by foreign anions, the reversal potential shifted towards the hyperpolarized level. The hyperpolarization was in the order CNS(-) > I(-) > NO(3) (-) > Br(-).4. When a part of Cl(-) in the bathing solution was substituted by Br(-), the inhibitory membrane conductance activated by GABA was increased as the concentration of Br(-) increased. The inhibitory membrane conductance decreased when one quarter or a half of Cl(-) was replaced by CNS(-), NO(3) (-) or I(-), but it increased again in higher concentrations of these anions.5. Ca(2+), Mg(2+) and Ba(2+) showed no appreciable effect on the inhibitory membrane conductance activated by GABA, while they decreased the conductance of the resting muscle membrane.6. It was suggested that, at the activated inhibitory membrane, there is an interaction between anions and the permeability of Cl(-) is decreased by the presence of foreign anions, such as CNS(-), I(-) and NO(3) (-). The present results support the idea that the activated inhibitory membrane is charged positively and anions penetrate the membrane interacting with the charge sites.