Effects Of Γ-aminobutyric Acid Research Articles

Chloride channels activated by γ-aminobutyric acid in normal bovine lactotrophs

The effect of γ-aminobutyric acid (GABA) on normal bovine lactotrophs was investigated using the patch clamp technique. GABA application (1–10 μM) induced fluctuations of membrane current, accompanied by an increase in inward current. The mean conductance of GABA-activated channels was estimated to be 19 pS from statistical analysis of these fluctuations whereas the mean conductance for GABA-activated single ion channels was 16 pS. The reversal potential of the current was near to the equilibrium potential for chloride ions. These results indicate that the one possible mechanism for inhibition of prolactin release induced by GABA is probably mediated by activation of chloride channels.

GABA enhances acetylcholine release from hippocampal nerve endings through a mechanism blocked by a GABA uptake inhibitor

The effect of γ-aminobutyric acid (GABA) on the release of [ 3H]acetylcholine ([ 3H]ACh) was investigated using superfused rat hippocampal synaptosomes. GABA enhanced the basal efflux of [ 3H]ACh. The effect of GABA was bicuculline-insensitive. Muscimol, (±)-baclofen or (−)-baclofen did not increase [ 3H]ACh release. The effect of GABA was counteracted by SK&F 89976 A (N-(4,4-diphenyl-3-butenyl)-nipecotic acid), a GABA uptake inhibitor. One possible interpretation of the results is that a GABA transport system is present on cholinergic terminals, suggesting that GABA and ACh may coexist in some rat hippocampus nerve endings. Another possibility is that the effect of GABA is mediated by a novel subtype of GABA receptor sensitive to SK&F 89976 A.

GABA-induced hypothermia in rats: Involvement of serotonergic and cholinergic mechanisms

1. 1. The effects of γ-aminobutyric acid (GABA) on body temperature of restrained rats has been studied. 2. 2. GABA (250–1000 mg/kg i.p.) caused a dose-dependent fall in BT of restrained rats at an ambient temperature of 18–22°C. 3. 3. The GABA-induced hypothermic response was attenuated by pretreatment with hexamethonium, p-chlorophenylalanine, methysergide, neostigmine and atropine (% MPE values: 27, 35, 51, 64 and 72 respectively). 4. 4. Pretreatment with methysergide and atropine was more potent than hexanmethonium and methysergide in inhibiting the GABA-induced hypothermia (% MPE = 68 and 47 respectively). 5. 5. The antagonism by neostigmine of GABA-induced hypothermia was attenuated by pretreatment with hexamethonium (7.5 mg/kg). 6. 6. Yohimbine and chlorimipramine potentiated GABA hypothermia (% MPE = −82 and −8 respectively). 7. 7. The data indicate that GABA-induced hypothermia may be mediated by serotonin and acetylcholine release. Muscarinic receptors may play an important role in the effect of GABA. The results support the hypothesis that the hypothermia induced by GABA is modulated by nicotinic receptors.

GABA B receptors in the rabbit uterus may mediate contractile responses

The effects of γ-aminobutyric acid (GABA) and related compounds on the contractility of isolated rabbit uterus were examined. GABA and baclofen (10 −6−10 −4 M) stimulated the spontaneous motility in a dose-dependent manner and showed cross-desensitization. The effect of baclofen was stereoselective for the (−)-enantiomer. Muscimol was ineffective, while bicuculline evoked marked contractions. Contractions elicited by submaximal electrical stimulation could be further increased by GABA and baclofen. The effects of GABA and baclofen on spontaneous contractility could not be antagonized by atropine, phentolamine, or tetrodotoxin. These findings indicate: (1) the presence of GABA B receptors in the rabbit uterus; (2) their involvement in the modulation of uterine contractility;; (3) the extraneuronal location of uterine GABA B receptors which are thus most probably on smooth muscle cells; and (4) a possible role of inhibitory GABA B receptors in the modulation of spontaneous movements of the uterus.

The action of picrotoxin on the interspike interval in rat supraoptic neurones

Effects of γ-aminobutyric acid (GABA) antagonists on cells of the supraoptic nucleus have been investigated in isolated slices of rat hypothalamus. Bicuculline had no effect on firing patterns or the minimum interspike interval. Picrotoxin reduced the minimum interspike interval and caused long-lasting bursts of high-frequency spikes, an effect that persisted even when synaptic transmission was blocked. We suggest, that in this case, picrotoxin is not acting primarily as a GABA antagonist but as a blocker of chloride channels, thereby altering the cell properties that regulate the minimum interval.

GABA mimetics decrease extracellular concentrations of 5-HIAA (as measured by in vivo voltammetry) in the dorsal raphe of the rat

The effect of γ-aminobutyric acid (GABA) mimetics on extracellular concentrations of 5-hydroxyindoleacetic acid (5-HIAA) (as measured by differential pulse voltammetry with carbon fiber electrodes) in the dorsal raphe´has been investigated in the rat. Systemic administration of dipropylacetamide decreased extracellular 5-HIAA to a similar extent, and within a comparable time course, in the dorsal raphe´and striatum. Similar results were obtained after intradorsal raphe´infusion of muscimol (100 ng). In contrast, local infusion of tetrodotoxin into the dorsal raphe´failed to alter serotonin metabolism in this area. It is concluded that GABA depresses serotonin metabolism not only in nerve endings, but also in dendrites (and/or cell bodies) of serotonergic neurons.

γ-Aminobutyric acid-induced depression of calcium currents of chick sensory neurons

The effects of γ-aminobutyric acid (GABA) on calcium currents were investigated in avian dorsal root ganglion (DRG) cells. GABA was applied to the vicinity of the cells by ejection pipettes using constant-pressure pulses. GABA concentrations between 5 and 100 μM reduced and slowed the calcium current in a dose-dependent manner. A contribution of K and Cl outward currents to the reduction of the inward current was minimized by using identical caesium chloride concentrations on both sides of the membrane. The onset of the effect was rapid and 80% of the effect was observed within 1 s. The attenuation of the Ca slope conductance by GABA was found to be independent of the membrane potential between −50 and +50 mV.

Effect of local GABA administration on rat ovarian blood flow, and on progesterone and estradiol secretion

The effect of γ-aminobutyric acid (GABA) — superfused onto the surface of the ovary — on ovarian blood flow and on estradiol-17-β (E 2) and progesterone (P) secretion was studied in anaesthesized, pseudopregnant rats. GABA, 0.5 μmol per 50 μl per animal, significantly reduced the blood pressure in the femoral artery, increased ovarian blood flow, enhanced the rate of E 2 release and markedly decreased P secretion. The present findings indicate that local GABAergic mechanisms may be involved in the regulation of ovarian blood flow and hormone secretion.

Effect of GABA and its antagonists, bicuculline and picrotoxin, on nerve cell discharges of the photosensory pineal organ of the frog, Rana esculenta

The effect of γ-aminobutyric acid (GABA) and its antagonists, bicuculline and picrotoxin, was studied on pineal neurons of the frog, Rana esculenta. The drugs were applied by microiontophoresis while monitoring the spontaneous activity and light-evoked responses of electrophysiologically identified achromatic (luminance) neurons of the pineal organ. Almost all neurons investigated were sensitive to GABA. The inhibitory action was characterized by its rapid onset and its reversibility. The GABA antagonists, bicuculline and picrotoxin, were able to antagonize the inhibitory action of the amino acid. The light-evoked inhibition of the maintained ganglion cell activity interfered with the GABA-induced inhibition, i.e. light reduced the strength of inhibition and shortened the effect of GABA. The investigation suggests a major role of GABAergic mechanisms in the ganglion cell output of pineal neurons.

γ-aminobutyric acid decreases levels of messenger ribonucleic acid encoding prolactin in the rat pituitary

The effect of γ-aminobutyric acid (GABA) on levels of messenger ribonucleic acid (mRNA) encoding prolactin (PRL) was studied in cultured anterior pituitary cells, in vitro and in intact rats, in vivo. As quantitated by hybridization to a 32P-labeled rat PRL complementary deoxyribonucleic acid (cDNA) probe, levels of PRL mRNA in cultured pituitary cells were decreased by about 50% following 3 days exposure to 10 −5 M GABA. This effect was mimicked by muscimol (10 −6 M) and antagonized by bicuculline (10 −5 M). An increase of endogenous GABA levels in vivo effected by injection of GABA transaminase blockers (aminooxyacetic acid, 20 mg/kg, twice daily; vinyl GABA, 800 mg/kg) into rats resulted in a similar decrease in rat PRL mRNA levels in the adenohypophysis 3–4 days following commencement of the drug treatment. These findings suggest that GABA might inhibit PRL gene expression by a direct action on lactotrophs of the adenohypophysis.

GABA receptors mediate cerebral vasodilation in the unanesthetized goat

The effects of γ-aminobutyric acid (GABA) and muscimol upon cerebral blood flow were evaluated in the unanesthetized goat. Cerebral blood flow was continuously measured by means of an electromagnetic flow probe chronically implanted on the internal maxillary artery after occlusion and thrombosis of the distal extracerebral vessels. Administration of GABA (1–100 μg) directly into the cerebral circulation produced dose-dependent increases in cerebral blood flow, without accompanying systemic effects. Muscimol mimicked the effects of GABA at doses 10 times lower. Administration of picrotoxin (1–3 mg) into the internal maxillary artery did not significantly change cerebral blood flow, but inhibited in a dose-dependent manner the vasodilation induced by GABA. Selective blockade of β-adrenergic or muscarinic cholinergic receptors by propranolol or atropine, respectively, did not modify the cerebrovascular response to the GABAergic agonists. These results indicate that GABA increases total cerebral blood flow, acting on specific receptor sites in the cerebral blood vessels. The absence of influence of picrotoxin on resting cerebral blood flow suggests that the GABAergic receptors are not tonically activated under physiological conditions.

Effect of γ-aminobutyric acid on cyclic nucleotides content of guinea-pig cerebral cortex slices

The effect of GABA and related drugs on 3'5' cAMP and 3'5' cGMP was investigated in slices of guinea-pig cerebral cortex, kept at rest or electrically stimulated. GABA 1 X 10(-4) - 1 X 10(-3) M raised the cAMP basal levels, bud reduced its increase due to electrical stimulation. These effects were antagonized by picrotoxin 1.6 X 10(-5) M. On the contrary, endogenous GABA did not influence cAMP and cGMP content. In fact, neither ethanolamine-O-sulphate 2 X 10(-3) M nor picrotoxin 1.6 X 10(-5) M affected the normal nucleotide values at all. Higher picrotoxin concentrations (3.2 - 8 X 10(-5) M), however, increased both cyclic nucleotides. Phentolamine counteracted the actions of both GABA 1 X 10(-4) M and picrotoxin 8 X 10(-5) M: therefore, the observed increase in cyclic nucleotides were due to norepinephrine release. Tetrodotoxin antagonized GABA but not picrotoxin metabolic effects. Thus it is suggested that the exogenous amino acid released norepinephrine through a sodium-dependent process, while picrotoxin released the monoamine directly from its storage sites. Clearly, attention must be paid when putative transmitters or their antagonists are added at high concentrations to isolated tissue to study the effect of endogenous compounds.

GABA B receptor-mediated stimulation of the contractility of isolated rabbit oviduct

The effects of γ-aminobutyric acid (GABA) and related compounds were examined on longitudinal and circular muscle preparations isolated from oviducts of virgin rabbits. GABA and baclofen stimulated the spontaneous motility in each type of preparation, which action could not be antagonized by bicuculline, phentolamine, atropine or tetrodotoxin. Muscimol was virtually ineffective. Our results indicate the presence of GABA B receptors in the rabbit oviductal musculature which mediate the contractile response.

RECENT ADVANCES IN BENZODIAZEPINE PHARMACOLOGY

A summary has been presented of recent developments in the field of benzodiazepine pharmacology. Details were presented of the interactions between two types of compounds which antagonise the effects of benzodiazepines, the β-carbolines and imidazodiazepine Ro 15-1788. The β-carbolines produce pharmacological effects which are opposite to those of the benzodiazepines, as they lower seizure threshold, have anxiogenic activity and decrease the effects of γ-aminobutyric acid (GABA). Ro 15-1788 prevents the effects of the benzodiazepines at doses which have no overt pharmacological effects, although there is evidence that it has weak partial agonist activity. It has been found that Ro 15-1788 also prevents the effects of the β-carbolines, both in vivo and in vitro and this discovery led to the suggestion that the benzodiazepine receptor may be unusual in that ligands of different types can produce opposite pharmacological effects. The support for this theory is discussed, as is also the possible involvement of benzodiazepine receptor activity in the effects of chronic treatment with benzodiazepines, barbiturates and ethanol.

The effects of γ-aminobutyric acid on gastrin and somatostatin release from isolated rat antral mucosa: R.F. Harty, P.A. Franklin, M.M. Wolfe and J.E. McGuigan. University of Florida College of Medicine, Gainesville, Florida USA

The effects of γ-aminobutyric acid on gastrin and somatostatin release from isolated rat antral mucosa: R.F. Harty, P.A. Franklin, M.M. Wolfe and J.E. McGuigan. University of Florida College of Medicine, Gainesville, Florida USA

Studies on the relation of γ-hydroxybutyric acid (GHB) to γ-aminobutyric acid (GABA): Evidence that GABA is not the sole source for GHB in rat brain

The effects of γ-aminobutyric acid (GABA)-α-oxoglutarate aminotransferase (GABA-T) inhibitors, l-glutamic acid decarboxylase (GAD) inhibitors, and antipetit mal anticonvulsants on γ-hydroxybutyric acid (GHB) and GABA were studied. Treatment with anticonvulsants and GABA-T inhibitors resulted in an increase in steady-state brain levels of both GHB and GABA. GAD inhibitors produced markedly decreased levels of brain GABA but no change in GHB concentrations. Studies of GHB derived exclusively from GABA showed that GABA-T inhibitors which produced an elevation of steady-state levels of GHB in brain also resulted in a decrease in GABA-derived GHB. Intracere-broventricular (i.c.v.) administration of GABA, putrescine, and 1,4-butanediol all produced significant elevations in brain GHB, but GABA-T inhibitors blocked this effect of GABA and putrescine. These data suggest that there may be another source for GHB in brain in addition to GABA and raise the possibility that 1,4-butanediol may be that source.

Angiotensin excites hippocampal pyramidal cells by two mechanisms

1. The mechanism of action of angiotensins was studied on CA 1 pyramidal cells in hippocampal slices of the rat. Extracellular field potentials, single-cell action potentials, and intracellular excitatory and inhibitory postsynaptic potentials (epsps and ipsps) were recorded. 2. Angiotensin II and Des-Asp1-angiotensin II added to the perfusion fluid caused a dose-dependent increase in extracellularly recorded epsps and synaptically evoked population spikes. Either the neurons were depolarized by angiotensins and their firing frequency of action potentials increased or the membrane potential was unaffected. 3. Local application of angiotensins caused a depolarization associated with a conductance increase which was resistant to synaptic isolation. 4. Evoked and spontaneous inhibitory postsynaptic potentials were reduced by angiotensins, but the effects ofγ-aminobutyric acid (GABA) on soma and dendrites were unchanged. 5. All angiotensin effects were blocked by the specific antagonist Sar1, Ala8-angiotensin II (saralasin). 6. It is concluded that angiotensins excite CA 1 pyramidal cells by a direct and a disinhibitory mechanism.

Comparison of the effects of GABA and chloride ion on the affinities of ligands for the benzodiazepine receptor

The effects of γ-aminobutyric acid (GABA), Cl − ion, and their combination were examined on the affinity for the benzodiazepine (BDZ) receptor of various ligands including BDZ's, putative endogenous ligands, and CL218872. When the binding was performed at 0°, the ligands could be classified into four types according to the action of the modulators. The affinity of the first type including some BDZ's increased with GABA, Cl − ion, or their combination while that of the second type consisting of 3-hydroxy-benzodiazepines, CL218872, and inosine increased only with GABA. In the third type, designated as the β-carboline type, only Cl − ion increased the affinity of the ligands. The fourth inactive type included pentylenetetrazole and nicotinamide and the affinities of these ligands were not affected by GABA or Cl − ion. At 37°, although GABA still retained the property to increase the affinity of prototype compounds of the first and second types, the effect of Cl − ion became somewhat ambiguous. Comparison of the present results with published data on the pharmacological and electrophysiological activities of the ligands suggested that the compounds with affinity increased by GABA act as BDZ-mimetics, while those belonging to types 3 and 4 act as antagonists. Also, our results seem to throw some additional light upon the relationships among the benzodiazepine receptor, the GABA receptor, and the chloride ionophore.

Multiple benzodiazepine receptors and their regulation by γ-aminobutyric

The interaction of propyl β-carboline-3-carboxylate (PCC) with benzodiazepine receptors in the cerebral cortex of the rat was investigated by direct measurements of [ 3H]PCC binding and by competitive inhibition of [ 3H]flunitrazepam (FLU) binding. Initial experiments showed that [ 3H]PCC binding exhibited characteristics of saturability, stereospecificity and a pharmacological specificity remarkably similar to that of [ 3H]FLU binding. Analysis of [ 3H]PCC binding isotherms and PCC/[ 3H]PCC competition curves revealed the presence of a small population of super high affinity PCC binding sites (K SH = 30–100 pM) which represents approximately 3–6% of the total sites. When measured by competitive inhibition of [ 3H]FLU binding, receptor occupancy by PCC was generally consistent with that determined by direct measurements of [ 3H]PCC binding. Analysis of the PCC/[ 3H]FLU competition curve revealed the presence of two major populations of high and low affinity PCC binding sites with dissociation constants of 0.54 and 10 nM and relative abundances of 52 and 45%, respectively. Collectively, the results of the [ 3H]PCC binding isotherm, PCC/[ 3H]PCC competition curve and PCC/[ 3H]FLU competition curve are internally consistent when rationalized in terms of three populations of benzodiazepine receptors - super high, high, and low affinity - each having different affinities for PCC and equal affinity for FLU. The effects of γ-aminobutyric acid (GABA) on PCC and FLU binding were investigated, and it was observed that GABA enhanced the binding of FLU to the various receptor subtypes whereas no significant effect of GABA on the binding of PCC was detected.

Effect of gamma-aminobutyric acid on growth hormone and prolactin secretion in man: influence of pimozide and domperidone.

To investigate the possibility that GH release induced by gamma-aminobutyric acid (GABA) in man is mediated by a dopaminergic mechanism(s), we evaluated the effect of two antidopaminergic compounds, pimozide and domperidone, on the plasma GH response to acute GABA administration (5 g, orally). Only the former compound can freely cross the blood-brain barrier. In 9 normal volunteers, GABA caused a significant increase of plasma GH levels (P < 0.0001 vs. GH levels in a group of 14 controls). In these subjects, pimozide (6 mg/day, orally, for 4 days) significantly blunted the GH elevation induced by GABA (P < 0.01). Unlike pimozide, in 8 additional subjects, domperidone (4 mg injected iv immediately before GABA administration) did not influence the GABA-induced GH response. GABA did not alter either baseline or pimozide-stimulated plasma PRL levels. Likewise, it did not significantly modify the brisk PRL rise after domperidone injection. These findings are consistent with the hypothesis that GABA-stimulated GH secretion is mediated via dopamine release at a suprapituitary level. With regard to PRL secretion, no GABA-dopamine interactions are readily apparent.