Inhibitory Effect Of GABA Research Articles

Gaba sensitivity of the somata and dendrites of pyramidal cells in isolated slices of rat hippocampus

During experiments on isolated slices slices of rat hippocampus the inhibitory action of γ-aminobutyric acid (GABA) was investigated on the excitation of field CA, pyramidal neurons, together with the effects of bicuculline, penicillin and thiopentone on this process. It was found that GABA effectively and reversibly reduced the amplitude of the antrodomic population spike in the area of both the somata and the dendrites of these cells. The sensitivity of apical dendrites to GABA exceeded that of the somata by one order, increasing in a proximal-distal direction. The somata of pyramidal neurons were marked by pronounced desensitization to GABA. Bicuculline and penicillin acted as GABA antagonists at all the levels of CA, pyramidal cells investigated. Bicuculline blocked the effects of GABA on somata and dendrites in almost equal measure. The antagonistic effects of penicillin were 10 times greater in the pyramidal layer than in the dendritic region. Thiopentone reinforced the inhibitory effects of GABA. The potentiating effects of thiopentone were exerted most strongly on the dendrites. It is postulated that the membrane of field CA, neurons contain two types of bicuculline-sensitive GABA receptors, differing in their location (mainly on the cell body or dendrite), their pharmacology, and degree of desensitization to GABA.

Inhibitory effect of GABA on phrenic discharge activity of rabbit through microinjection into nucleus tractus solitarii.

In 53 anesthetized, paralyzed and artificially ventilated rabbits with bilateral vagotomy and section of sinus nerves, the effects of GABA and related drugs on respiratory performances induced by microinjection of the drugs into nucleus tractus solitarii (NTS) were studied by recording phrenic nerve discharges. GABA significantly depressed respiratory output after injection, diazepam, a drug which potentiates GABAnergic transmission, produced the fame effect as GABA; GABA receptor antagonists, bicuculline methiodide and picrotoxin, augmented phrenic nerve discharges. Neither thyrotropin-releasing hormone nor methionine-enkephaline caused any change in the pattern of phrenic nerve discharges. The results suggest that central endogenous GABAnergic transmission system might be involved in inhibitory regulation of respiration in rabbits and that NTS is a possible acting site of GABAnergie system.

Inhibition of anaphylactic histamine releasein vitro by GABA

Inhibitory effect of GABA on anaphylactic histamine release in vitro is not mimicked by 2-aminoethansulphonic acid (taurine), an aminoacid unrelated to GABA neuro-transmission. Tetrodotoxin (TTX) 6 X 10(-7) M, a concentration known to block neuronal mechanism but not to modify muscle membrane and anaphylactic histamine release, strongly prevented the inhibition caused by GABA in the Schultz-Dale reaction and in anaphylactic histamine release. The inhibitory effect of GABA on anaphylactic reaction in vitro thus appears to be specific for this aminoacid and is neurogenic in nature, in that it requires integrity of neuronal mechanisms.

Calcium-activated potassium-currents and intracellular calcium-transients in bullfrog sympathetic neurons

Spontaneous activities from ventral and dorsal roots of the isolated perfused spinal cord of the bullfrog were inhibited by GABA. β-Alanine showed a strong and glycine a weaker inhibitory effect. The inhibitory effect of GABA was markedly reduced in a chloride-free medium, whereas glycine and β-alanine still showed an inhibitory effect similar to that seen in normal medium. Employing the sucrose-gap method, a marked depolarization in the dorsal root and a small but obvious hyperpolarization in the ventral root were found by the application of GABA. These results support the view that GABA is one of the transmitters involved in pre- and postsynaptic inhibition of the spinal cord. The postsynaptic hyperpolarizing effect of GABA on motoneurones would be caused directly through increased permeability of the membrane to chloride ion. The depolarizing effect of GABA on primary afferent terminals is discussed in connection with chloride dependency.

Membrane properties of the bullfrog dorsal root examined by a new single sucrose sucrose-GAP technique

Spontaneous activities from ventral and dorsal roots of the isolated perfused spinal cord of the bullfrog were inhibited by GABA. β-Alanine showed a strong and glycine a weaker inhibitory effect. The inhibitory effect of GABA was markedly reduced in a chloride-free medium, whereas glycine and β-alanine still showed an inhibitory effect similar to that seen in normal medium. Employing the sucrose-gap method, a marked depolarization in the dorsal root and a small but obvious hyperpolarization in the ventral root were found by the application of GABA. These results support the view that GABA is one of the transmitters involved in pre- and postsynaptic inhibition of the spinal cord. The postsynaptic hyperpolarizing effect of GABA on motoneurones would be caused directly through increased permeability of the membrane to chloride ion. The depolarizing effect of GABA on primary afferent terminals is discussed in connection with chloride dependency.

GABAB receptor mediated inhibition of field stimulation induced contractions of detrusor muscle strips from newborn rats.

The effect of GABA on field stimulation (0.1 Hz) induced contractions have been examined on detrusor strips from adult and newborn rats. Field stimulation induced contractions were unaffected by hexamethonium (10(-5) M) but almost suppressed by tetrodotoxin (5 X 10(-7) M) in both age groups. Atropine (3 X 10(-6) M) produced a marked inhibition of twitches in detrusor strips from newborn rats but only a slight (10% or less) inhibition in strips from adult animals. Neither GABA (10(-4), homotaurine (10(-3) M) or (+/-)-baclofen (10(-4] affected twitches in detrusor strips from adults rats. On the other hand both GABA and (+/-)-baclofen inhibited, to about the same extent (55-60% inhibition) twitches in detrusor strips from newborn animals. Homotaurine had no significant effect. The effects of both GABA (10(-4) M) and (+/-)-baclofen (10(-4) M) on twitches in detrusor strips from newborn rats were reduced if the preparations were previously exposed to a low concentration of GABA (5 X 10(-6) M) thus indicating desensitization. The inhibitory effect of GABA on twitches in newborn rats were concentration dependent in the range of 10(-7) to 10(-4) M. The GABAA receptor antagonist picrotoxin (10(-4) M) had no effect on the GABA concentration response curve whereas the GABAB receptor antagonist, homotaurine, produced an antagonism of the competitive type. The calculated pA2 value for the GABA-homotaurine interaction was 4.4 +/- 0.07. Neither GABA (10(-4) M) nor (+/-)-baclofen (10(-4) M) produced any significant inhibition of acetylcholine (2 X 10(-7) M) induced contractions of detrusor of strips from newborn rats, while atropine (3 X 10(-6) M) completely suppressed them. These results indicate that GABA inhibits field stimulation induced contractions of detrusor strips from newborn rats by activating prejunctional receptors (presumably of the GABAB subtype) which inhibit the release of acetylcholine from the postganglionic nerve endings.

Gamma-Aminobutyric acid and cholinergic transmission in the guinea-pig ileum.

1. The effects of GABA on release of acetylcholine and on contractility of the smooth muscle were studied in the myenteric plexus-longitudinal muscle preparation of the guinea pig. Acetylcholine was determined as radiolabelled transmitter from strips preloaded with 3H-choline. 2. GABA (1 - 300 microM) caused an increase in resting tension of smooth muscle as well as an increase in release of acetylcholine that was considerably reduced by tetrodotoxin. The facilitation by GABA of acetylcholine release exhibited a marked tachyphylaxis. The increase in muscle tension was clearly related to the increase in acetylcholine release. Muscimol (0.1 - 10 microM) also enhanced the release of acetylcholine. The effects of GABA on acetylcholine release and on longitudinal muscle contraction were competitively antagonized by bicuculline and picrotoxin. 3. GABA (1 - 300 microM) inhibited both release of acetylcholine and contractions evoked by electrical stimulation. Muscimol (1, 10 microM) did not modify the evoked release. The inhibitory effects of GABA were not antagonized by bicuculline or picrotoxin. 4. It is suggested that GABA facilitates and inhibits acetylcholine release, and in turn intestinal motility via two receptors with different pharmacological properties.

GABAergic control of anterior pituitary hormone secretion

Anatomical and biochemical studies have identified a hypothalamic tubero-infundibular GABAergic system, which plays a functional role on anterior pituitary hormone secretion. Experimental and clinical evidence support the presence of a dual component in the action of GABA; one mediated via the central nervous system and the other exerted directly at the anterior pituitary level. The two sites of action may be responsible for the excitatory and inhibitory effects of GABA on pituitary hormone and especially prolactin secretion. The future characterization of this system will provide a better understanding of the involvement of GABA in the physiology of anterior pituitary hormone secretion and will contribute to the developent of new pharmacological agents for the therapy of neuroendocrine disorders.

Inhibition of sympathetic preganglionic discharges by epinephrine and α-methylepinephrine

The present study was undertaken to determine the effect of iontophoretic applications of epinephrine (E) and its derivative α-methylepinephrine (mE) on the discharges of sympathetic preganglionic neurons (SPNs). Spontaneously active SPNs located in thoracic segment T2 were antidromically identified in White Carneaux pigeons anesthetized with urethane and immobilized with purified α-cobratoxin. All SPNs tested were inhibited by E, mE, several other catecholamines, clonidine, GABA, glycine and morphine. The inhibitory effects of E and mE but not those of GABA were antagonized by iontophoretic applications of the preferential α 2-antagonists piperoxane and yohimbine, but not by the α 1-antogonist praxosin or the β-antagonist sotalol when similarly applied. The inhibitory effects of GABA, glycine and morphine were respectively antagonized by bicuculline methiodide, strychnine and naloxone, but these antagonists failed to alter the action of E. It is concluded that (1) epinephrine and its α-methyl derivative inhibit the discharges of SPNs via the activation of α 2-receptors and(2) the epinephrine-induced inhibition does not result from the secondary release of GABA, glycine or opioid peptides from afferent terminals or interneurons.

Inhibitory effects of GABA, L-glutamic acid and nicotine on the potassium-evoked release of subsaance P in substantia nigra slices of the rat

Rat substantia nigra slices were superfused with a physiological medium containing a diluted substance P (SP) antiserum, bacitracin and serum albumin to measure SP released in superfusates. As shown by measuring the degradation of a SP-labelled derivative incubated with cerebellar slices, this medium prevented the enzymatic inactivation of SP. Potassium (K +, 50 mM) and veratridine (5 × 10 −5M) stimulated SP release and these effects were respectively prevented in absence of calcium and in presence of tetrodotoxin (5 × 10 −7 M). GABA (5 × 10 −5 M) nicotine (10 −6 M) and L-glutamic acid (5 × 10 −5 M) reduced the K + (50 mM)-evoked release of SP. In contrast, glycine (5 × 10 −5 M), oxotremorine (5 × 10 −5 M), D-glutamic acid (5 × 10 −5 M) and serotonin (5 × 10 −5 M) were without effect. Pempidine (10 − M) prevented the inhibitory effect of nicotine (10 − M) on the K +-evoked release of SP. Glutamic acid diethyl ester (10 −4 M) completely abolished the L-glutamic acid-induced inhibition of the K +-evoked release of SP. Picrotoxin (5 × 10 −5 M) did not influence the L-glutamic acid inhibitory effect excluding the intervention of GABAergic mechanisms.

Evidence for the involvement of a GABA-mediated inhibition in the hypovolaemia-induced vasopressin release.

The influence of GABA and of drugs, known to alter GABA-metabolism, on the hypovolaemia-provoked vasopressin release was investigated in rats. Blood volume was decreased without altering plasma osmolality or arterial blood pressure by i.p. injection of polyethylene glycol and the resulting plasma vasopressin concentration was measured using a radioimmunoassay. I.c.v. injections of GABA (0.4-2 mg) markedly suppressed the hypovolaemia-induced vasopressin release. The central inhibitory effect of GABA could not be related to appropriate changes in peripheral parameters believed to regulate vasopressin release (arterial blood pressure, renin-angiotensin system). Aminooxyacetic acid (9-81 mg kg-1, i.m.) and gamma-vinyl-GABA (1.5 g kg-1, i.p.), two potent inhibitors of GABA aminotransferase and known to increase brain GABA content, reduced vasopressin release to a comparable degree as did GABA (i.c.v.). On the other hand, 3-mercaptopropionic acid (10-90 mg kg-1, i.p.), an inhibitor of the GABA synthetizing enzyme glutamic acid decarboxylase, promoted the release of vasopressin when the rats were killed prior to the onset of convulsions. These results, on the whole, intimate the existence of a GABA-mediated inhibition in the central control of vasopressin release.

Characteristics of GABA binding sites in bovine pineal gland

In the absence of any known function of GABA in pineal gland, the possibility of its neuroinhibitory effects on pineal biochemical parameters including serotonin N-acetyltransferase (NAT) was tested. GABA (5–20 μM), but not other biogenic amines (histamine, dopamine, acetylcholine or melatonin), inhibited the norepinephrine (10 μM)-induced stimulation of NAT activity in bovine pineal expiants in culture without altering the basal NAT activity. The inhibitory effect of GABA, which was dose-dependent, could be blocked by N-methylbicuculline and was not seen in Cl-free medium. Binding studies on synaptic membranes prepared from bovine pineal gland revealed the presence of sodium dependent and sodium independent binding sites. The sodium independent GABA binding site had a K D value of 16.5 nM and a receptor density of 121 ± 3 fmoles per mg protein. The sodium dependent GABA binding site had a K D value of 0.677 μM and a receptor density of 4.47 ± 0.42 pmoles/mg protein. Sodium independent or sodium dependent binding sites exhibited neither positive nor negative cooperactivity. The exact functional interaction between GABAergic and adrenergic neurons in pineal gland, which may vary among mammalian species, is not clear and awaits further studies.

K+-evoked release of met-enkephalin from rat striatum in vitro: effect of putative neurotransmitters and morphine.

The effects of drugs on the K+-evoked release of met-enkephalin from superfused rat striatal slices were investigated using a specific radioimmunoassay. GABA, at concentrations of 50 microM and 100 microM, and the GABA agonist muscimol (50 microM), significantly inhibited the release. The inhibitory effect of GABA was reversed by picrotoxin suggesting that GABA inhibition is mediated by GABA receptors. Selected concentrations of the dopamine agonists apomorphine and ergonovine, as well as of haloperidol, acetylcholine, carbachol, noradrenaline, glutamic acid and substance P, had no effect on the release of metenkephalin. Increases in the evoked release (80%) and striatal enkephalin content (60%) were found in rats after chronic haloperidol administration, pointing to an increase in the synthesis and utilization of striatal enkephalin. No differences were found between the release from slices from morphine-tolerant/dependent and naive rats or after addition of naloxone to slices derived from tolerant/dependent animals. Selected concentrations of morphine and naloxone had no effect on release suggesting the absence of a mechanism for the regulation of enkephalin release involving autoreceptors.

Effects of intraventricular injection of gamma-aminobutyric acid (GABA) on plasma growth hormone and thyrotropin in conscious ovariectomized rats.

Control injections of saline ip or into the third ventricle (2 fil) failed to modify concentrations of GH or TSH in plasma of conscious, free-moving, ovariectomized (OVX) or OVX, estrogen- and progesteronetreated (OEP) rats. Intraventricular injection of y-aminobutyric acid (GABA) in doses ranging from 0.1–8 fimol induced a rapid elevation in plasma GH within 10 min of injection which was dose related and the elevation persisted for the 60-min duration of the study in both OVX and OEP rats. This effect was completely blocked by the GABA antagonist, bicuculline, which by itself had no effect on plasma GH in doses of 1 or 2 /imol ip. By contrast, GABA administration lowered plasma TSH, which became apparent within 10 min in OVX rats and persisted for the duration of the experiment and appeared to be dose related at the initial sampling time of 10 min. The inhibitory effect of GABA was lessprominent in OEP rats. Bicuculline, which by itself had no effect on plasma TSH, completely blocked the inhibtory effect...

The effects of GABA, picrotoxin and bicuculline on the spontaneous bioelectric activity of cultured cerebellar purkinje cells

Addition of GABA to the bathing medium in concentrations of 10(-6) to 10(-5) M abolished the spontaneous bioelectric activity of rat Purkinje cells in vitro. Lower concentrations of GABA (less than or equal to 10(-6) M) influenced the firing pattern of spontaneous discharges in such a way that the relative number of long interspike intervals increased. The inhibitory effect of GABA was reversible; the cells resumed normal activity after being transferred to a normal balanced salt solution. The action of GABA was antagonized by bicuculline and picrotoxin, whereas strychnine was unable to overcome the inhibition induced by GABA. Both bicuculline (10(-10) to 10(-6) M) and picrotoxin (10(-7) to 10(-4) M) exerted an excitatory effect on cultured Purkinje cells when added to the medium. This excitatory action suggests the existence of inhibitory synapses in cultures of rat cerebellum in which endogenous GABA is used as a transmitter. The ratio between the dose of picrotoxin and that of bicuculline necessary to produce maximal excitation was in the order of 10(3). Both substances showed an inhibitory action at high concentrations.

The chloride-dependent depression by GABA in the frog spinal cord

Spontaneous activities from ventral and dorsal roots of the isolated perfused spinal cord of the bullfrog were inhibited by GABA. β-Alanine showed a strong and glycine a weaker inhibitory effect. The inhibitory effect of GABA was markedly reduced in a chloride-free medium, whereas glycine and β-alanine still showed an inhibitory effect similar to that seen in normal medium. Employing the sucrose-gap method, a marked depolarization in the dorsal root and a small but obvious hyperpolarization in the ventral root were found by the application of GABA. These results support the view that GABA is one of the transmitters involved in pre- and postsynaptic inhibition of the spinal cord. The postsynaptic hyperpolarizing effect of GABA on motoneurones would be caused directly through increased permeability of the membrane to chloride ion. The depolarizing effect of GABA on primary afferent terminals is discussed in connection with chloride dependency.

D-Tubocurarine does not antagonize the inhibitory effects of GABA at the crustacean neuromuscular junction.

d-Tubocurarine does not antagonize the inhibitory effects of GABA at the crustacean neuromuscular junction.