Bicuculline-sensitive GABA Receptors Research Articles

Medicinal Chemistry and Molecular Pharmacology of GABA-C Receptors

GABA(C) receptors belong to the nicotinicoid superfamily of ionotropic receptors that include nicotinic acetylcholine receptors, bicuculline-sensitive GABA(A) receptors, strychnine-sensitive glycine receptors and 5HT3 serotonin receptors. The GABA(C) receptor concept arose from medicinal chemical studies of a conformationally restricted analog of GABA. Receptors matching the predicted properties of GABA(C) receptors were cloned from the retina. Post cloning studies revealed the unique physiology and pharmacology of these relatively simple homomeric receptors. Three subtypes of GABA(C) receptors have been cloned from mammalian sources and pharmacological differences between the rho1, rho2 and rho3 GABA(C) receptors have been described. There is evidence for functional GABA(C) receptors in the retina, spinal cord, superior colliculus, pituitary and the gut and their involvement in vision, aspects of memory and sleep-waking behaviour. This review concentrates on the medicinal chemistry and molecular pharmacology of GABA(C) receptor subtypes emphasising possible new investigational tools with which to investigate further GABA(C) receptor function. The most useful currently available ligands that show some GABA(C) receptor subtype selectivity are TPMPA, P4PMA, imidazole-4-acetic acid, 2-methyl-TACA and (+/-)-TAMP.

Morphological and electrophysiological evidence for an ionotropic GABA receptor of novel pharmacology.

Evidence from toxicological studies suggested that an ionotropic GABA receptor of novel pharmacology (picrotoxin-insensitive, bicuculline-sensitive) exists in the chick embryo retina. In this report, we provide direct morphological and electrophysiological evidence for the existence of such an iGABA receptor. Chick embryo retinas (14-16 days old) incubated in the presence of kainic acid showed pronounced histopathology in all retinal layers. Maximal protection from this toxicity required a combination of bicuculline and picrotoxin. Individual application of the antagonists indicated that a picrotoxin-insensitive, bicuculline-sensitive GABA receptor is likely to be present on ganglion and amacrine, but not bipolar, cells. GABA currents in embryonic and mature chicken retinal neurons were measured by whole cell patch clamp. GABA was puffed at the dendritic processes in the IPL. Picrotoxin (500 microM, in the bath) eliminated all (>95%) the GABA current in the majority of ganglion and amacrine cells tested, but many cells possessed a substantial picrotoxin-insensitive component. This current was eliminated by bicuculline (200 microM). This current was not a transporter-associated current, since it was not altered by GABA transport blockers or sodium removal. The current-voltage relation was linear and reversed near E(Cl), as expected for a ligand-gated chloride current. Both pentobarbital and lorazepam enhanced the picrotoxin-insensitive current. We conclude that chicken retinal ganglion and amacrine cells express a GABA receptor that is GABA-A-like, in that it can be blocked by bicuculline, and positively modulated by barbiturates and benzodiazepines, but is insensitive to the noncompetitive blocker picrotoxin. Understanding the molecular properties of this receptor will be important for understanding both physiological GABA neurotransmission and the pathology of GABA receptor overactivation.

Strong synergism between GABA(A) and glycine receptors on isolated carp third-order neurons.

A strong synergistic interaction between the bicuculline-sensitive GABA receptor (GABA(A) receptor) and the strychnine-sensitive glycine receptor was observed in third-order neurons acutely isolated from crucian carp retina, with the use of the whole-cell patch-clamp recording technique. In 58 of 153 cells, 10 microM GABA or glycine separately applied to amacrine/ganglion cells failed to induce any responses or only induced small currents (<20 pA), while co-application of these two chemicals resulted in much larger responses (403.05+/-319.98 pA). The current induced by the co-application was mediated by chloride channels, and both GABA(A) and glycine receptors were involved in the potentiation. The underlying mechanisms of this interaction and its possible physiological role are discussed.

Multitasking in the Olfactory System: Context-Dependent Responses to Odors Reveal Dual GABA-Regulated Coding Mechanisms in Single Olfactory Projection Neurons

Studies of olfaction have focused mainly on neural processing of information about the chemistry of odors, but olfactory stimuli have other properties that also affect central responses and thus influence behavior. In moths, continuous and intermittent stimulation with the same odor evokes two distinct flight behaviors, but the neural basis of this differential response is unknown. Here we show that certain projection neurons (PNs) in the primary olfactory center in the brain give context-dependent responses to a specific odor blend, and these responses are shaped in several ways by a bicuculline-sensitive GABA receptor. Pharmacological dissection of PN responses reveals that bicuculline blocks GABAA-type receptors/chloride channels in PNs, and that these receptors play a critical role in shaping the responses of these glomerular output neurons. The firing patterns of PNs are not odor-specific but are strongly modulated by the temporal pattern of the odor stimulus. Brief repetitive odor pulses evoke fast inhibitory potentials, followed by discrete bursts of action potentials that are phase-locked to the pulses. In contrast, the response to a single prolonged stimulus with the same odor is a series of slow oscillations underlying irregular firing. Bicuculline disrupts the timing of both types of responses, suggesting that GABAA-like receptors underlie both coding mechanisms. These results suggest that glomerular output neurons could use more than one coding scheme to represent a single olfactory stimulus. Moreover, these context-dependent odor responses encode information about both the chemical composition and the temporal pattern of the odor signal. Together with behavioral evidence, these findings suggest that context-dependent odor responses evoke different perceptions in the brain that provide the animal with important information about the spatiotemporal variations that occur in natural odor plumes.

Evidence that adenosine mediates the depression of spinal dorsal horn neurons induced by peripheral vibration in the cat

Nociceptive neurons in the dorsal horn of the cat spinal cord are depressed by vibration applied to the ipsilateral hind limb. The present study investigated the pharmacological properties of this depression because of the possibility that it represents the neural basis at the spinal level for the analgesic effects of vibration in humans. Experiments were done in cats anesthetized with sodium pentobarbital and acutely spinalized at the first lumbar level. Extracellular recordings were made from nociceptive neurons in the lower lumbar segments. The depression of these neurons induced by vibration to the hindlimb was attenuated by administration of the P 1-purinergic (adenosine) receptor antagonist, caffeine (20–60 mg/kg i.v.); the maximum attenuation was 100%. Effects of caffeine began within 2 min after the start of injection (1–3 min injection period), were greatest in the 10 min period after the end of injection and lasted for up to 2 hr. Importantly, another P 1-purinergic receptor antagonist, which does not cross the blood-brain barrier, 8-sulphophenyltheophylline (8–16 mg/kg), had no effect on the depression when given intravenously ( n = 5); however, when administered by iontophoresis 8-sulphophenyltheophylline blocked the depression in 2 of 6 units. Dipyridamole (1.0–2.0 mg/kg i.v.), an inhibitor of adenosine uptake, potentiated the depression in 2 of 5 cases. These results prompt us to suggest that depression induced by vibration may be mediated by adenosine via the activation of P 1-purinergic receptors. On the other hand, the GABA A antagonist, bicuculline, failed to attenuate vibration-induced depression when administered either intravenously (0.2–0.4 mg/kg; n = 5) or by iontophoresis ( n = 10) and the glycine antagonist, strychnine (0.2–0.6 mg/kg; n = 3) and the opiate antagonist, naloxone (0.1–0.4 mg/kg; n = 4) were similarly ineffective. These findings suggest that vibration-induced depression of these units occurs without involvement of bicuculline-sensitive GABA receptors, strychnine-sensitive glycine receptors and naloxone-sensitive opiate receptors. In view of the fact that vibration-induced depression is evoked synaptically, this study is the first to demonstrate in the central nervous system a synaptic response which is mediated by adenosine. In addition, we suggest that the analgesic effects of vibration in humans may be mediated at the spinal level by activation of P 1-purinergic receptors.

Facilitatory action of etomidate and pentobarbital on recurrent inhibition in rat hippocampal pyramidal neurons

Biochemical studies have shown that the non-barbiturate anesthetic etomidate can interact in a stereoselective, barbiturate-like fashion with the GABA/benzodiazepine receptor complex, enhancing both benzodiazepine and GABA binding, but its electrophysiological effects upon the mammalian CNS are largely unknown. The present investigations were designed to characterize the electrophysiological effects of etomidate on the recurrent GABAergic inhibitory pathway in the CA1 region of the rat in vitro hippocampal slice and to compare the actions of etomidate to those of pentobarbital. Electrical stimulation of the alveus elicited a biphasic hyperpolarizing response, consisting of an initial bicuculline-sensitive GABAergic IPSP. This was followed by a second component, termed the late hyperpolarizing potential (LHP), which is thought to reflect an increase in potassium conductance. Both pentobarbital (100 microM) and (+)-etomidate (10 microM) markedly increased the duration of the initial GABA-mediated IPSP, and frequently increased its amplitude as well. However, no significant effects of either of these drugs were observed on the LHP. Together with previous biochemical findings, our data suggest that the depressant effects of etomidate and barbiturates on the nervous system may reflect a common action upon a stereoselective receptor site intimately associated with bicuculline-sensitive GABA receptors and the chloride ion channel.

Gamma-aminobutyric acid receptors in the central nervous system

Recent research has raised a whole set of new and interesting points concerning the arrangement of GABA receptor sites. The most important of these is the separation of two distinct GABA receptor categories, namely bicuculline-sensitive and bicuculline-insensitive, which control the chloride and calcium conductance of the postsynaptic membrane. Information regarding the membrane and intracellular processes involved in activating GABAB receptors remains particularly limited as yet. Accordingly, findings from the literature maintain that calcium ions are not the sole transmitter of transmembrane current during activation of this category of receptor, while data from biochemical research suggests that the intracellular activity of cAMP and cGMP is changed when bicuculline-insensitive receptors are activated [15, 38]. It should be added that the physiological role played by these receptors is not yet known. The study of bicuculline-sensitive GABA receptor complexes using benzodiazepines, as well as their antagonists and reversible agonists, also offers considerable interest. Such research is particularly apposite in view of the widely discussed possibility of related endogenous-type substances existing and consequently of hitherto unknown inherent mechanisms controlling inhibitory processes within the CNS.

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.

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

1. 1. Structure-activity of primary afferent depolarising action (PAD) mediated by γ-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. 2. 2. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. 3. 3. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. 4. 4. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for l-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. 5. 5. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. 6. 6. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.

Liver and blood enzyme spectra of rats with experimental anthracosis on a diet supplemented with methionine and pyridoxine

The influence of Ro 15-1788 and bicuculline on the action of GABA-positive drugs (muscimol), GABA cethyl ester, piracetam and depakine and benzodiazepine tranquilizers (diazepam, phenazepam) on motivated aggression has been studied. It has been shown that Ro 15-1788 which has a weak antiaggressive effect selectively antagonizes the anti-aggressive effect of tranquilizers but not that of GABA-positive drugs. Bicuculline antagonizes antiaggressive activity of the drugs of both types. The action of these antagonists on the effect of the drugs under study as regards the analgetic activity of morphine was also studied. It has been shown that Ro 15-1788 antagonizes the potentiation of morphine analgesia caused by diazepam. At the same time Ro 15-1788 does not influence morphine analgesia potentiated by muscimol. Bicuculline removes the potentiation of morphine analgesia caused both by diazepam and muscimol it is concluded that bicuculline-sensitive GABA receptors modulate the antiaggressive effect of benzodiazepines and their influence on the analgetic action of opiates.

The effect of GABA on lumbar terminations of rubrospinal neurons in the cat spinal cord.

Although GABA and piperidine-4-sulphonic acid depolarize I a afferent terminations in the cat spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for a bicuculline-sensitive alteration by either gabamimetic of the electrical threshold of rubrospinal terminations in the spinal intermediate nucleus. The terminal axonal arborizations in the spinal cord of neurons in the red nucleus thus do not have GABA receptors similar to those on the cell bodies. The results are discussed in relation to the depolarizing action of GABA on some central neurons, and on neurons with peripheral cell bodies, and to probable differences in the intracellular chloride content of neurons having peripheral or central cell bodies, and thus of different embryological origin. A presynaptic depolarizing inhibitory process mediated by GABA appears to be confined to the terminals of primary afferent fibres in the mammalian central nervous system.

Angiotensin II-induced depression of Purkinje cell firing and possible modulatory action on GABA responses.

Effects of octapeptide angiotensin II (AII) were tested on cortical neurons of rat's cerebellum by means of microiontophoresis. It was observed that AII consistently depressed spontaneous firing of Purkinje cell, whereas other unidentified neurons were unaffected. When tested against responses of Purkinje cell to depressant putative neurotransmitters, namely, GABA, glycine, taurine, 5-hydroxytryptamine and noradrenaline, it was observed that AII specifically enhanced depressant action of GABA, while the responses to other substances were unaffected. Both AII-induced depression of cell firing and the AII-induced enhancement of GABA depression were antagonized by a specific GABA antagonist, bicuculline methochloride. We therefore suggest that AII exerts an inhibitory action on Purkinje cells through its modulatory action on bicuculline-sensitive GABA receptors.

THIP, a specific and clinically active GABA agonist

THIP is a potent and specific agonist at bicuculline-sensitive GABA receptors. THIP interacts selectively with medium/low-affinity GABA receptor sites in vitro , and it is capable of stimulating benzodiazepine binding in vitro with efficacies reminiscent of partial GABA agonists properties of THIP. THIP penetrates into the brain after administration to animals and man. It does not undergo significant phase I metabolic reaction and it is relatively non-toxic. THIP has potent non-opioid antinociceptive effects in animals, and it is a very active analgesic in man. THIP has anxiolytic properties and is capable of reducing monosynaptic T-reflexes in spastic patients.

3H-ACh release from guinea pig gallbladder evoked by GABA through the bicuculline-sensitive GABA receptor.

We investigated the effect of GABA on the spontaneous efflux of 3H-acetylcholine (ACh) from the isolated guinea pig gallbladder loaded with 3H-choline. Application of GABA (10(-5) M) caused a significant increase in the fractional rate of tritium efflux. This GABA-evoked efflux of ACh was inhibited by the perfusion of tetrodotoxin (10(-6) M) and Ca-free medium. Nipecotic acid (10(-4) M) did not affect the GABA-evoked release of ACh, indicating that ACh was not released by the entry of GABA into cholinergic nerve terminals. Bicuculline (10(-6) M) and furosemide (10(-6) M), the chloride ion channel blocker, inhibited the GABA-evoked ACh release. The application of muscimol (10(-5) M), but not baclofen (10(-5) M) also produced an increase in the fractional rate of ACh release. Thus, the GABA receptors involved in the increase of ACh release are bicuculline-sensitive. The GABA-evoked release of ACh was not altered by the perfusion with hexamethonium (10(-5) M), thus indicating the presence of GABA receptors on the postganglionic cholinergic neurons. These findings suggest that bicuculline-sensitive GABA receptors probably coupled to a Cl- ionophore are present on postganglionic cholinergic neurons and are involved in the increase of ACh release in guinea pig gallbladder.

The effect of GABA on the terminations of vestibulospinal neurons in the cat spinal cord

Although GABA and piperidine-4-sulphonate depolarize Ia afferent terminations in the lumbar spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for abicuculline-sensitive alteration by either GABAmimetic of the excitability of vestibulospinal terminations. This suggests that the terminal arborizations of vestibulospinal fibers, unlike their cell bodies. are devoid of GABA receptors having properties similar to those on either central neuron cell bodies, primary afferent cell bodies or their central terminations.

Effect of a ten-day course of fenibut and diazepam on gaba and benzodiazepine receptors in mouse brain

The clinical spectrum of action of fenibut (B-phenyl-GABA), which is used in clinical practice as a tranquilizer [2], differs in many respects from that of tranquilizers of the benzodiazepine series [3]. The writers showed previously that bicuculline-insensitive receptors take part in the mechanism of action of fenibut, whereas bicuculline-sensitive GABA receptors take part in the mechanism of action of diazepam [I]. Evidence has now been obtained to suggest that the tranquilizing effect of the benzodiazepines is closely linked with their effect on benzodiazepine receptors [7]. However, the role of benzodiazepine receptors in the mechanism of action of fenibut still remains unexplained.

γ-Aminobutyric acid inhibits motility of the isolated guinea-pig urinary bladder

When strips of guinea-pig urinary bladder were subjected to electrical transmural stimulation, the cholinergic but not the non-cholinergic components were inhibited. As this action was antagonized by bicuculline and furosemide, the possibility that GABA may inhibit the release of ACh through bicuculline-sensitive GABA receptors associated with the Cl − channel has to be considered.

Effects of some putative amino acid neurotransmitters on the stimulated TSH secretion in male rats.

The importance of several amino acids (glycine, L-glutamic acid, L-serine, taurine and beta-alanine) in the regulation of the stimulated secretion of TSH was studied in male rats using both peripheral and central administration of the amino acids. Glycine (10-200 mg/kg i.p.), L-glutamic acid (10-500 mg/kg i.p.) and L-serine (500 mg/kg i.p.) decreased significantly the cold-induced TSH secretion whereas beta-alanine (1-500 mg/kg i.p.) and taurine (10-100 mg/kg i.p.) were not effective. The effect of L-glutamic acid (100 mg i.p.) was partially antagonized by bicuculline (1 mg/kg i.p.) but not by picrotoxin (1 or 2 mg/kg i.p.). Only glycine (50 and 100 mg/kg i.p.) inhibited the TRH-stimulated TSH secretion. When the intracerebroventricular route was used, L-serine (50 micrograms/rat) decreased the TSH could response whereas glycine and L-glutamic acid (1-50 micrograms/rat) had no clear effect. We conclude that glycine, glutamate and serine inhibit the cold-induced TSH secretion in the male rat. The action of serine and glycine is possibly mediated through the periventricular hypothalamus and the anterior pituitary, respectively. The inhibition caused by glutamate seems to be partially mediated through the bicuculline-sensitive GABA receptors in the hypothalamus. Taurine and beta-alanine play no role in the control of rat TSH secretion.

Comparison of the analgesic actions of THIP and morphine

1. 1. The antinociceptive actions of intraperitoneally-administered 4,5,6,7-tetrahydroisoxazolo[5,4- c]pyridin-3-ol (THIP) and morphine were compared using three strains of mice. 2. 2. With the hot-plate assay, ED 50 values for the action of THIP were about 4 mg/kg in OF 1, CD 1 and NMRI strains, whereas ED 50 values for morphine varied among strains, being 6.8 mg/kg for OF 1, 16.9 mg/kg for CD 1, and about 29 mg/kg for NMRI mice; thus, the genetic control of the analgesic action of THIP appears to differ from that of morphine. 3. 3. The analgesic action of THIP in the hot-plate test was not blocked by naloxone, bicuculline, phentolamine or methysergide, but was partially reversed by a high dose of atropine, indicating that classic opiate-receptors, bicuculline-sensitive GABA-receptors, α-adrenoceptors and serotonin-receptors do not appear to mediate the action of THIP but that cholinergic receptors might be indirectly involved. 4. 4. THIP was about equipotent or more potent than morphine in the phenylbenzoquinone writhing test, evasion test, and traction test. 5. 5. Since the ED 50 values for THIP in OF 1 mice were similar for hot-plate, evasion and traction tests, the analgesic action of THIP might not be readily dissociated from its sedative or myorelaxant action.

Correlation between the function of GABAergic and cholinergic neurons in parasympathetic ganglion

We have already reported the occurrence of γ-aminobutyric acid (GABA) and glutamic acid decarboxylase and 3H-GABA uptake and release from the guinea pig urinary bladder. When we tested the effect of GABA on the motility and ACh release in the isolated urinary bladder O2 guinea pig, GABA inhibited dose-dependently, the cholinergic component of electrical stimulation (ES)- and nicotine-induced contractions, but not the non-cholinergic component of ES- and ACh-induced contractions. These effects of GABA were antagonized by bicuculline and furosemide. ES and nicotine evoked the tetrodotoxin-sensitive and Ca++-dependent release of 3H-ACh from the strips of urinary bladder preloaded with 3H-choline. Bicuculline at 10−5M enhanced the ACh release and contraction evoked by ES, under conditions of stimulation which produced a maximal release of GABA, -hereby suggesting that bicuculline antagonizes the inhibitory effect of endogenous GABA released during the stimulation. Nicotine-evoked release of ACh and contraction were inhibited by GABA and muscimol, but not by baclofen. These effects of 3ABA were antagonized by bicuculline and furosemide, but not by rhentolamine and propranolol. Thus, GABA may affect the motility of the guinea pig urinary bladder by inhibiting release of ACh through bicuculline-sensitive GABA receptors associated with the Cl- ion channel.