GABA receptor binding in frog spinal cord and brain

Characterization of the binding of [3H]CGP54626 to GABAB receptors in the male bullfrog (Rana catesbeiana)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate brain. GABA activates both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors in mammals. Whether non-mammalian vertebrates possess receptors with similar characteristics is not well understood. We used a mammalian GABA(B)-specific antagonist to determine the pharmacology of putative receptors in the brain of an anuran amphibian, the male bullfrog (Rana catesbeiana). Receptor binding assays with the antagonist [(3)H]CGP54626 revealed a single class of high affinity binding sites (with a K(D) of 2.97 nM and a B(max) of 2619 fmol/mg protein). Binding was time- and temperature-dependent, saturable and specific. Specific binding of [(3)H]CGP54626 was inhibited by several mammalian GABA(B) receptor agonists and antagonists. The rank order potency of agonists was: GABA = SKF97541 > (R)-Baclofen > 3-APPA. The rank order for antagonists was: CGP54626 = CGP55845 > CGP52432 > CGP35348. The GABA(A) receptor ligands muscimol and SR95531 had very low affinity for [(3)H]CGP54626 binding sites, while bicuculline compounds had no affinity. Binding of GABA was positively modulated by CGP7930. Taurine did not allosterically modulate GABA binding but did inhibit [(3)H]CGP54626 binding in a linear fashion. Bullfrog brain thus possesses binding sites with significant similarity to mammalian GABA(B) receptors. These receptors differ from mammalian receptors, however, in dissociation kinetics, ligand specificity and allosteric modulation.

Characterization of the GABA(A) receptor in the brain of the adult male bullfrog, Rana catesbeiana

Little is known about the properties of GABA receptors in the amphibian brain. The GABA(A) receptor is widespread in the mammalian brain, and can be specifically labeled with the receptor agonist [3H]muscimol. The binding of [3H]muscimol to membrane preparations from the brain of the bullfrog, Rana catesbeiana, was investigated in kinetic, saturation, and inhibition experiments to determine whether this species possessed a GABA(A)-like receptor. Binding of 20 nM [3H]muscimol to membranes was specific and could be displaced by 1 mM GABA. Association binding curves showed that steady state occurred rapidly, within 2 min, and dissociation occurred within 5 min. The receptor was saturable with a single, high-affinity binding site (K(D)=19.2 nM; B(max)=1.8 pmol/mg protein). Binding of [3H]muscimol was inhibited in a dose-dependent fashion by muscimol, GABA, bicuculline methiodide, and bicuculline (in order of potency). Baclofen (at doses from 10(-9) to 10(-3) M) failed to displace [3H]muscimol. The binding characteristics and ligand specificity of [3H]muscimol binding sites in the bullfrog brain support the hypothesis that this amphibian possesses a GABA(A)-like receptor protein similar to the GABA(A) receptor characterized in mammals.

Vertebrate cyclodiene insecticide resistance: role of gamma-aminobutyric acid and diazepam binding sites

Certain populations of the mosquitofish (Gambusia affinis) are highly resistant to cyclodiene and cyclodiene-type insecticides that competitively interact with the picrotoxinin binding site of the gamma-aminobutyric acid (GABA) receptor-ionophore complex in the central nervous system. Resistance involves a reduction in affinity of the picrotoxinin binding site. The present study reports that GABA receptor binding is increased in resistant brain membranes compared to membranes from susceptible fish at concentrations of free radioligand above 0.2 microM. The increase appears to be due to a greater number of binding sites (Bmax) in the resistant population. Diazepam binding affinity (Kd) and Bmax were not different in membranes from resistant fish compared to those from susceptible fish. Up-regulation of GABA binding sites in the resistant fish population may compensate for a possible reduction of GABAergic transmission caused by chronic environmental exposure to cyclodiene insecticides. However, a lack of cross-resistance to bicuculline (a competitive GABA antagonist) indicates that an increase in GABA sites is not a mechanism of cyclodiene resistance.

Actions of gamma-aminobutyric acid on rat supraoptic nucleus neurosecretory neurones in vitro

1. Intracellular recordings were obtained from thirty-eight rat supraoptic nucleus (s.o.n.) neurosecretory neurones in perfused hypothalamic explants. Changes in membrane potential and conductance were monitored following application of gamma-aminobutyric acid (GABA), and related agonists and antagonists. 2. GABA depressed action potential discharge of all of thirty-five s.o.n. neurones tested and induced either membrane hyperpolarization or depolarization. Neurones that displayed membrane hyperpolarization in response to lower GABA concentrations (30-300 microM) demonstrated a biphasic membrane voltage change with a later depolarizing phase as a response to higher concentrations (up to 3000 microM). 3. GABA (10-3000 microM) induced a prominent concentration-dependent increase in membrane conductance in all neurones. The critical slope for the log-log plot of [GABA] vs. GABA-induced membrane conductance was 1.7, indicating co-operativity in the GABA receptor-induced conductance change. 4. Muscimol (0.3-30 microM) potently mimicked all the effects of GABA. Bicuculline (1-100 microM) antagonized the effects of GABA and muscimol in a competitive manner. 5. Glycine and taurine (1-10 mM) had weak effects, although comparatively similar to those of GABA. These actions were blocked both by bicuculline (100 microM) and by strychnine (1 microM). At higher concentrations (greater than 10 microM), strychnine also antagonized the actions of GABA. 6. In recordings with potassium-acetate-filled micropipettes, the reversal potential of hyperpolarizing membrane voltage responses to GABA was -72.5 +/- 1.5 mV in close agreement (+/- 5 mV) with the reversal potential of inhibitory post-synaptic potentials (i.p.s.p.s) recorded in the same neurones. Depolarizing responses to GABA reversed polarity at -50 +/- 1.6 mV. In recordings with KCl-filled micropipettes, voltage responses to GABA were always depolarizing and reversed near -40.0 +/- 4.3 mV. Similarly, reduction of the concentration of chloride ions in the perfusion medium from 134 to 10.4 mM induced a positive shift of the GABA reversal potential by 40-50 mV. 7. From measurements of input resistance (Rin) and cell time constant (tau O), input capacitance (Cin; representing total membrane capacitance) was calculated as 78.9 +/- 2.1 pF. During responses to GABA or muscimol, decreased Rin was accompanied by a linearly related decrease in tau o indicating that these substances had no effect on the membrane capacitance of s.o.n. neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of putative glutamate receptors from insect skeletal muscles, insect central nervous system and rat brain

Binding of [3H]glutamate to housefly brain and honeybee brain and thoracic muscle membranes as well as to the American cockroach nerve cord was measured in Na+-free Tris-citrate buffer, 2.5 mM CaCl2, pH 7.4. The dissociation constants (KDS) ranged from 0.16 to 1.36 microM, and thoracic muscles had 2-4-fold higher density of receptors than brain tissue. The potent inhibitors of housefly brain binding were in decreasing order of effectiveness: L-glutamate greater than L-aspartate = L-cysteate = ibotenate greater than quisqualate greater than L-homocysteate greater than L-APB greater than L-APV greater than NMDA greater than D-APB greater than D-glutamate, with no inhibition by 100 microM of GDEE, dihydrokainate, D-APV, D-homocysteate or D-aspartate. The drug specificity of [3H]glutamate binding sites in housefly brain was generally similar to that of binding sites in housefly muscle, except that the former had a slightly higher affinity for L-APB, L-homocysteate and NMDA. [3H]Glutamate binding to insect tissues differed in its drug sensitivity from binding to rat brain. Binding to insect membranes was much less sensitive to L-APB, D-APB, APV, homocysteate, L-cysteate, quisqualate and ibotenate. However, the insect binding site was much more stereoselective for the L than D isomers of glutamate and aspartate, while the rat brain site was more stereoselective for APB. It is suggested that the observed [3H]glutamate binding to insect tissue is not to NMDA or kainate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-activity studies on the activity of a series of cyclopentane GABA analogues on GABAA receptors and GABA uptake

A series of analogues of gamma-aminobutyric acid (GABA) has been investigated for GABA mimetic activity on the isolated guinea-pig ileum, facilitation of [3H]diazepam binding in rat brain membranes and inhibition of [3H]GABA uptake from rat brain cortical slices. The derivatives tested include six racemic amino acids, all of which contain a 'GABA backbone' with the conformation restricted by a cyclopentane or cyclopentene ring system. From the more potent analogues, five optically pure compounds, including (+)-(4S)-4-aminocyclopent-1-ene-1-carboxylic acid and its (-)-4R enantiomer, have also been assessed as GABA agonists. Of the racemic analogues, 4-aminocyclopent-1-ene-1-carboxylic acid and trans-3-aminocyclopentane-1-carboxylic acid were the most potent at GABAA receptors, while most of the analogues had considerable activity on GABA uptake. The individual resolved isomers of 4-aminocyclopent-1-ene-carboxylic acid and of trans-3-aminocyclopentane-1-carboxylic acid displayed great specificity for GABA receptor and GABA uptake sites. For example (+)-(4S)-4-aminocyclopent-1-ene-1-carboxylic acid was approximately twice as potent as GABA and about 600 times more active than the (-)-4R isomer on the guinea-pig ileum, while it was not significantly active as a GABA uptake inhibitor at 500 microM. On the other hand, its (-)-4R isomer was selective for inhibiting GABA uptake with an IC50 equal to that of racemic nipecotic acid.

Pharmacological analysis of the central cardiovascular effects of four GABA analogues

The central cardiovascular effects of 4 structural analogues of GABA were investigated. The drugs were injected intracerebroventricularly (i.c.v.) in cumulative doses into pentobarbital-anaesthetized normotensive rats. Muscimol (0.01-10 micrograms/kg), THIP (0.01-100 micrograms/kg), kojic amine (0.1-100 micrograms/kg) and isoguvacine (0.1-100 micrograms/kg) produced dose-dependent hypotension and bradycardia. The maximal fall in the mean blood pressure was of about 35% of the initial values. These effects appears to be of central origin since the intravenous (i.v.) injection of the same doses of the drugs did not produce any similar cardiovascular modifications. The hypotensive effects of muscimol and kojic amine were antagonized partly by i.c.v. bicuculline. The combination of bicuculline and kainic acid almost completely prevented the blood pressure lowering effects of muscimol, kojic amine and isoguvacine. THIP however was only slightly antagonized by bicuculline and kainic acid. Atropine i.v. also prevented partly the cardiovascular effects of all these drugs. Thus, the mechanisms of the central cardiovascular actions of GABA analogues appear to be more complex than expected and variable from one drug to another. The involvement of GABA receptors of the A and B types and of cholinergic mechanisms in the hypotensive effect of the drugs is discussed.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a neuromodulatory role of GABA at the first synapse of the baroreceptor reflex pathway. Effects of GABA derivatives injected into the NTS

Microinjections of GABA and of the specific agonist of GABA receptors, muscimol, in the intermediate nucleus tractus solitarii (NTS) of pentobarbitone anaesthetized cats produced hypertension and tachycardia. The GABA receptor antagonist, bicuculline, had opposite effects and prevented those of muscimol. Therefore, a GABAergic system appears to modulate the cardiovascular regulation within the NTS. d,l-Baclofen also increased blood pressure and heart rate when injected into the same region, but this effect was not antagonized by bicuculline. The mechanism of this action of baclofen is discussed.

Avermectin B1a modulation of gamma-aminobutyric acid receptors in rat brain membranes

Avermectin B1a stimulates high-affinity binding of [3H]-gamma-aminobutyric acid (GABA) to receptors in washed rat brain membranes. Scatchard analysis of the data indicates that the drug does not significantly alter the apparent dissociation constant of GABA binding, but increases the detectable number of binding sites from 3.2 to 5.1 pmol/mg protein, (+)-Bicuculline completely blocks control and avermectin B1a-stimulated GABA binding, whereas picrotoxin antagonizes specifically the avermectin B1a-stimulated GABA binding. The avermectin B1a-stimulated GABA binding is also chloride ion-dependent, whereas GABA binding in the control is not. These observations suggest that the mechanism of avermectin B1a stimulation of GABA binding may involve the chloride ion channel.

The ventromedullary hypotensive effect of muscimol in the anaesthetized cat

Muscimol, a rigid analogue of GABA, was injected into the CNS of urethane-anaesthetized, normotensive cats. Bilateral microinjections of a small dose of muscimol (100 ng/kg, 0.5 microliter on each side) in the nucleus reticularis lateralis (NRL) induced hypotension. The marked fall in blood pressure obtained by injecting 1 microgram/kg of muscimol unilaterally into the NRL is completely reversed by subsequent local administration of bicuculline (5 microgram/kg, 0.5 microliter), a specific GABA antagonist. These data confirm that a GABAergic inhibitory system modulates the pressor tonic structures localized in the region of the NRL and that the anteroventral part of the medulla oblongata includes a trigger zone for the hypotensive action of muscimol.

Phylogenetic distribution of bicuculline-sensitive gamma-amino-butyric acid (GABA) receptor binding

Bicuculline-sensitive [3H]GABA receptor binding was studied in membrane fractions prepared from vertebrate whole brain or invertebrate cephalic ganglia. In tissue not treated with Triton X-100, a significant amount of bicuculline-displaceable [3H]GABA binding was detected in the brains of all vertebrates studied, with the hagfish brain binding over twice as much [3H]GABA as the spiny dogfish, the next oldest species. All other vertebrates bound similar amounts of [3H]GABA, being one-third to one-fourth that observed in the hagfish. In contrast, after Triton treatment, the hagfish displayed the least amount of bicuculline-sensitive [3H]GABA binding and, under those conditions, the amount of binding observed increased in an evolutionary fashion. No measurable bicuculline-sensitive GABA receptor binding was noted in any invertebrate studied. These results suggest that bicuculline-sensitive GABA receptors are present in the brains of all vertebrates and that during the course of evolution there developed a Triton-sensitive substance(s) whose presence modifies the kinetic properties of this receptor site.

Laminar distribution of putative neurotransmitter amino acids and ligand binding sites in the dog olfactory bulb

Coronal sections of frozen dog olfactory bulb have been dissected into four anatomically distinct layers. The laminar distribution of ten amino acids, the dipeptide carnosine, and nine [3H]ligand binding sites in these layers was determined. GABA and tyrosine levels were highest in the mitral cell-granule cell layer, and glutamate levels were slightly elevated in the glomerular layer. The distributions of all other amino acids did not show significant differences across the layers. Carnosine was predominantly localized in the fiber and glomerular layers. With the exception of quinuclidinyl benzilate, the [3H]ligand binding sites showed more discrete distributions. Muscimol, diazepam, kainic acid, and spiroperidol binding were predominantly localized in the mitral cell-granule cell layer, where clonidine binding was at a minimum. Dihydromorphine binding was high in both the fiber and the mitral cell-granule cell layers. Carnosine binding was maximal in the glomerular layer. The implications of these observations with regard to biochemical and neurophysiological data are discussed.

Ligand binding studies in the mouse olfactory bulb: identification and characterization of a L-[3H]carnosine binding site

Binding sites for the dipeptide L-carnosine (beta-alanyl-L-histidine) have been detected in membranes prepared from mouse olfactory bulbs. The binding of L-[3H]-carnosine was saturable, reversible and stereospecific and had a Kd of about 770 nM. The stereospecific binding of L-carnosine represented about 30% of the total binding at pH 6.8, and decreased markedly with increasing pH. Binding was stimulated by calcium, unaffected by zinc, magnesium or manganese and inhibited by sodium and potassium. Carnosine binding was sensitive to trypsin and phospholipases A and C, but not to neuraminidase. Nystatin and filipin, which interact with membrane lipids, also interferred with binding. Some peptide analogues of carnosine were potent inhibitors of binding, but a variety of drugs serving as potent inhibitors in other binding systems had no effect on carnosine binding. Carnosine binding to mouse olfactory bulb membranes was 15-fold higher than that seen in membranes prepared from cerebral hemispheres, 5-fold higher than that seen in membranes prepared from cerebral hemispheres, 5-fold higher than in cerebellum membranes and 3-fold higher than in membranes from spinal medulla and the olfactory tubercle-lateral olfactory tract area. Binding sites for 6 other radiolabeled receptor ligands were also detected in bulb membranes. Peripheral deafferentation of the olfactory bulbs by intranasal irrigation with ZnSO4 led to a loss greater than 90% of the L-[3H]carnosine binding in 4--5 days with much smaller losses in binding of the other 6 ligands over a 180-day observation period. This initial loss of carnosine binding after denervation was due to a loss of binding site stereo-specificity followed by a loss of binding sites. The characteristics of the carnosine binding site in olfactory bulb fulfil 6 of the 7 criteria considered relevant for a functional receptor.

Benzodiazepine receptor binding: the interactions of some non-benzodiazepine drugs with specific [3H] diazepam binding to rat brain synaptosomal membranes

The interaction of several non-benzodiazepine drugs with [3H] diazepam binding to benzodiazepine receptors in rat brain synaptosomal membranes was investigated. Baclofen, benzoctamine, hydroxyzine, chlorpromazine, haloperidol, imipramine, and amitriptyline displace specific [3H] diazepam binding, but the concentrations needed are too high to explain pharmacological effects of these drugs by an interaction with benzodiazepine receptors. The most potent non-benzodiazepine drug for inhibiting specific [3H] diazepam binding was methaqualone (IC50 value of 150 micrometer). It is suggested that interactions with benzodiazepine receptors may account for the anxiolytic and anticonvulsive side effects of this drug. The analeptic drug pentylenetetrazole interacts with benzodiazepine receptor binding with an IC50 value of about 1 mM, which is possibly too high to explain its convulsive properties by an antagonism at the benzodiazepine receptor.

Vertebrate GABA receptors

Physiologic-pharmacologic studies in vivo and with tissue cultures have revealed that synaptic GABA receptors exist in the vertebrate CNS. The GABA antagonist, bicuculline, can be used to detect synaptic GABA receptors in both the presence and absence of Na+, even though GABA binding to cerebral subcellular fractions occurs mainly to transport (uptake) receptors in the presence of Na+.

Conductance increases produced by glycine and gamma-aminobutyric acid in lamprey interneurones

1. Conductances of individual neurones in the isolated lamprey spinal cord were measured with separate intracellular electrodes for recording potentials and for passing current pulses during application of glycine or GABA (0.1-1.0 MM) in Ca-free bathing fluid. Large, reversible increases in conductance were produced in giant interneurones by both amino acids, but Müller axons and sensory dorsal cells were unaffected. 2. Conductance increases produced by glycine and by GABA were selective for Cl. Both conductance increases were linearly related to external Cl concentrations and repeated exposure to the amino acids in Cl-free fluid progressively reduced the conductance increases to less than 1% of their values in normal Cl. 3. Strychnine was a competitive antagonist of glycine, while GABA was antagonized competitively by bicuculline and non-competitively by picrotoxin. 4. The sensitivity of giant interneurones to glycine and GABA increased at low temperatures, in Na-free fluid, and after repeated exposure to the amino acids. Sensitization may have been produced by inhibition of uptake mechanisms for glycine and GABA in the spinal cord. 5. Discharges of interneurones recorded extracellularly were inhibited by bath-applied glycine and GABA, but directly elicited action potentials of axons were unaffected. Strychnine and Cl-free fluid in the presence of Ca produced seizures in lamprey spinal cord. 6. The conclusions of these experiments are that different receptors for glycine and for GABA are present on giant interneurones, that glycine is the better candidate for an inhibitory transmitter in the lamprey spinal cord, and that GABA produces effects similar to those which have been well studied in arthropod muscle.