Interaction of barbiturates with dihydropicrotoxinin binding sites related to the GABA receptor-ionophore system

Bidirectional effects of benzodiazepine receptor ligands against picrotoxin- and pentylenetetrazol-induced seizures

The dose response curves of picrotoxin-induced seizures and pentylenetetrazol-induced seizures were shifted to the right by the benzodiazepine (BZ) receptor agonist lorazepam, and to the left by the inverse agonists, DMCM, ZK 90886, FG 7142 and CGS 8216. The BZ receptor antagonists ZK 93426 and Ro 15-1788 had no effect on the dose response curves. The anticonvulsive action of lorazepam and the proconvulsive action of DMCM against picrotoxin-induced seizures and against pentylenetetrazol-induced seizures was inhibited by low doses of ZK 93426 and Ro 15-1788. These results indicate that the bidirectional effects of benzodiazepine receptor ligands on picrotoxin and pentylenetetrazol induced seizures is actually mediated through benzodiazepine receptors.

Enhancement of diazepam and gamma-aminobutyric acid binding by (+)etomidate and pentobarbital

(+)Etomidate and pentobarbital enhance [3H]diazepam and [3H]gamma-aminobutyric acid [( 3H]GABA) binding to cerebral cortex membranes. Both (+)etomidate and pentobarbital increase the affinity of [3H]diazepam for its binding sites. In contrast, they increase the Bmax of both the high- and low-affinity GABA receptor sites. The enhancement of [3H]diazepam and [3H]GABA by (+)etomidate and pentobarbital is blocked by GABA antagonists. These results indicate that hypnotic drugs such as (+)etomidate and pentobarbital, which are not structurally related, modulate diazepam and GABA binding sites via similar mechanisms.

The effect of ethanol on GABAergic transmission

One of the many actions of ethanol involves the GABAergic system. The interaction of ethanol with GABAergic neurons is a complex one involving both presynaptic and postsynaptic sites. Through a presumed fluidization of membranes after a single dose of ethanol, the available in vitro evidence suggests that ethanol disrupts the normal functioning of the GABA-benzodiazepine-chloride ionophore complex in a complicated manner involving a sequential activation of different active sites leading to the facilitation of GABA transmission. This finding has been supported in vivo using electrophysiological techniques. Presynaptic GABAergic neurons may experience a reduced activity, especially at low doses of ethanol. After chronic ethanol treatment, GABAergic transmission may be reduced, especially during an ethanol withdrawal syndrome. Also, other changes in the GABA-benzodiazepine-chloride ionophore complex suggest GABA transmission is suppressed postsynaptically. Drugs which enhance the actions of GABA may be suitable inhibitors of the ethanol withdrawal syndrome. In particular a new class of drugs, the triazolopyridazines, may be promising compounds for treatment of withdrawal with a more specific mode of action and fewer side effects.

Facilitation of diazepam action by anticonvulsant agents against picrotoxin induced convulsions

A subeffective dose (2 mg/kg) of diazepam produced only 50% protection against picrotoxin-induced (PTX) convulsions in rats. Simultaneous administration of GABA and other GABA-ergic substances such as piracetam and sidium valproate, which did not have any effect by themselves, potentiated diazepam action. The onset of convulsions and mortality due to PTX were significantly delayed. The other conventional anticonvulsants phenobarbitone, phenytoin and ethosuximide also enhanced the protective effect of diazepam. Inosine, a putative benzodiazepine ligand, also enhanced diazepam action. These observations are explained on the basis of data from in vitro studies indicating that GABA-ergic agents and barbiturates enhance both the number of benzodiazepine binding sites and benzodiazepine binding. The protective effect of clonidine, however, may be mediated by a different mechanism unrelated to the GABA-ergic system.

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 similarities between cyclodiene type insecticides and picrotoxinin in their action mechanisms

Evidence has been obtained to indicate that cyclodiene-type insecticides, e.g., heptachlor epoxide and gamma-BHC, mimic the action of picrotoxinin. These insecticides inhibit the GABA (gamma-aminobutyric acid)-stimulated chloride uptake in the coxal muscle of the American cockroach, and directly compete against [3H]a-dihydropicrotoxinin for binding in the rat brain synaptosomes. Moreover, several cyclodiene-resistant insect strains are also resistant to picrotoxinin. This cross-resistance is specific to picrotoxinin and does not extend to other neuroexcitants. These insecticides, like picrotoxinin, cause central nerve excitation by stimulating transmitter release. Similarity in molecular structures also has been pointed out. These results indicate that some of the nerve excitation symptoms that insecticides cause are likely due to their interaction with picrotoxinin receptor.

The interactions of ethanol with the benzodiazepine-GABA receptor-ionophore complex

Ethanol, barbiturates and benzodiazepines have similar pharmacological effects. All of these drugs facilitate the inhibitory transmission mediated by GABA. Drugs that facilitate GABAergic transmission are effective in alleviating ethanol withdrawal symptoms. Acute ethanol administration increases the number of low affinity GABA receptor sites, while, during ethanol withdrawal, the affinity of this site is decreased. This decreased affinity during withdrawal correlated with the audiogenic seizure activity. These results indicate that in vivo ethanol interacts with GABA receptors, and this interaction may be responsible for some of the effects of ethanol and for some symptoms of withdrawal. Ethanol, like pentobarbital, also enhances [3H]diazepam binding to the Lubrol-solubilized membrane fraction in vitro. This effect was dose-related and was blocked by picrotoxinin and other GABA antagonists. Enhancement of [3H]diazepam binding by various alcohols did not correlate with lipid:water partition coefficients. Ethanol also partially inhibited the binding of [3H]-alpha-dihydropicrotoxinin in the Lubrol-solubilized fraction. These results indicate that ethanol, like pentobarbital, may modulate the benzodiazepine binding component of the benzodiazepine-GABA receptor-ionophore complex via the picrotoxinin site. The possible interpretation of these results, in relation to the GABAergic transmission, is discussed.

Effect of barbiturates on the GABA receptor of cat primary afferent neurones

1. The effects of the barbiturate anaesthetics, pentobarbitone and thiopentone, on the membrane properties and the gamma-aminobutyric acid (GABA)-induced responses of cat primary afferent neurones were studied with intracellular recording and voltageclamp techniques.2. At low concentrations (10(-7)-10(-5) M) both barbiturates slightly enhanced and prolonged GABA-induced depolarizations or currents without affecting the membrane properties. At these concentrations, barbiturates have no effect on the apparent dissociation constant of the GABA-GABA receptor interaction or the reversal potential for GABA-induced depolarizations or currents.3. At high concentrations (10(-4)-10(-3) M) barbiturates produced a few millivolts reduction in the resting membrane potential. Voltage-clamp analysis revealed that the depolarization was associated with one of the three types of conductance change, i.e., an initial increase followed by a decrease (40% of neurones examined), only an increase (40%) and only a decrease (20%).4. Analysis in different ionic media indicated that the depolarization with a reduced membrane resistance is associated with an increased chloride conductance and that the one with an increased membrane resistance is accompanied by a reduction in potassium conductance. Bath-application of GABA (10(-3) M) or picrotoxin (10(-5) M) inhibited the increase in chloride conductance but not the reduction in potassium conductance.5. Barbiturates at these high concentrations initially caused a marked augmentation and prolongation of GABA responses; this was followed by a depression. The depressant action did not appear to be voltage-dependent. These actions of barbiturates were not accompanied by changes in the apparent dissociation constant of the GABA-current dose-response curve or the reversal potential for GABA currents. In addition, the single exponential decay of GABA current was not changed despite a marked prolongation of its decay time.6. Picrotoxin (10(-5) M) antagonized the depressant effect of barbiturates at high concentrations on GABA currents, and barbiturates (5 x 10(-6) M) reduced the inhibitory action of picrotoxin (5 x 10(-6) M) on the GABA-currents.7. From all these results, it is suggested that the site of barbiturate actions on GABA-responses is mainly the allosteric site (the ionic conductance regulatory subunit) but not the agonist recognition site or the chloride channels linked with GABA receptors.

Effects of sodium barbitone on learning and memory-storage of an appetitive and an aversive task

In order to test the effects of sodium barbitone on the acquisition and retention of an appetitively and an aversively reinforced behavior, mice were trained in a spatial discrimination Y-maze task. Learning was observed in both situations, with acquisition unimpaired by the drug. Sodium barbitone did, however, affect retention of both tasks in all groups treated with the drug before training. Results are discussed in light of the various modes of action of this drug, i.e., as an inhibitor of protein synthesis, as a blocker of catecholamine biosynthesis, with regard to its effects on paradoxical sleep and on gamma-amino-butyric acid (GABA).

Convulsant and anticonvulsant effects of opioids: relationship to GABA-mediated transmission

The convulsant benzodiazepine Ro 5-3663, bicuculline and picrotoxin induced electroencephalographic (EEG) and behavioural convulsions. In rabbits, the EEG modifications consisted, with increasing doses, of three different patterns: slow waves in the optic lead, spike- and wave-complexes in the sensorimotor cortex, and grand-mal generalized seizures. These EEG effects were terminated by administration of diazepam (1 mg/kg) and morphine (0.25-1.0 mg/kg). Naloxone, in doses of 5-10 mg/kg, potentiated the effects of the three convulsant drugs. This potentiating phenomenon was also antagonized by the administration of diazepam and morphine. In membrane preparations, obtained from rat cortex, deprived of endogenous modulators of [3H]GABA binding, naloxone but not morphine, was able to inhibit [3H]GABA binding to its specific recognition sites. These data agree with previous findings indicating a GABA-antagonistic effect of naloxone, and support the hypothesis that the anticonvulsant effect of morphine might be, at least in part, due to an increase in GABAergic activity at the synaptic level.

Molecular interactions of etazolate with benzodiazepine and picrotoxinin binding sites

Pyrazolopyridines, such as etazolate (SQ 20009), enhance [3H]diazepam binding to a Lubrol-solubilized fraction that has specific binding sites for 3H benzodiazepines, [alpha-3H]dihydropicrotoxinin (DHP) and [3H]muscimol. Etazolate enhancement of [3H]diazepam binding was inhibited by picrotoxinin. Furthermore, etazolate inhibited the [3H]DHP binding in a Lubrol-solubilized fraction with an IC50 value of 6-8 microM. These results provide evidence that etazolate, like pentobarbital, modulates benzodiazepine binding via the DHP-sensitive site of the benzodiazepine-GABA receptor-ionophore complex.

Comparison of the effects of ethylenediamine analogues and gamma-aminobutyric acid on cortical and pallidal neurones

1 The actions of ethylenediamine (EDA) and structurally related compounds were investigated by microiontophoresis in Wistar rats. 2 EDA inhibited, via a bicuculline-sensitive mechanism, the spontaneous firing rate of all cortical and pallidal cells tested. 3 The results with the analogues suggest that two amine groups are required for this neuronal depressant action whereas a carboxyl grouping is not. N-substitution reduces the depressant effect. The length of the molecule is also critical, more than 3 methylene components seriously reducing its effectiveness. A rigid analogue of EDA, piperazine, was also active. In addition the apparent transport numbers of EDA and gamma-aminobutyric acid (GABA) were calculated, showing a close similarity between the two. 4 The results are discussed wih respect to the possibility that EDA may represent a new class of GABA-mimetics, or may indicate the existence of a novel diamine receptor mediating bicuculline-sensitive inhibition in the rat CNS.

Antianalgesic action of thiamylal sodium in cats

The effects of thiamylal on the nociceptor-driven neural activity in the spinal cord were studied in decerebrate, non-anesthetized cats. Noxious stimulation was induced by the injection of bradykinin into the femoral artery and the neutral response in the lateral funiculus was measured by the multi-unit activity technique. The effects of thiamylal on the bradykinin-induced response were compared before and after the spinal cord transection, above the recording site. Thiamylal, 5 mg/kg i.v., potentiated the response significantly before the cord transection and depressed it after the transection. These findings indicate that the antianalgesic action of thiamylal is induced at the spinal cord level: although this anesthetic agent does have a direct intraspinal depressant action, the multisynaptic neural network of the supraspinal pain inhibition system is more susceptible to the actions of anesthetics, and the depression of this descending system by thiamylal results in a release of spinal cord nociceptive neural mechanisms from the supraspinal control.

The GABA postsynaptic membrane receptor-ionophore complex. Site of action of convulsant and anticonvulsant drugs

The function of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), has been implicated in the mode of action of many drugs which excite or depress the central nervous system. Many convulsant agents appear to block GABA action whereas anticonvulsants enhance GABA action. Some of these drug effects involve altered GABA-mediated synaptic transmission at the level of GABA biosynthesis, release from nerve endings, uptake into cells, and metabolic degradation. A greater number of agents of diverse classes appear to affect GABA action at the postsynaptic membrane, as determined from both electrophysiological and biochemical studies. The recently developed in vitro radioactive receptor binding assays have led to a wealth of new information about GABA action and its alteration by drugs. GABA inhibitory transmission involves the regulation, by GABA binding to its receptor site, of chloride ion channels. In this GABA receptor-ionophore system, other drug receptor sites, one for benzodiazepines and one for barbiturates/picrotoxinin (and related agents) appear to form a multicomponent complex. In this complex, the drugs binding to any of the three receptor categories are visualized to have an effect on GABA-associated chloride channel regulation. Available evidence suggests that the complex mediates many of the actions of numerous excitatory and depressant drugs showing a variety of pharmacological effects.

The presynaptic effects of valproic acid in the isolated frog spinal cord

The effects of the anticonvulsant valproic acid (n-dipropylacetate, DPA) on frog primary afferent fibers was examined with sucrose gap recordings from the dorsal roots. Addition of DPA to the superfusate consistently reduced the amplitude and duration of the dorsal root potential. In contrast, DPA augmented the depolarization of dorsal roots produced by GABA, beta-alanine and taurine. It also decreased afferent fiber 'desensitization' to GABA. DPA depressed the ability of K+ and the excitatory amino acids glutamate and aspartate to depolarize afferent fibers. In addition, the compounds decreased the amount of K+ released by tetanic stimulation of the dorsal root. The K+-evoked release of tritiated GABA from cord slices was initially reduced by exposure to DPA, but was then unaffected after a longer application of the anticonvulsant. On the other hand, the high affinity uptake of tritiated GABA and glycine were almost totally blocked by the addition of DPA to the incubating medium. In sum, DPA has complex effects on neuronal membranes. Some of these effects may serve to explain the anticonvulsant actions of this drug.

Dihydropicrotoxinin binding sites in mammalian brain: interaction with convulsant and depressant benzodiazepines

The specific binding of [3H] alpha-dihydropicrotoxinin to rat brain membranes was inhibited competitively and potently (IC50 congruent to 100 nM) by a convulsant benzodiazepine drug, RO5-3663. This compound did not inhibit high affinity flunitrazepam binding to the same tissue under similar conditions, and its reported pharmacological activity as an antagonist of GABAergic synaptic transmission, which resembles that of picrotoxinin, appears to involve the picrotoxinin binding sites. Other benzodiazepines such as diazepam, in micromolar concentrations, inhibited picrotoxinin binding in a stereospecific and chemically specific manner. However, the order of potency of a series of depressant benzodiazepines did not correlate well with pharmacological activities nor with reported activities for displacement of high affinity benzodiazepine 'receptor' binding sites (although heterogeneity of both picrotoxinin and benzodiazepine binding site populations may make difficult such comparisons). A comparison of benzodiazepine-displaceable benzodiazepine binding and benzodiazepine-displaceable picrotoxinin binding for different brain regions and subcellular fractions revealed a very similar though not identical distribution of these two classes of drug receptor, again suggesting that the two are not identical. Both classes of drug binding site also showed a very similar distribution to sodium-independent GABA receptor binding sites, which is consistent with other evidence that at least part of these 3 receptor types may be found at least sometimes coupled together in the postsynaptic membrane GABA receptor-ionophore complex.

Direct action of pentobarbitone in potentiating the responses to GABA of rat dorsal root ganglion neurones in vitro

Pentobarbitone (PB) was tested for effects on responses to GABA recorded intracellularly in rat dorsal root ganglion neurones. Concentrations of over 1 mM PB elicited small depolarizations, whereas at greater than or equal to 10 microM PB readily potentiated depolarizations and increased membrane conductance evoked by GABA. The GABA antagonists bicuculline and picrotoxin reduced PB-potentiated and equiamplitude control responses to the same degree. Since an action of PB on GABA transport is unlikely in this tissue, the PB effects probably occur at the receptor-ionophore complex.

Distinction between the effects of barbiturates, benzodiazepines and phenytoin on responses to gamma-aminobutyric acid receptor activation and antagonism by bicuculline and picrotoxin

1 Interactions of depressant and anticonvulsant drugs with the neuronal gamma-aminobutyric acid (GABA) receptor + effector system have been examined on afferent fibres to the rat cuneate nucleus in vitro. Three types of interaction have been measured: (a) potentiation of depolarizing responses to the GABA analogue, muscimol: (b) reduction in the potency of bicuculline as an antagonist of muscimol at the GABA receptor: (c) reduction in the potency of picrotoxin as an antagonist of muscimol acting on the effector mechanism. 2 Phenobarbitone reduced the potency of picrotoxin in doses which did not affect the potency of bicuculline and which caused only a small potentiation of muscimol. Pentobarbitone did not show such selectivity, a reduction in potency of picrotoxin always being accompanied by a reduction in potency of bicuculline and a substantial potentiation of muscimol. 3 Flurazepam and lorazepam both reduced the potency of picrotoxin without affecting that of bicuculline and with very little potentiation of muscimol. Phenytoin had no effect on the potency of picrotoxin whilst potentiating muscimol to the same extent as phenobarbitone. 4 The spectrum of drug activity in reducing the potency of picrotoxin correlates well with the reported anticonvulsant effects of these drugs against kindled amygdaloid seizures. Potentiation of muscimol and reduction of bicuculline potency appear more closely related to hypnotic properties.

Kainic acid lesions of striatum and decortication reduce specific [3H]sulpiride binding in rats, so D-2 receptors exist post-synaptically on corticostriate afferents and striatal neurons

Unilateral kainic acid lesions of rat striatum reduced specific striatal [3H]spiperone and [3H]sulpiride binding sites (Bmax) by 52 and 67% respectively compared with the intact side. The dissociation constant (KD) for [3H]spiperone binding was unchanged but that for [3H]sulpiride binding was reduced. Specific striatal [3H]spiperone and [3H]sulpiride binding was reduced by 22 and 37% respectively in unilateral decorticate animals, but there was no change in KD. Unilateral 6-hydroxydopamine lesions of the medial forebrain bundle caused no change in striatal [3H]spiperone binding sites or KD value, but produced a 27% increase in [3H]sulpiride binding sites with no change in KD. These data support the hypothesis of D-2 receptors located on cortico-striate glutamate fibres, but also indicate the presence of both D-1 and D-2 receptors on the cell bodies of striatal neurons.

The effect of anaesthetics on the uptake and release of gamma-aminobutyrate and D-aspartate in rat brain slices

1 The effect of various concentrations of thiopentone, pentobarbitone, methohexitone, hydroxydione, alphaxalone/alphadolone, ketamine, alpha-chloralose, and urethane on the transport of radiolabelled gamma-aminobutyric acid (GABA) and D-aspartate was investigated. 2 Uptake of the amino acids was weakly inhibited, if at all, by the anaesthetics and it is unlikely that such effects contribute significantly to their physiological function. 3 The spontaneous efflux of GABA and D-aspartate was not detectably altered by any of the drugs tested. 4 Thiopentone, pentobarbitone, methohexitone and hydroxydione inhibited K+-stimulated GABA and D-aspartate release. The other anaesthetics had no effect on K+-stimulated amino acid release. 5 The rank order of potency of the inhibitors of K+-stimulated amino acid release did not correlate with their anaesthetic potency. Furthermore not all inhibitors appeared to be very effective at anaesthetic concentrations. 6 It is concluded that although it is possible that inhibition of excitatory transmitter release may be involved in the anaesthetic action of some anaesthetics, for many of the substances tested in this study such as mechanism does not appear to be implicated.

Interaction of anticonvulsants with the barbiturate-benzodiazepine-GABA receptor complex

Unlike the anesthetic barbiturate pentobarbital and the anxiolytic pyrazolopyridine etazolate, which enhance [3H]diazepam binding to rat brain membranes, the anticonvulsant barbiturates phenobarbital and metharbital, and also chlormethiazole, at therapeutic concentrations (10-1000 muM), do not stimulate [3H]diazepam binding, but instead block the enhancement by both pentobarbital and etazolate. The same anticonvulsants at similar concentrations inhibit [3H]alpha-dihydropicrotoxinin (DHP) binding suggesting that these anticonvulsants compete for the same receptor sites as pentobarbital and etazolate, designated the barbiturate-picrotoxinin receptor component of the benzodiazepine-GABA receptor complex.

Pentobarbitone interference with inhibitory synaptic transmission in crayfish stretch receptor neurones

1. The effect of pentobarbitone (PB) on GABA-ergic inhibition was investigated in the isolated crayfish stretch receptor. The soma of the slowly adapting neurone was impaled with two micro-electrodes to give an accurate determination of membrane conductances. 2. Application of PB in concentrations from 10(-6) to 10(-3) M increased the rise time constant of the inhibitory post-synaptic potential (i.p.s.p.). The i.p.s.p. percentage amplitude and decay time constant were also increased in eight out of twelve neurones. On prolonged exposure, the percentage amplitude declined at a rate dependent upon the dose and the frequency of stimulation until the i.p.s.p. became undetectable. 3. The response to ionophoretically applied GABA remained essentially unaltered in the presence of PB, but the falling phase was prolonged by up to 8% in four of the ten neurones tested. Resting membrane conductance, i.p.s.p. driving force (i.p.s.p. reversal potential minus resting membrane potential), and parameters of the anti- and orthodromic action potential were not significantly affected. 4. Removal of PB after prolonged exposure usually caused an immediate increase in i.p.s.p. percentage amplitude but the i.p.s.p. rising phase remained slowed. 5. Application of excess extracellular GABA only affected the i.p.s.p. percentage amplitude after it had been reduced by PB. It transiently increased the attenuated i.p.s.p. percentage amplitude in the presence of PB, and after the removal of PB permanently increased the amplitude to its original value. 6. Nipecotic acid and cis-1,3-aminocyclohexane carboxylic acid, inhibitors of GABA re-uptake, slightly increased the i.p.s.p. percentage amplitude, and prolonged the falling phase but did not affect the rising phase. The percentage amplitude declined on prolonged exposure. 7. We conclude that PB has no electrophysiologically demonstrable post-synaptic action in the crayfish stretch receptor neurone, but it inhibits the presynaptic release and re-uptake of GABA.

Histidine modification with diethyl pyrocarbonate shows heterogeneity of benzodiazepine receptors

The effect of diethyl pyrocarbonate modification of histidine on the specific binding of [3H]diazepam and its enhancement with muscimol and (+/-)-pentobarbital was investigated. Diethyl pyrocarbonate treatment produced a dose-related inhibition of specific [3H]diazepam binding to rat brain membranes with a maximal inhibition of approximately 40% at 1 mM. Scatchard analysis of the binding data showed that diethyl pyrocarbonate, while having no effect on the affinity (Kd), decreased the binding capacity (Bmax) of diazepam from a control value of 1543 +/- 116 fmol/mg of protein to 789 +/- 79 fmol/mg of protein (mean +/- SD; P less than 0.005; n = 4). Under conditions in which approximately 40% of the diazepam binding sites were modified by diethyl pyrocarbonate treatment, the ability of muscimol and pentobarbital to enhance diazepam binding was not altered. These results suggest that a histidine residue is critical for a part (approximately 40%) of the benzodiazepine binding sites and that there may exist a heterogeneity of benzodiazepine binding sites. Furthermore, these results indicate that perhaps only a portion of the benzodiazepine binding sites are functionally coupled to the gamma-aminobutyric acid receptor-ionophore complex.

Pentobarbital enhances [3H]diazepam binding to soluble receptors at the benzodiazepine--GABA-receptor-ionophore complex

Pentobarbital enhances [3H]diazepam binding to soluble receptors in a dose-dependent and saturable manner. Half-maximal enhancement occurred at 110 micrometers pentobarbital. This enhancement was due to an increase in the affinity of diazepam to its receptor sites. Pentobarbital enhancement was blocked by picrotoxinin and bicuculline. These results are similar to those reported for membranes and suggest that various components of the benzodiazepine--GABA-receptor-ionophore complex are intimately associated.

Interaction of stereoisomers of barbiturates with [3H]alpha-dihydropicrotoxinin binding sites

Racemic depressant barbiturates inhibit the binding of gamma-aminobutyric acid antagonist [3H]alpha-dihydropicrotoxinin (DHP) to rat brain membranes with IC50 values ranging from 5 to 50 microM. The (-) isomers of pentobarbital and secobarbital were three to four-fold more potent that their (+) isomers in inhibiting [3H]DHP binding. In contrast, the (+) isomer of hexobarbital was a better inhibitor than (-) hexobarbital. The stereoisomers of 1-methyl-5-phenyl-5-propylbarbituric acid (MPPB), which show opposite pharmacological activity, inhibited [3H]DHP binding in a biphasic manner with a plateau at 4-100 nM MPPB. These results suggest heterogeneity of DHP binding sites. The possibility that depressant and convulsant barbiturates may act at the level of DHP site at the GABA synapse is discussed.

Solubilization of the picrotoxinin binding receptor from mammalian brain

The binding sites for alpha-dihydropicrotoxinin (DHP), which is a ligand for the picrotoxin-sensitive component at the benzodiazepine-gamma-aminobutyric acid-receptor-ionophore complex, has been solubilized from rat brain, using 1% Lubrol. A new assay, which involves precipitation of the [3H]DHP-soluble protein complex by gamma-globulin and polyethylene glycol (PEG), followed by centrifugation, is described. The solubilized material bound DHP to two sites with apparent affinities of 0.038 and 1.85 microM. The binding of DHP to the solubilized receptors was inhibited by convulsant and depressant drugs with potencies similar to those required for membrane receptors. The ability of barbiturates to inhibit DHP binding to both solubilized and membrane receptors strongly suggests that barbiturates may interact with the picrotoxin binding component. These data suggest that ligand recognition properties of the picrotoxinin binding are not altered by solubilization. The binding was abolished by urea and partially destroyed by heating the soluble extract at 65 degrees C for 30 min. This new method of measuring the binding of ligands to the solubilized receptors by PEG centrifugation might be used successfully in other solubilization studies.

GABA and baclofen potentiate the K+-evoked release of methionine-enkephalin from rat striatal slices

GABA potentiates the potassium-evoked release of methionine-enkephalin (ME) from slices of rat corpus striatum. This potentiation is observed only when a submaximal concentration (30 mM) of K+ is used to evoke release. The effect of GABA is dose-dependent between 100 and 100 micrometers. The basal release of ME is not altered by these concentrations of GABA. Before, but not muscimol, mimics the effect of GABA on the evoked release of ME. This effect is not stereoselective as both the (+)- and (-)-isomers of baclofen enhance ME release. Picrotoxin (100 micrometers) blocks the enhancement of ME release produced by both GABA and baclofen. Bicuculline methiodide (100 micrometers) does not block the effect of GABA. The effect of GABA on ME release may be mediated by an atypical GABA receptor which is activated by baclofen.