Stimulatory and protective effects of benzodiazepines on GABA receptors labeled with [3H]muscimol

Unequal efficacy of pyridinium oximes in acute organophosphate poisoning

The use of organophosphorus pesticides results in toxicity risk to non-target organisms. Organophosphorus compounds share a common mode of action, exerting their toxic effects primarily via acetylcholinesterase (AChE) inhibition. Consequently, acetylcholine accumulates in the synaptic clefts of muscles and nerves, leading to overstimulation of cholinergic receptors. Acute cholinergic crisis immediately follows exposure to organophosphate and includes signs and symptoms resulting from hyperstimulation of central and peripheral muscarinic and nicotinic receptors. The current view of the treatment of organophosphate poisoning includes three strategies, i.e. the use of an anticholinergic drug (e.g., atropine), cholinesterase-reactivating agents (e.g., oximes) and anticonvulsant drugs (e.g., benzodiazepines). Oximes, as a part of antidotal therapy, ensure the recovery of phosphylated enzymes via a process denoted as reactivation of inhibited AChE. However, both experimental results and clinical findings have demonstrated that different oximes are not equally effective against poisonings caused by structurally different organophosphorus compounds. Therefore, antidotal characteristics of conventionally used oximes can be evaluated regarding how close the certain substance is to the theoretical concept of the universal oxime. Pralidoxime (PAM-2), trimedoxime (TMB-4), obidoxime (LüH-6), HI-6 and HLö-7 have all been demonstrated to be very effective in experimental poisonings with sarin and VX. TMB-4 and LüH-6 may reactivate tabun-inhibited AChE, whereas HI-6 possesses the ability to reactivate the soman-inhibited enzyme. An oxime HLö-7 seems to be an efficient reactivator of AChE inhibited by any of the four organophosphorus warfare agents. According to the available literature, the oximes LüH-6 and TMB-4, although relatively toxic, are the most potent to induce reactivation of AChE inhibited by the majority of organophosphorus pesticides. Since there are no reports of controlled clinical trials on the use of TMB-4 in human organophosphate pesticide poisoning, LüH-6 may be a better option.

Inactivity of phosphoethanolamine, an endogenous GABA analog decreased in Alzheimer's disease, at GABA binding sites

Phosphoethanolamine (PE) is a metabolite of the phospholipid metabolism which is decreased in Alzheimer's disease brain. PE shows a strong structural similarity to the inhibitory neurotransmitter, GABA, and the GABAB receptor partial agonist, 3-amino-propylphosphonic acid. The ability of PE to compete for binding to GABAA and GABAB binding sites was investigated. GABAA sites were studied using [3H]SR-95531 and [3H]muscimol. GABAB sites were studied using [3H]GABA in the presence of isoguvacine to saturate GABAA sites. Total [3H]GABA binding was also examined. PE showed little activity at any of the GABA binding sites investigated. PE was most potent at GABAB sites, but the IC50 of 7.5 +/- 0.75 mM was considerably higher than its maximal physiologic concentration of approximately 1.5 mM. The efficient exclusion of PE from GABA binding sites may be an important physiologic mechanism in the control of inhibitory neurotransmission. The structural basis for this exclusion is discussed in reference to the GABAB partial agonist 3-amino-propylphosphonic acid.

Characterization of [3H]-imidazenil binding to rat brain membranes

1. The binding of [3H]-imidazenil, an imidazobenzodiazepine carboxamide, to rat cerebellar membranes was characterized at different temperatures. 2. Specific binding was linear with tissue concentrations and reached maximum after 90, 30 and 5 min incubation at 0, 21 and 37 degrees C, respectively. The binding was of high affinity, specific and saturable; non linear regression and Scatchard analysis of the data was compatible with the presence of a single population of receptor sites with Bmax of 0.74 +/- 0.020, 0.90 +/- 0.011 and 1.0 +/- 0.036 pmol mg-1 protein at 0, 21 and 27 degrees C, respectively. Binding affinity decreased with increasing temperature: Kd were 0.29 +/- 0.051 nM (0 degrees C), 1.0 +/- 0.080 nM (21 degrees C) and 2.4 +/- 0.38 nM (37 degrees C). 3. At all tested temperatures, [3H]-imidazenil binding was reversible and the Kd calculated from the dissociation and association rate constants approximated the equilibrium Kd. 4. In the presence of gamma-aminobutyric acid (GABA), Kd increased 4 fold at 0 degrees C, whereas Bmax increased, albeit slightly, at all temperatures. 5. Benzodiazepines (BZDs), imidazopyridines and methyl-beta-carboline-3-carboxylate (beta CCM) were effective inhibitors of [3H]-imidazenil binding. Conversely, GABAA antagonists, barbiturates, picrotoxin and peripheral BZD receptor ligands were devoid of any activity. 6. Comparing [3H]-imidazenil to [3H]-flumazenil binding in various brain areas, similar densities of recognition sites as well as like regional differences in the distribution of binding sites for both radioligands were observed (cortex = striatum > cerebellum > spinal cord). 7. The present results indicate that [3H]-imidazenil specifically binds to the BZD sites of GABAA receptors. Furthermore, the effects of GABA and temperature differentiate imidazenil from classicalBZDs. It is suggested that the characteristics of imidazenil binding may be relevant to the in vivo pharmacology of the drug.

Effects of 1-amino-5-bromouracil on the benzodiazepine-GABAA receptor complex

We investigated the effects of 1-amino-5-bromouracil on the benzodiazepine-gamma-aminobutyric acid (GABA)A receptor complex to elucidate its central action. 1-Amino-5-bromouracil neither displaced nor enhanced [3H]muscimol, [35S]t-butylbicyclophosphorothionate (TBPS), or [3H]dehydroepiandrosterone sulfate binding to the rat brain synaptosomal membranes. The anesthesia induced by 1-amino-5-bromouracil was potentiated by diazepam, pentobarbital, and muscimol, and was antagonized by picrotoxin but not by bicuculline. 1-Amino-5-bromouracil protected mice from picrotoxin-induced seizure and slightly ameliorated TBPS-induced seizure, but did not antagonize bicuculline-induced seizure. Diazepam antagonized both the bicuculline- and the picrotoxin-induced seizure, and pentobarbital antagonized the picrotoxin- and the TBPS-induced seizure. Our in vivo studies suggest that part of the central action of 1-amino-5-bromouracil is concerned with the benzodiazepine-GABAA receptor complex including the chloride channel.

Volatile anesthetics bidirectionally and stereospecifically modulate ligand binding to GABA receptors

Pharmacologically relevant concentrations of volatile anesthetics can bidirectionally modulate radioligand binding to GABAA receptors. In mouse cerebral cortex, halothane (a prototypic volatile anesthetic) increased [3H]muscimol (a GABA receptor agonist) binding while inhibiting the binding of a GABA receptor antagonist ([3H]SR 95531). These bidirectional effects of inhalational anesthetics on ligand binding to GABA receptors are effected through changes in the Bmax with no significant alterations in the KD of these radioligands. Moreover, the concentration dependent, bidirectional modulation of radioligand binding to GABA receptors by volatile anesthetics exhibited stereoselectivity. Thus, (+)-isoflurane was about twice as potent as the (-)-enantiomer in enhancing [3H]muscimol binding and approximately 50% more potent as an inhibitor of [3H]SR 95531 binding, respectively. The demonstration of a bidirectional, stereospecific modulation of radioligand binding to GABA receptors by inhalational agents is consistent with the presence of specific recognition sites for inhalational anesthetics on the GABAA receptor complex.

Benzodiazepine receptor binding following chronic treatment with naloxone, morphine and met-enkephalin in normal rats

The effects of chronic administration of naloxone, morphine and met-enkephalin on benzodiazepine (BDZ) receptor binding in rat brain were determined 2 and 50 days after treatments were accomplished. Two days after naloxone treatment (75 micrograms/h s.c. for 14 days), enhanced BDZ receptor binding was observed in cingulate, frontal, piriform, entorhinal and sensorimotor cortices; amygdala complex, hippocampus, substantia nigra and central gray. Two days after morphine treatment (20 mg/kg i.p. daily for 6 days), increased BDZ receptor binding was detected in cingulate, frontal, piriform, entorhinal and sensorimotor cortices; amygdala complex, hippocampus and substantia nigra. Two days after met-enkephalin treatment (10 micrograms/h i.c.v. for 6 days) enhanced BDZ receptor binding was shown only in sensorimotor cortex. No significant changes were observed 50 days after the treatments were completed. These data indicate an important interaction between GABAergic and opioid peptide systems.

Development of tolerance to anxiolytic effects of chlordiazepoxide in elevated plus-maze test and decrease of GABAA receptors

Repeated administration of benzodiazepines has been reported to produce tolerance in animals and humans. Using an elevated plus-maze test and an autoradiographic technique, we investigated whether repeated administration of chlordiazepoxide produced tolerance to its anxiolytic effects, and whether such repeated administration altered benzodiazepine and GABAA receptors. Tolerance to the anxiolytic effect of chlordiazepoxide was produced when it was administered at a dose of 30 mg/kg (i.p.) once a day for 10 and 14 days. In the quantitative autoradiographical study, although repeated chlordiazepoxide treatment had no effect on [3H]flunitrazepam and [3H]Ro 15-4513 binding to benzodiazepine receptors, such treatment reduced [3H]muscimol binding to GABAA receptors in the cortex, caudate putamen, and hippocampus. These results suggest firstly, the production of tolerance to the anxiolytic effects of chlordiazepoxide, and, secondly, that this tolerance may be due to the down-regulation of GABAA receptors, but not of benzodiazepine receptors.

Development of tolerance to amnesic effects of chlordiazepoxide in relation to GABAergic and cholinergic neuronal systems

Chronic administration of benzodiazepines has been reported to produce tolerance in animals and humans. We investigated whether benzodiazepines produce tolerance to the amnesic effects and effects on benzodiazepine receptors, GABAergic and/or cholinergic neuronal systems of repeated administration of chlordiazepoxide, using a passive avoidance task and autoradiographic techniques. Tolerance developed to the amnesic effect of chlordiazepoxide when the drug was administered at a dose of 30 mg/kg (i.p.) once a day for 14 days. Bicuculline (1.0 and 1.5 mg/kg), a GABAA receptor antagonist, did not induce amnesia in normal mice, but did so in chlordiazepoxide-tolerant mice. Muscimol (0.25 mg/kg), a GABAA receptor agonist, in combination with a low dose of chlordiazepoxide, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. Scopolamine, an acetylcholine receptor antagonist, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. In the autoradiographical study, although repeated treatment with chlordiazepoxide had no effect on [3H]flunitrazepam and [3H]Ro 15-4513 binding to benzodiazepine receptors, it decreased [3H]muscimol binding to GABAA receptors, with a decrease in affinity in the cortex and hippocampus. Furthermore, repeated administration of chlordiazepoxide increased [3H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors in the hippocampus. These results suggest that tolerance develops to the amnesic effects of chlordiazepoxide, and that tolerance may be due to down-regulation of GABAA receptors and/or up-regulation of acetylcholine receptors.

The role of GABAA and GABAB receptors in the development of amnesia

It has been demonstrated that a gradual spontaneous recovery of a previously unreproducible memory trace and the prevention of the antiamnesic effect of the blockade of the GABAA receptor, but not of the blockade of the benzodiazepine receptor or the chloride channel, take place with the development of amnesia against the background of the activation of the GABAB receptor by baclofen. The "neurochemical set" created by the activation of the GABAA receptor by muscimol in a dose of 1 mg/kg prevents the antiamnesic effect of the blockade of any component of the benzodiazepine-GABA-ionophore complex, while at a dose of muscimol of 0.5 mg/kg, the retrieval of the amnestic trace takes place only by blockade of the benzodiazepine receptor. Thus, the development of amnesia is determined by the functional state of inhibitory GABAergic systems of the brain, which governs the subsequent correction of the amnestic memory trace.

GABAergic and dopaminergic transmission in the rat cerebral cortex: effect of stress, anxiolytic and anxiogenic drugs

Benzodiazepines produce their pharmacological effects by regulating the interaction of GABA with its recognition site on the GABAA receptor complex. In fact, the anxiolytic effect of benzodiazepines may be considered the consequence of the activation of the GABAA receptors induced by these drugs. On the contrary, beta-carboline derivatives which bind with high affinity to benzodiazepine recognition sites modulate the GABAergic transmission in a manner opposite to that of benzodiazepines. Thus, these compounds reduce the function of the GABA-coupled chloride channel and produce pharmacological effects (anxiogenic, proconvulsant and convulsant) opposite to those of benzodiazepines. Taken together, these data strongly indicate that the GABAA receptor complex plays a major role in the pharmacology, neurochemistry and physiopathology of stress and anxiety. This conclusion is further supported by the finding that the function of the GABAA/benzodiazepine receptor complex may be modified by the emotional state of the animals before sacrifice. Accordingly, using an unstressed animal model, the 'handling-habituated' rats, it has been demonstrated that stress, like anxiogenic drugs, decreases the function of GABAA receptor complex, an effect mimicked by the in vivo administration of different inhibitors of GABAergic transmission and antagonized by anxiolytic benzodiazepines. Moreover, a long-lasting down regulation of GABAergic synapses can be obtained after repeated administration of anxiogenic, proconvulsant and convulsant negative modulators of GABAergic transmission. The latter finding further suggests that GABAergic synapses undergo rapid and persistent plastic changes when the GABAergic transmission is persistently inhibited. Finally, the evidence that the activity of mesocortical dopaminergic pathways is altered in opposite manner by drugs that either inhibit or enhance the GABAergic transmission indicates that GABA has a functional role in regulation of dopaminergic neurons in the rat cerebral cortex. Altogether these results suggest that cortical GABAergic and dopaminergic transmission play a major role in the pharmacology, neurochemistry and pathology of the emotional states and fear.

Functional coupling of GABAA receptors and benzodiazepine recognition site subtypes in the spinal cord of the rat

The interaction between GABAA receptors and benzodiazepine (BZD) recognition site subtypes in the spinal cord of the rat was investigated. Computer analysis of displacement curves for [3H]flunitrazepam [( 3H]FNT) binding by 2-oxo-quazepam (2OXOQ) indicated the presence of two subtypes of BZD recognition sites in this region. Type I sites accounted for approximately 25% of the total number of BZD recognition sites, the remainder being Type II sites. A similar proportion of Type I and Type II sites was obtained by Scatchard analysis of the saturation curves for [3H]FNT, [3H]2OXOQ and [3H]ethyl-beta-carboline-3-carboxylate [( 3H]beta CCE) binding. The in vitro addition of GABA (10(-8)-10(-4) M) to spinal cord membrane preparations produced an increase in the binding of [3H]FNT and [3H]2OXOQ. The maximal enhancement produced by GABA was 50 and 82% above control values for [3H]FNT and [3H]2OXOQ, respectively. In contrast, GABA stimulated both [3H]FNT and [3H]2OXOQ binding in the cerebellum to a similar extent. We also evaluated the effects of different ligands for BZD recognition sites on the binding of [3H]GABA to spinal cord membranes, as compared with brain areas containing a higher proportion ( greater than 30%) of Type I sites. Diazepam, quazepam and the beta-carboline, ZK 93423, enhanced the specific binding of [3H]GABA in a concentration-dependent manner (10(-7)-10(-5) M) in the cerebral cortex and hippocampus but not in the spinal cord and cerebellum. These results indicate that there is a regional variation in the interaction between GABA and BZD recognition sites in the central nervous system.

Functional coupling of gamma-aminobutyric acid (GABA)A and benzodiazepine type II receptors: analysis using purified GABA/benzodiazepine receptor complex from bovine cerebral cortex

Possible functional coupling between gamma-aminobutyric acid (GABA) and benzodiazepine receptors was examined using a purified GABA/benzodiazepine receptor complex. The purified receptor complex was obtained by 1012-S-acetamide adipic hydrazide Sepharose 4B affinity column chromatography, following the solubilization of synaptic membrane from the bovine cerebral cortex with Nonidet P-40. The binding of [3H]GABA to the purified GABA receptor was displaced significantly by muscimol and bicuculline, GABAA receptor agonists and antagonists, respectively, but not by baclofen, a GABAB receptor agonist. On the other hand, the binding of [3H]flunitrazepam to the purified benzodiazepine receptor was found to be displaced by microM ranges of CL 218,872, which is known to be sensitive to the benzodiazepine type II receptor. Furthermore, it was found that the binding of [3H]muscimol to these purified GABAA receptors was enhanced by benzodiazepines, while the binding of [3H]flunitrazepam to these benzodiazepine type II receptors was increased by GABA receptor agonists. These enhancements by GABA agonists and benzodiazepine agonists were found to be blocked by bicuculline and a benzodiazepine receptor antagonist, Ro15-1788, respectively. These results strongly suggest that the purified receptor may consist of GABAA and benzodiazepine type II receptors and possess a functional coupling of these sites, as shown in cerebral synaptic membranes.

The affinities, potencies and efficacies of some benzodiazepine-receptor agonists, antagonists and inverse-agonists at rat hippocampal GABAA-receptors

The abilities of some benzodiazepine-receptor agonists, antagonists and inverse agonists to modulate the inhibitory potency of the gamma-aminobutyric acid (GABA)A-receptor agonist, isoguvacine, on the CA1 population spike recorded from slices of rat hippocampus, were determined. Concentration-response curves were constructed of the extent to which the benzodiazepine-receptor ligands shifted the isoguvacine concentration-response curve to the left or right. These were compared to their displacement curves of [3H]-Ro15-1788 binding to rat hippocampal membranes under near physiological assay conditions. The above comparisons suggest that the effect on the potency of isoguvacine produced by the benzodiazepine-receptor agonists, diazepam and flunitrazepam, and the partial agonists, Ro16-6028 and Ro17-1812, closely parallels their degree of benzodiazepine-receptor occupancy. Thus, the partial agonists, Ro16-6028 and Ro17-1812, were unable to produce as large a maximum response as the full agonists, diazepam and flunitrazepam. The maximum effects produced by diazepam, flunitrazepam, Ro16-6028, Ro17-1812, the antagonist, propyl-beta-carboline-3-carboxylate, and the inverse agonist, methyl-6, 7-dimethyl-4-ethyl-beta-carboline-3-carboxylate (DMCM), on the potency of isoguvacine in the hippocampal slice corresponded to the change in their affinities produced by the addition of GABA in the radioligand binding studies (GABA-shift). This suggests that the changes in affinity of benzodiazepine-receptor ligands produced by GABAA-receptor activation reflects their ability to modify GABAA-receptor function. The benzodiazepine-receptor antagonists, Ro15-1788 and CGS 8216, had apparent agonist and inverse agonist effects, respectively, on the potency of isoguvacine. These effects occurred at concentrations above those required for saturation of the benzodiazepine-receptor, as labelled by [3H]-Ro15-1788, and were not in agreement with the absence of any effect of GABAA-receptor stimulation in the GABA-shift experiments. This indicates that these events are not mediated by an action at the classical benzodiazepine-receptor site. 6 It is concluded that hippocampal GABAA-receptor function can be allosterically modulated in a manner consistent with the agonist/inverse-agonist model of benzodiazepine-receptor activation, and that compounds exist with varying efficacies throughout this range.

In vitro studies of the effects of naloxone on the root potentials in the frog spinal cord: enkephalin-like effect on the recurrent presynaptic inhibition

Specific binding of [3H]naloxone was demonstrated in the frog spinal cord. In isolated and perfused frog spinal cord, naloxone increased the spontaneous discharges of the ventral root. Naloxone decreased the ventral root-dorsal root potential (VR-DRP) in a dose-dependent manner, and inhibited presynaptic inhibition of the ventral root reflex. Methionine-enkephalin also decreased the VR-DRP, and naloxone partially antagonized this effect. These results suggest the existence of enkephalinergic control of spinal motor activities and that naloxone has a partial agonistic effect in the frog spinal cord.

Mechanisms of depressant drug action/interaction

Whereas the effects of individual psychotropic drugs depend upon drug type, route of administration, dose, and frequency of use, as well as unique subject (patient) variables, the actions achieved by two or more psychotropics taken concurrently are complicated by their influence upon each other. Interactions may be antagonistic, additive, or synergistic and are frequently predictable, given a basic understanding of the kinetic and dynamic characteristics for each drug. This knowledge should enable rational interpretation, therapeutic intervention, and possible prevention of polydrug toxicity. Classically, pharmacodynamic drug interaction is described in terms of common receptor activation or antagonism. This limited view is inadequate in the present context and should be broadened to encompass all of the mechanistic elements that initiate, transduce, and amplify neuronal membrane action. Thus, although psychotropic drugs may compete for a limited number of specific binding sites, as the opiates do, they may also interact through allosteric mechanisms and nonspecific modulation of the receptor environment or subsequent effector cell mechanisms. Drugs in the depressant class often act synergistically in these ways. Through consideration of nonreceptor mediated interaction, we can more fully appreciate the potentiation that occurs between seemingly unrelated substances (e.g., antihistamines and ethanol) and the ability or lack thereof to medically treat such interactions specifically. The pharmacokinetic determinants of drug action provide many opportunities for synergy between psychotropic drugs. Each process is a fertile substrate. Absorption from the gastrointestinal tract is sensitive to drugs that alter peristaltic motility and glandular secretion. Those that inhibit motility tend to delay the rate, if not the extent, of absorption and consequently reduce peak intensity and prolong duration of the psychotropic effect. Serum albumin binding can be a vital point of interaction for drugs with high intrinsic binding affinity (e.g., 98% for methadone); displacement of a small amount of bound drug by a competing substance may increase the free drug concentration severalfold and thereby potentiate its actions(s). Psychotropic drug effects would last for days and even weeks, were it not for the body's ability to synthetically alter drug molecule configuration. This process takes place primarily in the liver where oxidative reactions are frequently catalyzed by the mixed function oxidase system.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of benzodiazepine recognition sites in the foot shock-induced decrease of low affinity GABA receptors in the rat cerebral cortex

The cerebral cortices of rats habituated to the handling manipulation that precedes sacrifice by guillotine (unstressed rats) have a higher number of low affinity GABA receptors than naive rats (stressed rats). Foot shock stress delivered to handling-habituated rats 5 min before sacrifice decreased the number of low affinity GABA receptors to the level found in naive animals, while leaving almost unchanged the [3H]GABA binding in the latter group. Since benzodiazepine (BZ) recognition sites are the target through which benzodiazepines modulate the emotional states of the animals, we investigated whether these receptors were involved in the action of foot shock stress on GABA binding. The in vitro addition of diazepam (5 X 10(-6) M) to cortical membranes from foot-shocked handling-habituated rats brought back the number of low affinity GABA receptors to the level found in cortical membranes from handling habituated rats. Moreover, the effect of foot shock on low affinity GABA receptors was completely antagonized in vivo by pretreatment with the specific benzodiazepine antagonist Ro15-1788 (30 mg/kg per os). Since the effect of foot shock on [3H]GABA binding is mimicked by the in vitro addition of beta-carbolines to cortical membranes from handling habituated rats, our working hypothesis is that an endogenous ligand for BZ recognition sites, possessing beta-carboline-like properties, is released during foot shock stress.

Photoaffinity labeling of the GABAA receptor with [3H]muscimol

Muscimol is one of the most potent agonist ligands at the gamma-aminobutyric acidA (GABAA) receptor. Analysis of its chemical structure showed it to be a candidate for photoaffinity labeling. In practice, UV irradiation at 254 nm both changed the UV spectrum of muscimol and induced an irreversible binding of [3H]-muscimol to rat cerebellar synaptosomal membrane. After 10 min of irradiation, using 10 nM [3H]muscimol, the specific portion of this binding was 270 fmol/mg protein. (Nonspecific binding was defined as that arising in the presence of 1 mM GABA.) Specific binding increased asymptotically up to 100 nM [3H]muscimol. Irradiation of the membranes themselves did not significantly alter the KD or Bmax of reversible [3H]muscimol binding. However, irradiation of [3H]muscimol reduced its capacity subsequently to photolabel the membranes by 86 +/- 3%. Dose-dependent inhibition of binding was observed with muscimol, GABA, and bicuculline methiodide; with 10 nM [3H]muscimol maximum inhibition was 70% of total labeling and the order of potencies of these three compounds was characteristic of labeling to the GABAA receptor. Baclofen, l-glutamate, and diazepam exerted no effect at high concentrations. SDS-PAGE of the photolabeled membranes indicated specific incorporation of radioactivity into two molecular-weight species. One failed to enter the separating gel, implying a molecular weight greater than 250,000 daltons (250 kD). The molecular weight of the other was identified by fluorography to be about 52,000 daltons (52 kD).

A study of the mechanism of cerebral hyperexcitability after irradiation

Whole-body irradiation at a lethal dose induced an early and transient perturbation of 3H-Muscimol specific binding in the mouse cerebellum. The density of high affinity binding sites was decreased and was associated with an increased affinity. This effect can explain early radio-induced cerebral hyperexcitability. Previous injection of cysteamine, which protects against cerebral hyperexcitability, also offers a good protection against this effect.

Stress and β-carbolines decrease the density of low affinity gaba binding sites

Cerebral cortex membranes from rats habituated to manipulations preceding decapitation (habituated rats) had 40% higher GABA binding than membranes from naive animals. Diazepam (5 X 10(-6) M), added to membranes from naive rats, increased GABA binding to the level of habituated rats, but failed to induce any further increase in membranes from the latter animals. Vice versa, beta-carbolines (FG 7142, beta-CCE, DMCM) added to membranes from habituated rats lowered GABA binding to the level of naive animals, but caused no further decrease in the membranes from this last group. Diazepam removed the effect of beta-carbolines in membranes from habituated rats. It is suggested that handling represents a stressful stimulus for naive animals and that stress lowers GABA binding by releasing an endogenous ligand for benzodiazepine receptors possessing similar properties to beta-carbolines. Finally, the results indicate that the emotional status of animals from which brain tissue is obtained should be considered when connections between GABA and benzodiazepine receptors are studied.

In vitro effects of melatonin on [3H]muscimol binding in rat brain

The possible interaction of melatonin with the GABAergic system at the receptor level was investigated. The effect of melatonin on the binding of the GABA agonist, [3H]muscimol, in crude synaptic membrane preparations was assayed. In fresh membrane preparations, melatonin increased [3H]muscimol binding in the rat striatum and frontal cortex. Treatment of membrane preparations with Triton X-100 appeared to abolish or depress the enhancing effect of melatonin. The use of Tris HCl as buffer appeared to be more effective than Tris-Citrate buffer suggesting a possible important ionic effect. The present findings suggest that melatonin is capable of enhancing [3H]muscimol binding in vitro, perhaps at the benzodiazepine-GABA-receptor-ionophore complex.

Benzodiazepine-GABA receptor-ionophore complex. Current concepts

The benzodiazepine--gamma-aminobutyric acid (GABA) receptor--ionophore system is an oligomeric complex, composed of at least three interacting components. These three components have been well characterized in vitro by radioreceptor binding assays. A variety of centrally acting anxiolytic, depressant, anticonvulsant and convulsant drugs, which affect GABAergic transmission, bind to one of the sites and modulate the binding of ligands at the other sites. Thus, depressant barbiturates, nonbarbiturate hypnotics (like etomidate) and pyrazolopyridines (like etazolate), while inhibiting the binding of alpha-dihydropicrotoxinin (DHP), enhance the binding of GABA and benzodiazepines. These enhancing effects are blocked by convulsant drugs that inhibit the binding of dihydropicrotoxinin and also by bicuculline. These interactions involving barbiturates and other modulatory drugs, exhibit stereoselectivity, anion dependence and brain regional selectivity. Several classes of drugs which facilitate GABAergic transmission appear to interact with the sites for GABA and benzodiazepines allosterically via the dihydropicrotoxinin site of the oligomeric complex. The GABA system has also been implicated in a variety of pathological conditions, including anxiety, seizure activity, movement disorders, cardiovascular control, pain and in drug dependence. Since most of the GABA agonists do not pass the blood-brain barrier, future trends in the pharmacology of GABA may be the development of drugs that will activate the GABA receptor system via picrotoxinin or benzodiazepine sites.

Diazepam stimulates the binding of GABA and muscimol but not THIP to rat brain membranes

Diazepam (10-1000 nM) enhanced the binding of [3H]GABA and of the monocyclic GABA agonist [3H]muscimol, but failed to alter binding of the bicyclic GABA agonist [3H]THIP to fresh, well washed rat brain membranes incubated at 2 degrees C. Although stimulation of [3H]diazepam binding by THIP was observed at higher incubation temperatures and in the presence of chloride ions, these measures did not induce a corresponding enhancement of [3H]THIP binding by diazepam. These results extend earlier observations of the unusual behavior of THIP as a selective GABA agonist, and emphasize that enhancement of benzodiazepine binding by GABA agonists is not necessarily reflected in a complementary manner by any action of benzodiazepines on the binding of GABA agonists.

Anisatin modulation of temperature-dependent inhibition [3H]muscimol binding by chloride ion

We investigated the effects of anisatin, a pure toxic substance isolated from Illicium anisatum, and chloride ion on [3H]muscimol binding to rat brain membranes at various temperatures (0-16 degrees C). Chloride ion (100 mM) itself slightly inhibited specific [3H]muscimol binding at 0-4 degrees C and this inhibition was more marked at 10 or 16 degrees C than at 0-4 degrees C. Anisatin further decreased [3H]muscimol binding inhibited by 100 mM chloride at 16 degrees C without affecting the basal specific binding. These effects of anisatin were also dependent on the temperature. Scatchard analysis revealed that the effects of chloride and chloride plus anisatin on [3H]muscimol binding were mainly due to reduction in the number of binding sites. The temperature- and chloride-dependent effects of anisatin were similar to those of picrotoxinin. Pre-treatment of membranes with 0.05% Triton X-100 markedly decreased the inhibitory effects of chloride and chloride plus anisatin. Our results indicate that anisatin affects the number of GABA receptor sites probably through the picrotoxin sensitive sites and that chloride ion not only modifies the GABA receptors but also plays an important role in the action of anisatin and picrotoxinin.

Enhancement of GABA binding by benzodiazepines and related anxiolytics

Several benzodiazepines (chlordiazepoxide, clonazepam, diazepam, midazolam, nitrazepam and oxazepam) produced a concentration-dependent enhancement of low affinity GABA binding to fresh, washed brain membranes in 50 mM Tris-citrate buffer at concentrations comparable to those displacing [3H]diazepam binding in vitro. The nonbenzodiazepine anxiolytics CL218872 and zopiclone also enhanced GABA binding, while the centrally inactive benzodiazepine Ro5-4864 failed to alter GABA binding. The benzodiazepine antagonist, Ro15-1788 did not alter GABA binding but potently antagonised stimulation of GABA binding by 100 nM diazepam. These pharmacological characteristics suggest that an enhancement of the binding of GABA to low affinity receptor sites may give rise to many of the in vivo actions of the benzodiazepines.

Molecular sizes of photolabeled GABA and benzodiazepine receptor proteins are identical

[3H]Muscimol was irreversibly incorporated into rat cerebellar membranes upon irradiation with ultraviolet light. GABA agonists and antagonists inhibited this incorporation. The reversible muscimol-binding decreased after photoaffinity-labeling of the membranes with muscimol. These results indicated that this irreversible incorporation is to the GABA receptor. Photolabeled GABA receptor protein showed its Mr of 50000 +/- 1000 in SDS-polyacrylamide gel electrophoresis. This molecular size is identical to that of the benzodiazepine receptor which was photolabeled with 3H-flunitrazepam.