Urinary and brain beta-carboline-3-carboxylates as potent inhibitors of brain benzodiazepine receptors

A three-state model of the benzodiazepine receptor explains the interactions between the benzodiazepine antagonist Ro 15-1788, benzodiazepine tranquilizers, beta-carbolines, and phenobarbitone

The potent benzodiazepine receptor ligands beta-carboline-3-carboxylic acid ethyl ester (beta-CCE) and the corresponding methylester (beta-CCM) administered i.v. depressed segmental dorsal root potentials in spinal cats, reversed the prolongation of dorsal root potentials by phenobarbitone, and abolished the depression of a motor performance task induced by phenobarbitone in mice; beta-CCE enhanced the low-frequency facilitation of pyramidal population spikes in the hippocampus of anaesthetized rats. These effects of beta-carbolines reflect a depression of GABAergic synaptic transmission and, thus, are diametrically opposed to the enhancing action of benzodiazepine tranquilizers. The specific benzodiazepine antagonist, Ro 15-1788, while not affecting dorsal root potentials, hippocampal population spikes or phenobarbitone-induced motor performance depression, abolished the effects of beta-CCE on the three parameters and similar effects of beta-CCM on the spinal cord and motor performance. A three-state model of the benzodiazepine receptor is proposed in which benzodiazepine tranquilizers act as agonists enhancing the function of the benzodiazepine receptor as a coupling unit between GABA receptor and chloride channel, beta-carbolines act as "inverse agonists" reducing this coupling function, and Ro 15-1788 represents a competitive antagonist blocking both the enhancing effect of agonists and the depressant effect of "inverse agonists" on GABAergic synaptic transmission.

Inhibition of [3H]diazepam and [3H]3-carboethoxy-beta-carboline binding by irazepine: evidence for multiple "domains" of the benzodiazepine receptor

The binding of [3H]diazepam and [3H]3-carboethoxy-beta-carboline was examined in rat brain synaptosomal membranes treated with irazepine, an alkylating benzodiazepine. Under incubation conditions that resulted in a 25-33% reduction in the Bmax of [3H]diazepam binding, only modest (less than 8.5%) reductions in the Bmax of [3H]3-carboethoxy-beta-carboline were observed. The differential effects of irazepine on the binding of these two compounds may be explained by the presence of multiple areas or "domains" on the benzodiazepine receptor.

Binding of beta-carbolines and caffeine on benzodiazepine receptors: correlations to convulsions and tremor

Compounds from both the beta-carboline (BC) and xanthine groups have been suggested to be the natural ligands for benzodiazepine (BZ) receptors. In this study we examined the effects of several BC's and caffeine, 1,3,7-trimethylxanthine, on the binding of 3H-flunitrazepam (3H-FZ) and beta-3H-carboline-3-carboxylic acid ethyl ester (3H-BCCE) to the BZ receptors of rat and mouse brain. In mice, convulsion-producing doses of caffeine (120 mg/kg intravenously) and harmane (30 mg/kg intravenously) lowered the specific binding of 3H-FZ in vivo by 12-31%. A tremorogenic dose of harmaline (30 mg/kg intravenously) increased binding by 31%. Caffeine and harmane also slightly decreased the in vivo binding of 3H-BCCE, a compound that binds preferentially to the cerebellar type of BZ receptors. Harmaline stimulated the binding of 3H-BCCE only in the forebrain. Both harmaline and harmane increased by 41-111% the amount of 3H-BCCE that was distributed to the brain. In vitro BC's and caffeine displaced 3H-FZ from receptors in the rat brain with various Ki values (4.7 to 206.9 microM). The antagonism for BZ binding was competitive and in Scatchard analysis produced linear plots. Exceptions were harmaline and caffeine in the forebrain: both exhibited curvilinear plots for 3H-FZ binding. Harmaline increased the binding, and caffeine decreased it by altering the affinity of a subgroup of BZ receptors. In the hindbrain both harmaline and caffeine inhibited binding and produced linear plots. BC-induced tremor and convulsions unveil a large number of spare receptors in the brain, and these seem to be of the cerebellar type of BZ receptors. In addition, our results show that tremorogenic and convulsive BC's act differently on BZ receptors: during harmaline-induced tremor the affinity of some BZ receptors is increased, while harmane-induced convulsions are connected to direct occupation of BZ receptors.

beta-Carbolines as selective monoamine oxidase inhibitors: in vivo implications

The inhibitory action of a range of beta-carbolines on human and rat monoamine oxidase (MAO) A and B has been studied. Concentrations of 5-hydroxytryptamine and phenylethylamine, approximately at their Km values, were used as substrates for MAO A and B respectively. A wide variation in selectivity was found, with harmaline being 10,000 times more potent an inhibitor of A than B whereas, using tetrahydro-beta-carboline and harmane, the difference was nearer to ten-fold. Of the carbolines which have been found endogenously, tetrahydro-beta-carboline, 6-methoxytetrahydro-beta-carboline and harmane are all sufficiently potent inhibitors of human MAO A, with I50 values of 5 X 10(-6), 10(-6), 5 X 10(-7) M respectively, for this property to be of possible physiological significance. Harmane, with an I50 of 5 X 10(-6) M, might also play a role as an inhibitor of MAO B.

Interaction of convulsive ligands with benzodiazepine receptors

The gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex, which is composed of distinct proteins embedded in the neuronal plasma membrane, is important for several effects of benzodiazepines, including protection afforded against convulsions. During structural modification of ethyl beta-carboline-3-carboxylate an agent was discovered which has high affinity for brain benzodiazepine receptors but which is a potent convulsant. Also in contrast to benzodiazepines, this type of benzodiazepine receptor ligand favors benzodiazepine receptors in the non-GABA-stimulated conformation, which may explain the convulsive properties.

Effects of melatonin on sleep and neurochemistry in the rat

1 The effects of intraperitoneally administered melatonin on sleep and brain neurochemistry in the rat were studied by use of EEG recording and standard fluorescence techniques. 2 Melatonin, 10 mg/kg, reduced time to sleep onset and time spent awake but increased both slow wave and paradoxical sleep. Qualitatively similar but smaller effects were produced by a dose of 2.5 mg/kg. 3 Neither dose of melatonin altered normal EEG patterns or disrupted normal sleep behaviour. 4 Melatonin, 20 mg/Kg, did not significantly alter concentrations of tryptophan, 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, noradrenaline or dopamine in any part of the brain. 5 it is concluded that the sleep promoting activity of melatonin cannot be related to gross changes in brain indoleamine and catecholamine levels.

Beta-carboline binding indicates the presence of benzodiazepine receptor subclasses in the bovine central nervous system

Receptor binding studies were performed with tritiated propyl beta-carboline-3-carboxylate ( [3H]PrCC), tritiated ethyl beta-carboline-3-carboxylate ( [3H]ECC), and tritiated flunitrazepam ( [3H]FNT) in membrane preparations from different regions of the bovine brain and retina. Specific binding in all regions investigated was associated with benzodiazepine receptor sites. However, not all benzodiazepine receptors in the regions investigated as determined by the specific binding of tritiated flunitrazepam ( [3H]FNT) are available for [3H]PrCC suggesting that specific [3H]PrCC binding labels only one subclass or subpopulation of the benzodiazepine receptor. This benzodiazepine receptor subclass is sensitive to GABAergic modulation and amounts for about 60% of the benzodiazepine receptors in bovine cortex, hippocampus, and retina but for about 80% of the benzodiazepine receptors in the bovine cerebellum. By contrast, specific [3H]ECC binding in the cerebellum and the hippocampus labeled the same number of benzodiazepine receptors as [3H]FNT, giving no evidence for a benzodiazepine receptor subclass specificity of this compound in the bovine CNS.

Peripheral metabolism of beta-carboline-carboxylic acid esters

Esters of beta-carboline-3-carboxylic acid have recently been identified as potent inhibitors of brain benzodiazepine receptors in vitro. Ethyl beta-carboline-3-carboxylate (beta-CCE), however, is a rather weak inhibitor in vivo of benzodiazepine receptors in mice. The ED50-value was 91 mg/kg intraperitoneally 35 min after administration (ED50 is that dose which inhibits by 50% the specific binding of 3H-flunitrazepam intravenously). ED50 for beta-CCE was 2-20 fold lower in mice pretreated with organophosphorus esterase inhibitors, concomitantly with the observation of strong inhibition of liver and kidney hydrolyzing activity, using 3H-propyl beta-carboline-3-carboxylate as substrate. The rat brain contains only approximately 0.1% of the hydrolyzing activity as compared to the liver. It is concluded that some esters of beta-carboline-3-carboxylate exhibit only weak effects on benzodiazepine receptors in living animals due to hydrolysis outside the brain.

Physical separation and characterization of two types of benzodiazepine receptors

Two distinct benzodiazepine receptors are solubilized differentially by various detergents. The receptor sites that resist solubilization, designated type I, are most highly concentrated in the cerebellum and corpus striatum whereas the more readily solubilized receptors, type II, are most enriched in the hippocampus. The type I receptors display higher affinity for beta-carboline esters and a triazolopyridazine whereas several benzodiazepines do not differentiate the two receptors. The type I receptors can be solubilized with 2% Triton X-100/1 M NaCl; they retain the same drug specificity as in the particulate state.

Urinary monoamine oxidase inhibitor and benzodiazepine withdrawal

A characteristic withdrawal reaction following the cessation of long-term benzodiazepine treatment in seven patients was accompanied by a significant increase in output of urinary monoamine oxidase inhibitor. This finding provides further support for the concept that the urinary inhibitor excretion pattern may be related to stress or anxiety.

Antagonism of the anticonflict effects of chlordiazepoxide by beta-carboline carboxylic acid ethyl ester, Ro 15-1788 and ACTH(4--10)

The antagonism of the anticonflict effect of chlordiazepoxide (CDP) by beta-carboline carboxylic acid ethyl ester (BCCE), Ro 15-1788 and ACTH(4--10) has been evaluated in the Geller-Seifter rat conflict test in which CDP increases punished (conflict), but not unpunished responding. BCCE (0.5--10 micrograms ICV) produced a dose-dependent reduction in the anticonflict activity of CDP. This was also significantly reduced by Ro 15-1788 (25 mg/kg IP) and a high dose of ACTH(4--10) (5 micrograms ICV). None of these test compounds had a marked direct effect on punished or unpunished responding in the dose used. These experiments provide further physiological support for the suggestion from binding studies that BCCE and Ro 15-1788 act on benzodiazepine receptors. However, the ability of ACTH(4--10) to reduce the anticonflict effect of CDP may be by some other, possibly opioid, mechanism.

Using Ro 15-1788 to investigate the benzodiazepine receptor in vivo: studies on the anticonvulsant and sedative effect of melatonin and the convulsant effect of the benzodiazepine Ro 05-3663

Both the anticonvulsant and sedative effects of diazepam (5 mg/kg) were reversed by subsequent administration of the suggested specific benzodiazepine antagonist Ro 15-1788. In contrast neither the seizure threshold raising or sedative effect of melatonin (200 mg/kg) was reversed by Ro 15-1788. Ro 15-1788 had no effect on the convulsant action of the benzodiazepine Ro 05-3663. These data therefore argue against the suggestion that melatonin produces its sedative and anticonvulsant effects in vivo by interacting with the benzodiazepine receptor, and also strengthens the suggestion that Ro 05-3663 does not act at this site. The use of Ro 15-1788 in demonstrating whether a drug acts in vivo at the benzodiazepine site to produce a pharmacological response is discussed.

The route and significance of endogenous synthesis of alkaloids in animals

There is now substantial evidence that several TIQs and beta-carbolines are present in vivo and increase during certain pathological conditions. It still remains to be determined, however, precisely what roles they play in endogenous functions and whether or not they are critical for the expression of these pathological conditions. Accumulating biochemical information continues to support the notion that these compounds can act as false transmitters. The exciting new findings, which will certainly receive a great deal more attention, concern the interaction of some of the beta-carbolines with the benzodiazepine receptor. Determining if a beta-carboline is an endogenous receptor ligand will attract further research interest on the theoretical and specifically clinically-directed levels. Biochemical, morphological, and behavioral data indicate that some of the condensation products can act as neurotoxins. Very few experiments have included an examination of long-term effects of exposure to one of these alkaloids, so the amount of information on this issue is limited. Chronic rather than acute administration of an alkaloid is more likely to mimic the pathological states in which these compounds are hypothesized to play a role. Biochemically, both the dopaminergic and serotonergic systems have been shown to be affected by chronic treatments with certain alkaloids. Progressive and long-term behavioral alterations also have been reported. Such changes may reflect an adaptation to an increase or decrease in activity of particular systems or a neurotoxic action.

Urinary MAO inhibitor in psychiatric illness

Normal human urine contains an endogenous monoamine oxidase inhibitor. We have now investigated its activity in urine samples from psychiatric patients in various diagnostic categories. Significantly higher values were observed in alcoholics recently withdrawn from ethanol, compared with controls. Inhibitory activity was not specifically related to primary affective disorders. Inhibitor output may be positively related to certain symptom clusters rather than to disease entities (i.e. alcohol withdrawal, agitation, and hyperkinesis). Significantly lower inhibitor output was also found in a small group of patients with chronic schizophrenia.

[3H]Diazepam displacing activity in human cerebrospinal fluid

Pooled human cerebrospinal fluid was separated by Sephadex G-50 chromatography. The presence of three peaks, A, B and C, was demonstrated by monitoring absorbance at 254 and 280 nm. All peaks showed [3H]diazepam displacing activity in the membrane receptor test. Peak B was further separated on Bio-Gel P-4. At least two major fractions free of salt and GABA in the molecular weight range of approximately 700--3600 were shown to displace [3H]diazepam in the receptor test. This activity was enhanced by a factor of 3 in the presence of 10 microM-GABA.

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.

[3H]Propyl beta-carboline-3-carboxylate as a selective radioligand for the BZ1 benzodiazepine receptor subclass

Ethyl beta-carboline-3-carboxylate (beta-CCE) is a mixed-type inhibitor of [3H]flunitrazepam ([3H]FNM) binding to benzodiazepine receptors in noncerebellar regions of rat brain. These findings may represent the presence of either receptor multiplicity or negative cooperativity among benzodiazepine receptors. [3H]Propyl beta-carboline-3-carboxylate ([3H]PrCC) has previously been shown to bind specifically to benzodiazepine receptors of rat cerebellum. In the present study we found no indication of the presence of true negative cooperativity among benzodiazepine receptors when [3H]PrCC was used as radioligand. However, we observed that [3H]PrCC labelled only 57% of [3H]FNM binding sites in rat hippocampus (Bmax values) and 71% in rat cerebral cortex, whereas the number of receptors labelled by both ligands was equal in the cerebellum. Hofstee analyses of the shallow inhibition curves seen in hippocampus and cerebral cortex when [3H]FNM binding was inhibited by beta-CCE indicate that beta-CCE and some other beta-carboline-3-carboxylate derivatives interact preferentially with a subclass of receptors, and that the percentage of this subclass is equivalent to the number of receptors labelled by [3H]PrCC. We conclude that [3H]PrCC at low concentration (0.3-0.4 X 10(-9) M) labels a subclass of benzodiazepine receptors, BZ1, while another class, BZ2 receptors, are not labelled by [3H]PrCC when filtration assays are used. By parallel determinations of the proportion between [3H]FNM and [3H]PrCC binding we calculated the percentage of BZ1 receptors in several regions of rat, guinea pig and calf brain and in mouse forebrain. The values ranged from approximately 50% in hippocampus to 90% in the guinea pig pons.

Ethyl beta-carboline carboxylate lowers seizure threshold and antagonizes flurazepam-induced sedation in rats

The mammalian central nervous system possesses specific high-affinity binding sites for the benzodiazepines and considerable evidence suggests that these binding sites are the pharmacological receptors through which these compounds act. Recently, ethyl beta-carboline-3-carboxylate (beta-CCE) has been identified in both human urine and rat brain. beta-CCE may be closely related to the endogenous ligand for the benzodiazepine receptor--it shows an affinity for the receptor of the same order as that of clonazepam, one of the most potent benzodiazepines, and is the first non-diazepinoid structure to be identified with an affinity in the nanomolar range. Furthermore, it is selective for the benzodiazepine receptor. Clinically and in animal studies, benzodiazepines have anti-convulsant, hypnotic and anxiolytic actions. We have therefore investigated whether beta-CCE exhibits any of these properties in rats. We report here that, in contrast to the benzodiazepines, beta-CCE lowers seizure threshold and reverses the sedative effect of flurazepam. If beta-CCE has a close structural relationship to the endogenous ligand, benzodiazepines may be antagonistic at the receptor site.

Anticonvulsant doses of inosine result in brain levels sufficient to inhibit [3H] diazepam binding

Several purines have been shown to be competitive inhibitors of [3H] diazepam binding. Inosine has also been shown to have benzodiazepine-like neurophysiologic, pharmacologic and behavioral effects, and to partially inhibit caffeine-induced seizures in mice. Using presumptive therapeutic doses of inosine, levels were determined in mouse brain at various times following injection. Inosine and hypoxanthine concentrations in brain increased several fold following inosine administration, indicating that inosine permeated the blood-brain barrier. The levels of inosine and hypoxanthine attained in brain were sufficient to inhibit by more than 50% the GABA-stimulated [3H] diazepam binding. These data suggest that the anticonvulsant properties of inosine are related to its interaction with the benzodiazepine receptor.

[3H]propyl beta-carboline-3-carboxylate binds specifically to brain benzodiazepine receptors

Ethyl beta-carboline-3-carboxylate has recently been isolated from human urine and it was proposed that derivatives of this compound might be related to an endogenous ligand for benzodiazepine receptors. In the present study we investigated high-affinity binding of [3H]propyl beta-carboline-3-carboxylate ([3H]PrCC) to rat brain membranes. [3H]PrCC binds specifically and with high affinity (half-maximal binding at ca. 1nM) to rat brain membranes. The regional and subcellular distributions of specific [3H]PrCC binding are similar, but not identical, to the distributions of [3H]flunitrazepam or [3H]-diazepam binding. The total numbers of binding sites labelled by [3H]PrCC and [3H]flunitrazepam in rat cerebellum are closely similar, and both ligands bind to cerebellar membranes in a mutually exclusive way. The pharmacological selectivity of [3H]PrCC and [3H]diazepam binding is almost identical. Binding of [3H]PrCC like binding of [3H]diazepam, can be increased in vitro by muscimol, GABA and SQ 20.009. Although subtle differences in binding characteristics were observed, these results indicate that [3H]PrCC and benzodiazepines bind to a common recognition site on benzodiazepine receptors.

beta-Carboline-3-carboxylic acid ethyl ester antagonizes diazepam activity

Analogous to the progression of events in the opiate receptor-enkaphalin area, the first reports that benzodiazepines have selective and specific high-affinity binding sites in brain have stimulated a search for the endogenous 'ligand' or substance that might normally act at these sites. Braestrup and co-workers have extracted from human urine a gamma-fraction (ref. 10) which they have recently identified as beta-carboline-3-carboxylic acid ethyl ester (beta CEE). They reported that this substance is extremely potent in displacing 3H-diazepam from brain binding sites and proposed that a beta-carboline-3-carboxylic acid derivative might, in part, be the endogenous ligand for the brain benzodiazepine receptor. We have examined several synthetically derived beta-carboline-3-carboxylic acid analogues and now present data obtained from testing only the beta CEE described by Braestrup et al. In addition to confirming these workers' observation that this compound is a potent displacer of 3H-diazepam from brain tissue, our pharmacological data indicate that beta CEE has activity that is opposite to, rather than similar to, that of diazepam.

Ethyl beta-carboline-3-carboxylate shows differential benzodiazepine receptor interaction

High-affinity binding of 3H-diazepam and 3H-flunitrazepam has provided evidence for the presence of benzodiazepine receptors on brain neurones. Pharmacological evidence showing a clear correlation between receptor affinity and in vivo pharmacological potency for several benzodiazepines and a link between benzodiazepine receptors and GABA (gamma-amino-butyric acid) receptors at the molecular level, indicates that these receptors are relevant to the pharmacological and clinical effects of benzodiazepines. In searching for possible endogeneous ligands for benzodiazepine receptors we have recently isolated ethyl beta-carboline-3-carboxylate (beta-CCE) found human urine and brain, and shown that beta-CCE has a higher affinity than diazepam for brain benzodiazepine receptors. beta-CCE itself is probably not present in the brain, but may be closely related to an endogenous benzodiazepine receptor ligand. We report here that beta-CCE, in contrast to benzodiazepines, can distinguish clearly between benzodiazepine receptors in cerebellum and hippocampus. This result strongly indicates that benzodiazepine receptors are not a single class of non-interacting entities. It has not been possible to determine whether two distinct receptors are present and/or whether true negative cooperativity exists among hippocampal, but not cerebellar, benzodiazepine receptors.