[3H] Ethyl beta-carboline-3-carboxylate binding to the benzodiazepine receptor is not affected by GABA

Bidirectional changes in the consumption of food produced by beta-carbolines

beta-Carboline derivatives provide examples of benzodiazepine receptor ligands which span the range: full agonist-partial agonist-antagonist-partial inverse agonist-full inverse agonist. Taken together, the effects of these compounds illustrate two important principles: firstly, the bidirectionality of effects which can be achieved using benzodiazepine receptor ligands; secondly, the selectivity of effects which are produced by partial agonists. Applied to the study of feeding processes, these principles imply that both hyperphagic and anorectic effects can be generated by actions of selected ligands at benzodiazepine receptors. Furthermore, they suggest that a hyperphagic effect may occur in the absence of side-effects (e.g., sedation, muscle-relaxation), which are characteristic of classical benzodiazepines. Experimental data in support of these predictions are presented. A microstructural approach to feeding behavior indicated that a benzodiazepine receptor agonist and an inverse agonist extend and abbreviate, respectively, the duration of individual bouts of eating. Preference for a saccharin solution was attenuated by the beta-carboline inverse agonist, FG 7142, but rejection of a quinine solution was not increased. Adrenalectomy had no effect on the anorectic effect of inverse agonists.

Hormonal interactions with benzodiazepine binding sites in vitro

Prostaglandin A1 and hormones like corticosteroids and DL-Thyroxin (T4) inhibit binding of [3H]RO 5-4864 and [3H] Clonazepam to their respective binding sites with inhibition constants in the low micromolar range. The corticosteroid Cortisone inhibits [3H] RO 5-4864, but not [3H] Clonazepam binding in a competitive manner with an inhibition constant of 4.3 +/- 0.7 microM, Prostaglandin A1 inhibits [3H] Clonazepam, but not [3H] RO 5-4864 binding in a competitive manner with an inhibition constant of 6 +/- 1.2 microM and DL-Thyroxin (T4) inhibits both [3H] RO 5-4864 and [3H] Clonazepam binding with inhibition constants of 12.1 +/- 2.2 and 1.6 +/- 0.4 microM respectively. While the inhibition of [3H] RO 5-4864 binding by DL-Thyroxin (T4) is competitive, the inhibition of [3H] Clonazepam binding is of the mixed type as indicated by Scatchard Plot.

Pre- versus postsynaptic localization of benzodiazepine and beta-carboline binding sites

[3H]Flunitrazepam (FNP) and [3H]methyl beta-carboline-3-carboxylate (MCC) binding was examined in soluble and particulate fractions from membranes solubilized with Triton X-100 or in subfractions of synaptosomal membranes obtained by a physical separation technique. Results using both methods demonstrate that benzodiazepine and beta-carboline sites reside on both pre- and postsynaptic membranes. Further, subfractionation experiments indicate that the binding sites for both ligands are unequally distributed within the synapse and among brain regions. For example, in cerebral cortical presynaptic membranes there are twice as many FNP as MCC sites whereas in postsynaptic membranes this ratio is reversed. The number of FNP and MCC sites are equal in the presynaptic fraction from cerebellum. The postsynaptic membranes derived from cerebellum have three times the number of FNP compared to MCC sites. In hippocampus this ratio varies between 1.5 and 2.8 in each subfraction. These results support the idea that benzodiazepine and beta-carboline binding sites represent different recognition sites.

Effects of various 2-amino-6-alkyldithiopurines on brain specific [3H]diazepam binding

Various derivatives of 2-amino-6- methylthiopurine with substituents at the 6-position of purine were tested for their abilities to displace [3H]diazepam binding to rat brain membranes. The potency was dependent on the carbon chain-length in the 6-position of purine. Among the derivatives tested, 2-amino-6-n- pentyldithiopurine had the highest potency, with a Ki value of 0.92 microM.

Benzodiazepine receptor ligands with positive and negative efficacy

Recent studies have shown that benzodiazepine receptors can be affected not only by benzodiazepine agonists and antagonists but also by a new class of ligands which produce effects opposite to those of benzodiazepines, that is they produce convulsions and anxiety. These ligands can be described as having a negative efficacy at the receptor; tentatively they are named "inverse agonists". Pharmacological experiments indicate that agonists, antagonists and inverse agonists comprise a whole continuum of agents with a graduated variety of efficacy at the receptor. Biochemical studies, supported by published electrophysiological data, indicate that the benzodiazepine receptor allosterically up- or down-regulates the gain in the GABAergic system depending on the nature of the ligand.

Studies on [3H]diazepam and [3H]ethyl-beta-carboline carboxylate binding to rat brain in vivo. I. Regional variations in displacement

The binding of [3H]diazepam and [3H]ethyl-beta-carboline carboxylate (beta-CCE) to rat brain membranes has been studied following injection of the ligand via a tail vein. "Ex vivo" binding was avoided by homogenising the tissue in an excess of unlabelled ligand. The dissociation rate constant for [3H]diazepam and [3H]beta-CCE was approximately 0.46 min-1 at 0 degree C. Displacement of [3H]diazepam by beta-CCE in vivo showed regional variation: the dose of beta-CCE required to inhibit 50% of [3H]diazepam binding in the cerebellum was one quarter of that required in the cortex, hippocampus, or striatum. However, when diazepam was used to displace [3H]beta-CCE in vivo the converse occurred: the dose needed for 50% inhibition in the cerebellum was more than four times that required in the other three regions. These findings support suggestions from in vitro experiments that two receptors exist with different affinities for benzodiazepines and beta-carbolines. The benzodiazepine receptor antagonist Ro 15-1788 did not differentiate between the two receptor subtypes.

The neuropharmacology of various diazepam antagonists

Recently, compounds which bind avidly to benzodiazepine binding sites have been shown to possess diazepam antagonist properties. For example, the benzodiazepine RO 15-1788 and the pyrazoloquinoline CGS 8216 can antagonize the anxiolytic, sedative, muscle relaxant and anticonvulsant properties of diazepam. The beta-carbolines have also been shown to antagonize several actions of diazepam. Other compounds including physostigmine, naloxone, bicuculline, picrotoxin, caffeine and theophylline, lack appreciable affinity for benzodiazepine binding sites but do antagonize at least some of the behavioral actions of diazepam. Their antagonist properties are probably the result of opposing pharmacological actions rather than direct receptor antagonism. Clinically, a potent safe diazepam antagonist could be used to reverse effects of diazepam overdose and to speed recovery of diazepam-treated patients after various out-patient procedures.

Benzodiazepine receptors in rat cerebral cortex and hippocampus undergo rapid and reversible changes after acute stress

Rats were submitted to forced swimming and were killed 15 min after initiation of the stress and at 1 h, 1 day and 4 days thereafter. Immediately after the stress there was a decrease of 30% in the density of [3H]flunitrazepam binding sites in the cerebral cortex and of 27% in the hippocampal formation, with no changes in all the other brain areas studied. These changes in the number of benzodiazepine receptors were also corroborated by the binding of [3H]ethyl-beta-carboline carboxylate. For both ligands there were no changes in affinity. These effects were selective for the benzodiazepine receptors and no changes in alpha 1, alpha 2 and beta adrenoceptors and in dopaminergic receptors were observed. One hour after the stress, the number of benzodiazepine receptors had recovered in the cerebral cortex (8% above the control) and had increased greatly in the hippocampal formation (+53%). One day after the stress, the [3H]flunitrazepam binding in the cerebral cortex reached the normal level but it was still slightly elevated (+16%) in the hippocampus. These results are discussed in relation to some contradictory findings in the literature and to the fact that the hippocampal formation is related to neural mechanisms underlying behavior and neuroendocrine regulation.

Suriclone: a new cyclopyrrolone derivative recognizing receptors labeled by benzodiazepines in rat hippocampus and cerebellum

Suriclone (RP 31,264), like zopiclone (RP 27,267), belongs to the family of cyclopyrrolones and is chemically entirely different from the benzodiazepines (BZDs). However, it possesses a pharmacological profile close to that of the BZDs and proved to be useful in therapeutics as an anxiolytic agent. In the present paper it is shown that suriclone possesses a high affinity for flunitrazepam binding sites and that tritiated suriclone binds specifically with high affinity in rat hippocampus (KD = 0.44 +/- 0.03 nM) and rat cerebellum (KD = 0.53 +/- 0.12 nM). Further, suriclone binding sites are recognized by BZDs or zopiclone, similarly in the two regions. The affinities of four BZD derivatives--nitrazepam, flunitrazepam, diazepam, and chlordiazepoxide--are similar for suriclone and flunitrazepam binding sites. Suriclone binding sites are, like flunitrazepam sites, protected from thermal inactivation by gamma-aminobutyric acid (GABA) (10 microM), but only flunitrazepam binding is enhanced by GABA. It could be postulated from this that suriclone interacts with a subpopulation of receptors that might be modulated differently from flunitrazepam binding sites. Our results indicate that suriclone could be a new probe for investigating the so-called BZD receptors.

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.

Blockade of 3-carbomethoxy-beta-carboline induced seizures by diazepam and the benzodiazepine antagonists, Ro 15-1788 and CGS 8216

The benzodiazepine antagonists Ro 15-1788 and CGS 8216 blocked the clonic and tonic convulsions elicited by 3-carbomethoxy-beta-carboline (beta-CCM). The PD50 values for Ro 15-1788, CGS 8216, and diazepam were: 2.0, 0.6, and 2.0 mg/kg, respectively. Neither Ro 15-1788 nor CGS 8216 potentiated the effect of a threshold convulsant dose of beta-CCM. Moreover, these benzodiazepine antagonists neither attenuated nor potentiated the tremorigenic actions of another beta-carboline, harmaline. Diazepam, however, considerably reduced the tremorigenic actions of this drug.

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.

An in vitro binding assay which differentiates benzodiazepine 'agonists' and 'antagonists'

An in vitro test which discriminates benzodiazepine "agonists' and 'antagonists' has been developed by exploiting the apparent differences in modulation of the benzodiazepine receptor by these classes of compounds. In the presence of 10 microM GABA, the potency of benzodiazepine 'agonists' (i.e., compounds which bind to the benzodiazepine receptor with a relatively high affinity and share pharmacologic properties with benzodiazepines) to displace [3H]3-carboethoxy-beta-carboline is significantly increased. In contrast, the potency of benzodiazepine 'antagonists' (compounds which have been demonstrated to antagonize some of the pharmacologic actions of benzodiazepines) is not altered by GABA. Several chemically diverse classes of compound have been examined in this test, and within the limited number of compounds examined, this test accurately predicts 'agonist' and 'antagonist' actions in vivo.

Interaction of avermectins with [3H]beta-carboline-3-carboxylate ethyl ester and [3H]diazepam binding sites in rat brain cortical membranes

The binding of [3H] beta-carboline-3-carboxylate ethyl ester ([3H] beta-CCE), a ligand for the benzodiazepine receptor in the mammalian CNS, to rat cortical membranes, is enhanced by avermectin B1a and its therapeutic formulation, Ivermectin. In contrast to the effects of the avermectins on [3H]diazepam binding, which involve changes in both receptor affinity and number, increases in beta-CCE binding, which are much less than those observed for the benzodiazepine ligand, involve only alterations in receptor number. This Bmax increase is bicuculline insensitive whereas Ivermectin effects on benzodiazepine binding are partially antagonized by GABA antagonist. The data suggest a differential interaction by the avermectins on benzodiazepine and beta-CCE binding sites in rat cortical membranes and indicate that these macrolide anthelmintics may be a useful tool for characterizing benzodiazepine/anxiolytic receptor subtypes.