Limited tryptic proteolysis of the benzodiazepine binding proteins in different species reveals structural homologies

Peptide mapping can be used to elucidate further the structural similarities of the benzodiazepine binding proteins in different vertebrate species. Crude synaptic membrane preparations were photoaffinity-labeled with [3H]flunitrazepam and subsequently degraded with various concentrations of trypsin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by fluorography allowed a comparison of the molecular weights of photolabeled peptides in different species. Tryptic degradation led to a common peptide of 40K in all species investigated, a finding indicating that the benzodiazepine binding proteins are structurally homologous in higher bony fishes and tetrapods.

Picrate and niflumate block anion modulation of radioligand binding to the gamma-aminobutyric acid/benzodiazepine receptor complex

The organic anions picrate (2,4,6-trinitrophenol) and niflumate (2-[[3-(trifluoromethyl)phenyl]-amino]-3-pyridinecarboxylate) were examined for their effects on radioligand binding to the gamma-aminobutyric acid (GABA)/benzodiazepine receptor complex. Neither organic anion produced the enhancement of [35S] t-butylbicyclophosphorothionate (TBPS) binding characteristic of anions (such as Cl- and Br-) known to permeate GABA-gated chloride channels. However, both picrate and niflumate potently (IC50 values between 66 and 531 and 30 and 155 microM, respectively) inhibited the effect of 100-200 mM concentrations of anions (I-, Br-, Cl-, SCN-, and F-) to increase the binding of [35S]TBPS to GABA-gated chloride channels. This inhibition resulted from a decrease in both the maximum number of binding sites and the apparent affinity (increased Kd) of [35S]TBPS. Niflumate was consistently more potent than picrate, but both organic anions exhibited the same sequence of relative potencies against smaller anions (I- greater than Br- greater than Cl- greater than SCN- greater than F-). This sequence was similar to that described for the relative permeabilities of these anions through GABA-gated chloride channels. Niflumate and picrate were potent inhibitors of Cl-, but not GABA-modulated radioligand binding to benzodiazepine receptors. These findings suggest that picrate and niflumate bind with high affinity at or near an anion binding site that may regulate the movement of anions through GABA-gated chloride channels and radioligand binding at this "supramolecular complex."

Differences in GABA activity between ethanol withdrawal seizure prone and resistant mice

Withdrawal seizure prone (WSP) and withdrawal seizure resistant (WSR) lines of mice have been genetically selected based on the severity of handling-induced convulsions after identical chronic ethanol exposure. The present experiments showed that naive WSP mice were more sensitive than WSR mice to a subconvulsant dose of picrotoxin, bicuculline or pentylenetetrazole as measured by the ability of these drugs to exacerbate handling-induced convulsions. This may reflect a difference between lines in the GABA-chloride channel. The density and affinity of [35S]t-butylbicyclophosphorothionate (TBPS) binding sites, a cage convulsant which binds to the picrotoxin site on the GABA-chloride channel, was measured in the frontal cortex, remainder of the cortex, cerebellum and hippocampus. The binding properties of [3H]flunitrazepam and the potency of gamma-aminobutyric acid (GABA) to enhance flunitrazepam binding was characterized in whole brain samples. There were no differences between lines. The behavioral results suggest a role for the GABA-chloride channel in the differential ethanol withdrawal seizure behavior of WSR and WSP mice, but this is not due to changes in receptor densities or affinities.

Effect of thyroid hormones on benzodiazepine receptors in neuron-enriched primary cultures

The effect of administration of thyroid hormones on central benzodiazepine receptors was investigated using neuron-enriched primary cultures obtained from the neopallium of 16-day-old embryonic rats. Addition of L-triiodothyronine for 3 days decreased the maximal number of benzodiazepine receptor binding sites without any change in affinity at 10(-5) and 10(-6) M. L-Thyroxine administered for 3 days had the same effect at 10(-5) M. No significant change was observed over periods of less than 3 days, a finding indicating that this inhibition was not a direct in vitro effect. This down-regulation seems to be a direct modulatory effect of thyroid hormones on cerebral cortical neurons. Addition for 3 days of D-thyroxine and D-triiodothyronine, which are physiologically inactive isomers of the thyroid hormones, did not induce any significant alterations in benzodiazepine receptors. The decrease in number of cerebral cortical neuronal benzodiazepine receptors due to L-isomers of thyroid hormones may be related to the convulsions and anxiety observed in thyrotoxicosis in humans.

Desensitization of adenosine A2 receptors in the striatum of the rat following chronic treatment with diazepam

Following prolonged treatment (7 days) with diazepam (10 mg/kg/day, using ALZET mini-osmotic pumps) in rats, the function of adenosine receptors was assessed in specific structures of the brain, using both agonist ligand binding and adenylate cyclase assays. Binding to A1 receptors was quantified using [3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine, a selective ligand at A1 receptors. Differences in the binding of this ligand and that of [3H]5'-N-ethylcarboxamide adenosine, which binds to both A1 and A2 subtypes of receptors with similar affinities, were used to quantify A2 receptors. Treatment with diazepam failed to alter the binding of [3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine in all areas of the brain studied. However, the binding of A2 receptors and A2 receptor-mediated stimulation of adenylate-cyclase were significantly attenuated in striatal membranes from diazepam-treated rats. Thus, the present study indicated that functional adenosine A2 receptors were desensitized after prolonged treatment with diazepam, since decreased agonist binding to A2 receptors paralleled an attenuation in the stimulation by adenosine of the activity of adenylate cyclase, an effect mediated by the A2 receptor. These results further indicate that the changes in adenosine A2 receptors correlated with significant short-lasting alterations in the sleep-wake cycle during the withdrawal of diazepam. The alterations in sleep-wakefulness did not correlate with the effect of diazepam on benzodiazepine receptors since no changes were observed in the binding of benzodiazepine receptors.

Isoquinoline and peripheral-type benzodiazepine binding in gliomas: implications for diagnostic imaging

Binding of the isoquinoline PK 11195 and of the benzodiazepines Ro5-4864 and flunitrazepam was compared in glioma cells and tissues. In human and rat glioma cell cultures [3H]PK 11195 bound with higher affinity (Kd = 14.01 and 15.76 nM, respectively) than either Ro5-4864 (Ki = 1200 and 84.9 nM, respectively) or flunitrazepam (Ki greater than 10,000 and = 848 nM, respectively). Autoradiograms of postmortem human brain sections containing glioma revealed that [3H]PK 11195 bound specifically to intact tumor cells and not to cells of normal cerebral cortex or necrotic areas of the tumor. Total [3H]Ro5-4864 or [3H]flunitrazepam binding to these sections was indistinguishable from nonspecific binding, and regions of tumor and normal brain could not be delineated. These results support the use of radiolabeled PK 11195 for clinical trials of imaging human gliomas by positron emission tomography.

The NMDA antagonists, CPP and CGS 19755, lack affinity for central benzodiazepine receptors

CPP (3-(2-carboxypiperazin-4-yl-propyl-1-phosphonic acid), a rigid analog of AP7 (2-amino-7-phosphonoheptanoate), previously shown to be a selective antagonist of the NMDA (N-methyl-D-aspartate) receptor (IC50 = 209 nM) has been reported to be exceptionally active (IC50 = 430 pM) at benzodiazepine binding sites. Re-examination of CPP, and the rigid AP5 analog, CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylic acid; 0.001-10,000 nM), showed that, as previously reported, neither compound affected the binding of [3H]flunitrazepam. These compounds are thus selective NMDA receptor antagonists.

Distribution of central omega 1 (benzodiazepine1) and omega 2 (benzodiazepine2) receptor subtypes in the monkey and human brain. An autoradiographic study with [3H]flunitrazepam and the omega 1 selective ligand [3H]zolpidem

The distribution of the central omega 1 (benzodiazepine1; BZD1) and omega 2 (BZD2) receptor subtypes has been studied autoradiographically in monkey and human brain sections using [3H]flunitrazepam (which binds indiscriminately to omega 1 and omega 2 subtypes) and [3H]zolpidem (which recognizes selectively the omega 1 subtype). Both ligands labeled an homogeneous population of binding sites (Kd values approximately equal to 1.5 nM and approximately equal to 5 nM, respectively) whose pharmacological characteristics were similar to those of central omega (BZD) receptors. Regional displacement studies in monkey brain showed that zolpidem was a more effective displacer of [3H]flunitrazepam from omega 1-than from omega 2-enriched areas. For example, it was 73-fold more potent in the cerebellum (omega 1-enriched) than in the dentate gyrus (omega 2-enriched). Zolpidem thus selectively recognizes omega 1 sites in primate brain. The autoradiographic distribution of [3H]flunitrazepam (omega 1 + omega 2) binding sites in primate brain was highly heterogeneous. Very high densities were observed in lamina IV of the neocortex (with higher densities in the occipital than in the frontal pole), the substantia innominata and molecular layer of the dentate gyrus. Intermediate densities were found in other neocortical laminae, cerebellum (molecular layer), claustrum, globus pallidus, caudate-putamen, nucleus accumbens, dentate gyrus (granular layer) and the majority of thalamic nuclei. Structures displaying low binding densities included the substantia nigra and the subthalamic nucleus. The regional distribution pattern of [3H]zolpidem binding sites was qualitatively similar to that of [3H]flunitrazepam but the relative density of the 3H-ligands differed in several brain regions. The relative density of omega 1 and omega 2 subtypes in each particular primate brain region was evaluated by measuring 1) the ratio of [3H]zolpidem to [3H]flunitrazepam binding; 2) [3H]flunitrazepam binding in the presence and in the absence of 100 nM zolpidem. With both approaches, a preferential enrichment in omega 1 sites was observed in lamina IV of sensorimotor cortical regions and in the extrapyramidal motor system (globus pallidus, ventral thalamic complex, subthalamic nucleus, substantia nigra and cerebellum). In contrast, omega 2 sites predominated in limbic areas (e.g., the dentate gyrus, amygdala, nucleus accumbens, cingulate and parahippocampal gyri) and in the anterior thalamic nucleus and caudate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased binding of [3H]muscimol and [3H]flunitrazepam in the rat brain under hypoxia

We examined the effects of in vivo hypoxia (10% O2/90% N2) on the gamma-aminobutyric acid (GABA)/benzodiazepine receptors and on glutamic acid decarboxylase (GAD) activity in the rat brain. Male Wistar rats were exposed to a mixture of 10% O2 and 90% N2 in a chamber for various periods (3, 6, 12, and 24 h). The control rats were exposed to room air. The brain regions examined were the cerebral cortex, striatum, hippocampus, and cerebellum. GABA and benzodiazepine receptors were assessed using [3H]muscimol and [3H]flunitrazepam, respectively. Compared with control values, GAD activity was decreased significantly following a 6-h exposure to hypoxia in all four regions studied. On the other hand, the numbers of both [3H]muscimol and [3H]flunitrazepam binding sites were increased significantly. The increase in receptor number tended to return to control values after 24 h. Treatment of the membrane preparations with 0.05% Triton X-100 eliminated the increase in the binding capacity. These results may represent an up-regulation of postsynaptically located GABA/benzodiazepine receptors corresponding to the impaired presynaptic activity under hypoxia.

Anion regulation of agonist and inverse agonist binding to benzodiazepine receptors

Binding of the benzodiazepine inverse agonist [3H]methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate [( 3H]DMCM) and the agonist [3H]flunitrazepam [( 3H]FNZ) was compared in rat cortical membranes. Halide ions enhanced [3H]DMCM binding three- to fourfold, increasing both the apparent affinity and the number of binding sites for this radioligand. The effect was present at both 0 and 37 degrees C. In contrast, the magnitude of halide stimulation of [3H]FNZ binding was much smaller, resulting solely from an increase in the apparent affinity for this radioligand, and was not observed at 37 degrees C. The potencies but not the efficacies of a series of anions to stimulate both [3H]DMCM and [3H]FNZ binding to benzodiazepine receptors were highly correlated with their relative permeabilities through gamma-aminobutyric acid (GABA)-gated chloride channels. Two stress paradigms (10 min of immobilization or ambient-temperature swim stress), previously shown to increase significantly the magnitude of halide-stimulated [3H]FNZ binding, did not significantly affect [3H]DMCM binding. Phospholipase A2 treatment of cortical membrane preparations was equipotent in preventing the stimulatory effect of chloride on both [3H]DMCM and [3H]FNZ binding. These data strongly suggest that anions modify the binding of [3H]DMCM and [3H]FNZ by acting at a common anion binding site that is an integral component of the GABA/benzodiazepine receptor chloride channel complex.

Quantitative autoradiography of major neurotransmitter receptors in the monkey striate and extrastriate cortex

In vitro autoradiography was used to determine the binding properties and distribution of 9 major neurotransmitter receptors and their subtypes in the striate (area 17 of Brodmann) and extrastriate (areas 18 and 19) cortex of 1 infant and 3 adult rhesus monkeys. Differences in total labeling and nonspecific labeling, as well as Kd and Bmax values, were determined for all cortical layers and sublayers in both cytoarchitectonic areas by Scatchard analysis of autoradiograms. Area 17 differed from area 18 in the laminar pattern and density of virtually every ligand examined, i.e., 3H-clonidine, 3H-prazosin, 125I-iodopindolol, 3H-quinuclidinyl benzilate, 3H-5-hydroxytryptamine, 3H-ketanserin, 3H-muscimol, 3H-flunitrazepam, and 3H-spiperone. Kd and Bmax values for each ligand were remarkably consistent across the 3 adult monkeys analyzed quantitatively. Particularly dramatic contrasts were observed with clonidine, 5-hydroxytryptamine, and ketanserin, which have high affinity for alpha 2-adrenergic, 5-HT1-, and 5-HT2-receptors, respectively. The differences in distribution of these ligands, especially clonidine and 5-hydroxytryptamine, correlated well with specific laminae and hence exhibited distinctly different patterns in areas 17 and 18. Other ligands, such as flunitrazepam and quinuclidinyl benzilate that bind to GABAergic and cholinergic receptors, were visually less discriminating both among layers and between regions. However, layer for layer, the Bmax values for quinuclidinyl benzilate were higher in area 17 than 18, indicating the subtle differences between areas may be revealed only by quantitative measures. Some ligands were particularly dense in layer I (iodopindolol in areas 17 and 18; 5-hydroxytryptamine in area 18), and others subdivided cortical layers that are otherwise cytoarchitectonically uniform (e.g., flunitrazepam and clonidine in layer VI of area 17), indicating that areal differences in ligand binding are not a simple read-out of cell-packing density but most likely reflect a genuine difference related to the neurotransmitters of intrinsic and extrinsic afferents in each area. The presence of binding sites in every layer of both areas for all ligands examined indicates that their distribution across laminae is quantitative and not all-or-none. No layer contained less than 50% of binding sites present in any other layer. These findings reveal that visual cortical areas differ in density and lamination of neurotransmitter receptors and presumably in their sensitivity to circulating levels of endogenous neurotransmitters and pharmacologically active compounds.

Brain GABAA/benzodiazepine binding sites and glutamic acid decarboxylase activity in depressed suicide victims

We have investigated the involvement of gamma-aminobutyric acid (GABA) in depression by quantitating benzodiazepine (BZ) binding sites, the ability of GABA to stimulate BZ binding and glutamic acid decarboxylase activity in frontal and temporal cortex obtained at postmortem examination from 21 suicide victims and 21 age- and sex-matched controls. We limited our study to suicide victims with clear evidence of depression, in the absence of symptoms of other psychiatric disorders. Thirteen of the depressed suicide victims had not been prescribed psychoactive drugs recently and none were found in their blood at postmortem; of the remaining 8 suicides, 6 were receiving antidepressant drugs, alone or in combination with other drugs. The number of BZ binding sites was significantly greater (by 18%) in the frontal cortex of the total group of depressed suicides compared to controls, but did not differ in the temporal cortex. The increase in the number of BZ binding sites in the frontal cortex was of similar magnitude when drug-free (16%), drug-treated (21%) and antidepressant-treated suicides (16%) were compared to matched controls, although the increase was only statistically significant for the drug-treated suicides. The Kd of BZ binding and the ability of GABA to stimulate BZ binding did not differ significantly between controls and the total, drug-free, drug-treated or antidepressant-treated suicides in either cortical area. Glutamic acid decarboxylase activity did not differ significantly between control and suicide groups, but was markedly reduced in subjects dying by carbon monoxide poisoning. The present study provides evidence for a greater number of BZ binding sites in the frontal cortex of depressed suicide victims, which could not clearly be attributed to drug treatment.

Persistence of species variation and regional heterogeneity of the apparent molecular masses of benzodiazepine-binding proteins after deglycosylation

Brain membrane preparations of different vertebrates were photoaffinity labeled with [3H]flunitrazepam and subsequently deglycosylated with endoglycosidase F and peptide N-glycopeptidase. SDS-polyacrylamide gel electrophoresis followed by fluorography revealed that each benzodiazepine-binding protein is deglycosylated in two steps, indicating that each protein has two glycosylation sites. Species variation of the apparent molecular masses of the benzodiazepine-binding proteins and regional heterogeneity in avians persist after deglycosylation. These results indicate that the alpha-subunit(s) of the GABA/benzodiazepine receptor has undergone electrophoretically detectable changes in its amino acid composition during vertebrate evolution. The existence of at least two different alpha-subunits in avians is further substantiated.

Benzodiazepine receptors in the human hippocampal formation: a pharmacological and quantitative autoradiographic study

The pharmacological characteristics and anatomical distribution of benzodiazepine receptors in the human hippocampal formation were studied in seven cases aged 4-68 years. The pharmacology of the receptors was studied by computerized, non-linear least squares regression analysis of [3H]flunitrazepam displacement by flunitrazepam, CL218,872 and ethyl beta-carboline-3-carboxylate binding to membranes and the anatomical localization of these receptors was demonstrated using quantitative autoradiography following in vitro labelling of cryostat sections with [3H]flunitrazepam. The pharmacological studies indicated that the human hippocampal formation contained equal numbers of benzodiazepine receptors with high affinity (Type I) and low affinity (Type II) for CL218,872 and ethyl beta-carboline-3-carboxylate. The autoradiograms demonstrated that the benzodiazepine receptors were distributed in a heterogeneous fashion throughout the major regions of the human hippocampal formation; the highest concentrations of receptors were present in the dentate gyrus (molecular layer) and field CA1 of Ammon's horn (strata pyramidale, oriens, lacunosum), with moderate concentrations in field CA2 of Ammon's horn (stratum pyramidale) and in regions of the subicular complex and entorhinal cortex, and with considerably lower densities in fields CA3 and CA4. Quantitative analyses of the autoradiograms showed that the regions containing the highest densities of receptors (molecular layer of dentate gyrus and the strata oriens, pyramidale and lacunosum of CA1) were enriched with Type 1 receptors whereas other regions of lower receptor densities were enriched with either Type I or Type II receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of gamma-aminobutyric acid/benzodiazepine receptor turnover in neuronal cells: evidence for nonlysosomal degradation

In a previous report we described the use of flunitrazepam as a photoaffinity label to monitor the turnover of the gamma-aminobutyric acid/benzodiazepine receptor complex in primary brain and spinal cord cell cultures [Science (Wash. D. C.) 226:857-860 (1984)]. In the present communication we have extended our studies on the kinetics of receptor turnover and have examined the mechanism of receptor degradation. There are approximately 60,000 irreversible binding sites per neuron, and photolabeling is stereospecific. Photolabeling does not demonstrably alter the kinetics of degradation, and a complete rate equation relating the kinetic constants for degradation to receptor number is described. The rapid phase of degradation is slowed at low temperature (Q10 = 5, which corresponds to an apparent energy of activation of 25 kcal/mol) and by inhibitors of ATP production (sodium azide, 2-deoxyglucose, and 2,4-dinitrophenol). The fast phase for the degradation of photolabeled receptor is not affected by lysosomotropic agents (methylamine, ammonium chloride, and chloroquine) or by elimination of horse serum and chick embryo extract from the growth medium. In contrast, overall protein degradation is inhibited by methylamine and enhanced in serum-free medium. The results suggest that the gamma-aminobutyric acid receptor complex is degraded through an energy-dependent nonlysosomal pathway.

Animal models of chronic anxiety and "fearlessness"

Three behavioral animal models have been described: a feline and a rodent model of chronic anxiety, and a rodent model of "fearless" behavior. The models have been obtained by pre- or perinatal exposure to diazepam (DZ) or RO 15-1788 which produced enduring postnatal deficits or enrichment, respectively, of brain benzodiazepine (BDZ) receptors. The receptor-deficient one-year-old cat progenies showed hyperarousal, unabated restless behavior, delayed acquisition of instrumentally conditioned behavior, bizarre escape responses and absence or reduced alpha-like EEG activity. The receptor-deficient rat progencies, studied at the age of 5-6 months, showed a reduction of time spent in deep slow wave sleep, and inability to habituate to novel environment, such as the radial arm maze. In the maze, the behavior of these progenies was characterized by delayed and incomplete exploratory activity often terminated by sudden escape, numerous fecal deposits and significantly more frequent than normal errors of "working memory." On the other hand, in all aspects, the receptor-enriched progenies were superior to the control animals as well as to the receptor-deficient group, particularly when the animals were challenged by novel and "intimidating" visual and/or auditory stimuli. In addition, 12 out of 51 receptor-deficient rats reared for 18 months developed mammary fibroadenomas, while no such tumors were found in the group of 44 vehicle-exposed control animals. Increased density and affinity of BDZ brain receptors was also observed after adult rats were treated with RO 15-1788 administered water for 7 or 14 days.

Distribution and binding parameters of GABA and benzodiazepine receptors in the cat motor thalamus and adjacent nuclear groups

Quantitative receptor binding autoradiography technique was utilized to study GABA and benzodiazepine receptors in the cat motor thalamus (ventral anterior, ventral medial and ventral lateral nuclei) and adjacent thalamic subdivisions. Binding parameters (Bmax and Kd) and distribution pattern of the binding sites for 3 tritiated ligands [3H]muscimol ([3H]MUS), [3H]flunitrazepam ([3H]FLU) and [3H]baclofen ([3H]BAC) were analyzed and compared using measurements from discrete and anatomically well-defined thalamic regions. There was little correlation in the regional distribution of the 3 binding sites. The concentration of [3H]BAC binding sites in thalamic nuclei of interest was very low, practically at the limit of resolution of the quantitative autoradiographic technique; whereas appreciable quantities of [3H]MUS and [3H]FLU binding sites were present in the motor and adjacent limbic nuclei of the thalamus. There was more difference between the nuclei in regard to the number of high affinity GABA receptors than benzodiazepine receptors. Moreover, the ratio of Bmax[3H]MUS/Bmax[3H]FLU varied from 2.2 to 4.4 in different thalamic regions suggesting the presence of a diverse population of GABAA and benzodiazepine receptors. The distribution pattern of the 3 binding sites was compared to the topography of GABAergic afferents of the basal ganglia origin and the frequency of GABAergic synapses formed by thalamic local circuit neurons (LCN) in the motor thalamus that were established earlier. It was concluded that in the cat motor thalamus: (1) none of the ligands studied appear to reveal the receptors associated with nigro- or pallidothalamic synapses; (2) [3H]MUS binding sites may be associated with the dendrodendritic contacts formed by LCN; and (3) the [3H]FLU binding sites are physically unrelated to [3H]MUS binding sites. The concentration of [3H]FLU and [3H]MUS binding sites in the midline nuclei and of [3H]MUS binding sites in the limbic nuclei was remarkably high. It was concluded that in addition to previously suggested limbic structures, the midline nuclei with their very high content of benzodiazepine receptors may be considered as a neuroanatomical substrate of certain forms of anxiety.

The apparent target size of rat brain benzodiazepine receptor, acetylcholinesterase, and pyruvate kinase is highly influenced by experimental conditions

Radiation inactivation is a method to determine the apparent target size of molecules. In this report we examined whether radiation inactivation of various enzymes and brain receptors is influenced by the preparation of samples preceding irradiation. The apparent target sizes of endogenous acetylcholinesterase and pyruvate kinase from rat brain and from rabbit muscle and benzodiazepine receptor from rat brain were investigated in some detail. In addition the target sizes of alcohol dehydrogenase (from yeast and horse liver), beta-galactosidase (from Escherichia coli), lactate dehydrogenase (endogenous from rat brain), and 5-HT2 receptors, acetylcholine muscarine receptors, and [35S] butyl bicyclophosphorothionate tertiary binding sites from rat brain were determined. The results show that apparent target sizes are highly influenced by the procedure applied for sample preparation before irradiation. The data indicate that irradiation of frozen whole tissue as opposed to lyophilized tissue or frozen tissue homogenates will estimate the smallest and most relevant functional target size of a receptor or an enzyme.

Differences in the biophysical properties of the benzodiazepine/gamma-aminobutyric acid receptor chloride channel complex in the long-sleep and short-sleep mouse lines

Significant differences in the thermal stability of benzodiazepine receptors were found in cerebral cortical membranes prepared from the long-sleep (LS) and short-sleep (SS) selected mouse lines. Thus, benzodiazepine receptors from LS mice were heat inactivated (55 degrees C) at a significantly faster rate than those from SS mice. Although gamma-aminobutyric acid (GABA) reduced the rate of heat inactivation in both lines, the more rapid rate of inactivation in the LS line was maintained. Furthermore, the potency of GABA to enhance [3H]flunitrazepam binding decreased threefold in membranes from LS mice as the incubation temperature was increased from 0 degrees to 37 degrees C, but was unaltered in membranes from SS mice. These differences in the biophysical properties of the benzodiazepine/GABA receptor chloride channel complex ("supramolecular complex"), together with a higher KD for t-[35S]butylbicyclophosphorothionate in membranes from LS compared to SS mice, suggest that the supramolecular complex may modulate the differential sensitivity to some depressants and convulsants in these lines.

Various proteins from rat brain, specifically and irreversibly labeled by [3H]flunitrazepam, are distinct alpha-subunits of the GABA-benzodiazepine receptor complex

gamma-Aminobutyric acid (GABA)-benzodiazepine receptors were purified from the brains of 5 to 10-day-old rats and photolabeled by [3H]flunitrazepam. After sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), 3 different proteins with apparent molecular weight 51,000, 53,000 and 59,000 Da specifically and irreversibly labeled by [3H]flunitrazepam were revealed by autoradiography. The same 3 proteins were recognized by the alpha-subunit-specific monoclonal antibody bd-28. In contrast, a different protein with apparent molecular weight 56,000 Da was recognized by the beta-subunit-specific antibody bd-17. These results indicate that the various proteins specifically labeled by [3H]flunitrazepam are different alpha-subunits and distinct and different from the beta-subunit of the GABA-benzodiazepine receptor complex.

The interaction of substituted benzamides with brain benzodiazepine binding sites in vitro

1. The interaction of substituted benzamides with brain benzodiazepine (BDZ) binding sites was examined by their ability to displace [3H]-flunitrazepam ([3H]-FNM) from specific binding sites in bovine cortical membranes in vitro. 2. Clebopride, Delagrange 2674, Delagrange 2335 and BRL 20627 displayed concentration-dependent displacement of [3H]-FNM with IC50 values of 73 nM, 132 nM, 7.7 microM and 5.9 microM, respectively. Other substituted benzamides including metoclopramide, sulpiride, tiapride, sultopride and cisapride were inactive at 10(-5) M. 3. Inhibition by clebopride and Delagrange 2674 of [3H]-FNM binding was apparently competitive and readily reversible. 4. In the presence of gamma-aminobutyric acid (GABA), the ability of diazepam and Delagrange 2674 to displace [3H]-Ro 15-1788 binding was increased 3.6 and 1.6 fold respectively, compared to the absence of GABA, while ethyl beta-carboline-3-carboxylate (beta CCE) and clebopride were less potent in the presence of GABA. 5. Diazepam was 30 fold less potent at displacing [3H]-Ro 15-1788 in membranes that had been photoaffinity labelled with FNM than in control membranes, whereas the potency of beta CCE did not differ. Clebopride and Delagrange 2674 showed a less than two fold loss of potency in photoaffinity labelled membranes. 6. The pattern of binding of clebopride and Delagrange 2674 in these in vitro tests is similar to that found previously with partial agonists or antagonists at BDZ binding sites. 7. Clebopride and Delagrange 2674 inhibited [3H]-FNM binding with similar potency in rat cerebellar and hippocampal membranes, suggesting they have no selectivity for BDZ1 and BDZ2 binding sites. 8. Clebopride and Delagrange 2674 are structurally dissimilar to other BDZ ligands and represent another chemical structure to probe brain BDZ binding sites.

Benzodiazepine interactions with GABAA receptors on chick ciliary ganglion neurons

gamma-Aminobutyric acidA (GABAA) receptors on chick ciliary ganglion neurons can be modulated by benzodiazepines and identified by radiolabeled benzodiazepine binding. Enhancement of submaximal GABA responses by benzodiazepines was demonstrated using a multibarrel pipette to construct complete benzodiazepine dose-response curves for single cells in culture. EC50 values of 22 +/- 5 nM, 1.1 +/- 0.3 microM, and 4.6 +/- 0.5 microM were obtained for flunitrazepam, clonazepam, and chlordiazepoxide, respectively. Chlordiazepoxide shifted the GABA dose-response curve to lower GABA concentrations without increasing the maximal response to GABA, demonstrating that benzodiazepines enhance the GABA response by increasing the receptor affinity for GABA. The imidazodiazepine Ro15-1788 potentiated the GABA response with an EC50 of 250 +/- 70 nM, and Ro5-4864 (chlorodiazepam) partially blocked the GABA response both in the presence and absence of chlordiazepoxide. Scatchard analysis of data from binding studies with [3H]flunitrazepam to ganglion membrane homogenates was consistent with the presence of a single class of high affinity sites with a KD of 34 +/- 6 nM and a Bmax of 145 +/- 26 fmol/mg of protein. Several lines of evidence indicated that the sites were associated with GABAA receptors. The KD of [3H]flunitrazepam binding was similar to the EC50 for flunitrazepam modulation of the GABA response. The level of [3H]flunitrazepam binding was enhanced approximately 50% over control levels by GABA. The binding was decreased both by clonazepam and by Ro5-4864 at concentrations similar to those required for the compounds to modulate the GABA response. These studies demonstrate that ciliary ganglion GABAA receptors are similar in major respects to GABAA receptors in the central nervous system but may differ in minor pharmacological properties.

Chronic benzodiazepine administration. II. Discontinuation syndrome is associated with upregulation of gamma-aminobutyric acidA receptor complex binding and function

A "withdrawal" or "rebound" syndrome may follow the abrupt discontinuation of chronic treatment with benzodiazepines. To assess the neurochemical basis of this syndrome, mice were treated with lorazepam, 2 mg/kg/day for 7 days, a course which produces tolerance and downregulation of receptor binding and function. Behavioral studies indicated that open-field activity in lorazepam-treated mice was similar to controls at 1 day postlorazepam. Activity was increased at 4 days postlorazepam, and had returned to control levels by day 7. Benzodiazepine receptor binding as measured by specific uptake of [3H]Ro15-1788 in several brain regions returned to control levels by day 2 postbrazepam but had increased above controls at day 4 postlorazepam. Binding returned to control levels by day 7. Similar results were observed using [3H]flunitrazepam in membrane preparations. A similar time course was also observed for the maximum binding of the chloride channel ligand t-[35S]butylbicyclophosphorothionate during this period. The IC50 value for muscimol inhibition of t-butylbicyclophosphorothionate binding was decreased at day 4, suggesting increased coupling between gamma-aminobutyric acid and the chloride channel. Muscimol-stimulated [36Cl-] up-take in cortical synaptoneurosomes was increased at day 4 postlorazepam. These results indicate that the benzodiazepine discontinuation syndrome is associated with upregulation of receptor binding and function at the gamma-aminobutyric acidA receptor complex.

Bicuculline up-regulation of GABAA receptors in rat brain

Effects of acute and subacute administration of bicuculline on [3H]muscimol, [3H]flunitrazepam, and t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to various brain regions were studied in Sprague-Dawley rats. Acute administration of bicuculline affected neither the KD nor the Bmax of the three receptor sites. In rats treated subacutely with bicuculline (2 mg/kg, i.p., daily for 10 days), [3H]muscimol binding was increased in the frontal cortex, cerebellum, striatum, and substantia nigra. Scatchard analysis revealed that subacute treatment of rats with bicuculline resulted in a significantly lower KD of high-affinity sites in the striatum and in a significantly lower KD of high- and low-affinity sites in the frontal cortex. In the cerebellum, two binding sites were apparent in controls and acutely treated animals; however, only the high-affinity site was defined in subacutely treated animals, with an increase in the Bmax value. Triton X-100 treatment of frontal cortical membranes eliminated the difference in [3H]muscimol binding between control and subacute bicuculline treatments. On the other hand, [3H]muscimol binding was significantly increased in the cerebellum from bicuculline-treated animals even after Triton X-100 treatment. The apparent Ki of bicuculline for the GABAA receptor was also decreased in the frontal cortex and the striatum following the treatment. However, subacute administration of bicuculline affected neither the KD nor the Bmax of [3H]flunitrazepam and [35S]TBPS binding in the frontal cortex and the cerebellum. These results suggest that GABAA receptors are up-regulated after subacute administration of bicuculline, with no change in benzodiazepine and picrotoxin binding sites.