Presence of peripheral-type benzodiazepine binding sites on human erythrocyte membranes

Pharmacological evaluation of [123I]-CLINDE: a radioiodinated imidazopyridine-3-acetamide for the study of peripheral benzodiazepine binding sites (PBBS)

PURPOSE

The study aims to evaluate the iodinated imidazopyridine, N',N'-diethyl-6-Chloro-(4'-[(123)I]iodophenyl)imidazo[1,2-a]pyridine-3-acetamide ([(123)I]-CLINDE) as a tracer for the study of peripheral benzodiazepine binding sites (PBBS).

MATERIALS AND METHODS

In vitro studies were performed using membrane homogenates and sections from kidney, adrenals, and brain cortex of Sprague-Dawley (SD) rats and incubated with [(123)I]-CLINDE. For in vivo studies, the rats were injected with [(123)I]-CLINDE. In competition studies, PBBS-specific drugs PK11195 and Ro 5-4864 and the CBR specific drug Flumazenil were injected before the radiotracer.

RESULTS

In vitro binding studies in adrenal, kidney, and cortex mitochondrial membranes indicated that [(123)I]-CLINDE binds with high affinity to PBBS, K(d) = 12.6, 0.20, and 3.84 nM, respectively. The density of binding sites was 163, 5.3, and 0.34 pmol/mg protein, respectively. In vivo biodistribution indicated high uptake in adrenals (5.4), heart (1.5), lungs (1.5), kidney (1.5) %ID/g at 6 h p.i. In the central nervous system (CNS), the olfactory bulbs displayed the highest uptake; up to six times the activity in blood. Pre-administration of unlabeled CLINDE, PK11195 and Ro 5-4864 (1 mg/kg) reduced the uptake of [(123)I]-CLINDE by 70-55% in olfactory bulbs. In the kidney and heart, a reduction of 60-80% ID/g was observed, while an increase was observed in the adrenals requiring 10 mg/kg for significant displacement. Flumazenil had no effect on uptake in peripheral organs and brain. Metabolite analysis indicated >90% of the radioactivity in the above tissues was intact [(123)I]-CLINDE.

CONCLUSION

[(123)I]-CLINDE displays high and selective uptake for the PBBS and warrants further development as a probe for imaging PBBS using single photon emission computed tomography (SPECT).

Imaging brain inflammation with [(11)C]PK11195 by PET and induction of the peripheral-type benzodiazepine receptor after transient focal ischemia in rats

[(11)C]PK11195 is used in positron emission tomography (PET) studies for imaging brain inflammation in vivo as it binds to the peripheral-type benzodiazepine receptor (PBR) expressed by reactive glia and macrophages. However, features of the cellular reaction required to induce a positive [(11)C]PK11195 signal are not well characterized. We performed [(11)C]PK11195 PET and autoradiography in rats after transient focal cerebral ischemia. We determined [(3)H]PK11195 binding and PBR expression in brain tissue and examined the lesion with several markers. [(11)C]PK11195 standard uptake value increased at day 4 and grew further at day 7 within the ischemic core. Accordingly, ex vivo [(3)H]PK11195 binding increased at day 4, and increases further at day 7. The PET signal also augmented in peripheral regions, but to a lesser extent than in the core. Binding in the region surrounding infarction was supported by [(11)C]PK11195 autoradiography at day 7 showing that the radioactive signal extended beyond the infarcted core. Enhanced binding was preceded by increases in PBR mRNA expression in the ipsilateral hemisphere, and a 18-kDa band corresponding to PBR protein was detected. Peripheral-type benzodiazepine receptor immunohistochemistry showed subsets of ameboid microglia/macrophages within the infarcted core showing a distinctive strong PBR expression from day 4. These cells were often located surrounding microhemorrhages. Reactive astrocytes forming a rim surrounding infarction at day 7 also showed some PBR immunostaining. These results show cellular heterogeneity in the level of PBR expression, supporting that PBR is not a simple marker of inflammation, and that the extent of [(11)C]PK11195 binding depends on intrinsic features of the inflammatory cells.

Strategy for improved [11C]DAA1106 radiosynthesis and in vivo peripheral benzodiazepine receptor imaging using microPET, evaluation of [11C]DAA1106

INTRODUCTION

The peripheral benzodiazepine receptor (PBR) has shown considerable potential as a clinical marker of neuroinflammation and tumour progression. [(11)C]DAA1106 ([(11)C]N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)-acetamide) is a promising positron emission tomography (PET) radioligand for imaging PBRs.

METHODS

A four-step synthetic route was devised to prepare DAA1123, the precursor for [(11)C]DAA1106. Two robust, high yielding methods for radiosynthesis based on [(11)C]-O-methylation of DAA1123 were developed and implemented on a nuclear interface methylation module, producing [(11)C]DAA1106 with up to 25% radiochemical yields at end-of-synthesis based on [(11)C]CH(3)I trapped. Evaluation of [(11)C]DAA1106 for in vivo imaging was performed in a rabbit model with microPET, and the presence of PBR receptor in the target organ was further corroborated by immunohistochemistry.

RESULTS

The standard solution method produced 2.6-5.2 GBq (n=19) of [(11)C]DAA1106, whilst the captive solvent method produced 1.6-6.3 GBq (n=10) of [(11)C]DAA1106. Radiochemical purities obtained were 99% and specific radioactivity at end-of-synthesis was up to 200 GBq/micromol for both methods. Based on radiochemical product, shorter preparation times and simplicity of synthesis, the captive solvent method was chosen for routine productions of [(11)C]DAA1106. In vivo microPET [(11)C]DAA1106 scans of rabbit kidney demonstrated high levels of binding in the cortex. The subsequent introduction of nonradioactive DAA1106 (0.2 micromol) produced considerable displacement of the radioactive signal in this region. The presence of PBR in kidney cortex was further corroborated by immunohistochemistry.

CONCLUSIONS

A robust, high yielding captive solvent method of [(11)C]DAA1106 production was developed which enabled efficacious in vivo imaging of PBR expressing tissues in an animal model.

The role of Ets transcription factors in the basal transcription of the translocator protein (18 kDa)

The translocator protein (18 kDa; TSPO), previously known as peripheral-type benzodiazepine receptor, is a high-affinity cholesterol- and drug-binding mitochondrial protein involved in various cell functions including steroidogenesis, apoptosis, and proliferation. TSPO is highly expressed in secretory and glandular tissues, especially in steroidogenic cells, and its expression is altered in certain pathological conditions such as cancer and neurological diseases. In this study, we characterized the regulatory elements present in the region of the TPSO promoter extending from 515 to 805 bp upstream of the transcription start site, an area previously identified as being important for transcription. Promoter fragments extending 2.7 kb and 805 bp upstream of the transcription start site were able to direct enhanced green fluorescent protein expression to Leydig cells of the testis, theca cells of the ovary, and cells of the adrenal cortex in transgenic animals. This expression pattern perfectly mimicked endogenous TSPO expression. Functional characterization of the 515-805 bp region revealed the presence of one specificity protein 1/specificity protein 3 (Sp1/Sp3) and two v-ets erythroblastosis virus E26 oncogene homologue (Ets) binding sites that are important for transcriptional activity in both MA-10 mouse Leydig tumor cells and NIH/3T3 whole mouse embryo fibroblasts. GA-binding protein alpha (GABPalpha), a member of the Ets family of transcription factors, was found to be associated with the endogenous TSPO promoter. We conclude that Sp1/Sp3 and members of the Ets family of transcription factors bind to specific binding sites in the TSPO promoter to drive basal TSPO gene transcription.

Characterization of two classes of benzodiazepine binding sites in Schistosoma mansoni

As we have recently shown that GABA should be considered a putative neurotransmitter inSchistosoma mansoni, the present work aimed to search for GABAAreceptors in adult worms using [3H]-flunitrazepam to label the allosteric benzodiazepine binding site which is classically present on GABAAreceptor complexes. We detected a large population (Bmax=8·25±1·1 pmol . mg protein−1) of high affinity (Kd=33·6±1·5 nm) binding sites for flunitrazepam. These sites harboured a singular pharmacological modulation that does not fit well with a mammalian central benzodiazepine receptor, mainly due to a very high affinity for Ro5-4864 and a very low affinity for clonazepam. We also detected a second population of benzodiazepine binding sites labelled with high affinity (IC50=85 nm) by [3H]-PK11195, a selective ligand of the mammalian peripheral benzodiazepine receptor. In conclusion, this work describes the pharmacological properties of a large population of central-like benzodiazepine receptors supporting their study as putative new targets for the development of anti-parasitic agents. We also describe, for the first time, the presence of peripheral benzodiazepine receptors in this parasite.

Translocator protein (18kDa): new nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function

The peripheral-type benzodiazepine receptor or recognition site (PBR) is a widely distributed transmembrane protein that is located mainly in the outer mitochondrial membrane. The PBR binds to high-affinity drug ligands and cholesterol. Many functions are associated directly or indirectly with the PBR, including the regulation of cholesterol transport and the synthesis of steroid hormones, porphyrin transport and heme synthesis, apoptosis, cell proliferation, anion transport, regulation of mitochondrial functions and immunomodulation. Based on these functions, there are many potential clinical applications of PBR modulation, such as in oncologic, endocrine, neuropsychiatric and neurodegenerative diseases. Although "PBR" is a widely used and accepted name in the scientific community, recent data regarding the structure and molecular function of this protein increasingly support renaming it to represent more accurately its subcellular role (or roles) and putative tissue-specific function (or functions). Translocator protein (18kDa) is proposed as a new name, regardless of the subcellular localization of the protein.

Nuclear location–dependent role of peripheral benzodiazepine receptor (PBR) in hepatic tumoral cell lines proliferation

PBR is involved in numerous biological functions, including steroid biosynthesis, mitochondrial oxidative phosphorylation and cell proliferation. The presence of PBR at the perinuclear/nuclear subcellular level has been demonstrated in aggressive breast cancer cell lines and human glioma cells where it seems to be involved in cell proliferation. In our study we investigated the presence of perinuclear/nuclear PBR in different hepatic tumor cell lines with regard to binding to [3H] PK 11195 and protein analysis. The results obtained by saturation binding experiments and scatchard analysis of perinuclear/nuclear PBR density in parallel with the results on the growth curves of the cell lines tested, indicate that the perinuclear/nuclear PBR density correlates inversely with cell doubling time. Moreover, the cell line with high perinuclear/nuclear PBR proliferated in response to PBR ligand, whereas that with low perinuclear/nuclear PBR did not. Our results reinforce the idea that the subcellular localisation of PBR defines its function and that this receptor could be a possible target for new strategies against cancer.

Peripheral benzodiazepine receptor and its clinical targeting

Tumor cell targeted therapies, by induction or enhancement of apoptosis, constitute recent promising approaches achieving more specific anti-tumor efficacy. The peripheral benzodiazepine receptor (PBR), which belongs to the permeability transition pore (PTP), the central regulatory complex of apoptosis, is a potential target. A number of findings argue in favor of the development of PBR targeting approaches: (i) overexpression of PBR has been described in a large range of human cancers, (ii) PTP-mediated regulation of programmed cell death is an apoptotic-inducing factor-independent check-point that could be modulated by various conventional cancer therapies, and (iii) PBR ligation enhances apoptosis induction in many types of tumors and reverses Bcl-2 cytoprotective effects. Altogether, these observations support the use of PBR-directed drugs, particularly PBR ligands such as Ro5-4864, in the treatment of human cancers.

Involvement of benzodiazepine receptors in neuroinflammatory and neurodegenerative diseases: evidence from activated microglial cells in vitro

Increased binding of a ligand for the peripheral benzodiazepine binding receptor is currently used in PET studies as an in vivo measurement of inflammation in diseases like multiple sclerosis and Alzheimer's disease. Although peripheral-type benzodiazepin receptors (PBRs) are abundant in many cell types and expressed in the CNS physiologically only at low levels, previous reports suggest that after experimental lesions in animal models and in human neurodegenerative/-inflammatory diseases upregulated PBR expression with increased binding of its ligand PK11195 is confined mainly to activated microglia in vivo/in situ. Because the functional role of the PBR is unknown, we confirm by immunohistochemistry and PCR (I) that this receptor is expressed on microglia in vitro and (II) that benzodiazepines modulate proliferation of microglial cells and the release of the inflammatory molecules nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) in cell culture supernatants of primary rat microglia. Compared to lipopolysaccharide-activated controls the release of NO was markedly decreased in cultures treated with benzodiazepines (clonazepam, midazolam, diazepam) and the PBR ligand PK11195. Moreover, release of TNF-alpha and proliferation was significantly inhibited in the benzodiazepine-treated groups. These findings link the in vivo data of elevated PBR levels in neurodegenerative/-inflammatory diseases to a functional role and opens up possible therapeutic intervention targeting the PBR in microglia.

Differential utilization of the promoter of peripheral-type benzodiazepine receptor by steroidogenic versus nonsteroidogenic cell lines and the role of Sp1 and Sp3 in the regulation of basal activity

The peripheral-type benzodiazepine receptor (PBR) is involved in many cellular functions, including steroidogenesis, oxidative processes, cellular proliferation, and apoptosis. Secretory and glandular tissues, especially steroid hormone-producing cells, are particularly rich in PBR. To understand the mechanisms of PBR expression and regulation, we established an mRNA expression profile in mouse tissues and cell lines and subsequently mapped the transcription start site and characterized the promoter of the gene. Our findings indicate that PBR tissue mRNA levels are relatively high in kidney, spleen, muscle, lung, adrenal gland, thymus, and stomach; are intermediate in pancreas, uterus, prostate, heart, and testis; and are low in brain and liver. Relatively high levels of PBR mRNA were also observed in the steroid-synthesizing MA-10 mouse Leydig tumor cells compared with adrenocortical Y1 mouse cells and nonsteroidogenic NIH-3T3 mouse fibroblasts, although PBR protein levels were much higher in both steroidogenic cells compared with fibroblasts. Transcription was initiated primarily at an adenine nucleotide 61 nucleotides upstream of the translation start site, but internal initiation was also observed. A 2.7-kb fragment of the mouse PBR promoter was cloned and sequenced. Sequence analysis revealed the absence of TATA or CCAAT boxes, but the presence of many putative transcription factor-binding sites, including Sp1/Sp3, AP2, Ik2, AP1, SOX, GATA, and SRY. Functional characterization revealed that two Sp1/Sp3 sites in the proximal promoter are important for basal activity in all cell lines tested and that the steroidogenic MA-10 and Y1 cells use different areas of the promoter compared with nonsteroidogenic NIH-3T3 cells.

Immunohistochemical assessment of the peripheral benzodiazepine receptor in human tissues

Exhaustive analysis of the location of the peripheral benzodiazepine receptor (PBR) both at the subcellular and the tissue level is warranted to gain a better understanding of its biological roles. To date, many studies have been performed in animal models, such as rat, mouse, and pig, that yielded important information. However, only a few reports were dedicated to the analysis of PBR expression in humans. To enlarge on previous studies, we investigated PBR expression in different human organs using the monoclonal antibody 8D7 that specifically recognized the human PBR. First, we performed electron microscopic analysis that for the first time unambiguously demonstrated the localization of the PBR on the outer mitochondrial membrane. Second, focusing our analysis on human tissues for which information on PBR expression is sparse (lung, stomach, small intestine, colon, thyroid, adrenal gland, pancreas, breast, prostate, ovary), we found that PBR exhibits selective localization. This characterization of PBR localization in human tissues should provide important insights for the understanding of PBR functions.

Design, synthesis and structure–affinity relationships of aryloxyanilide derivatives as novel peripheral benzodiazepine receptor ligands

Since the peripheral benzodiazepine receptor (PBR) has been primarily found as a high-affinity binding site for diazepam in rat kidney, numerous studies of it have been performed. However, the physiological role and functions of PBR have not been fully elucidated. Currently, we presented the pharmacological profile of two high and selective PBR ligands, N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl)acetamide (7-096, DAA1106) (PBR: IC(50)=0.28 nM) and N-(4-chloro-2-phenoxyphenyl)-N-(2-isopropoxybenzyl)acetamide (7-099, DAA1097) (PBR: IC(50)=0.92 nM). The compounds are aryloxyanilide derivatives, and identified with known PBR ligands such as benzodiazepine (1, Ro5-4864), isoquinoline (2, PK11195), imidazopyridine (3, Alpidem), and indole (5, FGIN-1-27) derivatives. The aryloxyanilide derivatives, which have been derived by opening the diazepine ring of 1, are a novel class as PBR ligands and have exhibited high and selective affinity for peripheral benzodiazepine receptors (PBRs). These novel derivatives would be useful for exploring the functions of PBR. In this paper, the design, synthesis and structure-affinity relationships of aryloxyanilide derivatives are described.

Inhibitory effect of diazepam on muscarinic receptor-stimulated inositol 1,4,5-trisphosphate production in rat parotid acinar cells

1. This study examined the effect of diazepam (DZP) on phosphoinositide turnover, which plays an important role in the regulation of salivary secretion, in rat parotid acinar cells. 2. DZP (10(-9) M to 10(-5) M), a potent agonist of both central- and peripheral-type benzodiazepine receptors, dose-dependently decreased inositol 1,4,5-trisphosphate IP3 production stimulated by carbachol, a muscarinic receptor agonist, in the cells. 3. DZP produced a maximum inhibitory response at a concentration of 10(-5) M, with IP3 production decreased to 63% of maximal levels. The concentration inducing half maximal inhibition of IP3 production was approximately 3.5 x 10 (-8) M. 4. An inhibitory response to DZP was produced by a short-term pretreatment (<3 min) of the cells and prevented by antagonist and competing ligand for the central- and peripheral-type benzodiazepine receptors, flumazenil and PK 11195, respectively. 5. DZP showed a non-competitive inhibition of carbachol-stimulated IP3 production. It did not directly inhibit the activities of GTP-binding regulatory proteins and phosphatidylinositol 4,5-bisphosphate-specific phospholipase C (PLC) in the parotid gland membranes, though choline chloride inhibited PLC activity. 6. DZP (10(-5) M) attenuated the increase in the intracellular Ca2+ concentration ([Ca(2+)](i)) in the cells following stimulation of the muscarinic and alpha(1)-adrenoceptors. 7. These results suggest that in the parotid acinar cells, DZP inhibits muscarinic receptor-stimulated IP3 production through benzodiazepine receptors and that PLC activity which produces IP3 is inhibited by chloride. The decreases in IP3 and [Ca(2+)](i) in the cells may be connected with the suppression of salivary secretion induced by DZP.

Brain plasticity and microglia: is transsynaptic glial activation in the thalamus after limb denervation linked to cortical plasticity and central sensitisation?

Microglia are a subset of tissue-macrophages that are ubiquitously distributed throughout the entire CNS. In health, they remain largely dormant until activated by a pathological stimulus. The availability of more sensitive detection techniques has allowed the early measurement of the cell responses of microglia in areas with few signs of active pathology. Subtle neuronal injury can induce microglial activation in retrograde and anterograde projection areas remote from the primary lesion focus. There is also evidence that in cases of long-standing abnormal neuronal activity, such as in patients after limb amputation with chronic pain and phantom sensations, glial activation may occur transsynaptically in the thalamus. Such neuronally driven glial responses may be related to the emergence central sensitisation in chronic pain states or plasticity phenomena in the cerebral cortex. It is suggested, that such persistent low-level microglial activation is not adequately described by the traditional concept of phagocyte-mediated tissue damage that largely evolved from studies of acute brain lesion models or acute human brain pathology. Due to the presence of signal molecules that can act on neurons and microglia alike, the communication between neurons and microglia is likely to be bi-directional. Persistent subtle microglial activity may modulate basal synaptic transmission and thus neuronal functioning either directly or through the interaction with astrocytes. The activation of microglia leads to the emergence of microstructural as well as functional compartments in which neurokines, interleukins and other signalling molecules introduce a qualitatively different, more open mode of cell-cell communication that is normally absent from the healthy adult brain. This 'neo-compartmentalisation', however, occurs along predictable neuronal pathways within which these glial changes are themselves under the modulatory influence of neurons or other glial cells and are subject to the evolving state of the pathology. Depending on the disease state, yet relatively independent of the specific disease cause, fluctuations in the modulatory influence by non-neuronal cells may form the cellular basis for the variability of brain plasticity phenomena, i.e. the plasticity of plasticity.

The peripheral benzodiazepine receptors: a review

Peripheral benzodiazepine receptors (PBRs) have been identified in various peripheral tissues as well as in glial cells in the brain. This review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. It deals with the pharmacological, structural and molecular characterization of the PBR, the proteins associated with the receptor (VDAC, ANC, PRAX-1) and their roles in cell growth and differentiation, cancer, steroid biosynthesis, and other physiological roles.

Up-regulation of mitochondrial peripheral benzodiazepine receptor expression by tumor necrosis factor alpha in testicular leydig cells. Possible involvement in cell survival

Porcine Leydig cells in primary cultures are resistant to tumor necrosis factor alpha (TNFalpha) cytotoxicity. Here we report that these cells can be rendered sensitive to TNFalpha killing by treatment with the translational inhibitor cycloheximide, suggesting the existence of proteins that can suppress the death stimulus induced by the cytokine. In search of these cytoprotective proteins, we focused on the constituents of the mitochondrial permeability transition pore (PT pore), whose opening has been shown to play a critical role in the TNFalpha-mediated death pathway. We found that TNFalpha up-regulated mRNA and protein expression of the mitochondrial peripheral benzodiazepine receptor (PBR), an outer membrane-derived constituent of the pore. A strong correlation was established between the resistance of the cells to TNFalpha killing and the density of PBR-binding sites. Concomitantly, TNFalpha down-regulated Bcl-2 mRNA and protein expression. As Bcl-2 has been shown to be an endogenous inhibitor of the PT pore, we hypothesize that the TNFalpha-induced up-regulation of PBR expression may compensate for the decrease in Bcl-2 levels to prevent the opening of the PT pore.

Relation of cell proliferation to expression of peripheral benzodiazepine receptors in human breast cancer cell lines

Peripheral benzodiazepine receptor (PBR) agonist [(3)H]Ro5-4864 has been shown to bind with high affinity to the human breast cancer cell line BT-20. Therefore, we investigated different human breast cancer cell lines with regard to binding to [(3)H]Ro5-4864 and staining with the PBR-specific monoclonal antibody 8D7. Results were correlated with cell proliferation characteristics. In flow cytometric analysis, the estrogen receptor (ER)-negative breast cancer cell lines BT-20, MDA-MB-435-S, and SK-BR-3 showed significantly higher PBR expression (relative fluorescence intensity) than the ER-positive cells T47-D, MCF-7 and BT-474 (P<0.05). Accordingly, BT-20 and MDA-MB-435-S had the highest capacity for binding [(3)H]-Ro5-4864, while the ER-positive cells exhibited only low binding of the benzodiazepine. PBR expression correlated inversely with cell doubling time (r = 0.78) and positively with Ki-67 expression (r = 0.77). The amount of mitochondria was significantly higher in cells with high PBR expression. As PBR could be demonstrated only after permeabilization of cells, PBR is suggested to be localized within the cytoplasm. Moreover, colocalization of PBR and mitochondria was shown by confocal microscopy analysis. The highest amounts of both PBR and mitochondria were found in cell lines with high mitotic activity. Therefore, it is concluded that the level of PBR is dependent on the number of mitochondria. PBR and its putative endogenous ligand diazepam-binding inhibitor are possibly involved in the regulation of cell proliferation of human breast cancer cell lines.

Diazepam inhibits forskolin-stimulated adenylyl cyclase activity in human tumour cells

Previous studies have shown that the benzodiazepine agonist, diazepam, suppresses adenylyl cyclase activity in rat brain, via a G protein-coupled benzodiazepine receptor. Since diazepam binding sites are also present in diverse non-neuronal tissues including tumour cells, its effects on adenylyl cyclase activity were examined in membranes from human MCF-7 (breast cancer) and M-6 (melanoma) cells. Diazepam caused a biphasic and concentration-dependent inhibition of forskolin-stimulated adenylyl cyclase activity in MCF-7 membranes. The first phase of inhibition, at picomolar to nanomolar drug concentrations (EC50=5.7 x 10(-12)M), is similar to the receptor mediated phase observed in the rat brain. At micromolar concentrations of diazepam (EC50= 1.8 x 10(-4)M), the steep decrease in adenylyl cyclase activity may involve a direct action on the enzyme itself, as detected previously in rat brain membranes. Diazepam-induced suppression of adenylyl cyclase activity was also detected in M-6 membranes. However, in contrast to MCF-7 findings, only micromolar concentrations of diazepam (EC50=5.2 x 10(-4)M) inhibited enzyme activity in M-6 membranes. These findings suggest that G protein-coupled benzodiazepine receptors, which mediate inhibition of the adenylyl cyclase-cAMP pathway in the brain, are also expressed in MCF-7 cells.

Decreased platelet peripheral-type benzodiazepine receptors in adolescent inpatients with repeated suicide attempts

BACKGROUND

Peripheral-type benzodiazepine receptors (PBR) are responsible for mitochondrial cholesterol uptake, the rate limiting step of steroidiogenesis. They have been shown to be increased after acute stress, and decreased during exposure to chronic stressful conditions, and in patients with generalized anxiety disorder and post-traumatic stress disorder. In view of the proven connection between adolescent suicidal behavior and stress, we hypothesized that PBR may be decreased in the suicidal adolescent population.

METHODS

We measured [3H] PK 11195 binding to platelet membrane in nine adolescent (age 13-20 years) inpatients with a history of at least three suicidal attempts and ten age-matched psychiatric inpatients with no history of suicide attempts. Suicidality was assessed with the Suicide Risk Scale (SRS), and symptom severity with the Beck Depression Inventory, State-Trait Anxiety Inventory (STAI), Overt Aggression Scale (OAS), and Impulsivity Scale (IS).

RESULTS

Suicide Risk Scale scores were significantly higher in the suicidal group. The suicidal group showed a significant decrease in platelet PBR density (-35%) compared to the controls (p < 0.005).

CONCLUSIONS

Our results of PBR depletion in adolescent suicide are in accordance with the findings in patients with generalized anxiety disorder and posttraumatic stress disorder and lend further support to the role of PBR in human response to chronic stress in adolescent suicide.

Peripheral-type benzodiazepine receptors in the regulation of proliferation of MCF-7 human breast carcinoma cell line

Peripheral-type benzodiazepine receptors (PBR) have been implicated in cell proliferation. The aim of the present study was to test the effect of the PBR ligands PK 11195 and Ro 5-4864 and the central-type benzodiazepine receptor ligand clonazepam on breast carcinoma cell proliferation, using [3H] thymidine incorporation. We then carried out a study to identify where the PBR-specific ligands Ro 5-4864 and PK 11195 act in the cell cycle, using flow cytometric analysis. We found PBR expression in the malignant breast cancer tumors, representing various levels of estrogen and/or progesterone receptors, as well as in the MCF-7 breast carcinoma cell line. PK 11195 and Ro 5-4864 inhibited cell proliferation at concentrations of 10(-5) to 10(-4) M, while clonazepam (the central-type benzodiazepine receptor-specific ligand) had no effect. In this same concentration range, PK 11195 and Ro 5-4864, in contrast to clonazepam, induced an accumulation of MCF-7 cells in both the G0-G1 and G2-M phases of the cell cycle. The present study demonstrates that PBR ligands play a role in regulating cell proliferation in the human breast carcinoma cell line MCF-7.

The expression of peripheral benzodiazepine receptors in human skin: the relationship with epidermal cell differentiation

The peripheral benzodiazepine receptor (PBR) is a protein of mitochondrial outer membranes utilizing porphyrins as endogenous ligands. PBR is part of a heteromeric receptor complex involved in the formation of mitochondrial permeability transition pores and in the early events of apoptosis. PBR may function as an oxygen-dependent signal generator; recent data indicate that these receptors may preserve the mitochondria of haematopoietic cell lines from damage caused by oxygen radicals. To identify PBRs in human skin, we used a specific monoclonal antibody directed against the C-terminus fragment of the human receptor. PBR immunoreactivity was found in keratinocytes, Langerhans cells, hair follicles and dermal vascular endothelial cells. Interestingly, confocal microscopic examination of skin sections revealed that PBR expression was strongly upregulated in the superficial differentiated layers of the epidermis. Ultrastructurally, PBRs were distributed throughout the cytoplasm but were selectively expressed on the mitochondrial membranes of epidermal cells. The elevated level of PBRs in the spinous layer was not associated with an increased number of mitochondria nor with an increased amount of mRNA as assessed by in situ hybridization on microautoradiographed skin sections. The present work provides, for the first time, evidence of PBR immunoreactivity in human skin. This mitochondrial receptor may modulate apoptosis in the epidermis; its increased expression in differentiated epidermal layers may represent a novel mechanism of natural skin protection against free radical damage generated by ultraviolet exposure.

Inhibitory regulation of amylase release in rat parotid acinar cells by benzodiazepine receptors

This study examined the influence of benzodiazepine receptors on amylase release from rat parotid acinar cells. Diazepam (10(-8)-10(-6) M), which is a potent agonist of both central- and peripheral-type benzodiazepine receptors, dose dependently decreased amylase release induced by isoprenaline and carbachol, which are beta-adrenoceptor and muscarinic receptor agonists, respectively. The maximum inhibitory response was obtained with 10(-6) M diazepam: amylase release was decreased to 57% (isoprenaline) and 39% (carbachol) of maximal levels, while these responses were completely inhibited by propranolol and atropine, respectively. Clonazepam and 7-chloro-1,3-dihydro-1-methyl-5-p-chlorophenyl)-2H-1,4-benzodiazepine-2- one (Ro 5-4864), which are selective agonists of central- and peripheral-type benzodiazepine receptors, respectively, also produced a significant and dose-dependent decrease in isoprenaline-induced amylase release. The inhibitory potency was diazepam > clonazepam > Ro 5-4864. Flumazenil and 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide (PK 11195), which are selective antagonists of central- and peripheral-type benzodiazepine receptors, respectively, dose dependently blocked the inhibition of isoprenaline-induced amylase release by diazepam. At a concentration of 10(-5) M, flumazenil and PK 11195 restored amylase release to approximately 75% of that in the presence of isoprenaline alone. The combination of both antagonists completely prevented the inhibition by diazepam. Similarly, the inhibitory responses of clonazepam and Ro 5-4864 were completely blocked by flumazenil and PK 11195, respectively. These results suggest that, in rat parotid acinar cells, benzodiazepines inhibit beta-adrenoceptor and muscarinic receptor-stimulated amylase release and that both central- and peripheral-type benzodiazepine receptors contribute to this inhibitory regulation.

PK11195 binding to the peripheral benzodiazepine receptor as a marker of microglia activation in multiple sclerosis and experimental autoimmune encephalomyelitis

Activated glial cells are implicated in regulating and effecting the immune response that occurs within the CNS as part of multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). The peripheral benzodiazepine receptor (PBR) is expressed in glial cells. We examined the utility of using in vitro and in vivo ligand binding to the PBR as a measure of lesion activity in autoimmune CNS demyelinating diseases. Applying a combined autoradiography and immunohistochemical approach to spinal cord and brain tissues from mice with EAE, we found a correlation at sites of inflammatory lesions between [3H]-PK11195 binding and immunoreactivity for the activated microglial/macrophage marker Mac-1/CD11b. In MS tissues, [3H]-PK11195 binding correlated with sites of immunoreactivity for the microglial/macrophage marker CD68, at the edges of chronic active plaques. Positron emission tomography (PET) imaging with [11C]-PK11195 showed ligand uptake only at sites of active MS lesions defined by magnetic resonance imaging criteria. Our results indicate the potential to develop markers suitable for both in vitro and in vivo use, which will serve to help correlate phenotypic and functional properties of cells which participate in disease or injury responses within the CNS.

Peripheral-type benzodiazepine receptors

1. The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the gamma-aminobutyric acid A receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues and is therefore called the peripheral type benzodiazepine receptor (PBR). 2. The first section of this review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. 3. The next section deals with the pharmacological, structural and molecular characterization of the PBR that has taken place in the past few years. 4. The final section describes the possible physiological role(s) of the PBR and identifies future work that would help deepen our understanding of the PBR and its function.

Multiple forms and locations for the peripheral-type benzodiazepine receptor

The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the neuronal gamma-aminobutyric acidA (GABAA) receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues, in blood cells, and in glial cells in the brain, but its functions remain unclear. Although these peripheral-type benzodiazepine binding sites (PBBS) have been localized to the mitochondrial outer membrane in many tissues, a growing body of evidence suggests that they may also exist on the plasma membrane. Plasma membrane PBBS have been described in heart, liver, adrenal, and testis and on hemopoietic cells. In rat liver, the two subcellular forms of PBBS are found separately in two different subpopulations of cells. The discovery of a plasma membrane fraction of PBBS clearly has implications for some of its putative functions, including steroidogenesis, mitochondrial respiration, heme metabolism, calcium channel modulation, cell growth, and immunomodulation. This commentary reviews the evidence for two locations for the PBBS and discusses the relevance of mitochondrial and plasma membrane forms with regard to structure, molecular biology, and proposed roles.

Two cellular and subcellular locations for the peripheral-type benzodiazepine receptor in rat liver

Determination of ligand binding properties of the peripheral benzodiazepine receptor (PBBS) in liver, in hepatocytes, and in nonparenchymal cells demonstrated the presence of receptor-specific high-affinity binding in both hepatocyte and nonhepatocyte cells. Density gradient centrifugation showed that the high-affinity receptor in hepatocytes was localised to mitochondria, whereas in nonhepatocytes it was not mitochondrial, but with a possible biliary epithelial cell plasma-membrane location. Both receptors showed the peripheral-type specific high-affinity binding of PK 11195 and Ro5 4864 and could be photolabelled as 18 kDa proteins with [3H]PK 14105. Immunocytochemistry showed the presence of acyl-CoA binding protein, a putative endogenous ligand for the receptor, in both cell locations. Some other properties of the PBBS were investigated in liver. Diphosphatidyl glycerol had a strong inhibitory effect on receptor binding in both liver and adrenal, with Ro5 4864 more sensitive to inhibition than PK 11195. However, whereas soybean lipid and phosphatidyl serine increased the binding of both ligands to adrenal receptor, these lipids had no effect on liver, suggesting that liver PBBS may differ from the well-characterised adrenal PBBS in some of its protein conformation. Modulators of mitochondrial respiration that also influence intermembrane contact site formation were found to elicit no marked stimulatory or inhibitory effects on PBBS ligand binding in liver, a result also found for adrenal mitochondria, suggesting that the extent of contact site formation does not influence ligand binding and that the hepatocyte receptor may not play a role in regulating mitochondrial respiration. These two cellular and subcellular locations of the PBBS in liver and the different effect of phospholipids compared to other peripheral tissues may be important for the role(s) of PBBS in liver and also for the multiple roles ascribed to the receptor and to peripheral-type benzodiazepine ligands.

Characterization of peripheral benzodiazepine type sites in a cultured murine BV-2 microglial cell line

It is known that the density of peripheral benzodiazepine receptors (PBR) increases after brain damage. Astrocytes are among the cell types where PBR ligand binding has been detected and may be involved in the response to neuronal injury and regeneration. Consistent with the hypothesis, the apparent density of PBR sites in astrocytes is increased by both cytokines and neurotoxins. However, microglia, the resident macrophages which represent 5-15% of glial cell populations have not been evaluated for the presence of the PBR. In the present study, we report the presence of [3H]Ro5-4864 binding in microglial cells. In particular, we used BV-2 cells, an immortalized cell line of murine microglial cells. High affinity binding of [3H]Ro5-4864 to a single site was detected in membranes prepared from BV-2 cells (KD = 4.4 nM, Bmax = 3,800 fmoles/mg protein). Various ligands for the PBR displaced [3H]Ro5-4864 binding with the following rank order of potencies: PK11195 = Ro5-4864 > FGIN-1-27 > triazolam = diazepam > beta-pro-pyl-beta-carboline-3-carboxylate = clonazepam > lorazepam = flurazepam >> chlordiazepoxide = clorazepate. Subcellular fractionationstudies indicate that the majority of the Ro5-4864 binding sites is in the mitochondrial fraction. The remainder is found in nonmitochondrial cell fractions. The [3H]Ro5-4864 binding observed on intact cells had characteristics similar to those found on membranes. The presence of a high density of PBRs in these cells establish the basis for additional investigations into their possible functional role, if any, in the microglial response to neuronal injury.

Down-regulation of hepatic peripheral-type benzodiazepine receptors caused by acute lead intoxication

In the present study we investigated the influence of acute lead poisoning upon the expression of benzodiazepine receptors. In addition, we examined if administration of PK 11195, an isoquinoline carboxamide derivative, to lead-poisoned rats could modulate the changes in receptor binding properties achieved by lead alone. Lead poisoning was ascertained by determination of urine delta-aminolevulinic acid levels and lead levels in rat livers. Scatchard analysis of saturation curves of [3H]PK 11195 binding to liver membranes of rats treated with lead alone or with both lead and PK 11195 showed and approximately two-fold decrease in receptor density in comparison with control groups. Peripheral benzodiazepine receptor density in kidneys and adrenals of poisoned rats was not changed by lead intoxication per se or by coadministration of PK 11195. Scatchard analysis of saturation curves of [3H]Ro 15-1788 binding in rat cerebral cortex tissue showed no difference in the receptor density between the various groups. The Kd values of all organs were in the nanomolar range (1-4 nM). We conclude that PK 11195 is not a protective agent of hepatic peripheral benzodiazepine receptors in lead intoxication. Moreover, it causes over-accumulation of lead in hepatocytes in an unknown mechanism of action.

New imidazo[1,2-b]pyridazine ligands for peripheral-type benzodiazepine receptors on mitochondria and monocytes

Several 6-chloro-2,3-disubstituted imidazo[1,2-b]pyridazines, selected from a number of synthetic imidazo[1,2-b]pyridazines which lacked significant binding activity at central benzodiazepine receptors, potently inhibit [3H]diazepam, [3H]Ro5-4864 and [3H]PK11195 binding to rat kidney mitochondrial membranes. In membrane preparations from cultures of THP-1 cells, a human monocytic leukaemia cell line, the isoquinoline carboxamide PK11195 is strongly bound but the benzodiazepine ligands, diazepam and Ro5-4864, are much more weakly bound. The imidazopyridazine compounds which bind strongly to mitochondrial benzodiazepine receptors are very potent displacers of [3H]PK11195 bound to the THP-1 membranes. It appears that the binding properties of these new imidazopyridazine ligands at 'peripheral-type' benzodiazepine receptors resemble those of the isoquinoline carboxamides more than those of the benzodiazepines.

Chronological study of peripheral benzodiazepine binding sites in the rat brain stab wounds using [3H] PK-11195 as a marker for gliosis

Chronological studies of the development of the peripheral benzodiazepine receptor sites were undertaken with the goal of evaluating the sensitivity of this marker for the study of the gliosis development in the injured brain. No significant increase in [3H] PK-11195 binding occurred in the rat brain stab wound one day following the puncture. A significant increase in the receptor density (Bmax) from the second day onward was observed. The Bmax reached its highest levels in the grey matter on the sixth day after a 23-gauge needle wound (8.75 +/- 0.09; pmol mg-tissue-1) and on the seventh day after an 18-gauge needle wound (8.98 +/- 0.31 pmol mg-tissue-1). In the white matter, the Bmax was greatest seven days after the wound (3.42 +/- 0.07; pmol mg-tissue-1; 23-gauge needle and 3.56 +/- 0.1 pmol mg-tissue-1 in the 18-gauge needle injury). Between 30 and 60 days after the wound, the Bmax was significantly lower than the Bmax observed between 6 and 14 days. The Bmax in the wound produced with needles was seven to eight times greater than the Bmax in the grey matter of the ipsilateral and contralateral cortices. Histological examination showed that there were no astrocytes or macrophages in the stab wound one day after the lesion. However, the glial fibrillary acidic protein positive cells and macrophages appeared on D3 after an injury. Gliosis, as measured by the PK-11195 binding, was also observed in the remote contralateral cortex. Data shows that PK-11195 binding is a very sensitive method of evaluating brain injury and could be of great value in studying progressive injuries in the living human brain in conjunction with positron emission tomography.

Enhancement by benzodiazepines of the inhibitory effect of adenosine on skeletal neuromuscular transmission

1. Interactions of benzodiazepines with adenosine on the neuromuscular transmission were studied in mouse diaphragm preparations. 2. In tubocurarine (0.6-0.8 microM)-partially paralyzed preparations, diazepam (35 microM) and Ro 5-4864 (3-30 microM), a peripheral type benzodiazepine receptor agonist, potentiated the inhibitory effect of adenosine on indirect twitch responses. 3. The central type receptor agonist, clonazepam did not affect the inhibitory effect of adenosine. 4. The peripheral benzodiazepine receptor antagonist, PK11195 (1-10 microM) attenuated the adenosine inhibition and antagonized the potentiation by Ro 5-4864. 5. Ro 5-4864 failed to enhance further the inhibitory effect of adenosine in the presence of dipyridamole, an adenosine uptake inhibitor that also potentiated adenosine inhibition. 6. Neither Ro 5-4864 nor PK 11195 affected the inhibition produced by a stable adenosine analogue, 2-chloroadenosine, which is not a substrate for the adenosine uptake system. 7. Ro 5-4864 did not affect endplate potentials (e.p.ps) in the absence of adenosine, but reduced the amplitude of e.p.ps in the presence of adenosine without affecting miniature e.p.ps. 8. It is suggested that benzodiazepines potentiate the adenosine-effected presynaptic inhibition of neuromuscular transmission by an inhibition of adenosine uptake through activation of peripheral type benzodiazepine receptors.

Cell surface expression of heat shock proteins in dog neutrophils induced by mitochondrial benzodiazepine receptor ligands

The effect of peripheral-type benzodiazepines on dog neutrophil stimulation was studied. Ro 5-4864 (a specific ligand of mitochondrial benzodiazepine receptor) and diazepam (which binds both to mitochondrial and central benzodiazepine receptors) did not show any direct toxic effect against neutrophils. PK 11195, a putative antagonist of the mitochondrial benzodiazepine receptor and an isoquinoline derivative, had a direct toxic effect at a concentration of 5 x 10(-5) M (72% of cells were viable). Ro 5-4864 (10(-6)-10(-4) M) and diazepam (10(-6)-2.5 x 10(-4) M) induced an intracellular oxidative stress in dog neutrophils. These compounds, in a micromolar range, also induced a concentration-dependent cell surface expression of heat shock protein (HSP) families. The percentages of positive cells that express these proteins were: 76.2% for HSP 27 kDa; 54.3% for HSP 72 kDa and 69.6% for HSP 90 kDa for Ro 5-4864 (10(-4) M), and 66.7% for HSP 27 kDa; 45.4% for HSP 72 kDa and 78.3 for HSP 90 kDa for diazepam (2.5 x 10(-4) M). It appears that this HSP expression, induced by peripheral-type benzodiazepines could be mediated by an intracellular oxidative stress.

Interaction of central and peripheral benzodiazepine sites in benzodiazepine tolerance and discontinuation

1. Chronic administration of benzodiazepines is associated with the development of tolerance and discontinuation effects in humans and in a mouse model. 2. Co-administration of compounds active at the "peripheral" benzodiazepine site may alter chronic benzodiazepine effects. 3. During chronic lorazepam administration, addition of the peripheral site antagonist PK11195 attenuates behavioral tolerance and receptor downregulation. 4. In mice treated with both lorazepam and PK11195, discontinuation effects were also attenuated compared to lorazepam alone. 5. Specificity of the action of PK11195 was confirmed by antagonism of its action by the peripheral-site agonist Ro5-4864.

Characterization of peripheral benzodiazepine receptors in rat prostatic adenocarcinoma

Using PK 11195, a high affinity ligand for peripheral benzodiazepine receptors (PBZr), binding sites in isolated mitochondrial (m-fraction) and microsomal fractions (p-fraction) from R-3327 Dunning AT-1 tumors, ventral and dorsolateral prostate were studied. Binding of PK 11195 in both m- and p-fractions from AT-1 tumors, but only in m-fraction from ventral and dorsolateral prostate, was specific, saturable, and of high affinity. The PBZr density in m-fraction from AT-1 tumor was 6-fold and 20-fold higher than that in ventral and dorsolateral prostate, respectively. The receptor density in p-fraction from AT-1 tumors was approximately 25% of that found in the m-fraction. Clear differences were observed in the competition by both diazepam and flunitrazepam for binding sites in m- and p-fractions from tumors. These data indicate that the receptors were not only localized to the mitochondria, but were also present in considerable amounts in the microsomal fractions. The unusually high amounts of receptors in the fast growing anaplastic prostatic tumor suggest their involvement in the regulation of cell proliferation and possibly in tumorigenesis.

Molecular basis for the different binding properties of benzodiazepines to human and bovine peripheral-type benzodiazepine receptors

The 18 kDa peripheral benzodiazepine receptor (PBR) can be labelled by benzodiazepines, such as Ro5-4864, and isoquinoline carboxamides such as PK11195. These two compounds are reversible competitive inhibitors of each other. However, while the binding affinity of Ro5-4864 varies enormously across species, PK11195 always displays high affinity, suggesting that their binding domains are overlapping but not identical. We report here that recombinant human and bovine PBR produced in yeast, a microorganism devoid of endogenous PBR, can be labelled with [3H]PK11195, but only the human receptor can be labelled with [3H]Ro5-4864. Furthermore, we identified, through the binding analysis of human-bovine chimaeric receptors, a region near the C-terminal end of the PBR, with only five non-conserved amino acids between human and bovine sequences, as responsible for the difference in high affinity binding of Ro5-4864 to the two receptors.

Peripheral-type benzodiazepine receptors

Since their first description as anomalous high affinity diazepam binding sites in rat peripheral tissues, the peripheral-type benzodiazepine receptor (PBR) has been increasingly studied to better understand nonneural effects of the benzodiazepines. The mammalian PBR is ubiquitously distributed with high concentrations in the outer mitochondrial membrane of secretory tissues. In regions of the brain, the density of PBR can equal or exceed the density of central-type benzodiazepine receptors. High affinity PK 11195 binding is diagnostic for the receptor while the affinity for benzodiazepines is species dependent. Recent cDNA cloning of a PBR component, the isoquinoline binding protein (IBP), shows no apparent sequence homology with any GABAA receptor subunits known to comprise central benzodiazepine receptor subtypes. The PBR seems at best only distantly related to CBRs. Recent advances in the pharmacology, biochemistry and molecular biology of the PBR are reviewed.

Distribution profile and properties of peripheral-type benzodiazepine receptors on human hemopoietic cells

The cellular localization of peripheral-type benzodiazepine receptors (PBRs) was characterized in several human blood cell subpopulations including erythrocytes, platelets, monocytes and polymorphonuclear neutrophils (PMN), B, NK, T8 and T4-cells. Pharmacological properties of the PBR were established by binding studies and PBR mRNA expression was measured by quantitative polymerase chain reaction based method. These data clearly indicate 1) the PBR is pharmacologically homogeneous in the various types of blood cells, 2) the rank order of PBR cell density is monocytes = PMN > lymphocytes >> platelets > erythrocytes, 3) the PBR appears to be transcriptionally regulated since mRNA levels are roughly correlated with PBR density.

The human "peripheral-type" benzodiazepine receptor: regional mapping of the gene and characterization of the receptor expressed from cDNA

A cDNA for the human "peripheral-type" benzodiazepine receptor (PBR) was isolated from a liver cDNA library. The 851-nucleotide probe hybridized with a approximately 1 kb mRNA in Northern blots of RNA extracted from various human tissues and cell lines. The human PBR probe was hybridized to DNA from a somatic cell hybrid mapping panel to determine that the gene maps to chromosome 22. With a regional mapping panel for chromosome 22, we localized the gene within band 22q13.31. The ligand-binding properties of the receptor expressed from the cDNA were examined in transient expression experiments and compared to the endogenous human PBR. The PBR ligand [3H]PK 11195 had high affinity for the expressed receptor in COS-1 cells, but the affinities of a pair of isoquinoline propanamide enantiomers differed remarkably in expressed and endogenous human PBR. These findings reveal that the host cell and/or post-translational modification may have an important influence on PBR function.

Localization of the peripheral-type benzodiazepine binding site to mitochondria of human glioma cells

Subcellular fractionation was performed on human U251 glioblastoma cultures. In all subcellular fractions, the binding of the peripheral benzodiazepine ligand, [3H]PK 11195, correlated with the specific activity of monoamine oxidase (r = 0.95, p less than 0.001) and succinate dehydrogenase (r = 0.93, p less than 0.001), two mitochondrial enzymes. The specific activity of plasma membrane and nuclear markers correlated poorly with the presence of PK 11195 binding sites. These data support the mitochondrion as the primary location of peripheral-type benzodiazepine binding sites (PBBS) in human glioma cells. Mitochondria-rich preparations were then assayed for [3H]Ro5-4964 binding. Six nM [3H]Ro5-4964 failed to specifically bind to human U251 mitochondria, but bound vigorously to mitochondria from rat C6 glioma. These data indicate that the low affinity of Ro5-4864 for PBBS in human glioma cells compared to those in rat is due to interspecies receptor variation rather than impaired drug transport into human cells.

Biochemical, physiological, and pathological aspects of the peripheral benzodiazepine receptor

The PBR is a mitochondrial protein composed of at least two subunits, an approximately 30-kDa subunit that contains the site for BZs and an approximately 18-kDa subunit that binds isoquinoline carboxamide derivatives. Porphyrins and diazepam binding inhibitor are putative endogenous ligands for these receptors, which are under neural and hormonal control. Alterations in the density of PBR seem to be a sensitive indicator of stress: up-regulation after acute stress and down-regulation induced by repeated stress. PBR-specific ligands are involved in the control of cell proliferation and differentiation, and their binding is increased in some cancer tumors. Numerous studies in various endocrine organs have revealed that PBR are located in specific regions or tissues in the organs. Furthermore, PBR densities in various organs subject to hormonal control are regulated by organotropic hormones. At least in some cases, BZ ligands do not exert a specific effect in an organ, but rather modulate the well-documented effects of that particular hormone. To the best of our knowledge, BZ ligand action in peripheral tissues is dependent on recognition of PBR, which may suggest a receptor-mediated action.

The mitochondrial benzodiazepine receptor: evidence for association with the voltage-dependent anion channel (VDAC)

Specific, high-affinity receptors for numerous drugs have recently been localized to mitochondrial membrane proteins. This review discusses the association of the mitochondrial receptor for benzodiazepines (mBzR) with the voltage-dependent anion channel (VDAC), indicating a possible auxiliary role for VDAC as a putative drug binding protein. The proposed subunit composition of the purified mBzR complex isolated from rat kidney mitochondria includes VDAC, which functions as a recognition site for benzodiazepines (e.g., flunitrazepam), the adenine nucleotide carrier (ADC), and an 18 kDa outer membrane protein identified by covalent labelling with the mBzR antagonists isoquinoline carboxamides (e.g., PK14105).

Characterization of [3H]Ro 5-4864 binding sites in rat vas deferens

The presence of benzodiazepine binding sites in rat vas deferens was detected using [3H]Ro 5-4864 as a radioligand. The binding of [3H]Ro 5-4864 to the mitochondrial sites is saturable, reversible, and temperature and time dependent. The association rate constant (k1) was 8.7 +/- 0.7 x 10(7) M-1 min-1, and the dissociation rate constant (k-1) was 0.031 +/- 0.003 min-1. The dissociation constant (KD) determined by saturation binding was 5.22 +/- 0.56 nM. The density of binding was 4,926 +/- 565 fmol/mg of protein. The Hill coefficient of binding was 0.99 +/- 0.01, an indication that [3H]Ro 5-4864 binds to a single site. The [3H]Ro 5-4864 binding was inhibited competitively by Ro 5-4864 and 2-hydroxy-5-nitrobenzyl-6-thioguanosine and noncompetitively by PK 11195, nitrendipine, alpha,beta-methylene-ATP, and carboxyatractyloside and was not affected by clonazepam, dicyclohexylcarbodiimide, or protoporphyrin IX. Our data indicate that [3H]Ro 5-4864 binding sites are not identical to those labeled by PK 11195. These binding sites are modulated by the ADP/ATP mitochondrial carrier, and an interaction of dihydropyridines and [3H]Ro 5-4864 binding sites in rat vas deferens is suggested.