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

The "peripheral-type" benzodiazepine (omega 3) receptor in hyperammonemic disorders

Increased levels of brain ammonia occur in both congenital and acquired hyperammonemic syndromes including hepatic encephalopathy, fulminant hepatic failure, Reye's syndrome and congenital urea cycle disorders. In addition to its effect on neurotransmission and energy metabolism, ammonia modulates the expression of various genes including the astrocytic "peripheral-type" benzodiazepine (or omega 3) receptor (PTBR). Increased expression of the isoquinoline carboxamide binding protein (IBP), one of the components of the PTBR complex, is observed in brain and peripheral tissues following chronic liver failure as well as in cultured astrocytes exposed to ammonia. Increased densities of binding sites for the PTBR ligand [3H]-PK11195 are also observed in these conditions as well as in brains of animals with acute liver failure, congenital urea cycle disorders and in patients who died in hepatic coma. The precise role of PTBR in brain function has not yet fully elucidated, but among other functions, PTBR mediates the transport of cholesterol across the mitochondrial membrane and thus plays a key role in the biosynthesis of neurosteroids some of which modulate major neurotransmitter systems such as the gamma-aminobutyric acid (GABA(A)) and glutamate (N-methyl-D-aspartate (NMDA)) receptors. Activation of PTBR in chronic and acute hyperammonemia results in increased synthesis of neurosteroids which could lead to an imbalance between excitatory and inhibitory neurotransmission in the CNS. Preliminary reports suggest that positron emission tomography (PET) studies using [11C]-PK11195 may be useful for the assessment of the neurological consequences of chronic liver failure.

Role of the peripheral-type benzodiazepine receptor in adrenal and brain steroidogenesis

The peripheral-type benzodiazepine receptor (PBR) has been demonstrated to be critical for steroidogenesis in all steroid-producing tissues. Here, we review the identification and characterization of the PBR, the evidence pointing to its function as a cholesterol pore involved in transporting cholesterol from the cytoplasm of steroid-producing cells into the inner mitochondrial membrane where it is metabolized, and the known mechanisms regulating its function. We present data on the functions of the PBR in the adrenal gland, a classical steroidogenic tissue, and in the brain, which has only recently been proven to be steroidogenic. Finally, we discuss other potential roles for the PBR in pathological conditions, including cancer, neurodegeneration, and neurotoxicity, and a broader role for the PBR in mediating intracellular cholesterol transport/compartmentalization, which may or may not be linked to steroid biosynthesis.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human colorectal cancer cells

The peripheral benzodiazepine receptor (PBR) has been implicated in growth control of various tumour models. Although colorectal cancers were found to overexpress PBR, the functional role of PBR in colorectal cancer growth has not been addressed to date. Using primary cell cultures of human colorectal cancers and the human colorectal carcinoma cell lines HT29, LS174T, and Colo320 DM we studied the involvement of PBR in the growth control and apoptosis of colorectal cancers. Both mRNA and protein expression of PBR were detected by RT-PCR and flow cytometry. Using confocal laser scanning microscopy and immunohistochemistry the PBR was localized in the mitochondria. The specific PBR ligands FGIN-1-27, PK 11195, or Ro5-4864 inhibited cell proliferation dose-dependently. FGIN-1-27 decreased the mitochondrial membrane potential, which indicates an early event in apoptosis. Furthermore, FGIN-1-27, PK 11195 or Ro5-4864 increased caspase-3 activity. In addition to their apoptosis-inducing effects, PBR ligands induced cell cycle arrest in the G(1)/G(0)-phase. Thus, our data demonstrate a functional involvement of PBR in colorectal cancer growth and qualify the PBR as a possible target for innovative therapeutic approaches in colorectal cancer.

Diazepam increases the hypothalamic-pituitary-adrenocortical (HPA) axis activity by a cyclic AMP-dependent mechanism

1. Previous studies in this laboratory have shown that diazepam behaves as a phosphodiesterase 4 (PDE 4) inhibitor. It has been reported that PDE-4 inhibitors activate the hypothalamic-pituitary-adrenocortical (HPA) axis in the rat. In the present study we have examined whether activation of the cyclic AMP-dependent protein kinase (PKA) is involved in the effect of diazepam on basal HPA axis activity. 2. Acute systemic administration of diazepam (10 mg kg(-1) i.p.) was found to increase the basal HPA axis activity, increasing the plasma concentrations of corticotrophin (ACTH) and corticosterone 30 min post injection. Diazepam also elevated cyclic AMP content of the hypothalamus. 3. Pretreatment of the animals with dexamethasone (1 mg kg(-1) s.c.) for 3 days completely abolished the effect of diazepam on HPA axis activity. 4. The antagonists of central and peripheral benzodiazepine receptors, flumazenil (10 mg kg(-1) i.p.) and PK 11195 (5 mg kg(-1) i.p.) did not affect the diazepam induced increase of HPA axis activity nor did they have an effect per se. 5. The increase in ACTH and corticosterone levels was significantly reduced by the cyclic AMP-dependent protein kinase (PKA) inhibitor, H-89, given either subcutaneously (5 mg kg(-1) s.c.) or intracerebroventricularly (i.c.v.; 28 microg in 10 microl). 6. The results indicate that diazepam can stimulate basal HPA axis activity in the rat by a cyclic AMP-dependent PKA mediated pathway.

The role of the peripheral benzodiazepine receptor in the apoptotic response to photodynamic therapy

Several previous studies have suggested that the peripheral benzodiazepine receptor (PBR) on the mitochondrial surface was an important target for photodynamic therapy (PDT). In this study we compared PBR affinity vs photodynamic efficacy of protoporphyrin-IX (PP-IX) and two structural analogs, PP-III and PP-XIII, using murine leukemia L1210 cells in culture. The results indicate that the three agents have approximately equal hydrophobicity, affinity for L1210 cells and ability to initiate photodamage leading to an apoptotic response. But only PP-IX had significant affinity for the PBR. These data indicate that the relationship between PDT efficacy and PBR affinity may hold only for sensitizers with the PP-IX configuration.

Mapping and fitting the peripheral benzodiazepine receptor binding site by carboxamide derivatives. Comparison of different approaches to quantitative ligand-receptor interaction modeling

The synthetic-computational approach to the study of the binding site of peripheral benzodiazepine receptor (PBR) ligands related to 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195, 1) within their receptor (Cappelli et al. J. Med. Chem. 1997, 40, 2910-2921) has been extended. A series of carboxamide derivatives endowed with differently substituted planar aromatic or heteroaromatic systems was designed with the aim of getting further information on the topological requisites of the carbonyl and aromatic moieties for interaction with the PBR binding site. The synthesis of most of these compounds involves Weinreb amidation of the appropriate lactone as the key step. The most potent compound, among the newly synthesized ones, shows a nanomolar PBR affinity similar to that shown by 1 and the presence of a basic N-ethyl-N-benzylaminomethyl group in 3-position of the quinoline nucleus. Thus, it may be considered the first example of a new class of water soluble derivatives of 1. Several computational methods were used to furnish descriptors of the isolated ligands (indirect approaches) able to rationalize the variation in the binding affinity of the enlarged series of compounds. Sound QSAR models are obtained by size and shape descriptors (volume approach) which codify for the short-range contributions to ligand-receptor interactions. Molecular descriptors which explicitly account for the electrostatic contribution to the interaction (CoMFA, CoMSIA, and surface approaches) perform well, but they do not improve the quantitative models. Moreover, useful hints for the identification of the antagonist binding site in the three-dimensional modeling of the receptor (direct approach) were provided by the receptor hypothesis derived by the pharmacophoric approach. The ligand-receptor complexes obtained provided a detailed description of the modalities of the interaction and interesting suggestions for further experiments.

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.

Flunitrazepam induces geometrical changes at the lipid-water interface

Flunitrazepam (FNTZ) effects on molecular packing and surface curvature in artificial model membranes were investigated. FNTZ, from the subphase under dipalmitoylphosphatidylcholine (dpPC) monolayers at the air-water interface, expanded the surface pressure-area isotherm and induced an increment in the limiting area; in this conditions, the collapse pressure of dpPC decreased, indicating a lowering in the stability of the monolayer. Thermodynamic-geometric correlations based on molecular parameters predicted a decrement in the aggregation number and stability, and an increase in the curvature of the self-aggregated structure of dpPC in aqueous medium in the presence of FNTZ. Accordingly, negative-staining electron microscopy of dpPC aqueous dispersions showed that the mean diameter of dpPC vesicles decreased 2 and 2.87 times in the presence of 10 nM and 50 µM FNTZ, respectively, compared with control samples. The release of a soluble marker entrapped in dpPC liposomes increased slightly respect to the control in the presence of FNTZ. In dpPC-dpPE mixed liposomes 50 µM FNTZ induced a decrement in the amount of the aminophospholipid exposed to the outer monolayer. Concluding, an FNTZ-induced expansion of dpPC-water interface region affected the constraints imposed on the lipid-water system by the molecular geometry, interacting free energies and entropy that determine the shape of a multimolecular structure. In liposomes composed of a pure phospholipid, the bilayer expansion leaded, through a structure instability, to reduce the liposome size; in mixed liposomes, phospholipid molecules translocation could be observed as another compensating mechanism of the initial perturbation. These results may be relevant for understanding benzodiazepines' effects non-mediated by membrane receptors.

Flunitrazepam partitioning into natural membranes increases surface curvature and alters cellular morphology

In recent studies, we showed that flunitrazepam (FNTZ) and other benzodiazepines interact with artificial phospholipid membranes locating at the polar head group region, inducing a membrane expansion, reducing the molecular packing and reorganising molecular dipoles. In the present paper we investigated the possibility that those phenomena could be transduced into changes in the curvature of membranes from natural origin. Hence we studied the effect of FNTZ on cellular morphology using human erythrocyte as a natural assay system. Shape changes of erythrocytes were evaluated by light microscopy and expressed as a morphological index (MI). FNTZ induced echinocytosis in a time-dependent manner with MI values significantly higher than those of control (without drug) or DMSO (vehicle) samples. Lidocaine, a local anesthetic known to induce stomatocytosis by incorporating in the inner monolayer, counterbalanced the concentration-dependent FNTZ crenating effects. FNTZ induced protective effects, compared with control and DMSO, against time-dependent hemolysis. Hypotonic-induced hemolysis, was also lowered by FNTZ in a concentration-dependent manner. Both antihemolytic effects suggested a drug-induced membrane expansion allowing a greater increase in cell volume before lysis. In such a complex system like a cell, curvature changes triggered by drug partitioning towards the plasma membrane, might be an indirect effect exerted through modifications of ionic-gradients or by affecting cytoskeleton-membrane linkage. In spite of that, the curvature changes can be interpreted as a mechanism suitable to relieve the tension generated initially by drug incorporation into the bilayer and may be the resultant of the dynamic interactions of many molecular fluxes leading to satisfy the spontaneous membrane curvature.

Diazepam induces FGF-2 mRNA in the hippocampus and striatum

starting by 6 h following diazepam injection and returning to approximately control values by 24 h. In situ hybridization showed elevated FGF-2 mRNA labeling in the hippocampal formation, mostly in the pyramidal layer of the CA1 and CA2 subfields and in the dentate gyrus hilar region. These results indicate that diazepam treatment up-regulates FGF-2 expression in select regions of the brain and suggest that GABA may promote neuroplasticity in concert with FGF-2.

The peroxisome proliferator perfluorodecanoic acid inhibits the peripheral-type benzodiazepine receptor (PBR) expression and hormone-stimulated mitochondrial cholesterol transport and steroid formation in Leydig cells

The peroxisome proliferator perfluordecanoic acid (PFDA) has been shown to exert an antiandrogenic effect in vivo by acting directly on the interstitial Leydig cells of the testis. The objective of this study was to examine the in vitro effects of PFDA and identify its site of action in steroidogenesis using as model systems the mouse tumor MA-10 and isolated rat Leydig cells. PFDA inhibited in a time- and dose-dependent manner the hCG-stimulated Leydig cell steroidogenesis. This effect was localized at the level of cholesterol transport into the mitochondria. PFDA did not affect either the total cell protein synthesis or the mitochondrial integrity. Moreover, it did not induce any DNA damage. Morphological studies indicated that PFDA induced lipid accumulation in the cells, probably due to the fact that cholesterol mobilized by hCG did not enter the mitochondria to be used for steroidogenesis. In search of the target of PFDA, we examined its effect on key regulatory mechanisms of steroidogenesis. PFDA did not affect the hCG-induced steroidogenic acute regulatory protein (StAR) levels. However, it was found to inhibit the mitochondrial peripheral-type benzodiazepine receptor (PBR) ligand binding capacity, 18-kDa protein, and messenger RNA (mRNA) levels. Further studies indicated that PFDA did not affect PBR transcription, but it rather accelerated PBR mRNA decay. Taken together, these data suggest that PFDA inhibits the Leydig cell steroidogenesis by affecting PBR mRNA stability, thus inhibiting PBR expression, cholesterol transport into the mitochondria, and the subsequent steroid formation. Moreover, this action of PFDA on PBR mRNA stability indicates a new mechanism of action of peroxisome proliferators distinct from the classic transcription-mediated regulation of target genes.

Lymphocyte peripheral benzodiazepine receptor mRNA decreases in obsessive-compulsive disorder

The relative content of peripheral benzodiazepine receptor (pBR) mRNA was examined by reverse transcriptase-polymerase chain reaction in lymphocytes of obsessive-compulsive disorder (OCD) patients, according to their clinical course of illness. pBR mRNA significantly decreased only in chronic OCD patients (n=8) as compared to controls (n=10), whereas no significant changes were observed in episodic OCD patients (n=7). We suggest that modulation of pBR gene expression might delineate a clinical heterogeneity in OCD.

The astrocytic ("peripheral-type") benzodiazepine receptor: role in the pathogenesis of portal-systemic encephalopathy

An increasing body of evidence supports the notion that activation of astrocytic (peripheral-type) benzodiazepine receptors contributes to the pathogenesis of the central nervous system symptoms which are characteristic of portal-systemic encephalopathy (PSE). Binding site densities for the PTBR ligand [3H-PK11195] are increased in autopsied brain tissue from PSE patients as well as in the brains of animals with experimental chronic liver failure. In the case of the animal studies, increased PTBR sites resulted from increased PTBR gene expression. Exposure of cultured astrocytes to ammonia or manganese (two neurotoxic agents which under normal circumstances are removed by the hepatobiliary system and which are found to accumulate in brain in PSE) results in increased densities of [3H-PK11195] binding sites. Activation of PTBR is known to result in increased cholesterol uptake and increased synthesis in brain of neurosteroids some of which have potent positive allosteric modulator properties on the GABA-A receptor system. Accumulation of such substances in the brain in chronic liver failure could explain the neural inhibition characteristics of PSE.

Increased expression of peripheral benzodiazepine receptors and diazepam binding inhibitor in human tumors sited in the liver

The peripheral benzodiazepine receptor system triggers intracellular metabolic events and has been associated with cell proliferation. Its endogenous ligand, the diazepam binding inhibitor, contributes to steroidogenesis by promoting cholesterol delivery to the inner mitochondrial membrane. The present study was undertaken to verify whether this system is altered in tumors sited in the liver. Peripheral benzodiazepine receptors and diazepam binding inhibitor were studied using immunocytochemistry and in situ hybridization in 9 human tumors sited in the liver, in liver hyperplasia, cirrhotic nodular regeneration, intestinal adenocarcinoma and in surrounding non-tumoral tissue. Immunocytochemical staining and in situ hybridization demonstrated that peripheral benzodiazepine receptors and diazepam binding inhibitor were more prominently expressed in neoplastic cells than in non-tumoral tissue. They were present in the same cells, suggesting that diazepam binding inhibitor may act in an intracrine manner in these cells. Higher peripheral benzodiazepine receptors and diazepam binding inhibitor expression in tumor cells suggest an implication of this system in the metabolism of neoplastic cells. Furthermore the evaluation of peripheral benzodiazepine receptor and diazepam binding inhibitor expression might be useful in evaluating malignancy and in diagnostic approaches of tumors in liver tissue.

Involvement of peripheral type of benzodiazepine receptor in social isolation stress-induced decrease in pentobarbital sleep in mice

Our previous studies have shown that central-type benzodiazepine (BZD) receptors (CBR) and neurosteroids capable of modulating GABA(A) receptor function are involved in the decrease of pentobarbital (PB)-induced sleep caused by social isolation stress in mice. In this study, to further clarify the mechanism underlying this decrease, we investigated the possible involvement of peripheral-type BZD receptors (PBR) which play an important role in neurosteroidogenesis in PB sleep in socially isolated mice. Socially isolated mice showed significantly shorter duration of PB-induced sleep than group-housed animals. When injected intracerebroventricularly (i.c.v.), FGIN-1-27 (FGIN, 25-100 nmol), a selective PBR agonist, and PK11195 (PK, 14-28 nmol), a PBR antagonist, and pregnenolone (PREG, 15-30 nmol), a neurosteroid precursor, dose-dependently normalized the PB sleep in isolated mice without having an effect on the group-housed animals. In contrast, pregnenolone sulfate (PS, 24 nmol), an endogenous neurosteroidal negative allosteric modulator of the GABA(A) receptor, reduced PB sleep in group-housed but not isolated mice. PS, at the same dose, significantly attenuated the effects of FGIN (100 nmol), PK (28 nmol) and PREG (30 nmol) in isolated mice, while FGIN (100 nmol), PK (28 nmol) and pregnenolone (30 nmol) significantly blocked the effect of PS (24 nmol) in group-housed mice. These results suggest that the PBR-mediated decrease in the genesis of neurosteroid(s) possessing a GABA(A) receptor agonistic profile is also partly involved in the down regulation of the GABA(A) receptor following long-term social isolation and contributes to the decrease of PB-induced sleep in isolation stressed mice.

Portacaval anastomosis causes selective alterations of peripheral-type benzodiazepine receptor expression in rat brain and peripheral tissues

There is a growing body of evidence to suggest that peripheral-type benzodiazepine receptors (PTBRs) and their endogenous ligands are implicated in the pathogenesis of end-organ failure in chronic liver disease. Portal-systemic encephalopathy, a major neuropsychiatric complication associated with chronic liver disease, results in activation of brain PTBR and probably in peripheral organs. In order to address these issues, PTBR mRNA was measured using semi-quantitative RT-PCR in extracts of cerebral cortex, kidney and testis of rats four weeks after end-to-side portacaval anastomosis and sham-operation (controls). Densities of PTBR sites were measured concomitantly by in vitro receptor binding using the selective PTBR ligand [3H]PK11195. Portacaval shunting resulted in a 2 to 3-fold increase in expression of PTBR in brain and kidney and a 37% reduction in expression in testis. Densities of [3H]PK11195 sites changed in parallel with the alterations of gene expression. These findings suggest that selective alterations of PTBR expression are implicated in the pathogenesis of peripheral tissue hypertrophy (kidney) and/or atrophy (testis) which accompanies portal-systemic shunting in chronic liver failure. In brain, activation of PTBR could result in an increase in the production of neurosteroids with potent inhibitory action in the CNS, which could contribute to the pathogenesis of portal-systemic encephalopathy.

Neuropharmacological profile of peripheral benzodiazepine receptor agonists, DAA1097 and DAA1106

Receptor binding and behavioral profiles of N-(4-chloro-2-phenoxyphenyl)-N-(2-isopropoxybenzyl)acetamide (DAA1097) and N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (DAA1106), novel, selective agonists for the peripheral benzodiazepine receptor (PBR) were examined. DAA1097 and DAA1106 inhibited [3H]PK 11195 binding to crude mitochondrial preparations of rat whole brain, with IC50 values of 0.92 and 0.28 nM. Likewise, DAA1097 and DAA1106 inhibited [3H]Ro 5-4864 binding to the same mitochondrial preparation, with IC50 values of 0.64 and 0.21 nM. In contrast, DAA1097 and DAA1106 did not inhibit [3H]-flunitrazepam, the central benzodiazepine receptor (CBR) ligand, binding to membranes of rat whole brain (IC50>10,000nM). Oral administration of DAA1097 and DAA1106 had anxiolytic effects in the mouse light/dark exploration test and in the rat elevated plus- maze test. Oral administration of DAA1106, diazepam and buspirone but not DAA1097 significantly increased sleeping time in hexobarbital-induced anesthesia in mice. The order of potency of potentiation of hexobarbital anesthesia was diazepam> buspirone> DAA1106> DAA1097. Oral administration of DAA1097 and DAA1106 but not diazepam and buspirone did not affect spontaneous locomotor activity in mice. These findings indicate that DAA1097 and DAA1106 are PBR selective ligands with potent anxiolytic-like properties, in laboratory animals.

Benzodiazepine localisation at the lipid-water interface: effect of membrane composition and drug chemical structure

The effect of membrane chemical composition and drug chemical structure on the localisation of several benzodiazepines (BZDs) (DZ, diazepam; CZ, clonazepam; CX, chlordiazepoxide) within model membranes was investigated. We used a spectrophotometric method presented in a previous paper (B.A. García, M.A. Perillo, Biochim. Biophys. Acta 1324 (1997) 76-84) based on the study of BZD acid-base equilibrium. 'Intrinsic pK' values (pKi) were calculated according to the theory of M.S. Fernández and P. Fromherz (J. Phys. Chem. 81 (1977) 1755-1761). Homogeneous media of known dielectric constant (dioxane 0-80% v/v in water) were used to construct a curve of DeltapKi (pKi-pKw) vs. dielectric constant (D) where DeltapKi values obtained in lipidic dispersions were interpolated. In heterogeneous media consisting of aqueous dispersions of Triton X-100 micelles we determined the relative localisation depth of BZDs according to their DTriton values (36, 37 and 62 for DZ, CX and CZ respectively) taking into account that lower D values correspond to deeper localisation. pKi determined in dispersions of dipalmitoylphosphatidylcholine (dpPC) and egg phosphatidylcholine (egg-PC) mixed multilamellar vesicles showed that, when cholesterol content increased from 0 to 20 mole%, D values decreased (from 59 to 40) in dpPC vesicles and increased (from 51 to 72) in egg-PC vesicles, indicating a tendency of BZDs to penetrate deeper into less ordered interfaces. These results should be considered to understand the non-specific pharmacological effects of BZDs as well as to evaluate the actual relevance of their pharmacological concentrations.

Peripheral type benzodiazepine receptor in T lymphocyte rich preparation

Some types of mood disorders and drugs are suggested to affect peripheral type benzodiazepine receptors (PBR), but their mechanisms are unclear. The isolation of pure lymphocytes is requisite for the investigation of the function of PBR on lymphocytes, since platelets and monocytes also have many PBR. The objective of this study was to establish a method of binding assay for PBR using pure T lymphocytes. Mononuclear cells and T lymphocytes were prepared by using a density gradient material and magnetic beads, respectively. The cells were analyzed using flow cytometry and a counting chamber. Binding studies were performed using T lymphocytes from 10 normal volunteers. The T lymphocytes were incubated with [3H]PK11195, harvested on glass fiber filters, and counted with a plate scintillation counter. The binding data were analyzed by the Scatchard method. With the magnetic bead technique, pure T cells were selected that contained only 1.5% monocytes and platelet/cell ratio of 1.4. The Scatchard plot of the data indicated that only one type of specific binding site was involved in the binding. The dissociation constant (Kd) was 3.8+/-1.3nM (mean+/-SD), and the Bmax was 379+/-124 fmol//10(6) cells (mean+/-SD). The density gradient- magnetic beads technique can be used as an appropriate method of preparation of T cells for PBR binding assay.

Cardiac effects of benzodiazepine receptor agonists and antagonists in the isolated rat heart: a comparative study

Effects of PK 11195 and flumazenil on cardiac responses to diazepam, clonazepam and zolpidem were compared. Coronary flow rate was increased at relatively low doses of diazepam and decreased at higher doses. Clonazepam induced a dose-dependent increase, and zolpidem a decrease of coronary flow rate. PK 11195 reduced the diazepam-induced increase of coronary flow rate, and flumazenil was ineffective. Neither antagonist evoked substantial changes in the decrease of coronary flow rate. PK 11195, and less so flumazenil, antagonized the clonazepam-induced increase. PK 11195 and flumazenil only in their highest doses suppressed and respectively potentiated the zolpidem-induced decrease. Inotropy showed a biphasic response in the presence of diazepam, i.e. an initial transient decrease, followed by a dose-dependent increase in two steps. Clonazepam induced a similar response. Zolpidem increased the inotropy. The negative inotropic response induced by diazepam did not change significantly in the presence of PK 11195 or flumazenil. The positive inotropic response was suppressed by PK 11195, and less so by flumazenil. The negative response to clonazepam was antagonized by both PK 11195 and flumazenil; the positive response was not significantly changed. In the presence of lower doses of PK 11195, the zolpidem-induced response was potentiated, whereas higher doses produced reversal; flumazenil potentiated the response. In conclusion, the results support earlier suggestions, involving receptor mechanisms with cardiac effects of benzodiazepines. Both agonists and antagonists (inter)act in a different manner, suggesting that rather ambiguous receptor mechanisms are involved in benzodiazepine effects in the heart.

Peripheral-type benzodiazepine receptors in diagnostic subtypes of schizophrenic patients

The peripheral-type benzodiazepine receptor (pBZD-R; also called the omega-3 receptor or the mitochondrial benzodiazepine receptor) seems to play a critical role in the production of neurosteroids, which are able to alter the electrical properties of neuronal membranes and thus the firing patterns of neurons. Putative endogenous ligands are the diazepam-binding inhibitor and its processing products, as well as porphyrins, some of them, in the case of porphyria, are well known to give rise to certain aspects of neuropsychiatric disorders, such as schizophrenic-like symptoms. Previous findings of altered benzodiazepine binding sites in post-mortem brain samples and platelets from small samples of schizophrenic patients have been inconclusive. Therefore we investigated characteristic binding parameters (Bmax, Kd) of the granulocytic pBZD-R by using the selective ligand PK11.195 in 53 subjects, fulfilling ICD-10 and DSM-IV criteria of schizophrenia. The binding parameters in our total group of 53 schizophrenic patients did not differ from those in healthy subjects. However, Bmax values were significantly reduced in schizophrenic patients with predominantly negative symptoms (residual type) compared to schizophrenic patients with predominantly positive symptoms, i.e. paranoid (-50%) and catatonic subtype (-38%). Moreover, only residual type schizophrenics exhibited a significantly reduced binding capacity compared to healthy subjects (-38%). More studies are warranted to clarify the functional significance of this binding site in the pathogenesis of negative symptoms.

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.

Up-regulation of peripheral benzodiazepine receptor system in hepatocellular carcinoma

Increased number of peripheral benzodiazepine receptors (PBRs) have been found in some tumors outside the liver. The present study was to verify whether the PBR system is altered in hepatocellular carcinoma (HCC). The levels of endogenous benzodiazepine-like compounds (BZDs), measured by radioreceptor binding technique after HPLC purification and the endogenous ligand for PBRs, termed diazepam binding inhibitor (DBI), measured by radioimmunoassay utilizing a specific antibody for human DBI, were studied in the blood of 15 normal subjects, 12 liver cirrhosis and 10 patients with HCC. The levels of BZDs in serum were increased hundred fold in liver cirrhosis patients and slightly elevated in HCC patients. DBI was found to be increased in HCC patients. The binding recognition sites for PBRs (Bmax) were increased 4 to 7 fold in HCC tissue in comparison with that found in non-tumoral liver tissue (NTLT). On the contrary the concentrations of DBI were found to be significantly decreased in HCC tissue in comparison with the respective NTLT. These results seem to suggest an implication of PBRs and of their putative endogenous ligands in the metabolism of these neoplastic cells and possibly in their proliferation. The up-regulation of PBRs found in HCC tissue seems to indicate an increased functional activity of these receptors and opens up the possibility of new pharmacological and diagnostic approaches while the changes in the circulating endogenous ligands for the above receptors might be envisaged as early markers of tumorigenesis in liver cirrhosis.

Peripheral benzodiazepine receptor messenger RNA is decreased in lymphocytes of generalized anxiety disorder patients

BACKGROUND

The aim of this study was to assess whether the decrease of peripheral benzodiazepine receptor (pBR) number in peripheral blood mononuclear cells (PBMC), previously observed in patients with generalized anxiety disorder, is paralleled by changes in the relative content of messenger RNA (mRNA) encoding pBR.

METHODS

Eight patients with a DSM-III-R diagnosis of generalized anxiety disorder were examined before, during, and after 2'-chloro-N-desmethyl-diazepam treatment. Eight healthy subjects were analyzed in parallel. The relative content of pBR mRNA was determined by reverse-transcriptase-polymerase chain reaction, using beta-actin as internal standard. Kinetic binding properties of pBR were measured using 3H-PK11195 as a ligand.

RESULTS

pBR and pBR mRNA were significantly decreased in untreated generalized anxiety disorder patients as compared to controls (by 45% and 70%, respectively). Both pBR density and mRNA levels returned to control values during treatment or after withdrawal, which also coincided with recovery from anxiety.

CONCLUSIONS

These results suggest that the turnover rate of pBR is reduced in PBMC of generalized anxiety disorder patients, and that this change occurs at the transcriptional level.

Assessment of the peripheral benzodiazepine receptors in human gliomas by two methods

This study was designed to evaluate the density of peripheral benzodiazepine receptor (PBR) sites as a function of tumor malignancy in human gliomas, and to compare the results obtained with autoradiographic and liquid scintillation measurements performed on the same tissue specimens. In vitro binding of [3H]PK-11195[1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoguinol ine carboxamide] to human gliomas in radioligand binding studies revealed a significantly higher level (about 3 fold) of PBR binding sites in both low grade and high grade gliomas as compared to normal cortex. The Bmax (mean +/- SD) of high and low grade gliomas, when entire tissue sections were measured by autoradiography, was 5.5 +/- 0.3 pmol/mg-tissue (n = 5) and 1.8 +/- 0.9 pmol/mg-tissue (n = 6), respectively, although it was evident that there was area of hot spots in the high grade tumors. This difference was significant (p < 0.05; two-tailed t-test). Similarly, the KD values (dissociation constant; nM) between the high (KD = 20.4 +/- 1.3 nM) and low (KD = 14.3 +/- 2.1 nM) grade gliomas were significantly different. A significant difference in binding site density (Bmax) between the two types of gliomas was also obtained in liquid scintillation measurements. The hot spot areas which showed the most intense binding of [3H]PK-11195 had KD of 24.5 +/- 1.0 nM and Bmax of 6.2 +/- 0.42 pmol/mg-tissue, values significantly higher (p < 0.05, two-tailed t-test) than those obtained when the entire tissue section was measured. The data on the Bmax/KD ratios presented here suggest that it might be possible to differentiate high from low grade gliomas in human by in vivo imaging with 11C-labelled PK-11195.

Characterization of binding sites for the omega3 receptor ligands [3H]PK11195 and [3H]RO5-4864 in human brain

The kinetics and pharmacology of the isoquinoline and benzodiazepine binding sites of the omega3 or peripheral-type benzodiazepine receptors were studied using the specific ligands [3H] 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2H-1,4-benzodiazepin -2-one ([3H]PK11195) and [3H]1-(2-Chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarb oxamide ([3H]RO5-4864), respectively. Binding of both ligands was saturable, reversible, displayed nanomolar affinity, and best fit to a single site model. Occipital cortex and cerebellum displayed highest and lowest densities of binding sites respectively; for both ligands. Bmax values of [3H]PK11195 were several-fold higher than that of [3H]RO5-4864 in all regions studied consistent with their binding to distinct subunits of the human peripheral-type benzodiazepine receptor heteromeric complex. However, the isoquinoline and benzodiazepine ligands were found to be mutually competitive at nanomolar concentrations suggesting allosteric interactions between these two sites. Competition binding experiments showed that the binding of both ligands was displaced by diazepam with Ki values in the nM range, and by clonazepam in the microM range. The novel peripheral-type benzodiazepine receptor ligand 2-(4-fluorophenyl)-N,N-di-n-hexyl-1H-indole-3-acetamide (FGIN1-27) displaced only [3H]PK11195 binding with high potency. Heterogeneity of the two sites is observed, manifested by their differential susceptibility towards detergents and alcohols. Histidine residue modification by diethylpyrocarbonate treatment abolished only [3H]PK11195 binding but had no effect on [3H]RO5-4864 binding. These studies demonstrate that the isoquinoline and benzodiazepine sites on the peripheral-type benzodiazepine receptor in human brain manifest many pharmacological characteristics that are distinct from each other and from rodent brain peripheral-type benzodiazepine receptors.

Partitioning of flunitrazepam into model membranes studied by temperature controlled gel filtration chromatography

The use of gel filtration chromatography with Sephadex as a separation medium was used in order to study flunitrazepam (FNTZ) partitioning into artificial model membranes consisting of dipalmitoyl-phosphatidylcholine (dpPC) vesicles, under controlled temperature conditions. In this system two phenomena are taking place simultaneously: the ligand-liposome interaction and the lipid self-aggregation to form the liposome. The liposome-FNTZ interaction was evidenced by the non-enantiography of the first derivative of FNTZ elution peak in frontal chromatography through Sephadex G-75. On the other hand, the presence of FNTZ reduced liposomes mean size and increased their size dispersion as evidenced by molecular filtration through Sephadex G-200. The dpPC-buffer FNTZ partition coefficient determined in zonal chromatography through Sephadex G-10 increased about 33% when the temperature rose above the temperature of dpPC transition from the liquid crystalline to the fluid phase. Gel filtration chromatography seems a suitable technique to study lipid liposome-FNTZ interactions at a qualitative level. In addition, this technique has the advantage over other methods of giving the possibility of observing the mutual effects exerted between the drug and the self-aggregating structure.

Peripheral-type benzodiazepine receptor ligands and serum steroid hormones

The peripheral-type benzodiazepine receptors (PBR) are involved in various cellular functions, including steroidogenesis. The impact of these receptor ligands has been demonstrated mainly in steroidogenic cells. The aim of the present study was to assess in intact female rats the effect of chronic (21 days) administration of the PBR ligands PK 11195 (15 mg/kg) and Ro 5-4864 (5 mg/kg), the mixed ligand diazepam (5 mg/kg), and the central benzodiazepine receptor ligand clonazepam (1 mg/kg) on PBR binding characteristics in steroidogenic (ovary and adrenal) and non-steroidogenic (uterus and kidney) organs, as well as on serum hormonal steroids (estradiol, progesterone, and corticosterone). Selective and mixed PBR ligands up-regulated PBR density in the two steroidogenic organs, while Ro 5-4864 also induced elevation of the receptor density in the non-steroidogenic organs. In contrast to Ro 5-4864, PK 11195 treatment down-regulated renal PBR. Clonazepam elevated adrenal PBR. On the serum hormonal level, Ro 5-4864 suppressed estradiol secretion. The other ligands did not affect hormonal steroid levels. It appears that in female rats, at least at these doses and dosing schedules, there is no correlation between the impact of chronic in vivo exposure to these agents on PBR density and ovarian and adrenal hormone levels.

Mapping the peripheral benzodiazepine receptor binding site by conformationally restrained derivatives of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3- isoquinolinecarboxamide (PK11195)

A synthetic-computational approach to the study of the binding site of peripheral benzodiazepine receptor (PBR) ligands related to 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxam ide (PK11195, 1) within their receptor has been developed. A wide series of conformationally restrained derivatives of 1 has been designed with the aim of probing the PBR binding site systematically. The synthesis of these compounds involves palladium-catalyzed coupling and amidation as the key steps. Twenty-nine rigid and semirigid derivatives of 1 were tested in binding studies using [3H]-1, and most of these showed PBR affinities in the nanomolar range. The essential role of the carbonyl moiety as a primary pharmacophoric element in the recognition by and the binding to PBR has been confirmed, and the restricted range of the carbonyl orientations, which characterizes the most potent ligands, points to a specific hydrogen-bonding interaction, mainly directed by the geometrical factors, when the electronic ones are fulfilled. Moreover, the fundamental importance of the short-range dispersive interactions in the modulation of the binding affinity and, hence, in the stabilization of the ligand-receptor complex, emerged from the QSAR models reported.

Impact of pregnancy and lactation on GABA(A) receptor and central-type and peripheral-type benzodiazepine receptors

The effect of pregnancy and lactation on GABA(A) receptor and central- and peripheral-type benzodiazepine receptors (CBR and PBR, respectively) was studied in female Sprague-Dawley rats. Pregnancy was associated with increased CBR density (on day 19) in the hippocampus and with decreased [3H]Ro 15-1788-specific binding in the hypothalamus during pregnancy and lactation. A similar decrease in [3H]PK 11195-specific binding was observed in the hypothalamus and pituitary. An increase in PBR density in the ovary and uterus was observed during pregnancy, while adrenal PBR density was down-regulated during pregnancy and lactation. It seems that the hormonal changes occurring during pregnancy and lactation play a role in the regulation of CBR and PBR in discrete tissues.

The stimulatory effect of the octadecaneuropeptide (ODN) on cytosolic Ca2+ in rat astrocytes is not mediated through classical benzodiazepine receptors

Diazepam-binding inhibitor has been initially isolated from the rat brain from its ability to compete with benzodiazepines for their receptors. We have recently shown that the octadecaneuropeptide (diazepam-binding inhibitor-(33-50) or ODN) induces an increase in cytosolic free Ca2+ concentration ([Ca2+]i) in astroglial cells. The purpose of the present study was to determine whether central-type benzodiazepine receptors or peripheral-type benzodiazepine receptors are involved in the response of cultured rat astrocytes to ODN. The mixed central-/peripheral-type benzodiazepine receptor ligand flunitrazepam (10(-10) to 10(-6) M), the specific peripheral-type benzodiazepine receptor agonist Ro5-4864 (10(-10) to 10(-6) M) and the peripheral-type benzodiazepine receptor 'antagonist' PK 11195 (10(-9) to 10(-6) M) all induced a dose-dependent increase in [Ca2+]i. At high doses (10(-7) to 10(-5) M), the central-type benzodiazepine receptor agonist clonazepam also mimicked the stimulatory effect of ODN on [Ca2+]i. However, the [Ca2+]i rise induced by ODN was blocked neither by PK 11195 nor by the central-type benzodiazepine receptor antagonist flumazenil (10(-6) M each). Binding of [3H]flunitrazepam to intact astrocytes was displaced by low concentrations of the peripheral-type benzodiazepine receptor ligands flunitrazepam, Ro5 4864 and PK 11195, and by high concentrations of clonazepam. In contrast, ODN did not compete for [3H]flunitrazepam binding in intact cells. These data indicate that the effect of ODN on Ca2+ mobilization in rat astrocytes is mediated by high affinity receptors which are not related to classical benzodiazepine receptors.

Localization of flunitrazepam in artificial membranes. A spectrophotometric study about the effect the polarity of the medium exerts on flunitrazepam acid-base equilibrium

In the present paper we tried to test the hypothesis that nonspecific flunitrazepam-membrane interactions are consistent with drug molecules accommodated between lipid molecules, becoming an integral part of the bilayer. We developed a spectrophotometric method to determine FNTZH+ equilibrium dissociation constant and applied it to the study of the acid-base equilibria of this drug in homogeneous media of different polarity. In these conditions, pK decreased with the decrement in the dielectric constant (D) of the media. These results, analyzed under the light of the theory developed by Fernandez and Fromherz (1977; J. Phys. Chem. 81, 1755-1761) let us infer that flunitrazepam is localized a region with D = 60. This D value is lower that Dwater = 78 and higher than D of hydrocarbon chains zone (D = 2-5) and would correspond to D of the region of polar groups. This result is compatible with the hypothesis.

Peripheral benzodiazepine receptor in cholesterol transport and steroidogenesis

Steroidogenesis begins with the metabolism of cholesterol to pregnenolone by the inner mitochondrial membrane cytochrome P450 side-chain cleavage (P450scc) enzyme. The rate of steroid formation, however, depends on the rate of cholesterol transport from intracellular stores to the inner mitochondrial membrane and loading of P450scc with cholesterol. In previous in vitro studies, we demonstrated that a key element in the regulation of cholesterol transport is the mitochondrial peripheral-type benzodiazepine receptor (PBR). We also showed that the polypeptide diazepam binding inhibitor (DBI), an endogenous PBR ligand, stimulates cholesterol transport and promotes loading of cholesterol to P450scc in vitro, and that its presence is vital for hCG-induced steroidogenesis by Leydig cells. Based on these data and the observations that i) the mitochondrial PBR binding and topography are regulated by hormones; ii) the 18-kDa PBR protein is functionally coupled to the mitochondrial contact site voltage-dependent anion channel protein; iii) the 18-kDa PBR protein is a channel for cholesterol, as shown by molecular modeling and in vitro reconstitution studies; iv) targeted disruption of the PBR gene in steroidogenic cells dramatically reduces the ability of the cells to transport cholesterol in the mitochondria and produce steroids; v) endocrine disruptors, with known anisteroidogenic effect, inhibit PBR ligand binding; and vi) in vivo reduction of adrenal PBR expression results in reduced circulating glucocorticoid levels, we conclude that PBR is an indispensable element of the steroidogenic machinery.

Molecular basis of peripheral vs central benzodiazepine receptor selectivity in a new class of peripheral benzodiazepine receptor ligands related to alpidem

Alpidem (1), the anxiolytic imidazopyridine, has nanomolar binding affinity for both the central benzodiazepine receptor (CBR) and the peripheral benzodiazepine receptor (PBR). A novel class of PBR ligands related to alpidem has been designed by comparing the interaction models of alpidem with PBR and CBR. Several compounds in this class have shown high selectivity for PBR vs CBR, and the selectivity has been discussed in terms of interaction models. The binding behavior of the three selected compounds was extensively studied by competition and saturation assays, and the results suggest that they are capable of recognizing two sites labeled by [3H]PK11195. The molecular structure of one of the most active compounds (4e) has been determined by X-ray diffraction and compared with that of alpidem. Molecular modeling studies suggest that the bioactive conformation of 4e is likely to be very similar to the conformation found in the crystal.

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

Direct interactions of androgenic/anabolic steroids with the peripheral benzodiazepine receptor in rat brain: implications for the psychological and physiological manifestations of androgenic/anabolic steroid abuse

The peripheral benzodiazepine receptor (PBR) is a mitochondrial protein involved in regulating steroid synthesis and transport. We report here the effects of androgenic/anabolic steroids (AAS) on the binding of the PBR-specific ligand [3H] PK11195 to male rat brain cortical synaptoneurosomes. Two synthetic AAS, stanozolol and 17beta-testosterone cypionate (17beta-cyp), significantly inhibited 1 nM [3H] PK11195 binding at concentrations greater than 5 and 25 microM, respectively. Stanozolol was the most effective inhibitor, reducing [3H] PK11195 binding by up to 75%, compared to only 40% inhibition by 17beta-cyp, at 50 microM AAS concentration. Two other AAS, 17alpha-methyltestosterone and nortestosterone decanoate, were incapable of inhibiting [3H] PK11195 binding at concentrations up to 50 microM. On the basis of Scatchard/Rosenthal analysis, [3H] PK11195 binds to two classes of binding sites, and the inhibition of [3H] PK11195 binding by stanozolol appears to be allosteric, primarily reducing binding to the higher affinity [3H] PK11195 binding site. These results, in combination with earlier studies indicating the direct effects of AAS on the function of additional central nervous system receptor complexes, suggest that the behavioral and psychological effects of AAS result from the interactions of AAS with multiple regulatory systems in the brain.

Cardiovascular characterization of pyrrolo[2,1-d][1,5]benzothiazepine derivatives binding selectively to the peripheral-type benzodiazepine receptor (PBR): from dual PBR affinity and calcium antagonist activity to novel and selective calcium entry blockers

The synthesis and cardiovascular characterization of a series of novel pyrrolo[2,1-d][1,5]-benzothiazepine derivatives (54-68) are described. Selective peripheral-type benzodiazepine receptor (PBR) ligands, such as PK 11195 and Ro 5-4864, have recently been found to possess low but significant inhibitory activity of L-type calcium channels, and this property is implicated in the cardiovascular effects observed with these compounds. In functional studies both PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxa mide) and Ro 5-4864 (4'-chlorodiazepam) did not display selectivity between cardiac and vascular tissue. Therefore, several 7-(acyloxy)-6-arylpyrrolo[2,1-d][1,5]benzothiazepines, potent and selective peripheral-type benzodiazepine receptor ligands recently developed by us (3, 7-20), were subjected to calcium channel receptor binding assay. Some of these compounds showed an unexpected potency in displacing the binding of [3H]nitrendipine from L-type calcium channels, much higher than that reported for PK 11195 and Ro 5-4864 and equal to or higher than that of reference calcium antagonists such as verapamil and (+)-cis-diltiazem. Specifically, in rat cortex homogenate, our prototypic PBR ligand 7-acetoxy-6-(p-methoxyphenyl)pyrrolo[2,1-d][1,5]benzothiazepine (3) showed an IC50 equal to 0.13 nM for inhibition of [3H]nitrendipine binding. Furthermore, in functional studies this compound displayed a clear-cut selectivity for cardiac over vascular tissue. Comparison of calcium antagonist activity on guinea pig aorta strips with the negative inotropic activity, determined by using isolated guinea pig left atria, revealed that 3 displayed higher selectivity than the reference (+)-cis-diltiazem. Thus, the pyrrolobenzothiazepine 3 might represent a new tool for characterizing the relationship between the PBR and cardiac function. Furthermore, we have also investigated the structural dependence of binding to PBR and L-type calcium channels, and this study allowed us to identify a new class of potent calcium channel blockers selective for cardiac over vascular tissue, with no affinity for PBR. A number of structure-activity relationship trends have been identified, and a possible explanation is advanced in order to account for the observed differences in selectivity. Three structural features, namely, (i) the saturation of the C(6)-C(7) double bond, with a consequent higher molecular flexibility, (ii) the presence of a substituent in the benzofused ring, and (iii) a basic side chain at C-10 of the pyrrolobenzothiazepine ring system, were found to be responsible for potent L-type calcium channel antagonism and clear-cut selectivity for cardiac over vascular tissue. Among the synthesized compounds the pyrrolobenzothiazepine 62 was found to be the most promising selective calcium channel blocker. Additionally, the molecular structure determination of the key intermediate 48 by X-ray diffraction, molecular modeling, and NMR analysis is reported.

Characterization of peripheral benzodiazepine receptors in purified large mammal pancreatic islets

In this work, we evaluated the biochemical properties of peripheral benzodiazepine receptors (PBRs) in the porcine endocrine pancreas and their role in insulin release. Binding of [3H]1-(2-chlorophenyl-N-methyl-1-methyl-propyl)-3-isoquinolinecarboxa mide ([3H]PK-11195), a specific ligand of PBRs, to islet membranes was saturable and Scatchard's analysis of saturation curve demonstrated the presence of a single population of binding sites, with a dissociation constant (Kd) value of 4.75 +/- 0.70 nM and a maximum amount of specifically bound ligand (Bmax) of 4505 +/- 502 fmol/mg of proteins. The pharmacological profile of PBRs was determined as the ability of PK-11195 and several benzodiazepine compounds to displace [3H]PK-11195 from these binding sites. The rank order of potency yielded the following affinity results: PK-11195 > 7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4-benzodiazepine-2 -on (Ro 5-4864) > diazepam > or = flunitrazepam >> flumazenil. Secretion studies demonstrated that PK-11195 (1 and 10 microM) and Ro 5-4864 (10 and 50 microM) significantly potentiated insulin secretion from freshly isolated porcine islets at 3.3 mM glucose. This potentiating effect was not observed at 16.7 mM glucose concentration nor by the addition of clonazepam. These results show the presence of PBRs in purified porcine pancreatic islets and suggest an implication of PBRs in the mechanisms of insulin release.

Cognitive impairments of alcoholic cirrhotic patients: correlation with endogenous benzodiazepine receptor ligands and increased affinity of platelet receptors

OBJECTIVES

To determine whether differences in cognitive function between alcoholic and non-alcoholic cirrhotic patients relate to differences in endogenous ligands for the benzodiazepine receptor and/or benzodiazepine binding.

METHODS

Seventeen grade-I hepatic encephalopathic patients (nine alcoholic, eight non-alcoholic) were compared with 10 matched controls on plasma concentrations of endogenous ligands for the neuronal benzodiazepine receptor, benzodiazepine binding in platelets, and performance on tests of cognitive function.

RESULTS

Both groups of patients were impaired on verbal recall and on reaction time tasks compared with controls; alcoholic patients were also impaired on Reitan's trails test and digit cancellation. Four of the 17 patients had detectable concentrations of endogenous benzodiazepine ligands and they were more impaired than other patients on trails and cancellation tests. The groups did not differ in the density of benzodiazepine platelet receptors, but receptor affinity was higher in alcoholic patients than in controls; furthermore, receptor affinity correlated with the time to complete the cancellation task and with reaction time.

CONCLUSION

Alcoholic cirrhotic patients may have enhanced concentrations of ligands for neuronal and peripheral benzodiazepine receptors and these may contribute to cognitive impairments in these patients.

Pathophysiology and management of the serotonin syndrome

OBJECTIVE

To review the symptoms, pathophysiology, and treatment of the serotonin syndrome (SS).

DATA SOURCES

A MEDLINE search (1957-1995) of the English-language literature pertaining to the SS was performed. Additional literature was obtained from reference lists of pertinent articles identified through the search.

STUDY SELECTION AND DATA EXTRACTION

All articles were considered for possible inclusion in the review. Pertinent information, as judged by the authors, was selected for discussion.

DATA SYNTHESIS

The SS, an occasionally fatal disorder, is characterized by symptoms such as mental status changes, seizures, myoclonus, and blood dyscrasias. Both the central and peripheral serotonergic systems and several serotonin receptor types are involved in the symptomatology of the SS. The pathogenesis of SS may be due to endogenous as well as iatrogenic deficits in peripheral serotonin metabolism, a stimulus for release of serotonin, and interactions with other neurotransmitter systems. Lorazepam, serotonin-blockers, and nitroglycerin have been used successfully to treat SS.

CONCLUSIONS

The SS is increasingly recognized and reported in the literature. Clinical and basic science research have increased our understanding of the pathophysiology, conditions, and agents that may predispose to the development of the syndrome. Newer treatment strategies are discussed.

Recruitment of peripheral-type benzodiazepine receptors after acute stress in chick forebrain membranes: action of Triton X-100

A significant increase in the number of measurable [3H]Ro 5-4864 receptors was found in forebrain membranes of chicks submitted to 15 min of acute swim stress compared to non-stressed chicks. In addition, low subsolubilizing concentrations of Triton X-100 caused a significant increase in the measurable [3H]Ro 5-4864 receptor number in forebrain membranes from non-stressed chicks. However, this increase caused by Triton X-100 was not observed when tested in forebrain membranes from stressed chicks. In all cases the affinity remained unchanged. These results suggest that: (1) acute stress and Triton X-100 induce receptor increase by enhancing [3H]Ro 5-4864 accessibility to a pool of receptors not detected before stress or in the absence of detergent; (2) the pool of non-measured receptors represents about a third of the total in control chicks; (3) the increments are not additive and could involve receptors coming from the same non-measured pool; (4) the receptor increase during a short time of stress could be explained by recruitment of receptors but not by an increase in the receptor protein biosynthesis; (5) stress induces a maximal recruitment of measurable [-3H]Ro 5-4864 receptors.

Benzodiazepine receptors increase induced by stress and maze-learning performance in chick forebrain

Two-day-old chicks were selected on their second escape performance in a one-trial, maze-learning task, and termed high-performance (H-P), moderate-performance (M-P), and low-performance (L-P) chicks. The learning degree was expressed by the escape time improvement being respectively the 64, 46, and 24%. Then, the three selected groups were maintained to reach 15 days of age and then submitted to acute swimming stress, and [3H]flunitrazepam and [3H]Ro 5-4864 receptor bindings were performed on synaptosomal/mitochondrial membranes from forebrain. The receptor number for both radioligands in stressed high-performance chicks was significantly higher than in stressed low-performance chicks. The results suggest that higher performance chicks were more susceptible than lower performance chicks to acute stress associated to increase of both central and peripheral type benzodiazepine receptors, probably due to differences in the degree of endogenous emotionality.

Identification of three transcriptional regulatory elements in the rat mitochondrial benzodiazepine receptor-encoding gene

The sequence upstream from the first exon in the rat mitochondrial benzodiazepine receptor-encoding gene (MBR) was analyzed for transcriptional promoter activity by three techniques: promoter deletion analysis in vectors containing the gene cat encoding chloramphenicol acetyltransferase, electrophoretic mobility shift analysis (EMSA) and DNase I protection assay. All three methods are in uniformity with the identification of at least three regulatory elements corresponding to locations -51/-33, -267/-249 and -555/-526. The most distal and proximal domains are positive-acting, whereas the element at -267/-249 acts in a negative manner. The positive-acting -51/-33 element contains the middle of three consensus Sp1-recognition sequences found in this region of the gene. Binding of Y1 cell nuclear protein to a DNA fragment corresponding to this region of the gene is competed by a synthetic oligodeoxyribonucleotide bearing the consensus Sp1-binding site sequence. These studies provide the first reported functional evidence localizing transcriptional elements of MBR.

Acute liver failure and hyperammonemia increase peripheral-type benzodiazepine receptor binding and pregnenolone synthesis in mouse brain

We investigated the role of brain peripheral-type benzodiazepine receptors (PBRs) and pregnenolone (a product of PBRs activation) in hepatic encephalopathy (HE)/hyperammonemia. Administration of the hepatotoxin, thioacetamide, or ammonium acetate to mice for 3 days significantly increased the number of brain PBRs (138-146% of control) and the affinity of the ligands for these receptors (2-fold). The total content of pregnenolone and its rate of synthesis in brain of the experimental animals were significantly increased. Our results suggest a novel integrated mechanism by which ammonia-induced activation of PBRs leads to elevated levels of pregnenolone-derived neurosteroids which are known to enhance GABA-ergic neurotransmission. This mechanism may play a pivotal role in pathogenesis of HE.

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.