Characterization of benzodiazepine receptors in the bovine pineal gland: evidence for the presence of an atypical binding site

Tetrapyrroles as Endogenous TSPO Ligands in Eukaryotes and Prokaryotes: Comparisons with Synthetic Ligands

The 18 kDa translocator protein (TSPO) is highly 0conserved in eukaryotes and prokaryotes. Since its discovery in 1977, numerous studies established the TSPO’s importance for life essential functions. For these studies, synthetic TSPO ligands typically are applied. Tetrapyrroles present endogenous ligands for the TSPO. Tetrapyrroles are also evolutionarily conserved and regulate multiple functions. TSPO and tetrapyrroles regulate each other. In animals TSPO-tetrapyrrole interactions range from effects on embryonic development to metabolism, programmed cell death, response to stress, injury and disease, and even to life span extension. In animals TSPOs are primarily located in mitochondria. In plants TSPOs are also present in plastids, the nuclear fraction, the endoplasmic reticulum, and Golgi stacks. This may contribute to translocation of tetrapyrrole intermediates across organelles’ membranes. As in animals, plant TSPO binds heme and protoporphyrin IX. TSPO-tetrapyrrole interactions in plants appear to relate to development as well as stress conditions, including salt tolerance, abscisic acid-induced stress, reactive oxygen species homeostasis, and finally cell death regulation. In bacteria, TSPO is important for switching from aerobic to anaerobic metabolism, including the regulation of photosynthesis. As in mitochondria, in bacteria TSPO is located in the outer membrane. TSPO-tetrapyrrole interactions may be part of the establishment of the bacterial-eukaryote relationships, i.e., mitochondrial-eukaryote and plastid-plant endosymbiotic relationships.

Translocator protein (Tspo) gene promoter-driven green fluorescent protein synthesis in transgenic mice: an in vivo model to study Tspo transcription

Translocator protein (TSPO), previously known as the peripheral-type benzodiazepine receptor, is a ubiquitous drug- and cholesterol-binding protein primarily found in the outer mitochondrial membrane as part of a mitochondrial cholesterol transport complex. TSPO is present at higher levels in steroid-synthesizing and rapidly proliferating tissues and its biological role has been mainly linked to mitochondrial function, steroidogenesis and cell proliferation/apoptosis. Aberrant TSPO levels have been linked to multiple diseases, including cancer, endocrine disorders, brain injury, neurodegeneration, ischemia-reperfusion injury and inflammatory diseases. Investigation of the functions of this protein in vitro and in vivo have been mainly carried out using high-affinity drug ligands, such as isoquinoline carboxamides and benzodiazepines and more recently, gene silencing methods. To establish a model to study the regulation of Tspo transcription in vivo, we generated a transgenic mouse model expressing green fluorescent protein (GFP) from Aequorea coerulescens under control of the Tspo promoter region (Tspo-AcGFP). The expression profiles of Tspo-AcGFP, endogenous TSPO and Tspo mRNA were found to be well-correlated. Tspo-AcGFP synthesis in the transgenic mice was seen in almost every tissue examined and as with TSPO in wild-type mice, Tspo-AcGFP was highly expressed in steroidogenic cells of the endocrine and reproductive systems, epithelial cells of the digestive system, skeletal muscle and other organs. In summary, this transgenic Tspo-AcGFP mouse model recapitulates endogenous Tspo expression patterns and could be a useful, tractable tool for monitoring the transcriptional regulation and function of Tspo in live animal experiments.

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.

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.

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.

Modulation of melphalan resistance in glioma cells with a peripheral benzodiazepine receptor ligand-melphalan conjugate

Peripheral benzodiazepine receptors (PBRs) are located on the outer membrane of mitochondria, and their density is increased in brain tumors. Thus, they may serve as a unique intracellular and selective target for antineoplastic agents. A PBR ligand-melphalan conjugate (PBR-MEL) was synthesized and evaluated for cytotoxicity and affinity for PBRs. PBR-MEL (9) (i.e., 670 amu) was synthesized by coupling of two key intermediates: 4-[bis(2-chloroethyl)-amino]-L-phenylalanine ethyl ester trifluoroacetate (6) and 1-(3'-carboxylpropyl)-7-chloro-1,3- dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one (8). On the basis of receptor-binding displacement assays in rat brain and glioma cells, 9 had appreciable binding affinity and displaced a prototypical PBR ligand, Ro 5-4864, with IC50 values between 289 and 390 nM. 9 displayed differential cytotoxicity to a variety of rat and human brain tumor cell lines. In some of the cell lines tested including rat and human melphalan-resistant cell lines, 9 demonstrated appreciable cytotoxicity with IC50 values in the micromolar range, lower than that of melphalan alone. The enhanced activity of 9 may reflect increased membrane permeability, increased intracellular retention, or modulation of melphalan's mechanisms of resistance. The combined data support additional studies to determine how 9 may modulate melphalan resistance, its mechanisms of action, and if target selectivity can be achieved in in vivo glioma models.

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.

Benzodiazepines influence melatonin secretion of the pineal organ of the trout in vitro

The effect of benzodiazepines (BZP) on melatonin release was investigated in the pineal gland of the rainbow trout, Oncorhynchus mykiss, maintained under in vitro perifusion culture conditions. Melatonin and the methoxyindoles 5-methoxytryptophol (5-MTOL), 5-methoxyindoleacetic acid (5-MIAA), and 5-methoxytryptamine (5-MT) were determined directly in samples of the superfusion medium by HPLC with electrochemical detection. Melatonin release was significantly increased by addition of diazepam and clonazepam in a dose-related and reversible manner. The effects of benzodiazepines were more pronounced in light-adapted pineal organs, when melatonin secretion is low, than under scotopic conditions. When the perifusion medium was replaced by a medium containing low calcium, high magnesium concentrations, melatonin release was considerably decreased by 70% in light-adapted and 20% in dark-adapted pineal organs. Addition of diazepam to low Ca2+, high Mg(2+)-medium reversed the decrease of melatonin release and produced a clear rise in its secretion rate. Addition of the BZP antagonist flumazenil to the perifusion medium slightly decreased melatonin release in the light- and dark-adapted state, whereas the peripheral receptor antagonist PK 11195 did not alter melatonin release. The effect of diazepam is reduced by simultaneous addition of flumazenil to the superfusion medium, suggesting that the effects of diazepam are receptor-mediated. The methoxyindoles 5-MTOL, 5-MIAA, and 5-MT showed no significant changes of their release pattern after diazepam application in light- and dark-adapted pineal organs. These results suggest that BZP can influence melatonin production and release by an intrapineal action possibly on the melatonin synthesizing photoreceptor cell.

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.

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.

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.

Central and peripheral benzodiazepine receptors: involvement in an organism's response to physical and psychological stress

The present review discusses the current knowledge of the molecular pharmacology and neuroanatomical and subcellular localization of both the central benzodiazepine/GABA-chloride ionophore receptor complex and the peripheral benzodiazepine receptor. It then reviews all of the literature to date on how these two receptor sites are modulated by environmental stress. The possible role of these sites in learning and memory is also discussed. Finally, a theoretical model is presented which examines the differential, and perhaps complementary, alterations of these two sites in an organism's response to stress.

Toxicokinetics of Ro 5-4864, lindane and picrotoxin compared

The effects produced by IP administration of these three agents in the rat were compared because of in vitro evidence that each modulates the picrotoxinin site of the GABAA receptor. For each, hypothermia had the lowest threshold and convulsions the next, with hypophagia produced only by the highest dose of either Ro 5-4864 or lindane. Convulsant effects had a shorter latency and a shorter duration than did hypothermia. Hypophagia, when present, lasted the longest. Myoclonus was the seizure type with the lowest threshold for all three agents. At the highest dose, lindane produced a high incidence of maximal clonic (hopping) seizures, whereas Ro 5-4864 and picrotoxin produced a high incidence of maximal tonic seizures instead. On a mole/kg basis, picrotoxin was 40 times more effective than the other two agents and produced seizures which started later, peaked later, and persisted longest. Ro 5-4864 and lindane were effective at equimolar concentrations and, in combination, produced effects which suggested either dose-addition or synergism. The data are consistent with the hypothesis that the toxic effects of both Ro 5-4864 and lindane may be attributable, at least in part, to an action at a subpopulation of GABAA receptors.

Peripheral-type benzodiazepine receptors in human cerebral cortex, kidney, and colon

The specific binding of [3H]PK 11195 and [3H]Ro 5-4864 to human cerebral cortex, kidney, and colon membranes was studied in order to determine whether peripheral type benzodiazepine receptors (PBR) characteristics located in human tissues are similar to those located in calf or rat tissues. While [3H]PK 11195 (0.05-10 nM, final concentration) bound with high affinity (KD about 2 nM) to human cerebral cortex, kidney, and colon membranes, yielding maximal numbers of binding sites of 255 +/- 23, 1908 +/- 28, and 1633 +/- 98 fmol/mg protein, respectively, the specific binding of [3H]Ro 5-4864 (1.25-40 nM, final concentration), was barely detectable (nonspecific binding about 90% of the total binding). Furthermore, unlabeled PK 11195 was two orders of magnitude more potent than unlabeled Ro 5-4864 in displacing [3H]PK 11195 specific binding from human cerebral cortex and kidney membranes. These results indicate that PBR binding characteristics located in human tissues are similar (but not identical) to those located in calf tissues, but not to those located in rat tissues.

GABA as a presumptive paracrine signal in the pineal gland. Evidence on an intrapineal GABAergic system

GABA is present in the pineal gland of several mammals, where it is synthesized in situ as well as taken up from the circulation. This article reviews available information suggesting a local, physiological role of pineal GABA. Both the pinealocytes and the glial pineal cells have the capacity to take up GABA from the extracellular space. The GABA synthesizing enzyme glutamic decarboxylase (GAD) is detectable in the pineal gland; in the bovine pineal GAD exhibits "neuronal-like" properties. By employing a specific antibody against GABA, about 15% of pinealocytes gave a positive reaction in bovine pineal glands. After a depolarizing stimulus, GABA was released from bovine and rat pineal glands by both Ca2(+)-dependent and Ca2(+)-independent processes. By employing neuronal and glial GABA uptake inhibitors, most 3H-GABA release in bovine pineal gland could be attributed to a "neuronal" (presumably pinealocyte) compartment. Several components of the GABA type A receptor supramolecular complex (i.e., GABA binding sites, central-type benzodiazepine binding sites, Cl- ionophore), as well as a minor population of GABA type B receptor sites, were detected in bovine and human pineal glands. In the rat pineals, GABA is released by norepinephrine (NE) acting through alpha 1-adrenoceptors. Physiological concentrations of GABA, by its effect on type A receptor sites, impaired NE-induced melatonin release; by acting on GABA type B receptors, it decreased NE release. Another presumable presynaptic effect of GABA (i.e., to augment maximal velocity and to decrease affinity of NE uptake) was mediated by type A receptor sites. It is proposed that pre- and postsynaptic activity of GABA in the pineal does not differ from that found for GABA interneurons in local circuits of the brain.

Presynaptic effects of gamma-aminobutyric acid on norepinephrine release and uptake in rat pineal gland

The effect of tau-aminobutyric acid (GABA) on pineal norepinephrine (NE) release was examined in vitro in the rat pineal gland. Exposure of pineal explants previously loaded with 3H-NE to 1-100 microM GABA caused a dose-dependent decrease of 3H-NE release triggered by 60 mM K+, with a threshold GABA concentration of 1 microM and IC50 of about 10 microM. The inhibitory effect of GABA was mimicked by the type B GABA agonist baclofen, displaying a similar dose-response relationship as GABA. The type A GABA agonist muscimol increased depolarization-induced 3H-NE release, while the co-incubation with GABA and the type A receptor antagonist bicuculline augmented significantly GABA's depressive effect on 3H-NE release. Bicuculline alone brought about a significant decrease of 3H-NE release. Neither GABA, nor baclofen, muscimol or bicuculline, modified the spontaneous pineal 3H-NE efflux. Assessment of 3H-NE uptake at a low NE concentration (0.5 microM) indicated that GABA decreased it in a dose-dependent manner (IC50 = 100 microM) through an effect blocked by bicuculline and mimicked by muscimol but not by baclofen; at a 5 microM-3H-NE concentration a bicuculline-sensitive GABA augmentation of uptake was found. A kinetic analysis study of the pineal NE uptake process indicated that GABA augmented both Vmax and Km of transmitter uptake. These results indicate that GABA may be a significant regulatory signal for rat pineal sympathetic synapses.

Species differences and heterogeneity of solubilized peripheral-type benzodiazepine binding sites

The pharmacological characteristics of digitonin-solubilized peripheral-type benzodiazepine binding sites (PBS) from kidney membranes of various species were investigated to determine whether the species differences and heterogeneity observed in membrane-bound binding sites would be maintained after solubilization. [3H]PK 11195 (0.05 to 10 nM) bound with high affinity to rat, guinea pig, calf, and cat kidney solubilized preparations yielding maximal numbers of binding sites (Bmax) of 3,593 +/- 381, 25,645 +/- 1,795, 1,327 +/- 141, and 2,446 +/- 148 fmol/mg protein, respectively, and equilibrium dissociation constant (KD) values of 1.74 +/- 0.18, 2.15 +/- 0.15, 0.85 +/- 0.09, and 1.02 +/- 0.06 nM, respectively. On the other hand, the respective Bmax and KD values for [3H]Ro 5-4864 (1.25 to 40 nM) were 2,688 +/- 275, 14,182 +/- 1,134, 144 +/- 23 and 205 +/- 17 fmol/mg protein (about 75, 55, 11, and 8%, respectively, of that of [3H]PK 11195) and 13.8 +/- 1.5, 14.6 +/- 1.1, 10.6 +/- 1.7, and 19.9 +/- 1.2 nM. Unlabeled Ro 5-4864 was two orders of magnitude more potent in displacing [3H]PK 11195 binding from rat kidney solubilized preparations than from calf kidney solubilized preparations, whereas the potency of unlabeled PK 11195 in displacing [3H]PK 11195 binding from both rat and calf kidney solubilized preparations was almost identical. Analysis of these displacement data revealed that PK 11195 bound to a single population of binding sites (nH approximately equal to 1.0), whereas Ro 5-4864 bound to two populations of binding sites (nH less than 1.0) in both rat and calf kidney solubilized preparations. These results indicate that PBS species differences and heterogeneity observed in membrane-bound binding sites are retained in the soluble state and are probably attributable to variations in the molecular structure of PBS rather than to differences in membrane environment.

Differential binding properties of the peripheral-type benzodiazepine ligands [3H]PK 11195 and [3H]Ro 5-4864 in trout and mouse brain membranes

High-affinity binding sites for [3H]PK 11195 have been detected in brain membranes of rainbow trout (Salmo gairdneri) and mouse forebrain, where the densities of receptors were 1,030 and 445 fmol/mg of protein, respectively. Ro 5-4864 (4'-chlorodiazepam) was 2,200-fold less potent as a competitor of [3H]PK 11195 binding in the piscine than the murine membranes. Investigation of the regional distribution of these sites in trout yielded a rank order of density of spinal cord greater than olfactory bulb = optic tectum = rhombencephalon greater than cerebellum greater than telencephalon. This site in trout shared some of the characteristics of the peripheral-type benzodiazepine receptor (PTBR) (also known as the mitochondrial benzodiazepine receptor) in rodents, i.e., high affinity for PK 11195 and the endogenous ligand protoporphyrin IX, but was unique in the low affinity of Ro 5-4864 (41 microM) and diazepam and the relatively high affinity of the calcium channel ligand diltiazem and two central benzodiazepine ligands, CGS 8216 and CGS 9896. The differential affinity for the two prototypic PTBR ligands in trout is similar to that previously observed in calf and human brain membranes. Structural differences for the trout sites are indicated by the relative inability of diethyl pyrocarbonate to modify histidine residues of the binding site in trout as compared with mouse membranes. Heterogeneity of binding of the two prototypic PTBR ligands in mouse brain membranes was indicated by additivity studies, equilibrium competition experiments, and saturation isotherms, which together support the hypothesis that Ro 5-4864 discriminates between two [3H]PK 11195 binding sites having high (nanomolar) and low (micromolar) affinity, respectively.

gamma Aminobutyric acid uptake, release, and effect on 36Cl--influx in bovine pineal gland

Two apparent affinities for Na+-dependent, 3H-GABA uptake were found in bovine pineal fragments in vitro i.e., a high affinity uptake (Km = 37 +/- 5 microM) and a low affinity uptake (Km = 435 +/- 50 microM). GABA or the neuronal and glial GABA uptake inhibitor nipecotic acid was significantly more effective than the inhibitor of the GABA glial uptake beta-alanine to decrease pineal 3H-GABA uptake. High K+ concentration release 3H-GABA in superfused bovine pineals, no differences in 3H-GABA release among fragments taken from medial, proximal or distal pineal regions being apparent. Superfusion of pineal fragments in the absence of Ca2+ but in the presence of EGTA, Mg2+ or verapamil decreased significantly 3H-GABA release induced by K+. In every case a Ca2+-independent pineal GABA release was found. Preincubation with GABA or nipecotic acid, but not with beta-alanine, blunted subsequent 3H-GABA release. GABA increased 36Cl--influx in pineal homogenates, an effect blocked by picrotoxin. Incubation of pineal homogenates in the presence of aminooxyacetic acid decreased Vmax of glutamic acid decarboxylase, without modifying its Km. These results are compatible with a transmitter or modulator role of GABA in bovine pineal gland.

[3H]AHN 086 acylates peripheral benzodiazepine receptors in the rat pineal gland

AHN 086, an isothiocyanato derivative of Ro 5-4864 (4'-chlorodiazepam), inhibits radioligand binding to peripheral benzodiazepine receptors with characteristics of an irreversible (acylating) ligand. We now report that [3H]AHN 086 labels a approximately 30 kDa protein in the rat pineal gland determined by both SDS-polyacrylamide gel electrophoresis and gel filtration high-performance liquid chromatography of digitonin-solubilized membranes. Specific incorporation of [3H]AHN 086 into this protein was inhibited by preincubating membranes with excess AHN 086. Moreover, significant specific binding of [3H]AHN 086 was not observed in either bovine pineal gland (which does not possess high-affinity binding sites for Ro 5-4864) or ovalbumin. These findings suggest that the approximately 30 kDa protein labeled by [3H]AHN 086 in rat pineal gland is associated with peripheral benzodiazepine receptors in this tissue.

Heterogeneity between rat and calf peripheral-type benzodiazepine binding sites: differential sensitivity to Triton X-100

The effect of various detergents treatment on the specific binding of [3H]PK 11195 (2nM) to peripheral-type benzodiazepine binding sites (PBS) in calf and rat kidney, adrenal gland, and cerebral cortex membranes was studied. At a concentration of 0.025%, Triton X-100 increased [3H]PK 11195 specific binding to calf kidney, adrenal gland, and cerebral cortex membranes by 20-40%. At the same concentration, Triton X-100 scarcely affected specific binding of [3H]PK 11195 to rat cerebral cortex but decreased binding to rat kidney and adrenal gland membranes by 20-30%. At a concentration of 0.05% of Triton X-100, [3H]PK 11195 specific binding to calf kidney, adrenal gland, and cerebral cortex membranes was increased by 10-20%; whereas [3H]PK 11195 specific binding to rat kidney, adrenal gland, and cerebral cortex membranes was decreased by more than 40%. The increase in [3H]PK 11195 specific binding to calf kidney membranes following Triton X-100 (0.05%) treatment was apparently due to an increase in the binding affinity of PBS, since the density remained unaltered; whereas, the decrease in [3H]PK 11195 specific binding to rat kidney membranes was due to a decrease in both binding affinity and density of PBS. On the other hand, the detergents 3- [(3- cholamidopropyl)- dimethylammonio] - 1 - propane sulfonate (CHAPS), Tween 20, deoxycholic acid, and digitonin have a similar effect on [3H]PK 11195 specific binding to PBS in both calf and rat kidney membranes.

Limitations of the benzodiazepine receptor nomenclature: a proposal for a pharmacological classification as omega receptor subtypes

At present, the nomenclature of benzodiazepine (BZ) receptors is based on historical association with the BZ structure. However, it is mainly through the new compounds chemically unrelated to BZ that the central and peripheral subtypes of BZ receptors have been characterized. We therefore propose the nomenclature of a Greek letter omega, as omega 1, omega 2, and omega 3 to designate the central BZ1, BZ2, and peripheral BZ receptors, respectively. Among the several classes of non-BZD drugs with affinity for different receptors, the imidazopyridines provide a valuable tool for the characterization of omega receptor subtypes. Most BZ are nonselective ligands for the central omega 1 and omega 2 receptors, while selectivity for omega 1 receptor subtypes is present in several non BZ chemical series: imidazopyridines (zolpidem), triazolopyridazines (CL 218872), betacarbolines (beta-CCE), and pyrazoloquinolines (CGS 8216). Selective ligands for the omega 2 subtype are not available so far. The so-called peripheral BZ receptor is also present in the central nervous system; therefore, the proposed nomenclature of omega 3 receptors resolves this paradox because it does not designate location and is defined in terms of pharmacological specificity. Selective ligands for omega 3 receptors include the BZ Ro 5-4864, and the isoquinolinecarboxamide PK 11195, while the imidazopyridine alpidem is the ligand with the highest affinity for this receptor subtype.

Cellular and molecular mechanisms controlling melatonin release by mammalian pineal glands

1. The pineal gland is regulated primarily by photoperiodic information attaining the organ through a multisynaptic pathway initiated in the retina and the retinohypothalamic tract. 2. Norepinephrine (NE) released from superior cervical ganglion (SCG) neurons that provide sympathetic innervation to the pineal acts through alpha1- and beta 1- adrenoceptors to stimulate melatonin synthesis and release. 3. The increase in cyclic AMP mediated by beta 1-adrenergic activation is potentiated by the increase in Ca2+ flux, inositol phospholipid turnover, and prostaglandin and leukotriene synthesis produced by alpha 1-adrenergic activation. 4. Central pinealopetal connections may also participate in pineal control mechanisms; transmitters and modulators in these pathways include several neuropeptides, amino acids such as gamma-aminobutyric acid (GABA) and glutamate, and biogenic amines such as serotonin, acetylcholine, and dopamine. 5. Secondary regulatory signals for pineal secretory activity are several hormones that act on receptors sites on pineal cells or at any stage of the neuronal pinealopetal pathway.