Chronic ethanol exposure increases peripheral-type benzodiazepine receptors in brain

The effect of chronic ethanol exposure, withdrawal from chronic exposure and short-term ethanol exposure on mouse brain peripheral-type benzodiazepine receptors (PBR) and on PBR from selected peripheral tissues was studied. Male C57BL/6J mice were fed an ethanol-containing liquid diet for 7.75 or 9 days under conditions which produced physical dependence. Control mice received the diet with isocaloric carbohydrates substituted for ethanol. The binding of [3H]Ro5-4864 to PBR was increased in brain membranes, but not heart or kidney membranes, of mice exposed to ethanol for 7.75 days. Scatchard plot analysis indicated that the increase was due to an increase in the apparent number of binding sites and not to a change in receptor affinity. The same results were obtained in ethanol-dependent mice that were withdrawn from the ethanol for 12 h prior to binding determinations. The binding of [3H]PK-11195 to brain PBR was likewise increased in mice made physically dependent on ethanol after 9 days exposure. In contrast to the effects of chronic exposure using the liquid diet, repeated short-term exposure to ethanol following a procedure known to cause functional tolerance (3.6 g/kg i.p. once daily for 4 days) did not significantly affect the binding of [3H]Ro5-4864 to brain when compared to saline-injected and naive control mice. The results are consistent with previous findings and suggest that ethanol exposure causes time-dependent changes in brain PBR that may be linked to the development of physical dependence. However, further studies are necessary to determine whether the increase in brain PBR sites of alcohol-dependent mice is causally related to the development of alcohol dependence.     

Characterization of stress-induced alterations in [H] Ro5-4864 binding to peripheral benzodiazepine receptors in rat heart and kidney

Inescapable tailshock has been shown to elicit a tissue specific decrease in the density of peripheral benzodiazepine receptors (PBR). We have now explored possible mechanisms that may be responsible for this phenomenon. An 80 minute session of inescapable tailshock produced a reduction in the binding of [³H] Ro5-4864 to renal membranes at 0, 1 and 2 hr after stress, with values returning to control (naive) levels within 24 hr. In cardiac membranes, statistically significant reductions were observed only at 2 and 4 hr after stress. The role of the pituitary-adrenal axis and the sympathetic nervous system in this phenomenon was assessed by subjecting adrenalectomized, hypophysectomized, 6-OHDA-treated or control (sham-operated or saline-treated) rats to inescapable shock. Neither adrenalectomy, hypophysectomy, nor 6-OHDA pretreatment altered the stress-induced reduction in renal PBR. However, the stress-induced decrease in renal PBR was blocked by pretreatment with clonazepam (1 mg/kg), a potent anxiolytic with low affinity for PBR.     

Sexual dimorphism of stress-induced changes in renal peripheral benzodiazepine receptors in rat.

Adult male and female rats were exposed to either inescapable shock or no treatment. In vitro [3H]Ro 5-4864 (4'-chlorodiazepam) binding (1 nM) to peripheral benzodiazepine receptors (PRB) in both CNS and peripheral tissues indicated no gender differences in olfactory bulb, heart, lung or adrenal gland but a significant effect was observed in renal tissue. Female rats showed an attenuated stress-induced reduction (23%) in PBR in comparison to males (55%). This difference was shown to be an alteration of Bmax and not kD by Scatchard analysis. These data are the first demonstration of a sexual dimorphism in environmentally-induced alterations in PBR.     

Inescapable shock reduces [3H]Ro 5-4864 binding to "peripheral-type" benzodiazepine receptors in the rat

[3H]Ro 5-4864 binding to "peripheral-type" benzodiazepine receptors was examined in brain and peripheral tissues of rats subjected to inescapable tailshocks. Two hours after a session of 80 (five-second) inescapable tailshocks, a significant reduction in [3H]Ro 5-4864 (10 mM) binding was observed in membranes from kidney (31%), cerebral cortex (29%), heart (19%) and pituitary (17%) compared to tissues from naive animals. In contrast, inescapable shock did not effect [3H]Ro 5-4864 binding to hippocampal, lung, or adrenal membranes. Scatchard analyses of [3H]Ro 5-4864 binding to renal membranes demonstrated that this session of tailshock reduced the density (Bmax) of "peripheral-type" benzodiazepine receptors without effecting the apparent affinity (Kd) of the radioligand for these sites. The effects of graded stress on [3H]Ro 5-4864 binding to cerebral cortex and kidney were investigated using 5, 20, or 80 (five-second) inescapable shocks. In cerebral cortical membranes, sessions of either 5 or 20 shocks did not affect, while 80 shocks reduced (29%) [3H]Ro 5-4864 (10 mM) binding. In renal membranes, 5 shocks significantly increased (35%), 80 shocks significantly decreased [3H]Ro 5-4864 (10mM) binding (31%). These findings demonstrate that the density of "peripheral-type" benzodiazepine receptors in both peripheral tissues and the central nervous system can be rapidly modulated by stress.     

Impact of psychological dynamics of stress on the peripheral benzodiazepine receptor.

In an attempt to dissociate the relative impact of psychological vs. physiological concomitants of stress on the peripheral benzodiazepine receptor (PBR), the influence of stressor controllability and predictability was investigated in rats. In addition, the effect of a purely psychological stressor, contextually conditioned fear, was examined. The response of the PBR in rats confronted with a naturalistic threat, a cat, was also tested. Various peripheral and CNS tissues were analyzed. Specific binding of [3H]Ro 5-4864 was significantly reduced in the kidneys of subjects receiving either controllable or uncontrollable shock. Similar changes were seen in the kidneys of subjects receiving either predictable or unpredictable shock. Mean [3H]Ro 5-4864 binding in lung was reduced following both predictable and unpredictable shock, but only the reduction in the predictable shock group reached significance. Controllability appeared to protect against the stress-induced reduction in [3H]Ro 5-4864 binding in lung. Contextually conditioned fear only affected PBR in the olfactory bulb, and exposure to a cat was without effect. These data suggest that the PBR responds only to potent stressors, and psychological influences on the PBR are tissue specific.     

Interactions of lipids with peripheral-type benzodiazepine receptors

Peripheral-type benzodiazepine receptors (PBRs) are present at high densities in the rat kidney distal tubule. [3H]RO 5-4864 binding to PBRs in kidney membranes is inhibited by several unidentified low molecular weight hydrophobic compounds in urine and serum. We tested representative hydrophobic compounds from several lipid classes for ability to inhibit binding to rat kidney PBRs of two high affinity ligands, [3H]RO 5-4864 and [3H]PK 11195. Unsaturated fatty acids and alcohols inhibited [3H]RO 5-4864 binding with half-maximal inhibition occurring at 3 X 10(-6) M to 10(-4) M. Inhibitory potency increased with the degree of unsaturation. Phospholipids inhibited [3H]RO 5-4864 in the same concentration range, with inhibitory potency in this case dependent both upon an unsaturated fatty acid moiety and upon the polar head group. Phosphatidylethanolamine was the most potent phospholipid tested (IC50 = 2 X 10(-6) M), whereas phosphatidylcholine was not inhibitory. Although phospholipids inhibited both [3H]RO 5-4864 and [3H]PK 11195 binding equally, unsaturated fatty acids had a much greater inhibitory effect upon [3H]RO 5-4864 than upon [3H]PK 11195 binding. Similar effects were obtained with digitonin-solubilized PBRs. These data demonstrate that in our experiments PBR binding was inhibited by specific lipids and that binding of proposed agonist (RO 5-4864) and antagonist (PK 11195) ligands was differentially affected by unsaturated fatty acids.     

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

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

Peripheral benzodiazepine receptor ligands: mitochondrial transmembrane potential depolarization and apoptosis induction in rat C6 glioma cells

The peripheral benzodiazepine receptor (PBR) is a component of a multiprotein complex, located at the contact site between the inner and outer mitochondrial membranes, which constitutes the mitochondrial permeability transition (MPT)-pore. The opening of the MPT-pore, leading to the transmembrane mitochondrial potential (DeltaPsi(m)) dissipation, is a critical event in the mechanism of apoptosis. In the present work, we investigated the ability of the specific PBR ligands, PK 11195 or Ro5-4864, to affect mitochondrial potential and to induce apoptotic cell death in rat C6 glioma cells. Both specific ligands inhibited cell survival in a dose- and time-dependent manner, as assessed by MTS conversion assay, whereas the non-site selective ligand Diazepam or the low-affinity benzodiazepine Clonazepam showed no significant effects. After cell exposure to PK 11195 or Ro5-4864 we evidenced typical alterations of apoptotic cell death such as DNA fragmentation and chromatin condensation assessed by flow cytometric and transmission electron microscopy (TEM) analysis, respectively. Activation of the "effector" caspase-3 confirmed the ability of specific PBR ligands to induce apoptosis. Moreover, PK 11195 and Ro5-4864 induced a decrease of DeltaPsi(m), as evidenced by JC-1 flow cytometry analysis. Our data demonstrate the pro-apoptotic effects of specific PBR ligands on rat C6 glioma cells.     

Novel irreversible ligands specific for "peripheral" type benzodiazepine receptors: (+/-)-, (+)-, and (-)-1-(2-chlorophenyl)-N-(1-methylpropyl)-N- (2-isothiocyanatoethyl)-3-isoquinolinecarboxamide and 1-(2-isothiocyanatoethyl)-7-chloro-1,3-dihydro-5-(4-chlorophenyl )-2H-1,4-benzodiazepin-2-one

Novel ligands that bind irreversibly and selectively to "peripheral" type benzodiazepine receptors (PBR) have been prepared. These compounds inhibit radiolabeled binding to PBR in the nanomolar range. The 2-isothiocyanatoethyl analogue of Ro 5-4864 (1-methyl-7-chloro-1,3-dihydro-5-(4-chlorophenyl)-2H-1,4-benzodiazepi n-2-one) (5, AHN 086) was synthesized in three steps from desmethyl Ro 5-4864. The (+/-) (11a, AHN 070), R-(-) (11b), and S-(+) (11c) 2-isothiocyanatoethyl derivatives of PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carboxami de) were each prepared in three steps from PK 11209 (1-(2-chlorophenyl)-3-isoquinolinecarboxylic acid, 6). All four compounds inhibited radioligand binding to the PBR in brain and kidney. The R-(-) stereoisomer 11b was observed to be approximately 2.5-fold more potent than its enantiomer 11c; this is the first report of stereoselectivity in the isoquinoline series of ligands selective for the PBR. Furthermore, pH dependency studies showed that, at lower pH, change in the affinities for the PBR ligands is a property of the receptor, substantiating the hypothesis that a histidine moiety on the PBR is the most likely site for covalent bond formation, whereas, at higher pH, the observed changes in affinities can be attributed to properties of the compounds. All four of these novel ligands are potentially useful tools in the investigation of the PBR.     

Characterization of peripheral-type benzodiazepine recognition sites in the rat spinal cord

The binding of [3H]Ro 5-4864 to membranes prepared from spinal cord of the adult rat has been studied in vitro. At 4 degrees C, the binding of [3H]Ro 5-4864 reached equilibrium by 120 min, and was rapidly reversible (dissociation t0.5 = 21 min). The [3H]Ro 5-4864 bound with a high affinity (Kd approximately equal to 3 nM) to a single, saturable population of binding sites (Bmax = 27 pmol/g tissue wet weight). Activation of receptors for gamma-aminobutyric acid with 10 microM muscimol did not alter these binding parameters. The drugs Ro 5-4864, diazepam and flunitrazepam were potent inhibitors of this binding (Kis of 10(-9)-10(-8) M) whereas clonazepam, CL 218,872 and Ro 15-1788 were weak inhibitors (Kis greater than 10(-5) M). A comparison of the binding of [3H]Ro 5-4864 in spinal cord with that in other areas of the CNS revealed that whereas the binding affinity was similar in all regions, membranes from spinal cord contained a slightly greater number of binding sites than cerebral cortex and cerebellum, and approximately one-third of the number present in the olfactory bulb. The characteristics of the binding of [3H]Ro 5-4864 obtained in this study are consistent with this ligand binding to peripheral-type benzodiazepine recognition sites in membranes from spinal cord.     

Angiotensin II rapidly modulates the renal peripheral benzodiazepine receptor.

The effects of acute exposure to angiotensin II (AII) on the renal peripheral benzodiazepine receptor were studied in rats. As little as 37.5 micrograms of AII injected s.c. over an 80 min period caused immediate reductions in [3H]Ro5-4864 binding. Scatchard analysis revealed that the reduction in [3H]Ro5-4864 binding induced by AII was due to a drop in receptor density or Bmax. The influence of AII on the peripheral benzodiazepine receptor is similar to that of stress.     

Differential effects of anxiogenic central and peripheral benzodiazepine receptor ligands in tests of learning and memory

Previous research has demonstrated that low doses of anxiogenic central benzodiazepine receptor (CBR) ligands, the beta-carbolines, improve performance in various learning and memory tests in animals if administered prior to training. The present experiments compared the effect of a beta-carboline (FG 7142) with that of a pharmacologically distinct anxiogenic compound, a peripheral benzodiazepine receptor (PBR) ligand, 4-chlorodiazepam (Ro5-4864), in two tests of learning and memory in rats. As expected, FG 7142 significantly improved performance in a passive avoidance test. Ro5-4864 was without effect. In a shuttlebox escape test, Ro5-4864 significantly impaired performance while FG 7142 had no effect. The effect of Ro5-4864 was antagonized by the specific peripheral benzodiazepine receptor antagonist, PK 11195. These results indicate that the differential impact of CBR and PBR anxiogenic ligands on performance in aversively-motivated learning tests may be a reflection of their distinct pharmacologies.     

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

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

Environmentally induced changes in peripheral benzodiazepine receptors are stressor and tissue specific

The stress-induced changes in peripheral benzodiazepine receptors (PBR) can be observed in a number of different tissues, depending upon the nature and chronicity of the aversive experience. In addition, virtually all stress procedures that cause rapid changes in PBR simultaneously increase the physical activity or metabolic rate of the subjects. The present study analyzed the contributions of rapid alterations in activity or metabolic rate with and without aversive stimulation and their subsequent impact on PBR. Mechanically induced increases in activity by forced running stress results in a significant reduction in [³H]Ro 5-4864 binding to PBR in olfactory bulb, opposite to the PBR changes in this tissue following forced cold-water swim stress. Pharmacological induction of increased locomotor activity as well as metabolic rate by d-amphetamine causes a significant increase in cardiac PBR binding, again, opposite to the response typically observed following inescapable shock stress. Finally, administration of the anxiogenic beta-carboline, FG-7142, causes increases in both hippocampus and adrenal gland PBR binding reminiscent of acute noise stress exposure. These experiments demonstrate that increased locomotor activity or metabolic rate alone is not a necessary and sufficient condition for previous stress-induced changes in PBR. Conversely, increased metabolic rate coupled with an aversive stimulus appears to be an important factor for inducing stress-like changes in PBR. This data, coupled with previous reports, suggests that rapid alterations in these sites are Stressor and tissue dependent. Finally, we propose that the PBR may be involved in many aspects of the stress response including: a) a biowarning system in adrenal gland, b) participation in stress-induced hypertension via renal PBR, and c) a modulator of stress-induced immunosuppression and subsequent recovery of function or recuperation by actions on immune cells.     

The effect of chronic treatment with peripheral benzodiazepine receptor ligands on behavior and GABAA/benzodiazepine receptors in rat

Summary Rats were twice daily (2 × 10 mg/kg, i.p.) treated for three weeks with the peripheral benzodiazepine (BZ) receptor ligands Ro 5-4864 (4-chlorodiazepam) and PK 11 195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carboxamide). After the first injection there were no differences between the drug-treated and control animals in behavioral tests. After 10 days treatment, the number of sniffings was increased in Ro 5-4864-treated rats. After the last injection, sniffings and ambulations were decreased in PK 11 195-treated animals. The number of rearings and groomings remained unchanged throughout the treatment, and there were no changes in the results in the elevated plus-maze test. Apparently these compounds are devoid of anxiolytic and anxiogenic effects at moderate doses.The effect of 72 a h withdrawal from the above mentioned chronic treatment on peripheral and central BZ receptors as well as on GABA_A receptors was studied with receptor binding techniques using ³H-Ro 5-4864, ³H-flumazenil and ³H-muscimol, respectively, as ligands. The number of GABA_A and central BZ receptors was lower after Ro 5-4864 treatment, as was the effect of progesterone-induced stimulation of ³H-muscimol binding. The number of peripheral BZ receptors was decreased after Ro 5-4864 and PK 11 195 treatments in the olfactory bulb but not in the cerebral cortex.The chronic treatment with peripheral BZ receptor ligands ] Ro 5-4864 and PK 11 195 produced only little behavioral effects. Ro 5-4864, often presented as an agonist of peripheral BZ receptors, was behaviorally inactive. PK 11 195, often considered to be an antagonist of Ro 5-4864 developed a small sedative action during chronic treatment. The withdrawal from chronic treatment with these ligands similarly affected peripheral BZ receptors but only Ro 5-4864 affected GABA_A/BZ receptor complex in the CNS.The present data support the idea that Ro 5-4864 has independent of peripheral BZ receptors effects on GABA, receptors while PK 11 195 is rather a partial agonist than antagonist of peripheral BZ receptors. Correspondence to V Saano at the above address     

Changes in neurotransmitter receptors and neurobehavioral variables in rats co-exposed to lead and ethanol.

The influence of lead (10 mg/kg) and ethanol (10% v/v, in drinking water) administration, either alone or in combination for 8 weeks was investigated on the uptake of lead in tissues, dopamine, benzodiazepine and cholinergic (muscarinic) receptors, motor activity, number of fighting episodes and several selected lead-sensitive biochemical indices in young rats. Lead or ethanol treatment did not elicit any appreciable influence on body weight gain, tissue weight, and food intake. However, a decreased amount of mean water intake was noticed in animals ingesting ethanol plus lead. Both lead and ethanol when administered individually exacerbate the decrease of blood delta-aminolevulinic acid dehydratase (ALAD) activity while lead and ethanol administered together exhibited a pronounced inhibition of ALAD activity. A significant increase in hepatic GSH levels, hepatic and brain MDA levels was also observed in animals co-exposed to lead and ethanol compared to lead alone treated rats. Simultaneous exposure to lead and ethanol also resulted in a significantly more pronounced decrease in binding of [3H]fluintrazepam in membranes prepared from the fronto-cortical region compared to the corresponding controls. The binding of [3H]spiroperidol to striatal and [3H]quinuclidinyl benzylate (QNB) to cerebellar membranes remained unaltered in all the exposed animals. Spontaneous locomotor activity and aggressive behaviour increased significantly in the group treated with both lead and ethanol compared to the control group. The lead concentrations in blood, liver, kidney and brain were significantly higher in rats exposed simultaneously to lead and ethanol compared to lead alone exposed group. The results indicate more pronounced neurotoxic and neurobehavioural changes in animals co-exposed to lead and ethanol; however, the exact mechanism or site of its action is still not clear.     

Peripheral-type benzodiazepine receptor (PBR) and PBR drug ligands in fibroblast and fibrosarcoma cell proliferation: role of ERK, c-Jun and ligand-activated PBR-independent pathways

Peripheral-type benzodiazepine receptor (PBR) is a 18-kDa high-affinity drug and cholesterol binding protein, that has been implicated in several physiological processes, such as cholesterol transport and mitochondrial respiration. Specific PBR ligands regulate cell proliferation, although their action is controversial and probably cell-type specific. The aim of the present study was to examine the expression of PBR in cells of mesenchymal origin, i.e. human fibroblasts and fibrosarcoma cells, as well as its role in the regulation of their proliferation. Both mesenchymal cell types express high levels of PBR, localized exclusively in mitochondria. PBR-specific drug ligands, the isoquinoline carboxamide PK 11195 and the benzodiazepine Ro5-4864, at relative high concentrations (10(-4)M), exert a strong inhibitory effect on cell proliferation by arresting the cells at the G0/G1 phase of the cell cycle, while no apoptotic cell death was observed. In normal fibroblasts, this inhibition was correlated with a decrease in the activation of the cell cycle markers ERK and c-Jun. PBR knockdown by RNA inhibition did not affect the proliferation of either cell type and did not influence the inhibitory effect of PK 11195 and Ro5-4864 on cell growth. These data suggest that in fibroblasts and fibrosarcoma cells PBR drug ligands inhibit cell proliferation in a PBR-independent manner. These results are in contrast to data reported on cells of epithelial origin, suggesting that the origin of the cells is crucial in defining the role of PBR in their proliferation, and raise caution in the commonly made assumption that PBR mediates cell functions affected by PBR drug ligands.     

Repeated administration of diazepam reduces [3H]Ro 5-4864 binding in cerebral cortex of rats.

Binding of [3H]Ro 5-4864 to mitochondrial membrane preparations of rat cerebral cortex was measured after repeated (5 days) IV administration of diazepam (10 mg/kg daily) and clonazepam (2.5 mg/kg daily). The B(max) value for [3H]Ro 5-4864 was significantly reduced in rats treated with diazepam (-49%) but not in those treated with clonazepam. These findings suggest the involvement of peripheral-type binding sites in the development of rapid tolerance to the sedative effects of benzodiazepines. A downregulation of [3H]Ro 5-4864 (-65%) was also observed after repeated administration of Ro 5-4864 (4 mg/kg daily), thus confirming that this compound behaves as an agonist at its own recognition sites.     

Preservation of “peripheral” benzodiazepine receptors: Differential effects of freezing on [3H]Ro 5-4864 and [3H]PK 11195 binding

A statistically significant decrease in the density of peripheral benzodiazepine receptors was observed in renal membranes of rats beginning 2 weeks after adrenalectomy when compared with sham-operated controls. This decrease in peripheral benzodiazepine receptor density was manifest as a decrease in the B_max of two ligands [³H]Ro 5-4864 and [³H]PK 11195, without accompanying changes in their apparent affinity (K_d) for this site. Similar changes were not seen in another aldosterone-sensitive organ, the submandibular salivary gland. The decrease in peripheral benzodiazepine receptor density in observed in adrenalectomized rat renal membranes was restored to control levels after 1 week of aldosterone administration using a dose (12.5 μg/kg/day) that had no effect on peripheral benzodiazepine receptor density in sham-operated animals. In contrast, dexamethasone administration (50 μg/kg/day, 1 week) had no effect on renal peripheral benzodiazepine receptor density when administered to either adrenalectomized or sham-operated rats. Further, adrenal demedullation had no effect on renal peripheral benzodiazepine receptor density or affinity. The decrease in peripheral benzodiazepine receptor density was localized to the renal cortex and the outer stripe of the medulla by gross dissection of renal slices and renal tissue section autoradiography. The specific effect of adrenalectomy on renal peripheral benzodiazepine receptor density, the lack of direct effect of aldosterone on [³H]Ro 5-4864 binding, and the localization of the change in peripheral benzodiazepine receptor density to the renal cortex and outer stripe suggests that these changes may reflect an adaptation of the renal nephron (possibly the distal convoluted tubule, intermediate tubule and/or the collecting duct) to the loss of mineralocorticoid hormones.