Elsevier

Brain Research

Volume 772, Issues 1–2, 24 October 1997, Pages 203-208
Brain Research

Research report
Peripheral-type benzodiazepine receptor ligands and serum steroid hormones

https://doi.org/10.1016/S0006-8993(97)00815-9Get rights and content

Abstract

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.

Introduction

Peripheral-type benzodiazepine receptors (PBR) have been identified in various peripheral tissues such as kidney, uterus, ovaries, and adrenal, as well as in the central nervous system (for review, see Gavish et al. 15, 16and Weizman and Gavish [41]). This class differs from the central-type benzodiazepine receptors (CBR) by their lack of coupling to the γ-aminobutyric acid (GABA) receptors and the chloride ion channels 24, 37. In rodents, but not in humans or other species, the PBR bind with high affinity Ro 5-4864 (4′-chlorodiazepam). PBR also bind with high affinity the isoquinoline carboxamide derivative PK 11195, but exhibit very low affinity for clonazepam; the reverse is true with regard to CBR. The benzodiazepines diazepam and flunitrazepam bind to both PBR and CBR.

However, more recent studies suggest that Ro 5-4864 actions may be due to allosteric interactions with the CBR/GABA receptor complex by binding to the chloride ion channel-coupled t-butylbicyclophosphorothionate site [17]. In addition, evidence for the PBR-specific actions of PK 11195 is also controversial 17, 38. Thus, it is possible that Ro 5-4864 and PK 11195 also have a dual (CBR/PBR) action.

PBR are composed of three subunits: a binding site for PK 11195, a voltage-dependent anion channel, and an adenine nucleotide carrier, with molecular weights of 18, 32, and 30 kDa, respectively [26]. The site for isoquinoline binding has been purified to apparent homogeneity from rat adrenal mitochondria [3]. The protein that includes this site has a molecular weight of 18 kDa. A full-length complementary DNA comprising 781 base pairs and encoding this protein has been cloned and encodes a 169-amino acid open reading frame with five putative transmembrane regions [33]. A three-dimensional model of the PBR has been built using molecular dynamics simulations [8].

PBR seem to be involved in intermediary metabolism [2], metabolism of heme [35], steroidogenesis 28, 30, 43, cell proliferation [36], immunomodulation 9, 14, 32, 44, and response to stress 13, 14, 19, 40, 41, 42. It has been suggested that the effects of PBR ligands on such diverse cellular processes as cell differentiation, immune function, and steroid biosynthesis are mediated via modulation of the mitochondrial channels [20].

Despite extensive literature about the in vitro effects of PBR ligands on cellular function, there is a paucity of information about the in vivo effects of chronic exposure of agents active at the PBR on the expression of this receptor and the serum levels of steroid hormones. Two studies have demonstrated that concurrent administration of the PBR ligand PK 11195 attenuates behavioral and neurochemical effects of chronic diazepam and lorazepam administration in rodents and can prevent the development of behavioral tolerance 25, 27.

The aim of the present study was to investigate the impact of chronic treatment with peripheral and CBR ligands on the pharmacodynamic characteristics of PBR in kidney, uterus, ovaries, and adrenal, as well as serum levels of progesterone, estradiol, and corticosterone in female rats.

Section snippets

Materials

[3H]PK 11195 (85.0 Ci/mmol) was purchased from New England Nuclear, Boston, MA, USA. Unlabeled PK 11195 was kindly donated by Dr. A. Bouvier, Rhône-Poulenc Santé, Vitry-sur-Seine, France. Ro 5-4864, diazepam, and clonazepam were kindly supplied by Drs. H. Gutmann and E. Kyburz, Hoffmann-La Roche, Basel, Switzerland. Lumax was purchased from Lumac, Schaesburg, the Netherlands. All other reagents were supplied by Merck, Darmstadt, Germany.

Animals

Adult female Sprague–Dawley rats (180–200 g) were housed

Results

The effects of chronic administration of the various ligands on PBR density in the kidney, uterus, ovaries, and adrenal are shown in Fig. 1Fig. 2Fig. 3Fig. 4. Ro 5-4864 administration resulted in significant increases of [3H]PK 11195 binding in all the tissues examined (kidney, +34%, P<0.001; uterus, +72%, P<0.001; ovaries, +122%, P<0.001; adrenal, +149%, P=0.001). Chronic exposure to PK 11195 did not consistently affect the receptor density in the various tissues. A slight but significant

Discussion

The present study demonstrated a uniform up-regulatory effect of Ro 5-4864 on the four tissues examined and a significant suppressive effect of the PBR ligand on serum estradiol levels. In contrast, all the other ligands (PK 11195, diazepam, and clonazepam) affected the PBR densities in an inconsistent manner. Another interesting finding was the sensitivity of ovary to chronic exposure to pure and mixed PBR ligands. Chronic administration of these agents, but not the CBR ligand clonazepam,

Acknowledgements

This research was supported in part by the Technion V.P.R. Fund–Charles Krown Research Fund. We thank Ruth Singer for editing the manuscript.

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