Brain α2-adrenoceptors in monoamine-depleted rats: increased receptor density, G coupling proteins, receptor turnover and receptor mRNA

Authors

  • Catalina Ribas,

    1. Laboratory of Neuropharmacology, Associate Unit of the Institute Cajal/CSIC, Department of Biology, University of the Balearic Islands, Cra. Valldemossa Km 7.5, E-07071 Palma de Mallorca, Spain
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  • Antonio Miralles,

    1. Laboratory of Neuropharmacology, Associate Unit of the Institute Cajal/CSIC, Department of Biology, University of the Balearic Islands, Cra. Valldemossa Km 7.5, E-07071 Palma de Mallorca, Spain
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  • Xavier Busquets,

    1. Laboratory of Neuropharmacology, Associate Unit of the Institute Cajal/CSIC, Department of Biology, University of the Balearic Islands, Cra. Valldemossa Km 7.5, E-07071 Palma de Mallorca, Spain
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  • Jesús A García-Sevilla

    Corresponding author
    1. Laboratory of Neuropharmacology, Associate Unit of the Institute Cajal/CSIC, Department of Biology, University of the Balearic Islands, Cra. Valldemossa Km 7.5, E-07071 Palma de Mallorca, Spain
      Unité de Recherche Clinique, Département de Psychiatrie, HUG Belle-Idée (Le Salève), 2 Chemin du Petit-Bel-Air, CH-1225 Chêne-Bourg/Genève, Switzerland
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Unité de Recherche Clinique, Département de Psychiatrie, HUG Belle-Idée (Le Salève), 2 Chemin du Petit-Bel-Air, CH-1225 Chêne-Bourg/Genève, Switzerland

Abstract

  • This study was designed to assess the molecular and cellular events involved in the up-regulation (and receptor supersensitivity) of brain α2-adrenoceptors as a result of chronic depletion of noradrenaline (and other monoamines) by reserpine.

  • Chronic reserpine (0.25 mg kg−1 s.c., every 48 h for 6 – 14 days) increased significantly the density (Bmax values) of cortical α2-adrenoceptor agonist sites (34 – 48% for [3H]-UK14304, 22 – 32% for [3H]-clonidine) but not that of antagonist sites (11 – 18% for [3H]-RX821002). Competition of [3H]-RX821002 binding by (−)-adrenaline further indicated that chronic reserpine was associated with up-regulation of the high-affinity state of α2-adrenoceptors.

  • In cortical membranes of reserpine-treated rats (0.25 mg kg−1 s.c., every 48 h for 20 days), the immunoreactivities of various G proteins (Gαi1/2, Gαi3, Gαo and Gαs) were increased (25 – 34%). Because the high-affinity conformation of the α2-adrenoceptor is most probably related to the complex with Gαi2 proteins, these results suggested an increase in signal transduction through α2-adrenoceptors (and other monoamine receptors) induced by chronic reserpine.

  • After α2-adrenoceptor alkylation, the analysis of receptor recovery (Bmax for [3H]-UK14304) indicated that the increased density of cortical α2-adrenoceptors in reserpine-treated rats was probably due to a higher appearance rate constant of the receptor (Δr=57%) and not to a decreased disappearance rate constant (Δk=7%).

  • Northern- and dot-blot analyses of RNA extracted from the cerebral cortex of saline- and reserpine-treated rats (0.25 mg kg−1, s.c., every 48 h for 20 days) revealed that reserpine markedly increased the expression of α2a-adrenoceptor mRNA in the brain (125%). This transcriptional activation of the receptor gene expression appears to be the cellular mechanism by which reserpine induces up-regulation in the density of brain α2-adrenoceptors.

British Journal of Pharmacology (2001) 132, 1467–1476; doi:10.1038/sj.bjp.0703963

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