Brain alpha(2)-adrenoceptors in monoamine-depleted rats: increased receptor density, G coupling proteins, receptor turnover and receptor mRNA.
1. This study was designed to assess the molecular and cellular events involved in the up-regulation (and receptor supersensitivity) of brain alpha(2)-adrenoceptors as a result of chronic depletion of noradrenaline (and other monoamines) by reserpine. 2. Chronic reserpine (0.25 mg kg(-1) s.c., every 48 h for 6 - 14 days) increased significantly the density (B(max) values) of cortical alpha(2)-adrenoceptor agonist sites (34 - 48% for [(3)H]-UK14304, 22 - 32% for [(3)H]-clonidine) but not that of antagonist sites (11 - 18% for [(3)H]-RX821002). Competition of [(3)H]-RX821002 binding by (-)-adrenaline further indicated that chronic reserpine was associated with up-regulation of the high-affinity state of alpha(2)-adrenoceptors. 3. 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 (Galphai(1/2), Galphai(3), Galphao and Galphas) were increased (25 - 34%). Because the high-affinity conformation of the alpha(2)-adrenoceptor is most probably related to the complex with Galphai(2) proteins, these results suggested an increase in signal transduction through alpha(2)-adrenoceptors (and other monoamine receptors) induced by chronic reserpine. 4. After alpha(2)-adrenoceptor alkylation, the analysis of receptor recovery (B(max) for [(3)H]-UK14304) indicated that the increased density of cortical alpha(2)-adrenoceptors in reserpine-treated rats was probably due to a higher appearance rate constant of the receptor ((Delta)r=57%) and not to a decreased disappearance rate constant ((Delta)k=7%). 5. 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 alpha(2a)-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 alpha(2)-adrenoceptors.
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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D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D000068438:Brimonidine Tartrate; D003000:Clonidine / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D019329:Idazoxan / Q000031:analogs & derivatives* / Q000378:metabolism; D008297:Male; D008566:Membranes / Q000378:metabolism; D009638:Norepinephrine / Q000378:metabolism*; D011804:Quinolines / Q000494:pharmacology; D011810:Quinoxalines / Q000378:metabolism; D012333:RNA, Messenger / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D012110:Reserpine / Q000494:pharmacology; D014316:Tritium
D018759:Adrenergic Uptake Inhibitors; D000317:Adrenergic alpha-Antagonists; D011804:Quinolines; D011810:Quinoxalines; D012333:RNA, Messenger; D018341:Receptors, Adrenergic, alpha-2; D014316:Tritium; D000068438:Brimonidine Tartrate; C000470:EEDQ; D012110:Reserpine; C058525:2-methoxyidazoxan; D019204:GTP-Binding Proteins; D003000:Clonidine; D009638:Norepinephrine; D019329:Idazoxan; D004837:Epinephrine