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Br J Pharmacol. Apr 2001; 132(7): 1467–1476.
PMCID: PMC1572698

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

Abstract

  1. 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.
  2. 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.
  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 (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.
  4. 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%).
  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 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.
Keywords: α2-Adrenoceptors, agonist and antagonist binding sites, G coupling proteins, receptor turnover, α2a-adrenoceptor mRNA, reserpine, monoamine depletion, rat brain

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