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 invol ved 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) increas ed significantly the density (B-max values) of cortical alpha (2)-adrenocep tor agonist sites (34-48% for [H-3]-UK14304, 22-32% for [H-3]-clonidine) bu t not that of antagonist sites (11-18% for [H-3]-RX821002). Competition of [H-3]-RX821002 binding by (-)-adrenaline further indicated that chronic res erpine was associated with up-regulation of the high-affinity state of alph a (2)-adrenoceptors. 3 In cortical membranes of reserpine-treated rats (0.25 mg kg(-1) s.c., eve ry 48 h for 20 days), the immunoreactivities of various G proteins (G alpha i(1/2), G alphai(3), G alphao and G alphas) were increased (25-34%). Becaus e the high-affinity conformation of the alpha (2)-adrenoceptor is most prob ably related to the complex with G alphai(2) proteins, these results sugges ted 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 recover y (B-max for [H-3]-UK14304) indicated that the increased density of cortica l alpha (2)-adrenoceptors in reserpine-treated rats was probably due to a h igher appearance rate constant of the receptor (Deltar = 57%) and not to a decreased disappearance rate constant (Deltak=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 fo r 20 days) revealed that reserpine markedly increased the expression of alp ha (2)-adrenoceptor mRNA in the brain (125%). This transcriptional activati on of the receptor gene expression appears to be the cellular mechanism by which reserpine induces up-regulation in the density of brain alpha (2)-adr enoceptors.