Partial to complete antagonism by putative antagonists at the wild‐type α_2C‐adrenoceptor based on kinetic analyses of agonist : antagonist interactions

Abstract

1 Activation of the recombinant human a 2C -adrenoceptor (a 2C AR) by (7)-adrenaline in CHO-K1 cells transiently co-expressing a chimeric G aq/i1 protein induced a rapid, transient Ca 2+ response with a high-magnitude followed by a low-magnitude phase which continued throughout the recorded time period (15 min). 2 Activation of the a 2C AR by various a 2 AR agonists revealed the following rank order of highmagnitude Ca 2+ response [E max (%) versus 10 mM (7)-adrenaline]: UK 14304 (102+4)=talipexole (101+3)=(7)-adrenaline (100)=d-medetomidine (98+1)4oxymetazoline (81+4)^clonidine (75+5). 3 The methoxy-(RX 821002) and ethoxy-derivatives (RX 811059) of idazoxan and the dexefaroxan analogue atipamezole were fully e ective as antagonists of both the high-and the low-magnitude Ca 2+ response. However, though acting as full antagonists of the high-magnitude response, the further putative a 2 AR antagonists idazoxan (27%), SKF 86466 (29%) and dexefaroxan (59%) reversed the low-magnitude response only partially. 4 In conclusion, kinetic analyses of agonist : antagonist interactions at the a 2C AR demonstrate a wide spectrum of partial to complete antagonism of the low-magnitude Ca 2+ response for structurally related a 2 AR ligands.

α2A‐Adrenergic Receptor Signaling Underlies Synergistic Enhancement of Ethanol‐Induced Behavioral Impairment by Clonidine

“…First, it is possible that RX821002, acting as an inverse agonist (Murrin et al., 2000), further suppressed the clonidine component of the LORR response to clonidine‐ethanol combination. Second, the ability of RX821002 to block α 2C AR, which was demonstrated in vitro (Pauwels and Colpaert, 2000), makes it difficult to unequivocally rule out a role for α 2C AR in the clonidine‐ethanol behavioral interaction. α 2C AR, found at a lower density in the brain (Bucheler et al., 2002), makes a small contribution to behavioral effects such as inhibition of locomotor activity and hypothermia (Hein, 2006; Lahdesmaki et al., 2003).…”

Real‐time analysis of dopamine: antagonist interactions at recombinant human D2long receptor upon modulation of its activation state

“…This observation exemplifies that a weak antagonism cannot be solely explained by the putative presence or, for that matter, absence of the ligand's intrinsic activity. Similar observations have been made for closely related compounds at the wild‐type α 2C ‐adrenoceptor (Pauwels & Colpaert, 2000). Remarkably, bromerguride and also (+)‐UH 232, when applied prior to dopamine, demonstrated a lower Ca 2+ response as compared to their intrinsic Ca 2+ response at the unoccupied D 2long receptor.…”

“…A chimeric G αq/o protein was constructed by exchanging the last five amino acids (Glu 355 ‐Tyr‐Asn‐Leu‐Val) of a mouse G αq protein (Genbank accession number: M55412) by those corresponding to a G αo protein (Gly‐Ile‐Gly‐Leu‐Tyr) and subsequent mutation of the fourth last residue into an Ile. This was realized by inserting the respective nucleotide sequence on the reverse oligonucleotide primer used in a PCR reaction on cloned wild‐type G αq protein cDNA (Pauwels & Colpaert, 2000). The pertussis‐toxin resistant mutant G αo Cys 351 Ile protein and G α15 protein were obtained as described (Dupuis et al ., 1999).…”

“…Activation of these receptor mutants was further investigated by real‐time analysis of Ca 2+ responses in the co‐presence of either a chimeric G αq/o protein (Conklin et al ., 1993) or the promiscuous G α15 protein (Wilkie et al ., 1991). These G α proteins have been shown to couple G protein‐coupled receptors efficaciously to the cellular Ca 2+ signalling pathway (Pauwels et al ., 2000b; Pauwels & Colpaert, 2000). Ca 2+ mobilization by dopamine D 2 receptors has previously been demonstrated in CHO‐K1 and Ltk − fibroblast cells stably expressing the receptor (Hayes et al ., 1992; Liu et al ., 1992).…”

Hypothermic responses to infection are inhibited by α2-adrenoceptor agonists with possible clinical implications

“…UK14304-18 has some selectivity for a 2A receptors, 18 whereas RX811059A has high affinity at a 2 -receptors with poor subtype selectivity. 19 The doses of the agonists used were chosen based on two criteria: doses were of the same magnitude as those currently used for sedation and sedative withdrawal in HDU patients (1 -10 mg kg 21 ) 1 2 4 and the doses used had previously been demonstrated to affect central noradrenergic release in microdialysis studies of rat brain. 20…”