Actions of α2 adrenoceptor ligands at α2A and 5-HT1A receptors: the antagonist, atipamezole, and the agonist, dexmedetomidine, are highly selective for α2A adrenoceptors

Abstract

This study examined the activity of chemically diverse alpha2 adrenoceptor ligands at recombinant human (h) and native rat (r) alpha2A adrenoceptors compared with 5-HT1A receptors. First, in competition binding experiments at h alpha2A and h5-HT1A receptors expressed in CHO cells, several compounds, including the antagonists 1-(2-pyrimidinyl)piperazine (1-PP), (+/-)-idazoxan, benalfocin (SKF 86466), yohimbine and RX 821,002, displayed preference for h alpha2A versus h5-HT1A receptors of only 1.4-, 3.6-, 4-, 10- and 11-fold, respectively (based on differences in pKi values). Clonidine, brimonidine (UK 14304), the benzopyrrolidine fluparoxan and the guanidines guanfacine and guanabenz exhibited intermediate selectivity (22- to 31-fold) for h alpha2A receptors. Only the antagonist atipamezole and the agonist dexmedetomidine (DMT) displayed high preference for alpha2 adrenoceptors (1290- and 91-fold, respectively). Second, the compounds were tested for their ability to induce h5-HT1A receptor-mediated G-protein activation, as indicated by the stimulation of [35S]GTPgammaS binding. All except atipamezole and RX 821,002 exhibited agonist activity, with potencies which correlated with their affinity for h5-HT1A receptors. Relative efficacies (Emax values) were 25-35% for guanabenz, guanfacine, WB 4101 and benalfocin, 50-65% for 1-PP, (+/-)-idazoxan and clonidine, and over 70% for fluparoxan, oxymetazoline and yohimbine (relative to 5-HT = 100%). Yohimbine-induced [35S]GTPgammaS binding was inhibited by the selective 5-HT1A receptor antagonist WAY 100,635. In contrast, RX 821,002 was the only ligand which exhibited antagonist activity at h5-HT1A receptors, inhibiting 5-HT-stimulated [35S]GTPgammaS binding. Atipamezole, which exhibited negligeable affinity for 5-HT1A receptors, was inactive. Third, the affinities for r alpha2A differed considerably from the affinities for h alpha2A receptors whereas the affinities for r5-HT1A differed much less from the affinities for h5-HT1A receptors. This affected markedly the affinity ratios of certain compounds. For example, (+/-)-idazoxan was only 3.6-fold selective for h alpha2A versus h5-HT1A but 51-fold selective for r alpha2A versus r5-HT1A receptors. Conversely, yohimbine was tenfold selective for h alpha2A versus h5-HT1A adrenoceptors but 4.2-fold selective for r alpha2A versus r5-HT1A receptors. Nevertheless, both atipamezole and DMT were highly selective for both rat and human alpha2A versus rat or human 5-HT1A receptors. In conclusion, these data indicate that: (1) the agonist DMT and the antagonist atipamezole are the ligands of choice to distinguish alpha2-mediated from 5-HT1A-mediated actions, whilst several of the other compounds show only low or modest selectivity for alpha2A over 5-HT1A receptors; (2) caution should be exercised in experimental and clinical interpretation of the actions of traditionally employed alpha2 ligands, such as clonidine, yohimbine and (+/-)-idazoxan, which exhibit marked agonist activity at 5-HT1A receptors.

Yohimbine impairs extinction of cocaine-conditioned place preference in an α2-adrenergic receptor independent process

“…Next, we tested the effects of atipamezole, a more specific ␣ 2 -AR antagonist, which does not have subtype selectivity (Newman-Tancredi et al 1998). In contrast to yohimbine, atipamezole failed to alter cocaine CPP extinction and showed a trend toward impairing long-term fear extinction.…”

“…and s.c.) in saline vehicle in a volume of 10 mL/kg body weight 30-35 min prior to extinction testing at a dose of 2.5 mg/kg or 5 mg/kg, based upon extinction facilitating doses in C57Bl6/j mice (Cain et al 2004). For cocaine CPP and fear extinction, atipamezole was administered s.c. in saline vehicle in a volume of 10 mL/kg body weight 30-35 min prior to extinction testing at a dose of 3 mg/kg, based upon studies in rats and mice (Seppala et al 1994;Newman-Tancredi et al 1998;Millan et al 2000;Powell et al 2005;Risbrough and Geyer 2005).…”

RX 821002 as a Tool for Physiological Investigation of α2‐Adrenoceptors

“…In common with many á 2 -ligands, it binds with moderate affinity (20-30 nM) to these sites (42,79,112). There is a greater separation of affinities between rat á 2 and 5-HT 1A receptors than for their human counterparts (79, Table 1).…”

“…However, clinical trials with á 2 -adrenoceptor antagonists have, for various reasons, not been positive (79) and the only current clinical use of such drugs is to reverse the effects of agonists. Nonetheless, the effort put into designing selective antagonists of á 2 -adrenoceptors has yielded a number of compounds, including RX 821002, that can, when used with care, indicate physiological activity at á 2 -adrenoceptors.…”

“…1) was developed by Reckitt and Coleman in the late1970s and represented something of a breakthrough in á 2 -adrenoceptor antagonist design, giving better á 2 /á 1 selectivity than yohimbine (13,31). This agent exhibits some lack of specificity, being a high affinity ligand at imidazoline receptors (see below) and a partial agonist at 5-HT 1A -receptors (79). Subsequent development of the idazoxan molecule showed that bulky substitutions at the 2-position of the benzodioxanyl ring gave improved potency and selectivity at the á 2 -receptor.…”