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

This study examined the activity of chemically diverse alpha(2) adrenoceptor ligands at recombinant human (h) and native rat (r) alpha(2A) adrenoceptors compared with 5-HT1A receptors. Fit-st, in competition binding experiments at ha,, 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 ha,, versus h5-HT1A receptors of only 1.4-, 3.6-, 4-, 10- and 11-fold, respectively (based on differences in pK(i) values). Clonidine, brimonidine (UK 14304), the benzopyrrolidine fluparoxan and the guanidines guanfacine and guanabenz exhibited intermediate selectivity (22- to 31-fold) for h alpha(2A) receptors. Only the antagonist atipamezole and the agonist dexmedetomidine (DMT) displayed high preference for alpha(2) 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 [S-35]GTP gamma S binding. All except atipamezole and RX 821,002 exhibited agonist activity, with potencies which correlated with their affinity for h5-HT1A receptors. Relative efficacies (E-max values) were 25-35% for guanabenz, guanfacine, WE 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 [S-35]GTP gamma S 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 [S-35]GTP gamma S binding. Atipamezole, which exhibited negligeable affinity for 5-HT1A receptors, was inactive. Third , the affinities for r alpha(2A) differed con considerably from the affinities for h alpha(2A) 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 ha,, versus h5-HT1A but 51-fold selective for r alpha(2A), versus r5-HT1A receptors. Conversely, yohimbine was tenfold selective for ha,, versus h5-HT1A adrenoceptors but 4.2-fold selective for ra,A ver sus r5-HT1A receptors. Nevertheless, both atipamezole and DMT were highly selective for both rat and human alpha(2A) versus rat or human 5-HT1A, receptors. In conclusion, these data indicate that: (1) the a,agonist DMT and the antagonist atipamezole are the ligands of choice to distinguish alpha-mediated from 5-HT1A-mediated actions, whilst several of the other compounds show only low or modest selectivity for alpha(2A) over 5-HT1A receptors; (2) caution should be exercised in experimental and clinical interpretation of the actions of traditionally employed alpha(2) ligands, such as clonidine, yohimbine and (+/-)-idazoxan, which exhibit marked agonist activity at 5-HT1A receptors.