Partial agonism at the human α2A-autoreceptor: role of binding duration

Abstract

Temperature-induced changes of affinity and efficacy of the alpha2-adrenoceptor full agonist UK14,304 and the partial agonists clonidine and guanfacine were investigated to elucidate the mechanism of partial agonism at the terminal alpha2-autoreceptor. The effect of temperature on the efficacy of the substances was tested in 3H-noradrenaline release experiments at 37 degrees C and at room temperature. Human neocortical slices were prelabeled with 3H-noradrenaline, superfused, and stimulated electrically under autoinhibition-free conditions. Furthermore, saturation binding experiments with human neocortical synaptosomes were performed at 37 degrees C and 17 degrees C to evaluate the influence of temperature on the affinity of 3H-clonidine and 3H-UK14,304. Temperature-induced changes of the association and dissociation rate constants of 3H-UK14,304 and 3H-clonidine were assessed in corresponding kinetic binding experiments. Our experiments reveal that clonidine and guanfacine lose efficacy when the temperature is lowered, whereas no change was noted for the full agonist UK14,304. Moreover, the affinity of clonidine and guanfacine was shown to decrease at lower temperature. Kinetic experiments indicated that the loss of affinity observed for 3H-clonidine at 17 degrees C is due to a marked reduction of the association rate. The loss of efficacy of the partial agonists is most likely related to the short binding duration; partial agonists do not bind long enough to the receptor to mediate a maximum response. The discrepancy between the time required to elicit a maximum response and the actual binding time may be greater for partial agonists at lower temperatures, thus, causing the intrinsic activity to decline.

BAY60-6583 Acts as a Partial Agonist at Adenosine A2B Receptors

“…Another theory implies that partial agonists may not bind long enough at the receptor to induce a maximum response (Hoeren et al, 2008). Our data indicate that BAY60-6583 may stabilize a receptor conformation that does not fully activate the G protein.…”

A Comparison of the Anorectic Effect and Safety of the Alpha2-Adrenoceptor Ligands Guanfacine and Yohimbine in Rats with Diet-Induced Obesity

“…Guanfacine is a long-acting [ 13 ], partial α 2 -adrenoceptor agonist [ 11 ]. It activates α 2A -adrenoceptors located mainly presynaptically [ 14 ].…”

“…Guanfacine is a selective agonist of α 2 -adrenoceptors [ 10 , 11 ], which in many experimental models acts differently than other agonists, e.g. clonidine [ 12 , 13 ].…”

Drug‐Target Residence Time—A Case for G Protein‐Coupled Receptors

“…and Hoeren et al. at the adenosine A 2A receptor and at the α 2A ‐adrenoceptor . Importantly, such a finding again emphasizes that equilibrium models and assays alone are not sufficient to describe a dynamic signaling system; instead, a combination of kinetic models alongside the traditional affinity determination might be preferred to better capture the nature of efficacy and to develop efficacious drugs for GPCRs …”