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

Temperature-induced changes of affinity and efficacy of the alpha(2)-adrenoceptor full agonist UK14,304 and the partial agonists clonidine and guanfacine were investigated to elucidate the mechanism of partial agonism at the terminal alpha(2)-autoreceptor. The effect of temperature on the efficacy of the substances was tested in H-3-noradrenaline release experiments at 37 degrees C and at room temperature. Human neocortical slices were prelabeled with H-3-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 H-3-clonidine and H-3-UK14,304. Temperature-induced changes of the association and dissociation rate constants of H-3-UK14,304 and H-3-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 H-3-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.