NEW INSIGHTS INTO RECEPTOR THEORY, AS PROVIDED BY AN ARTIFICIAL PARTIAL AGONIST MADE-TO-MEASURE

In the present study a mixture of a full agonist (noradrenaline) and a full antagonist (yohimbine) was used to mimic the effects of a partia l agonist (clonidine) on alpha(2)-autoreceptor-mediated regulation of noradrenaline release in order to learn more about the shape of concen tration-response curves in the absence and presence of spare receptors . The sigmoidal shape of the cloud of single experimental data points may be reflected by different curve fits based on either descriptive o r mechanistic mathematical models. Only mechanistic models allow the i nterpretation of the relationship between occupancy of receptors and i nduced response. The experiments were performed in rat neocortex and i n rabbit hippocampus tissue where electrical field stimulation with 4 pulses/100 Hz of slices prelabelled with [H-3]noradrenaline elicited t he release of noradrenaline. A receptor reserve was found in the rabbi t hippocampus and quantified from the concentration-response curve of noradrenaline in this tissue using a mechanistic general response func tion, developed to reflect the condition of spare receptors. The mixtu re of noradrenaline and yohimbine, NA-Yoh, (three parts to one part), corrected by the different affinities to the alpha(2)-autoreceptors, w as designed to mirror the quantified proportion of 75% non-spare and 2 5% spare alpha(2)-autoreceptors. In the spare receptor-free rat cortex NA-Yoh acted like a typical partial agonist, as clonidine, with nearl y the same EC(50) K-d in this case) as the full agonist noradrenaline, but with a maximum effect significantly lower than that of noradrenal ine. In the rabbit hippocampus, however, the same maximum effect was o btained with NA-Yoh, noradrenaline and clonidine. In this tissue the E C(50) of the logistic fit of the concentration-response relationship o f noradrenaline was lower than that of NA-Yoh and the correspending sl ope parameter of the noradrenaline curve was significantly higher than unity, indicating the descriptive character of this curve fit which d oes not allow an interpretation of the link between occupation and res ponse. NA-Yoh and clonidine, however, yielded slope parameters of arou nd unity. Therefore, the logistic EC(50)'s of NA-Yoh and of clonidine may be interpreted mechanistically as K-d's which was confirmed by the estimation of their K-d,'s using the general response function. In co ntrast, the general response function yielded a K-d of noradrenaline s ignificantly higher than its logistic EC(50). The observation that the mixture of a pure agonist and a pure antagonist shows the properties of a partial agonist, suggests that the efficacy of the agonist-antago nist continuum is the conditional probability p (O less than or equal to p less than or equal to 1) to activate a receptor given it occupies this receptor, where p is unity for a full agonist and zero for a ful l antagonist. Activation of a receptor, however, leads to a response o nly when this receptor is a non-spare receptor. In a system with a pos sible receptor reserve the use of both an ( (artificial) partial agoni st and a full agonist, which induce different slope parameters in the logistic fits of their concentration-response curves, allows us to qua ntify the receptor reserve of the full agonist by nonlinear regression analysis of the concentration-response curve of the full agonist with the general response function.