Currently, there are no known drugs which can mimic or prevent dopamine's actions at just one of the five currently known dopamine [DA] receptor subtypes. The use of clonal cell lines provides the only way in which drug effects may be studied on one dopamine receptor subtype at a time. Overall, we will establish which characteristics of drug molecules are responsible for preferential affinity and high efficacy at D1A vs. D2A DA receptors. We have two specific aims: to determine the affinity and efficacy of a set of 6 - 8 structurally diverse DAergic drugs in a cell line [Ltk - D1A] which selectively expresses human D1A receptors coupled to stimulation of cAMP accumulation; and to determine the affinity and efficacy of a set of 6 - 8 structurally diverse DAergic drugs in a cell line [C6 - D2A] which selectively expresses rat D2A receptors coupled to inhibition of cAMP accumulation stimulated by forskolin. To aid in selection of appropriate agents to test and to integrate our data from the cAMP studies, we will use a new method for computer- assisted drug design called CoMFA (comparative molecular field analysis). CoMFA offers significant advantages over other approaches to computer-assisted drug design. CoMFA begins with a conformational analysis of a set of compounds for which receptor affinity has been experimentally determined. Thus, one of the significant advantages of our CoMFA-based approach is the prediction of the most probable conformation of the receptor-bound agent. Based upon the predicted conformations of receptor-bound agents, the second phase of a CoMFA study involves construction of the electrostatic and steric interaction fields which complement the predicted drug conformation. These interaction fields reflect the essential chemical features and geometric contours of the receptor site. Our results will build a powerful picture of the essential electrical field properties of drugs which determine their affinity and efficacy at D1A vs. D2A receptors. This information will allow the synthesis of novel DAergic agents which should have desired properties of affinity and efficacy at D1A and D2A receptors. We will then be able to use these novel agents to test directly our hypotheses concerning drug electrical field properties which account for specific DAergic functions at each receptor. Our work should lead to improved design of DA agonist drugs which can bind to D1A and D2A DA receptors with the same relative affinities as DA itself. Our results should also lead to an improved knowledge of both the chemical and geometric (i.e., shape) requirements for effective binding to D1A and D2A receptors. Finally, our results should establish a general method [combining cell biological and computer-based techniques] for the molecular study of affinity and efficacy at the other known and to be discovered DA receptors.
