Multidimensional analysis of ligand binding data: application to opioid receptors.

The existence of distinct mu and delta opioid receptors is now well accepted. Most investigators favor the hypothesis that these receptors are physically distinct and that the enkephalins are only 2-10 fold selective for the delta receptor. Rothman and Westfall (Mol. Pharmacol. 21:548-557) recently challenged this hypothesis, proposing that at least some population of mu and delta receptors coexist in an opioid receptor complex and that the enkephalins are at least 100 fold selective for the delta receptor. In this paper we describe a generally applicable method we have used to design and analyze ligand binding experiments which distinguish between the two different models.