ABSTRACT
This chapter focuses on binding selectivity, which is the prerequisite for selective labeling by techniques such as receptor autoradiography. The isolation of receptor proteins by conventional biochemical methods and the identification of the genes encoding these receptor proteins by molecular biological techniques enable the use of additional methods for the localization of receptors in tissue. A method where opioid drug interactions with their receptors are directly measured using radioactive ligands was described by Avram Goldstein. A critical element of this approach was the introduction of a method for distinguishing receptor-bound radioligand from that associated with nonreceptor components of the tissue. Opioid agonists act on the gastrointestinal system to produce contraction of the intestinal smooth muscle, inhibition of gastrointestinal transit, and reduction of fluid secretion into the intestinal lumin. The investigation of opioid receptor-mediated regulation of gastrointestinal activity by the central nervous system (CNS) has been greatly facilated by the availability of peptides highly selective for specific opioid receptor types.
