Problems and Approaches in Studying Membrane Opioid Receptors

Considerable progress has been made in recent years in understanding opioid receptors at the molecular level. Several laboratories have reported purification of opioid receptors by standard procedures involving solubilization of brain membranes in nondenaturing detergent, followed by affinity chromatography and other fractionation methods. In addition, several alternative approaches have been used with some degree of success. Several investigators have labeled receptors with covalent ligands, then characterized them under denaturing conditions such as SDS gel electrophoresis. Antibodies have been prepared to several pure or partially pure opioid receptor preparations that can be used to (1) map the brain regional distribution of opioid receptors, (2) determine the role of specific regions of the receptor molecules in ligand binding and in interaction with other functional molecules, (3) compare the structural features of different opioid receptor types, and (4) purify receptors further. One group also has reported cloning of the cDNA for an opioid-binding protein using antibodies as affinity ligands to purify receptors from solubilized material and the application of gene cloning techniques. The successful use of these varied approaches ensures that future progress in this field will be rapid. The purification, reconstitution, and detailed structural characterization of different opioid receptor types appears feasible within a few years. The next challenge in the molecular understanding of these receptors will be to identify and characterize the second messenger systems in the cell membranes that mediate their pharmacological functions.