Endogenous opioid peptides play a major role in brain function, and opiate drugs are both clinically important and socially abused. Based on pharmacological and ligand binding studies, receptors for these compounds have been divided into five major classes: mu, delta, kappa, sigma, and epsilon. However, physico-chemical evidence for distinct opiate binding proteins is lacking and thus little is known of the physical and chemical characteristics which differentiate these receptor types. Derivatives of opiate alkaloids and opioid peptides will be synthesized which will irreversibly bind to mu, delta, and kappa receptor types. The derivatives will be analyzed for binding affinity, receptor selectivity, and irreversibility by competition studies with labeled reversible mu, delta, and kappa receptor probes. The tritiated or radioiodinated form of suitable ligands will then be synthesized. Brain membrane homogenates and cryostat-cut, slide-mounted thin sections of brain will be treated with these compounds. Labeled membranes will be solubilized with detergents and the extracts subjected to electrophoretic analysis. Proteins labeled by mu, delta, and kappa receptor probes will be characterized with regards to their molecular weights, isoelectric points, and peptide maps to determine the degree of similarity. Finally, a structural model for the receptor, Type 1 receptor model, will be tested. This model involves interacting mu and delta binding sites either in a protein complex or binding domains on a single polypeptide chain. This will be accomplished by ascertaining the proximity of binding sites to each other in the membrane by analyzing the effect of protein crosslinking reagents on the electrophoretic-mobilities of the labeled receptor proteins. It is anticipated that this approach will lead to chemical identification of the receptor types and will define the molecular basis of opiate receptor heterogeneity. In addition, the irreversibly binding opiate ligands produced in this project may prove useful in pharmacological and behavioral studies of opiate action.
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