Kinetics of ?-funaltrexamine binding to wild-type and mutant ?-opioid receptors expressed in Chinese hamster ovary cells

Receptor reserve reflects differential intrinsic efficacy associated with opioid diastereomers

Structure-activity relationships built around receptor binding or cell-based assays are designed to reveal physiochemical differences between ligands. We hypothesized that agonist receptor reserve may provide a unique approach to distinguish structurally-related agonists exhibiting similar functional characteristics. An intracellular calcium activation assay in Chinese Hamster Ovary (CHO) cells expressing cloned human mu-opioid receptors was developed. We examined two isomers exhibiting indistinguishable receptor binding and in vitro potency profiles. Oxymorphone, a clinically-available congener of codeine has at least two active diastereomeric metabolites (6alpha- and 6beta-oxymorphols) found to be similar for mu-opioid receptor binding affinity (K(d) = 15 versus 14 nM) and calcium activation (EC(50) = 22 versus 14 nM). Calcium activation was then inhibited in CHO cells in a concentration-dependent manner using the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Under these conditions, approximately 10-fold greater receptor reserve was found for 6alpha-oxymorphol compared to 6beta-oxymorphol. This difference between the oxymorphols corresponded to a rank order of intrinsic efficacy (Emax): DAMGO > oxymorphone = 6alpha-oxymorphol = oxycodone > 6beta-oxymorphol. In addition, 6alpha-oxymorphol exhibited greater relative potency than the 6beta-oxymorphol in mouse tail-flick, hot-plate and phenylquinone writhing antinociceptive assays, regardless of route of administration. Thus the beta-FNA/calcium model provides a novel, cell-based approach to distinguish structurally related mu-opioid agonists, and in the specific case of the oxymorphols, receptor reserve differences provided a means to bridge functional in vitro and in vivo models.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.