FUNCTIONAL-ANALYSIS OF CLONED OPIOID RECEPTORS IN TRANSFECTED CELL-LINES

Opioids modulate numerous central and peripheral processes including p ain perception, neuroendocrine secretion and the immune response. The opioid signal is transduced from receptors through G proteins to vario us different effecters. Heterogeneity exists at all levers of the tran sduction process. There are numerous endogenous ligands with differing selectivities for at least three distinct opioid receptors (mu, delta , kappa) G proteins activated by opioid receptors are generally of the pertussis toxin-sensitive Gi/Go class, but there are also opioid acti ons that are thought to involve Gq and cholera toxin-sensitive G prote ins. To further complicate the issue, the actions of opioid receptors may be mediated by G-protein alpha subunits and/or beta gamma subunits . Subsequent to G protein activation several effecters are known to or chestrate the opioid signal. For example activation of opioid receptor s increases phosphatidyl inositol turnover, activates K+ channels and reduces adenylyl cyclase and Ca2+ channel activities. Each of these ef fecters shows considerable heterogeneity. In this review we examine th e opioid signal transduction mechanism. Several important questions ar ise: Why do opioid ligands with similar binding affinities have differ ent potencies in functional assays? To which Ca2+ channel subtypes do opioid receptors couple? Do opioid receptors couple to Ca2+ channels t hrough direct G protein interactions? Does the opioid-induced inhibiti on of vesicular release occur through modulation of multiple effecters ? We are attempting to answer these questions by expressing cloned opi oid receptors in GH(3) cells. Using this well characterized system we can study the entire opioid signal transduction process from ligand-re ceptor interaction to G protein-effector coupling and subsequent inhib ition of vesicular release.