Research on opioid tolerance continues to be inspired by the conviction that elucidation of the adaptations causally associated with this condition will facilitate the development of pharmacotherapies for its amelioration and thus enhance the medicinal usefulness of narcotics. Opioid tolerance is protective. It has been associated with a wide spectrum of neurochemical changes believed to be adaptive. Adaptations to the persistent presence of morphine that have thus far been identified generally fall into two main categories, those that result in the actual loss of opioid receptor-mediated signaling and those that result in the apparent loss of this activity via its masking. Several noted deficiencies in these formulations prompted our laboratory to hypothesize complimentary strategies of adaptation to chronic morphine. In contrast to prevailing models of tolerance, we proposed that chronic morphine induces the emergence of new, but not lost, signaling sequelae. Specifically, we demonstrated adaptations that facilitated the emergence of opioid receptor- coupled GPY stimulation of adenylyl cyclase (AC) that did not require altered opioid receptor G protein coupling. In the current proposal, we will add a new dimension to our formulation that tolerance results, in part, from the altered consequences of continued opioid receptor GJG0 signaling to include enhanced coupling to Gs. This has heretofore been controversial but is now of compelling interest based on our recent biochemical demonstration of ^-opioid receptor (MOR) Gs association, which is markedly enhanced following chronic morphine. The specific objectives are to (1) Determine the Gp isoforms that participate in tolerance- associated enhanced MOR Gs coupling (2) Determine the ability of chronic morphine, and mechanisms thereof, to decrease phosphorylation of GSa, and thereby enhance its association with MOR, (3) Determine the influence of GSa phosphorylation on its interaction with AC, (4) Determine the influence of chronic morphine on MOR Gs functional coupling and (5) Determine the effect of targeted alteration of signaling equilibria on previously defined neurochemical adaptations to chronic morphine. This will establish that tolerance adaptations are not hard wired but depend on initial steady state conditions. Collectively, proposed studies will help establish a model of tolerance that incorporates a dynamic mosaic of convergent interrelated changes in signaling molecules in an attempt to reestablish functional equilibria.
|Chakrabarti, Sumita; Chang, Andrew; Gintzler, Alan R (2010) Subcellular localization of mu-opioid receptor G(s) signaling. J Pharmacol Exp Ther 333:193-200|
|Gintzler, Alan R; Chakrabarti, Sumita (2004) Chronic morphine-induced plasticity among signalling molecules. Novartis Found Symp 261:167-76; discussion 176-80, 191-|