The long-term objective of this proposal is to identify regulators of G protein signaling (RGS) proteins as therapeutic targets for medications to treat the misuse of mu-opioids and other drugs of abuse. RGS proteins are GAPS (G protein accelerating proteins) that negatively regulate signaling through G protein-coupled receptors, including receptors for mu-opioids and dopamine. For example, the rewarding effects of morphine and cocaine are increased in mice lacking the gene for one particular RGS protein (RGS-9). RGS proteins also serve to control the severity of the withdrawal response seen on removal of chronic morphine. On the other hand they may promote tolerance. These findings suggest the ability to stimulate RGS activity would decrease the rewarding effects of opioids and stimulants and decrease the severity of withdrawal symptoms. Conversely, an inhibitor of RGS protein action might be a useful adjunct to morphine analgesia, without promoting tolerance. The potential of RGS proteins as drug targets is exciting but does depend upon a better understanding of the specificity of RGS action since there are >30 proteins with RGS action. Our previous studies suggest that RGS proteins by acting as modulators also control which signaling pathways are activated: an effect that could lead to selectivity of pharmacological action. The present proposal has 3 specific aims (1) to study how RGS protein GAP activity can selectively regulate signaling, (2) to identify the RGS proteins that modulate signaling through the mu-opioid receptor and (3) to examine the consequences of this modulation on opioid efficacy and potency. The work will study mu-opioid receptor-G protein coupling and signaling to adenylyl cyclase, increases in intracellular calcium, inwardly rectifying K+ channels and the MAP kinase pathway. Roles for RGS proteins will be determined using Ga proteins that are insensitive to the GAP activity of all RGS proteins and techniques including RNAi to identify which specific RGS proteins are responsible for the modulation and control of mu-opioid signaling. An understanding of roles for RGS proteins in the actions of mu-opioids may lead to improvements in the treatment of pain as well as treatment of opioid and stimulant abuse.
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