The exposure of an opioid receptor to opioid peptides or opiate alkaloids initiates a biological response. Continuous or repeated exposure to opioids causes decreased sensitivity to the drug leading to reduced cellular response; this desensitization response is regulated by multiple mechanisms. Acute opioid treatment results in rapid desensitization by functional uncoupling of the receptor from the effector system. In addition, this treatment results in rapid internalization of the receptor into intracellular compartments. Chronic opioid treatment results in longer desensitization due to receptor down-regulation with a net loss of binding sites from the cell. Relatively little is known about the molecular mechanisms underlying these events. The long term goals of the proposed studies are to delineate the molecular mechanisms that regulate opioid receptor desensitization and opioid tolerance. The objective of the studies proposed in this grant application is to explore the agonist-mediated events that lead to receptor internalization and degradation. We have applied modern molecular biological, cell biological and biochemical methods to characterize domains of the mouse delta opioid receptor involved in these events. In the studies carried out during the previous funding period, we have shown that the first step in opioid receptor down-regulation process is the rapid internalization upon agonist treatment. We have also shown that delta opioid receptors exist as dimers and that agonist treatment causes rapid monomerization. Furthermore, receptor monomerization precedes internalization and the C-terminal tail of the receptor is important for receptor dimerization and internalization. In this application studies are proposed to explore the molecular mechanisms of opioid receptor internalization and dimerization in order to understand the functional significance of these events.
The specific aims are: (i) to characterize delta opioid receptor trafficking in neuronal cells, (ii) to explore the role of dimerization in opioid receptor function, and (iii) to explore differences among the opioid receptor types in receptor trafficking and dimerization. The studies described in this grant application will provide critical information on the early agonist-mediated events that modulate opioid receptor function. Elucidation of the cellular pathways involved in modulation of receptor desensitization is a compelling strategy for identifying appropriate pharamacological interventions for drug addiction.
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