Opioids are potent analgesics, but their application is limited by the development of
tolerance (the increase in doses required to achieve the same effect) after chronic or
repetitive usage. Because tolerance developed much more easily for the analgesia effect
than for side effects (e.g. respiration depression), it is difficult to be overcome by simply
increasing the doses of opioids. On the other hand, the identification of opioid receptors
and endogenous agonists suggests the involvement of opioid pathways in the central
nervous system. Thus exploring the mechanisms of tolerance development has been the
focus of a vast number of laboratories for decades.
Several hypotheses on tolerance development have been proposed. For example,
because of the correlation between receptor internalization and tolerance development,
receptor internalization has been considered as an inhibitor of tolerance. In addition, the
involvement of δ-opioid receptor preproenkephalin, Ca2+/calmodulin-dependent protein
kinase II, Protein Kinase C and β-arrestin2 has been suggested by knockout experiments. However, there is no universal explanation for tolerance development.
The long-term goal of my studies is to understand the mechanism of tolerance
development. Taking advantage of the observations that opioids have different abilities to
induced tolerance and signaling events, I have proposed is that agonists induce different
levels of tolerance by inducing different signaling events (agonist-selective signaling).
Because of the inconsistency between the time courses of signaling cascades (usually
seconds to hours) and the tolerance development (usually hours to days), It is suggested that the agonist-selective regulation on gene expression transduces the signals from the
agonist-selective signaling to agonist-selective tolerance development.
Hence, in my studies, the different abilities of agonists to initiate signaling events
(ERK phosphorylation, receptor desensitization on intracellular calcium release) were
compared. Then several determinants (receptor phosphorylation, cholesterol-rich lipid
raft microdomain, receptor palmitoylation) for the agonist-selective signaling were
identified. The final portion of my studies is to explore how agonist-selective regulation
on gene expression (NeuroD and miR-190) results from the agonist-selective signaling
and to determine whether the agonist-selective regulation on gene expression can
contribute to the different levels of tolerance induced by agonists.