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Abstract
Delta and mu opioid receptors (DORs and MORs) are inhibitory G protein-coupled receptors
that reportedly cooperatively regulate the transmission of pain messages by substance
P and TRPV1-expressing pain fibers. Using a DOReGFP reporter mouse we now show that
the DOR and MOR are, in fact, expressed by different subsets of primary afferents.
The MOR is expressed in peptidergic pain fibers, the DOR in myelinated and nonpeptidergic
afferents. Contrary to the prevailing view, we demonstrate that the DOR is trafficked
to the cell surface under resting conditions, independently of substance P, and internalized
following activation by DOR agonists. Finally, we show that the segregated DOR and
MOR distribution is paralleled by a remarkably selective functional contribution of
the two receptors to the control of mechanical and heat pain, respectively. These
results demonstrate that behaviorally relevant pain modalities can be selectively
regulated through the targeting of distinct subsets of primary afferent pain fibers.