in vivofluorescent deltorphin (Fluo-DLT) internalization assay was used to assess the distribution and regulation of pharmacologically available δ opioid receptors (δORs) in the rat lumbar (L4-5) spinal cord. Under basal conditions, intrathecal injection of Fluo-DLT resulted in the labeling of numerous δOR-internalizing neurons throughout dorsal and ventral horns. The distribution and number of Fluo-DLT-labeled perikaryal profiles were consistent with that of δOR-expressing neurons, as revealed by in situhybridization and immunohistochemistry, suggesting that a large proportion of these cells was responsive to intrathecally administered δOR agonists. Pretreatment of rats with morphine for 48 hr resulted in a selective increase in Fluo-DLT-labeled perikaryal profiles within the dorsal horn. These changes were not accompanied by corresponding augmentations in either δOR mRNA or125I-deltorphin-II binding levels, suggesting that they were attributable to higher densities of cell surface δOR available for internalization rather than to enhanced production of the receptor. Unilateral dorsal rhizotomy also resulted in increased Fluo-DLT internalization in the ipsilateral dorsal horn when compared with the side contralateral to the deafferentation or to non-deafferented controls, suggesting that δOR trafficking in dorsal horn neurons may be regulated by afferent inputs. Furthermore, morphine treatment no longer increased Fluo-DLT internalization on either side of the spinal cord after unilateral dorsal rhizotomy, indicating that μOR-induced changes in the cell surface availability of δOR depend on the integrity of primary afferent inputs. Together, these results suggest that regulation of δOR responsiveness through μOR activation in this region is linked to somatosensory information processing.