Regulation of an opioid-binding protein in NG108-15 cells parallels regulation of δ-opioid receptors

An opioid-binding protein has recently been purified from bovine brain and cloned, and its cDNA sequence has been obtained. Indirect evidence suggests that this protein has a role in opioid-receptor function. However, because direct testing of its function by expression of its cDNA has not yet been possible and because its structure bears no resemblance to G protein-coupled receptors, the role of this protein in opioid-receptor activity is still in question. An antibody raised to a portion of the predicted amino acid sequence of opioid-binding cell-adhesion molecule (OBCAM) specifically labeled the surface of NG108-15 cells, as visualized by immunofluorescence with confocal microscopy. Furthermore, chronic treatment of these cells with opioid agonist, which down-regulates opioid receptors, reduced OBCAM immunoreactivity (ir). Down-regulation of both opioid receptors and OBCAM-ir was greatest after chronic treatment of NG108-15 cells with δ-opioid agonists, as well as with nonselective agonists such as etorphine, whereas other agonists including [D-Ala²-N-MePhe⁴-Glyol]enkephalin, morphine, levorphanol, dynorphin A-(1-13), and U-50,488H were less effective or ineffective. Chronic treatment of NG108-15 cells with muscarinic agonists had no effect on OBCAM-ir. Furthermore, NG108-15 cells transfected with an antisense construct to OBCAM have a reduced density of opioid-binding sites as well as reduced OBCAM-ir. Taken together, these results strongly suggest that OBCAM has a role in opioid-receptor function in NG108-15 cells.