Dimerization of the δ Opioid Receptor:

Dimerization of G-protein-coupled receptors has been increasingly noted in the regulation of their biological activity. However, its involvement in agonist-induced receptor internalization is not well understood. In this study, we examined the ability of mouse δ-opioid receptors to dimerize and the role of receptor dimerization in agonist-induced internalization. Using differentially (Flag and c-Myc) epitope-tagged receptors we show that δ-opioid receptors exist as dimers. The level of dimerization is agonist dependent. Increasing concentrations of agonists reduce the levels of dimer with a corresponding increase in the levels of monomer. Interestingly, morphine does not affect the levels of either form. It has been shown that morphine, unlike other opioid agonists, does not induce receptor internalization. This suggests a relationship between the ability of agonists to reduce the levels of dimer and to induce receptor internalization. The time course of the agonist-induced decrease of δ-opioid receptor dimers is shorter than the time course of internalization, suggesting that monomerization precedes the agonist-induced internalization of the receptor. Furthermore, we found that a mutant δ-opioid receptor, with a 15-residue C-terminal deletion, does not exhibit dimerization. This mutant receptor has been shown to lack the ability to undergo agonist-induced internalization. These results suggest that the interconversion between the dimeric and monomeric forms plays a role in opioid receptor internalization. Dimerization of G-protein-coupled receptors has been increasingly noted in the regulation of their biological activity. However, its involvement in agonist-induced receptor internalization is not well understood. In this study, we examined the ability of mouse δ-opioid receptors to dimerize and the role of receptor dimerization in agonist-induced internalization. Using differentially (Flag and c-Myc) epitope-tagged receptors we show that δ-opioid receptors exist as dimers. The level of dimerization is agonist dependent. Increasing concentrations of agonists reduce the levels of dimer with a corresponding increase in the levels of monomer. Interestingly, morphine does not affect the levels of either form. It has been shown that morphine, unlike other opioid agonists, does not induce receptor internalization. This suggests a relationship between the ability of agonists to reduce the levels of dimer and to induce receptor internalization. The time course of the agonist-induced decrease of δ-opioid receptor dimers is shorter than the time course of internalization, suggesting that monomerization precedes the agonist-induced internalization of the receptor. Furthermore, we found that a mutant δ-opioid receptor, with a 15-residue C-terminal deletion, does not exhibit dimerization. This mutant receptor has been shown to lack the ability to undergo agonist-induced internalization. These results suggest that the interconversion between the dimeric and monomeric forms plays a role in opioid receptor internalization. The opioid receptor family, a member of the superfamily of G-protein-coupled receptors (GPCRs), 1The abbreviations used are: GPCR, G-protein-coupled receptor; DOR, δ-opioid receptor; MDOR, DOR tagged with c-Myc; FDOR, DOR tagged with the Flag epitope; CHO, Chinese hamster ovary; DSP, dithiobis-(succinimidylpropionate); PAGE, polyacrylamide gel electrophoresis; DADLE, d-Ala2,d-Leu5enkephalin; DSLET, d-Ser2Leu-enkephalin-Thr; DPDPE, d-Pen2,Pen5enkephalin; DAMGO, d-Ala2, N-Me-Phe4,Gly5-olenkephalin. consists of three receptor types: δ, ॖ, and κ. The opioid receptors transmit the signals induced by binding of opioid peptides and opiate alkaloids, such as morphine. Continuous or repeated exposure to opioid ligands causes decreased sensitivity to the drug and reduced cellular response; this response is regulated by multiple mechanisms. Long-term exposure to opioid ligands causes receptor down-regulation as a result of the receptor degradation (1Law P.-Y. Hom D.S. Loh H.H. Mol. Pharmacol. 1982; 22: 1-4Crossref PubMed Scopus (1) Google Scholar, 2Law P.-Y. Hom D.S. Loh H.H. Mol. Pharmacol. 1983; 24: 413-424PubMed Google Scholar, 3Law P.-Y. Hom D.S. Loh H.H. J. Biol. Chem. 1984; 259: 4096-4104Abstract Full Text PDF PubMed Google Scholar, 4Cvejic S. Trapaidze N. Cyr C. Devi L.A. J. Biol. Chem. 1996; 271: 4073-4076Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). Short-term treatment with opioid ligands causes rapid loss of receptors from the surface of the cell as a result of the receptor endocytosis (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar, 6Sternini C. Spann M. Anton B. Keith D.E. Bunnett N.G. von Zastrow M. Evans C.J. Brecha N.C. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 9241-9246Crossref PubMed Scopus (289) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). Both of these effects require the intact C-terminal tail of the receptor (4Cvejic S. Trapaidze N. Cyr C. Devi L.A. J. Biol. Chem. 1996; 271: 4073-4076Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). Although deletion of the C-terminal tail substantially reduces the extent of both down-regulation and rapid internalization, point mutations within this region reduce the extent of internalization without affecting down-regulation, suggesting that these two responses are differentially regulated (7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). Different opioid ligands induce different effects on rapid internalization of the opioid receptors. Morphine, unlike most of the opioid agonists, does not induce rapid internalization of the opioid receptors (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar, 8Arden J.R. Segredo V. Wang Z. Lameh J. Sadee W. J. Neurochem. 1995; 65: 1636-1645Crossref PubMed Scopus (285) Google Scholar). An exact mechanism of the opioid receptor internalization is not known, although it has been suggested that the rapid endocytosis of the receptors is mediated through the classic endocytic pathway (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). Possible events that would precede the receptor internalization, such as the interaction of the receptor with an adapter protein or another receptor, have not yet been determined. A number of pharmacological studies suggest the existence of opioid receptor dimers. Dimeric analogs of morphine and enkephalin exhibit higher affinity for δ- and ॖ-opioid receptors in membranes (9Hazum E. Chang K.-J. Leighton H.J. William Lever O. Cuatrecasas P. Biochem. Bioph. Res. Commun. 1982; 104: 347-353Crossref PubMed Scopus (53) Google Scholar). They also have severalfold greater potency than their monomeric forms in the guinea pig ileum assay, suggesting the involvement of opioid receptor dimerization in their function. Apart from homodimerization, several pharmacological studies have suggested the existence of heterodimers between opioid receptor subtypes (10Bowen W.D. Gentleman S. Herkenham M. Pert C.B. Proc. Natl. Acad. Sci. U. S. A. 1981; 78: 4818-4822Crossref PubMed Scopus (150) Google Scholar, 11Rothman R.B. Danks J.A. Jacobson A.E. Burke Jr., T.R. Rice K.C. Tortella F.C. Holaday J.W. Eur. J. Pharmacol. 1986; 124: 113-119Crossref PubMed Scopus (82) Google Scholar, 12Rothman R.B. Bykov V. Long J.B. Brady L.S. Jacobson A.E. Rice K.C. Holaday J.W. Eur. J. Pharmacol. 1989; 160: 71-82Crossref PubMed Scopus (79) Google Scholar). Since ॖ-receptor ligands inhibit the binding of δ-ligands in both a competitive and noncompetitive manner, Rothman et al. (11Rothman R.B. Danks J.A. Jacobson A.E. Burke Jr., T.R. Rice K.C. Tortella F.C. Holaday J.W. Eur. J. Pharmacol. 1986; 124: 113-119Crossref PubMed Scopus (82) Google Scholar,12Rothman R.B. Bykov V. Long J.B. Brady L.S. Jacobson A.E. Rice K.C. Holaday J.W. Eur. J. Pharmacol. 1989; 160: 71-82Crossref PubMed Scopus (79) Google Scholar) have divided δ-receptors into two subtypes, those that are associated with ॖ-receptors and those that are not, further supporting the notion of opioid receptor dimerization. The dimerization of growth factor receptors has been an extensively studied phenomenon (for review, see Ref. 13Heldin C.-H. Cell. 1995; 80: 213-223Abstract Full Text PDF PubMed Scopus (1440) Google Scholar). Exposure to ligand induces the dimerization of receptors, leading to their autophosphorylation, a step that is necessary for the following intracellular signaling. The dimerization of GPCRs and the role of dimerization in the function of these receptors are not well understood. Most of the evidence suggesting the existence of GPCRs as dimers is from pharmacological studies (14Kobilka B.K. Kobilka T.S. Daniel K. Regan J.W. Caron M.G. Lefkowitz R.J. Science. 1988; 240: 1310-1316Crossref PubMed Scopus (609) Google Scholar, 15Monnot C. Bihoreau C. Conchon S. Curnow K.M. Corvol P. Clauser E. J. Biol. Chem. 1996; 271: 1507-1513Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar, 16Maggio R. Vogel Z. Wess J. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: PubMed Scopus Google Scholar). with several GPCRs show that the of two mutant receptors, not or results in receptors that and signals (14Kobilka B.K. Kobilka T.S. Daniel K. Regan J.W. Caron M.G. Lefkowitz R.J. Science. 1988; 240: 1310-1316Crossref PubMed Scopus (609) Google Scholar, 15Monnot C. Bihoreau C. Conchon S. Curnow K.M. Corvol P. Clauser E. J. Biol. Chem. 1996; 271: 1507-1513Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar, 16Maggio R. Vogel Z. Wess J. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: PubMed Scopus Google Scholar). These that a is by interaction between receptor and et al. S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar) have shown that the receptor and that a from the of the receptor S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). The of dimerization is by the that this also of in membranes S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). However, the role of dimerization in receptor internalization not examined in these is evidence for the existence of opioid receptor dimers or In this study, we used and of the tagged mouse δ-opioid receptor with either Flag or to these receptors to dimers. also examined opioid peptides and opiate agonists and the levels of receptor Furthermore, we the time course of the agonist-induced in the levels of dimer and it to the time course of the agonist-induced receptor internalization. we used a mutant of the δ-opioid receptor to the role of the C-terminal tail in dimer found that δ-opioid receptors exist as dimers. The level of dimerization is agonist and an intact C-terminal tail of the δ-opioid receptor is for receptor dimerization. The Flag epitope-tagged DOR as (4Cvejic S. Trapaidze N. Cyr C. Devi L.A. J. Biol. Chem. 1996; 271: 4073-4076Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). epitope-tagged δ-opioid receptor by an Flag epitope-tagged DOR as the The for the Flag with the the The deletion mutant the to a region from to of Flag epitope-tagged δ-opioid receptor that into the used to a corresponding region of the receptor with the Flag epitope-tagged or deletion mutant receptors and their binding to internalization, and down-regulation as (4Cvejic S. Trapaidze N. Cyr C. Devi L.A. J. Biol. Chem. 1996; 271: 4073-4076Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). the with and or with of DOR the R. J.A. K. in Scholar). and as either or mutant δ-opioid receptor either with different of ligand for or or with the of the ligand for different time in The following ligands and the of the the to and without or with in in the with either for for or for The by with for The with and with and for The cell for and the used for further course of agonist-induced in the level of δ-opioid receptor dimers. δ-opioid receptor to or for the with the DSP, and The to and to as The and of are on The from multiple as and the of the dimer to the determined. The from three to the of protein as on The to with in and and for with the in and for and with for with in in for the membranes with for The the used as by the to the This in the of the Flag epitope-tagged DOR in in the δ-opioid receptor. the of DOR in and used for of the from the either of the two tagged a of in with the receptor in the of the receptor in from the of the receptor in to the different extent of in these two cell of the a to a and two or three of membranes and the in of the from or DOR with the by for by with protein for an The by for and the with the The in and The to and with The receptor as the and the δ-opioid receptors we used the Flag epitope-tagged δ-opioid receptor. have shown that the receptors in these with affinity and to (4Cvejic S. Trapaidze N. Cyr C. Devi L.A. J. Biol. Chem. 1996; 271: 4073-4076Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). of cell with the a protein of The of this protein to the for δ-opioid receptor C.J. Keith D.E. K. Science. PubMed Scopus Google Scholar, K. C. Proc. Natl. Acad. U. S. A. PubMed Scopus Google Scholar). Interestingly, we also an of The that this higher protein is to the suggested that it the δ-opioid receptor with another cell in the of higher concentrations of in the protein This suggested that the protein by a interaction of two and that this interaction to the of the of the receptor to a of events the cell we of in intact used DSP, a that through their or the of has been extensively used to the of L. J.R. J. J. Biol. PubMed Scopus Google Scholar). found that treatment of intact with interaction between two that than of the receptors as the A of the dimeric to the monomeric two different used is a the the δ-opioid receptor with either a protein or a reduce the of the receptor to as by the the protein is another δ-opioid receptor, the of the would to reduce to the protein is this protein would not and would not to reduce the of the to found that in the of a and an suggesting that the protein is another receptor and that the δ-opioid receptors exist as dimers. the δ-opioid receptor dimerization by receptor with in The of the dimeric to the monomeric in with a level is to the of these two forms in levels of receptor These results that in δ-opioid receptors exist both in monomeric and dimeric forms and that the of these forms is of the level of receptor the of δ-opioid receptor we used and of differentially epitope-tagged δ-opioid receptors. a δ-opioid receptor tagged with the with the Flag epitope-tagged receptor in or The receptors from the both and to and with in dimeric forms in both and and not in or The dimeric forms also in both and A of and a of in both and The protein the receptor form. The of the protein is not and further It is that this the receptor with a the of this is with such a A of is in three this of the with in the The that and as of a dimer that the δ-opioid receptors exist as a the agonist the levels of the dimeric of δ-opioid receptor, with of for to The of with of decreased the levels of δ-opioid receptor dimer and the levels of This in an agonist-induced decrease in the of the dimeric to the monomeric form. decrease of dimer to by DADLE, induced with of in the of the not show a in the dimer to of with DSLET, DPDPE, or substantially reduced the of the dimeric to the monomeric DAMGO, a ॖ-opioid receptor not the level of either δ-opioid receptor of the with not affect the dimer to these results suggest that the of δ-opioid receptors that exist in dimeric by either a agonist or an agonist and that this is by the Morphine, another opioid to the δ-opioid receptor and induce its to C. J. Pharmacol. 1995; Google Scholar, Evans C. Loh H.H. J. Pharmacol. 271: Google Scholar). other agonists, morphine not affect the levels of δ-opioid receptor dimer Morphine, as as is not to induce rapid internalization of either δ- or ॖ-opioid receptor (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar). This suggests that the ability of an agonist to induce interconversion of dimers to is with its ability to induce receptor internalization. The time course of in the levels of dimeric treatment with a decrease in the levels of this and a corresponding increase in the levels of the monomeric This decrease is with a treatment with DADLE, an decrease in the levels of dimer exposure induced a further decrease in the dimer to for different time with in the levels of the dimeric it has been shown that the δ-opioid receptor in and other rapid agonist-induced internalization (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar, 7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). The of receptor internalization is on the level of its with a or level of the receptor exhibit agonist-induced internalization with a of and with a level of the receptor exhibit internalization with a of This is in to the of for the of the dimer to These results show that treatment with induces a decrease in the levels of dimeric than it induces receptor internalization, suggesting that the monomerization of the receptor precedes its internalization. have that a 15-residue C-terminal deletion of the δ-opioid receptor substantially reduces the extent of agonist-induced internalization of the receptor (7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). further a relationship between the levels of receptor dimerization and agonist-induced internalization, we examined the forms of receptor in this The treatment of with not result in a in the levels of the dimeric Furthermore, treatment with in the or in the of on the level of either of two forms The of the dimeric to the monomeric in the mutant with substantially than the in the and it the mutant with these results suggest that the deletion of the C-terminal δ-opioid receptor dimer The of the receptor in this the that lack of dimerization is to deletion of the in by The that the mutant a loss of agonist-induced in the levels of dimeric or monomeric and a in agonist-induced internalization suggests that these two require of the receptor and that interconversion between the two receptor forms have an role in internalization. Furthermore, this result suggests that the existence of the receptor in the monomeric is not for internalization. In this we found that in the δ-opioid receptors exist as dimers. The existence of GPCRs in dimeric is suggested by pharmacological of that not transmit a in of receptors (14Kobilka B.K. Kobilka T.S. Daniel K. Regan J.W. Caron M.G. Lefkowitz R.J. Science. 1988; 240: 1310-1316Crossref PubMed Scopus (609) Google Scholar). of two different point of the receptors binding C. Bihoreau C. Conchon S. Curnow K.M. Corvol P. Clauser E. J. Biol. Chem. 1996; 271: 1507-1513Abstract Full Text Full Text PDF PubMed Scopus (203) Google Scholar). The existence of dimers is also shown by the of two receptor in the C-terminal receptor between the receptor and the receptor; of either of these receptors not result in binding their in the of a number of binding R. Vogel Z. Wess J. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: PubMed Scopus Google Scholar). The existence of forms of and other GPCRs in has been also shown S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, P. S.R. Biochem. Bioph. Res. Commun. 1996; PubMed Scopus Google Scholar, J. A. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, E. L. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, B. R. J. Biol. Chem. Full Text PDF PubMed Google Scholar). growth factor receptors C.-H. Cell. 1995; 80: 213-223Abstract Full Text PDF PubMed Scopus (1440) Google GPCRs exist in a dimeric in the of agonist treatment shown by this and P. S.R. Biochem. Bioph. Res. Commun. 1996; PubMed Scopus Google Scholar, J. A. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, E. L. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, B. R. J. Biol. Chem. Full Text PDF PubMed Google Scholar). This suggests that although dimerization for the the exact role of dimers different in these two receptor found agonist-induced decrease in the level of δ-opioid receptor dimers. This with both and opioid agonists and it is et al. S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar) have shown that agonist the dimeric of receptor. The between these two to the between these two receptors in the regulation of dimeric these to the different studies in the cell cell and other events that agonist binding are the studies with receptor with membranes S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). does not induce in the levels of the dimeric of the δ-opioid receptor or its internalization used in concentrations that are higher than its and (5Keith D.E. Murray S.R. Zaki P.A. Chu P.C. Lissin D.V. Kang L. Evans C.J. von Zastrow M. J. Biol. Chem. 1996; 271: 19021-19024Abstract Full Text Full Text PDF PubMed Scopus (481) Google Scholar, Evans C. Loh H.H. J. Pharmacol. 271: Google Scholar). This a relationship between ability of an agonist to induce of dimers and to induce receptor internalization. The time course of in the level of dimers with a increase in is shorter than the time course of internalization suggesting that the monomerization precedes internalization Furthermore, that inhibit receptor internalization, such as or (7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google on agonist-induced in the level of Cvejic and L. A. supporting the notion that monomerization precedes internalization. It is that binding of an agonist induces monomerization of the receptor, interaction with adapter in endocytosis or other in signaling. The interconversion between dimeric and monomeric forms is agonist and agonists that not have the ability to induce this interconversion also lack the ability to induce internalization. The of GPCRs has been shown to for their dimerization. of and receptors their dimerization S. C. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, P. S.R. Biochem. Bioph. Res. Commun. 1996; PubMed Scopus Google dimerization of the receptor on the of the intracellular R. P. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus (93) Google Scholar). found that the deletion of the C-terminal of the δ-opioid receptor its dimerization. A for the of this mutant to dimerize is that it forms by with or with other to the induced by the lack of a of the a of the receptor would to ligands and transmit the would not to of its to to in found that this mutant receptor has the ability to to and that it forms on the cell surface in response to agonist treatment (7Trapaidze N. Keith D.E. Cvejic S. Evans C.J. Devi L.A. J. Biol. Chem. 1996; 271: 29279-29285Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar). A between the lack of internalization and the increase in receptor on the surface of the cell has been suggested by et E. Chang K.-J. Cuatrecasas P. Science. PubMed Scopus Google Scholar, E. Chang K.-J. Cuatrecasas P. PubMed Scopus Google Scholar, E. Chang K.-J. Cuatrecasas P. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). In these it shown that treatment of with morphine or that induce induces of of opioid receptors, as by binding of Although interconversion between dimeric and monomeric forms precedes internalization, the existence of the receptor in monomeric not to for its internalization. The C-terminal deletion mutant of δ-opioid receptor that in the monomeric is to undergo rapid internalization, suggesting that are for receptor internalization. These regulated by the signals by the of the receptor that is in the interconversion between the dimeric and monomeric The that opioid receptors exist in dimeric of the function of this receptor. receptors have been into on the of their involvement in the of J. Pharmacol. Google Scholar, Eur. J. Pharmacol. PubMed Scopus Google Scholar). this it has been suggested that δ-opioid receptors a with the not (for review, see Ref. J.R. J. Pharmacol. Sci. 1993; Full Text PDF PubMed Scopus Google Scholar). morphine treatment a of δ-opioid receptors that is a of the (11Rothman R.B. Danks J.A. Jacobson A.E. Burke Jr., T.R. Rice K.C. Tortella F.C. Holaday J.W. Eur. J. Pharmacol. 1986; 124: 113-119Crossref PubMed Scopus (82) Google Scholar, 12Rothman R.B. Bykov V. Long J.B. Brady L.S. Jacobson A.E. Rice K.C. Holaday J.W. Eur. J. Pharmacol. 1989; 160: 71-82Crossref PubMed Scopus (79) Google Scholar). The existence of a receptor is also by ligand binding studies membranes and of the results show that the the opiate receptor the binding of either or agonists for the receptors that are of the R.B. Jacobson A.E. Rice K.C. Herkenham M. PubMed Scopus Google R.B. Long J.B. Bykov V. Jacobson A.E. Rice K.C. Holaday J.W. J. Pharmacol. 1988; Google Scholar). in the affinity and of agonists and for the monomeric and dimeric forms of the receptors and their ability to transmit signals the in the responses The of different opioid receptor subtypes heterodimers to the in of the two different subtypes of the receptors are In dimerization of is as an mechanism for the regulation of these receptors. the dimerization of opioid receptors to the role of this of interaction in the function of opioid receptors and of the of GPCRs in Trapaidze for