Ž . European Journal of Pharmacology 371 1999 235–240 www.elsevier.nlrlocaterejphar Differential desensitization of human d-opioid receptors by peptide and alkaloid agonists Stephane Allouche ) , Mikael Roussel, Nicolas Marie, Philippe Jauzac ´ ¨ Laboratory of Biochemistry A, UniÕersity of Caen, C.H.U. Cote de Nacre, 14033 Caen Cedex, France ˆ Received 3 December 1998; received in revised form 9 March 1999; accepted 12 March 1999 Abstract The efficacy of different opioid agonists to induce acute desensitization of the human d-opioid receptor-mediated inhibition of cAMP accumulation was investigated in the neuroblastoma cell line SK-N-BE, which endogenously expresses these receptors. While etorphine, a non-selective alkaloid agonist, caused 50% desensitization after a 30-min incubation, the same treatment in the presence of the selective Žw 2 5 x . Ž . peptide agonists, DPDPE D-Pen ,D-Pen enkephalin and deltorphin I Tyr-D-Ala-Phe-Asp-Val-Val-Gly , almost totally desensitized the d-opioid receptor-mediated inhibition of adenylyl cyclase. When SK-N-BE cells were prechallenged either with alkaloid or peptide agonist, we observed a cross-desensitization that was less marked when cells were pretreated with peptide agonists and then challenged with etorphine. Taken together, these results demonstrate that human d-opioid receptors are differentially desensitized by alkaloid and peptide agonists. q 1999 Elsevier Science B.V. All rights reserved. Keywords: d-Opioid receptor; Alkaloid agonist; Peptide agonist; Desensitization; Adenylyl cyclase inhibition 1. Introduction Opioid agonists, such as morphine or its derivatives, are Ž currently used in the management of chronic pain Herz, . 1993 . Pharmacological and molecular data led to the identification of three major classes of opioid receptors, Ž . namely m, d and k for review, see Reisine, 1995 . While the m-opioid receptor represents the primary target of Ž morphine, which produces antinociception Matthes et al., . 1996 , d-opioid receptors have also been demonstrated to Ž mediate analgesia Sanchez-Blazquez et al., 1997; Narita . et al., 1997a . These receptors have been reported to be Ž critical in the development of tolerance to morphine Suzuki . et al., 1997 , which could be initiated by the desensitiza- Ž . tion of opioid receptors Loh et al., 1988 . Phosphorylation of opioid receptors by second messen- Ž . ger-dependent kinases Narita et al., 1997b or by G Ž .Ž protein-coupled receptor kinases GRK Pei et al., 1995; . Hasbi et al., 1998 was demonstrated to be the major step w 2,5 x w AbbreÕiations: DPDPE, D-Pen enkephalin and Deltorphin I, Tyr- x D-Ala-Phe-Asp-Val-Val-Gly ) Corresponding author: Tel.: q33-231-064-870; Fax: q33-231-064- 985 triggering desensitization by uncoupling the receptors from their associated G proteins. Recent reports have pointed out that opioid agonists have distinct potencies to desensi- Ž . Ž tize and internalize m- Blake et al., 1997a , d- Bot et al., . Ž . 1997 as well as k-opioid receptors Blake et al., 1997b when transfected in host cells. In the current study, we investigated the regulation of adenylyl cyclase by human d-opioid receptors, endoge- nously expressed in the neuroblastoma cell line SK-N-BE Ž . Polastron et al., 1994 , in the presence of a non-selective alkaloid agonist, etorphine, and in the presence of selective peptide agonists, DPDPE and deltorphin I. Our major findings indicate that the d-opioid receptors are more rapidly desensitized by peptide agonists than by etorphine. 2. Material and methods 2.1. Cell culture SK-N-BE cells were cultured in Dulbecco’s modified Ž . Eagle’s medium DMEM supplemented with 10% foetal calf serum, 2 mM L-glutamine and 1% antibiotic–anti- mycotic solution. 0014-2999r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0014-2999 99 00180-6