Allosteric modulation of metabotropic glutamate receptor 5 affects phosphorylation, internalization, and desensitization of the μ-opioid receptor

Recent evidence suggests that opioid analgesia and tolerance can be modulated by metabotropic glutamate receptors. Therefore, we studied the functional coupling and desensitization of the mu-opioid receptor (MOR) in human embryonic kidney (HEK) 293 cells which co-express metabotropic glutamate receptor 5 (mGluR5). As demonstrated by the D-Ala(2),N-MePhe(4),Gl-ol(5)-enkephalin (DAMGO)-induced inhibition of intracellular cAMP level and by binding studies, the co-expression of mGIuR5 had no substantial effect on the agonist binding sites and functional coupling of the MOR. However, in MOR/mGIuR5 co-expressing cells, the non-competitive mGluR5 antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) decreases the DAMGO-induced MOR phosphorylation, internalization, and desensitization, whereas non-selective competitive mGluR antagonists or agonists had no effects. These findings indicate that an allosteric modulation of mGIuR5 can affect the agonist-induced MOR signalling and regulation. As a mechanistic basis for the observed effects we suggested an interaction/hetero-dimerization of MOR and mGluR5, which is supported by the DAMGO-induced co-internalization of MOR and mGIuR5 and by the increase of MPEP binding sites (B-max) and a change of the binding affinity (K-D) Of mGIuR5 receptors after the co-expression of MOR. In addition, co-immunoprecipitation experiments revealed evidence for an interaction between MOR and mGIuR5 which is facilitated by MPEP treatment. (C) 2008 Elsevier Ltd. All rights reserved.