title: Change in functional selectivity of morphine with the development of antinociceptive tolerance.
abstract: OBJECTIVE
  Opioids, such as morphine, are the most effective treatment for pain but their efficacy is diminished with the development of tolerance following repeated administration. Recently, we found that morphine activated ERK in opioid-tolerant but not in naïve rats, suggesting that morphine activation of μ-opioid receptors is altered following repeated morphine administration. Here, we have tested the hypothesis that μ-opioid receptor activation of ERK in the ventrolateral periaqueductal gray (vlPAG) is dependent on dynamin, a protein implicated in receptor endocytosis.
  
  
  METHODS
  Rats were made tolerant to repeated microinjections of morphine into the vlPAG. The effects of dynamin on ERK activation and antinociception were assessed by microinjecting myristoylated dominant-negative dynamin peptide (Dyn-DN) or a scrambled control peptide into the vlPAG. Microinjection of a fluorescent dermorphin analogue (DERM-A594) into the vlPAG was used to monitor μ-opioid receptor internalization.
  
  
  RESULTS
  Morphine did not activate ERK and Dyn-DN administration had no effect on morphine-induced antinociception in saline-pretreated rats. In contrast, morphine-induced ERK activation in morphine-pretreated rats that was blocked by Dyn-DN administration. Dyn-DN also inhibited morphine antinociception. Finally, morphine reduced DERM-A594 internalization only in morphine-tolerant rats indicating that μ-opioid receptors were internalized and unavailable to bind DERM-A594.
  
  
  CONCLUSIONS
  Repeated morphine administration increased μ-opioid receptor activation of ERK signalling via a dynamin-dependent mechanism. These results demonstrate that the balance of agonist signalling to G-protein and dynamin-dependent pathways is altered, effectively changing the functional selectivity of the agonist-receptor complex.
  
  
  BACKGROUND
  This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.
authors.affiliation: Department of Neurological Surgery, Oregon Health & Science University, Portland, OR, USA.
authors.forename: T A
authors.initials: TA
authors.lastname: Macey
authors.forename: E N
authors.initials: EN
authors.lastname: Bobeck
authors.forename: K L
authors.initials: KL
authors.lastname: Suchland
authors.forename: M M
authors.initials: MM
authors.lastname: Morgan
authors.forename: S L
authors.initials: SL
authors.lastname: Ingram
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D004361:Drug Tolerance / Q000502:physiology*; D034281:Dynamins / Q000494:pharmacology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D006358:Hot Temperature; D008297:Male; D009020:Morphine / Q000494:pharmacology* / Q000627:therapeutic use; D018847:Opioid Peptides / Q000494:pharmacology; D010146:Pain / Q000188:drug therapy / Q000378:metabolism; D010487:Periaqueductal Gray / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D018847:Opioid Peptides; D017450:Receptors, Opioid, mu; C030058:dermorphin; D009020:Morphine; D048049:Extracellular Signal-Regulated MAP Kinases; D034281:Dynamins
keywords: ERK; endocytosis; functional selectivity; ligand bias; morphine; morphine tolerance; opiate; opioid; periaqueductal gray