title: Pharmacological consequence of the A118G μ opioid receptor polymorphism on morphine- and fentanyl-mediated modulation of Ca²⁺ channels in humanized mouse sensory neurons.
abstract: BACKGROUND
  The most common functional single nucleotide polymorphism of the human OPRM1 gene, A118G, has been shown to be associated with interindividual differences in opioid analgesic requirements, particularly with morphine, in patients with acute postoperative pain. The purpose of this study was to examine whether this polymorphism would modulate the morphine and fentanyl pharmacological profile of sensory neurons isolated from a humanized mouse model homozygous for either the 118A or 118G allele.
  
  
  METHODS
  The coupling of wild-type and mutant μ opioid receptors to voltage-gated Ca channels after exposure to either ligand was examined by employing the whole cell variant of the patch-clamp technique in acutely dissociated trigeminal ganglion neurons. Morphine-mediated antinociception was measured in mice carrying either the 118AA or 118GG allele.
  
  
  RESULTS
  The biophysical parameters (cell size, current density, and peak current amplitude potential) measured from both groups of sensory neurons were not significantly different. In 118GG neurons, morphine was approximately fivefold less potent and 26% less efficacious than that observed in 118AA neurons. On the other hand, the potency and efficacy of fentanyl were similar for both groups of neurons. Morphine-mediated analgesia in 118GG mice was significantly reduced compared with the 118AA mice.
  
  
  CONCLUSIONS
  This study provides evidence to suggest that the diminished clinical effect observed with morphine in 118G carriers results from an alteration of the receptor's pharmacology in sensory neurons. In addition, the impaired analgesic response with morphine may explain why carriers of this receptor variant have an increased susceptibility to become addicted to opioids.
authors.affiliation: Department of Anesthesiology, Penn State College of Medicine, Hershey, Pennsylvania, USA.
authors.forename: Saifeldin
authors.initials: S
authors.lastname: Mahmoud
authors.forename: Annika
authors.initials: A
authors.lastname: Thorsell
authors.forename: Wolfgang H
authors.initials: WH
authors.lastname: Sommer
authors.forename: Markus
authors.initials: M
authors.lastname: Heilig
authors.forename: Joan K
authors.initials: JK
authors.lastname: Holgate
authors.forename: Selena E
authors.initials: SE
authors.lastname: Bartlett
authors.forename: Victor
authors.initials: V
authors.lastname: Ruiz-Velasco
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005260:Female; D005283:Fentanyl / Q000494:pharmacology*; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009020:Morphine / Q000494:pharmacology*; D020641:Polymorphism, Single Nucleotide; D017450:Receptors, Opioid, mu / Q000235:genetics*; D011984:Sensory Receptor Cells / Q000187:drug effects* / Q000502:physiology; D012668:Trigeminal Ganglion / Q000187:drug effects / Q000502:physiology
chemical_list: D000701:Analgesics, Opioid; D015220:Calcium Channels; D017450:Receptors, Opioid, mu; D009020:Morphine; D005283:Fentanyl