title: Ventilation and the Response to Hypercapnia after Morphine in Opioid-naive and Opioid-tolerant Rats.
abstract: BACKGROUND
  Opioid-related deaths are a leading cause of accidental death, with most occurring in patients receiving chronic pain therapy. Respiratory arrest is the usual cause of death, but mechanisms increasing that risk with increased length of treatment remain unclear. Repeated administration produces tolerance to opioid analgesia, prompting increased dosing, but depression of ventilation may not gain tolerance to the same degree. This study addresses differences in the degree to which chronic morphine (1) produces tolerance to ventilatory depression versus analgesia and (2) alters the magnitude and time course of ventilatory depression.
  
  
  METHODS
  Juvenile rats received subcutaneous morphine for 3 days (n = 116) or vehicle control (n = 119) and were then tested on day 4 following one of a range of morphine doses for (a) analgesia by paw withdraw from heat or (b) respiratory parameters by plethysmography-respirometry.
  
  
  RESULTS
  Rats receiving chronic morphine showed significant tolerance to morphine sedation and analgesia (five times increased ED50). When sedation was achieved for all animals in a dose group (lowest effective doses: opioid-tolerant, 15 mg/kg; opioid-naive, 3 mg/kg), the opioid-tolerant showed similar magnitudes of depressed ventilation (-41.4 ± 7.0%, mean ± SD) and hypercapnic response (-80.9 ± 15.7%) as found for morphine-naive (-35.5 ± 16.9% and -67.7 ± 15.1%, respectively). Ventilation recovered due to tidal volume without recovery of respiratory rate or hypercapnic sensitivity and more slowly in morphine-tolerant.
  
  
  CONCLUSIONS
  In rats, gaining tolerance to morphine analgesia does not reduce ventilatory depression effects when sedated and may inhibit recovery of ventilation.
authors.affiliation: From the Department of Anesthesiology and Pain Medicine, University of Washington School of Medicine, Seattle, Washington (M.J.E., C.C.G., T.N.K., C.S., G.W.T.); and Pulmonary and Critical Care Medicine, Veterans Affairs Puget Sound Health Care System, Seattle, Washington (M.J.E.).
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Emery
authors.forename: Chase C
authors.initials: CC
authors.lastname: Groves
authors.forename: Timothy N
authors.initials: TN
authors.lastname: Kruse
authors.forename: Chen
authors.initials: C
authors.lastname: Shi
authors.forename: Gregory W
authors.initials: GW
authors.lastname: Terman
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D005260:Female; D006935:Hypercapnia / Q000188:drug therapy*; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D010146:Pain / Q000188:drug therapy; D051381:Rats; D017207:Rats, Sprague-Dawley; D012119:Respiration / Q000187:drug effects*
chemical_list: D000701:Analgesics, Opioid; D009020:Morphine