title: Ketamine coadministration attenuates morphine tolerance and leads to increased brain concentrations of both drugs in the rat.
abstract: OBJECTIVE
  The effects of ketamine in attenuating morphine tolerance have been suggested to result from a pharmacodynamic interaction. We studied whether ketamine might increase brain morphine concentrations in acute coadministration, in morphine tolerance and morphine withdrawal.
  
  
  METHODS
  Morphine minipumps (6 mg·day(-1) ) induced tolerance during 5 days in Sprague-Dawley rats, after which s.c. ketamine (10 mg·kg(-1) ) was administered. Tail flick, hot plate and rotarod tests were used for behavioural testing. Serum levels and whole tissue brain and liver concentrations of morphine, morphine-3-glucuronide, ketamine and norketamine were measured using HPLC-tandem mass spectrometry.
  
  
  RESULTS
  In morphine-naïve rats, ketamine caused no antinociception whereas in morphine-tolerant rats there was significant antinociception (57% maximum possible effect in the tail flick test 90 min after administration) lasting up to 150 min. In the brain of morphine-tolerant ketamine-treated rats, the morphine, ketamine and norketamine concentrations were 2.1-, 1.4- and 3.4-fold, respectively, compared with the rats treated with morphine or ketamine only. In the liver of morphine-tolerant ketamine-treated rats, ketamine concentration was sixfold compared with morphine-naïve rats. After a 2 day morphine withdrawal period, smaller but parallel concentration changes were observed. In acute coadministration, ketamine increased the brain morphine concentration by 20%, but no increase in ketamine concentrations or increased antinociception was observed.
  
  
  CONCLUSIONS
  The ability of ketamine to induce antinociception in rats made tolerant to morphine may also be due to increased brain concentrations of morphine, ketamine and norketamine. The relevance of these findings needs to be assessed in humans.
authors.affiliation: Institute of Biomedicine, Pharmacology, University of Helsinki, Finland.
authors.forename: T O
authors.initials: TO
authors.lastname: Lilius
authors.affiliation: Institute of Biomedicine, Pharmacology, University of Helsinki, Finland.
authors.forename: V
authors.initials: V
authors.lastname: Jokinen
authors.affiliation: Haartman Institute, Department of Clinical Pharmacology, University of Helsinki, Finland.
authors.forename: M S
authors.initials: MS
authors.lastname: Neuvonen
authors.affiliation: Haartman Institute, Department of Clinical Pharmacology, University of Helsinki, Finland.|HUSLAB, Helsinki University Central Hospital, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Niemi
authors.affiliation: Institute of Biomedicine, Pharmacology, University of Helsinki, Finland.|Department of Anaesthesia, Intensive Care Medicine, Emergency Medicine and Pain Medicine, Pain Clinic, Helsinki University Central Hospital, Finland.
authors.forename: E A
authors.initials: EA
authors.lastname: Kalso
authors.affiliation: Institute of Biomedicine, Pharmacology, University of Helsinki, Finland.
authors.forename: P V
authors.initials: PV
authors.lastname: Rauhala
mesh_terms: D000700:Analgesics / Q000378:metabolism / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D001921:Brain / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D004359:Drug Therapy, Combination; D004361:Drug Tolerance; D007279:Injections, Subcutaneous; D007649:Ketamine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism; D009020:Morphine / Q000378:metabolism / Q000494:pharmacology*; D009022:Morphine Derivatives / Q000378:metabolism; D010146:Pain / Q000188:drug therapy; D010147:Pain Measurement / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D053719:Tandem Mass Spectrometry
chemical_list: D000700:Analgesics; D000701:Analgesics, Opioid; D009022:Morphine Derivatives; D007649:Ketamine; D009020:Morphine; C018108:morphine-3-glucuronide; C033419:norketamine