title: Intrathecal atipamezole augments the antinociceptive effect of morphine in rats.
abstract: BACKGROUND
  Opioid analgesics are effective in the treatment of chronic pain, but they have serious adverse effects such as development of tolerance and dependence. Adrenergic α(2) agonists and μ-opioid receptor agonists show synergistic potentiation and cross-tolerance in spinal analgesia, whereas α(2)-adrenergic antagonists have shown pronociceptive effects. However, at ultralow doses, spinal α(2)-adrenergic antagonists have been reported to paradoxically enhance opioid antinociception. New data have suggested a functional μ-opioid-α(2)-adrenoceptor complex, which may help in interpreting the paradoxical effect of the α(2)-adrenergic antagonists. In the present study we assessed the effects of low doses of atipamezole, a nonselective α(2)-adrenergic antagonist, on both systemic and spinal morphine antinociception and tolerance.
  
  
  METHODS
  Antinociception was assessed in male Sprague-Dawley rats using hotplate, tail-flick, and paw pressure tests. Spinal or systemic opioid tolerance was induced for 4 days. The effects of both intrathecal and subcutaneous atipamezole on acute morphine-induced antinociception and established morphine tolerance were studied.
  
  
  RESULTS
  Systemic or spinal atipamezole itself did not produce antinociception at the doses studied (subcutaneous 0.03, 0.3, 3 μg/kg or intrathecal 0.1, 1, 10 ng). The combined administration of spinal morphine and 1 ng of atipamezole increased the antinociceptive effect of acute spinal morphine 30 minutes after the administration of test drugs in the tail-flick test. Furthermore, 10 ng of intrathecal atipamezole attenuated established morphine tolerance 30 minutes after the administration of test drugs in the tail-flick test. However, subcutaneous atipamezole had no significant effect on systemic morphine antinociception, and it did not attenuate morphine tolerance.
  
  
  CONCLUSIONS
  Spinal coadministration of low doses of atipamezole augmented the antinociceptive effect of morphine in naïve and tolerant rats. Heterodimerization of μ-opioid- and α(2A)-adrenoceptors with consequent changes in function and interaction could explain these results. This also suggests an interesting explanation for the variability in opioid response and tolerance in patients experiencing stress or having an increased noradrenergic tone due to other causes, e.g., drugs.
authors.affiliation: Institute of Biomedicine, Pharmacology, University of Helsinki, Helsinki, Finland. tuomas.lilius@helsinki.fi
authors.forename: Tuomas O
authors.initials: TO
authors.lastname: Lilius
authors.forename: Pekka V
authors.initials: PV
authors.lastname: Rauhala
authors.forename: Oleg
authors.initials: O
authors.lastname: Kambur
authors.forename: Sami M
authors.initials: SM
authors.lastname: Rossi
authors.forename: Antti J
authors.initials: AJ
authors.lastname: Väänänen
authors.forename: Eija A
authors.initials: EA
authors.lastname: Kalso
mesh_terms: D058669:Adrenergic alpha-2 Receptor Antagonists / Q000008:administration & dosage*; D000701:Analgesics, Opioid / Q000008:administration & dosage*; D000818:Animals; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004361:Drug Tolerance; D006358:Hot Temperature; D007093:Imidazoles / Q000008:administration & dosage*; D007278:Injections, Spinal; D007279:Injections, Subcutaneous; D008297:Male; D009020:Morphine / Q000008:administration & dosage*; D010146:Pain / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control*; D010147:Pain Measurement; D017288:Pain Threshold / Q000187:drug effects; D011312:Pressure; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
chemical_list: D058669:Adrenergic alpha-2 Receptor Antagonists; D000701:Analgesics, Opioid; D007093:Imidazoles; C050701:atipamezole; D009020:Morphine