title: Increased impulsive action in rats: effects of morphine in a short and long fixed-delay response inhibition task.
abstract: BACKGROUND
  Impulsive action is mediated through several neurochemical systems, although it is not clear which role each of these plays in the inability to withhold inappropriate responses. Manipulations of the opioid system alter impulsive action in rodents, although the effects are not consistent across tasks. Previously, we speculated that these discrepancies reflect differences in the cognitive mechanisms that control responding in each task.
  
  
  OBJECTIVE
  We investigated whether the effect of morphine, a mu opioid receptor (MOR) agonist, on impulsive action depends on the ability of the subjects to time the interval during which they must inhibit a response.
  
  
  METHODS
  Male Long-Evans rats were trained in a response inhibition (RI) task to withhold responding for sucrose during a 4- or 60-s delay; impulsive action was assessed as increased responding during the delay. The rats were tested following an injection of morphine (0, 1, 3, 6 mg/kg). In a subsequent experiment, the effects of morphine (6 mg/kg) plus the MOR antagonist naloxone (0, 0.3, 1, 3 mg/kg) were investigated.
  
  
  RESULTS
  Morphine increased impulsive action, but had different effects in the two conditions: the drug increased the proportion of premature responses as the 4-s interval progressed and produced a general increase in responding across the 60-s interval. Naloxone blocked all morphine-induced effects.
  
  
  CONCLUSIONS
  The finding that morphine increases impulsive action in a fixed-delay RI task contrasts with our previous evidence which shows no effect in the same task with a variable delay. Thus, MORs disrupt impulsive action only when rats can predict the delay to respond.
authors.affiliation: Department of Psychology, Queen's University, Kingston, ON, K7L 3N6, Canada.
authors.forename: Megan K
authors.initials: MK
authors.lastname: Mahoney
authors.forename: Mason M
authors.initials: MM
authors.lastname: Silveira
authors.forename: Mary C
authors.initials: MC
authors.lastname: Olmstead
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D007175:Impulsive Behavior / Q000523:psychology*; D007266:Inhibition, Psychological; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009270:Naloxone / Q000008:administration & dosage / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000008:administration & dosage / Q000494:pharmacology; D051381:Rats; D020318:Rats, Long-Evans; D011930:Reaction Time / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000037:antagonists & inhibitors; D013395:Sucrose / Q000008:administration & dosage; D013997:Time Factors
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D009270:Naloxone; D013395:Sucrose; D009020:Morphine