title: Naltrexone attenuation of conditioned but not primary reinforcement of nicotine in rats.
abstract: BACKGROUND
  Opioid neurotransmission has been implicated in reinforcement-related processes for several drugs of abuse, including opiates, stimulants, and alcohol. However, less is known about its role in the motivational effects of nicotine and nicotine-associated environmental cues.
  
  
  OBJECTIVE
  This study investigated whether pretreatment with naltrexone, an opioid receptor antagonist, alters conditioned incentive salience of nicotine cues under two conditions: cue-induced reinstatement of nicotine-seeking after extinction and cue-maintained responding during extinction. The effect of naltrexone on nicotine self-administration during the maintenance phase was also examined.
  
  
  METHODS
  Male Sprague-Dawley rats were trained in daily 1-h sessions to self-administer nicotine (0.03 mg/kg/infusion, i.v.) on a fixed-ratio 5 schedule and associate a conditioned stimulus (CS) with each nicotine delivery. Once responding was extinguished by saline substitution for nicotine and omission of the CS, the reinstatement tests were conducted following subcutaneous administration of naltrexone (0, 0.25, 1, 2 mg/kg). In separate groups of rats, naltrexone (0, 2 mg/kg) was chronically given before each extinction sessions, where responses on the active lever resulted in presentations of the CS without nicotine infusion (saline substitution). Self-administration/naltrexone tests were conducted in different groups of rats receiving similar nicotine self-administration training.
  
  
  RESULTS
  Naltrexone significantly attenuated the CS-reinstated responding on the active, previously nicotine-reinforced lever in the reinstatement tests and the CS-maintained active lever responding during the extinction tests. In contrast, neither acute nor chronic naltrexone produced an effect on nicotine self-administration behavior.
  
  
  CONCLUSIONS
  These results indicate that activation of opioid receptors is implicated in mediation of the conditioned incentive properties of nicotine cues but not in the maintenance of nicotine self-administration. Therefore, these findings suggest that opioid receptor antagonists might have clinical potential for prevention of smoking relapse associated with exposure to environmental cues.
authors.affiliation: Division of Neurobiology and Behavior Research, Department of Psychiatry and Human Behavior, University of Mississippi Medical Center, 2500 N. State St., Jackson, MS 39216, USA. xliu@psychiatry.umsmed.edu
authors.forename: Xiu
authors.initials: X
authors.lastname: Liu
authors.forename: Matthew I
authors.initials: MI
authors.lastname: Palmatier
authors.forename: Anthony R
authors.initials: AR
authors.lastname: Caggiula
authors.forename: Alan F
authors.initials: AF
authors.lastname: Sved
authors.forename: Eric C
authors.initials: EC
authors.lastname: Donny
authors.forename: Maysa
authors.initials: M
authors.lastname: Gharib
authors.forename: Sheri
authors.initials: S
authors.lastname: Booth
mesh_terms: D000818:Animals; D003216:Conditioning, Operant / Q000187:drug effects*; D003463:Cues; D004305:Dose-Response Relationship, Drug; D005108:Extinction, Psychological / Q000187:drug effects; D005502:Food; D008297:Male; D009271:Naltrexone / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009538:Nicotine / Q000037:antagonists & inhibitors* / Q000494:pharmacology*; D018722:Nicotinic Agonists / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012054:Reinforcement, Psychology; D012646:Self Administration
chemical_list: D009292:Narcotic Antagonists; D018722:Nicotinic Agonists; D009271:Naltrexone; D009538:Nicotine