title: Association between initial morphine intake and body weight change, acoustic startle reflex and drug seeking in rats.
abstract: BACKGROUND
  Although chronic use of opiates can induce physical dependence and addiction, individual differences contributing to these symptoms are largely unknown.
  
  
  OBJECTIVE
  Using intravenous morphine self-administration (MSA), we investigated whether individual differences in drug intake are associated with weight change, acoustic startle reflex (ASR), pre-pulse inhibition (PPI), and drug seeking during spontaneous withdrawal.
  
  
  METHODS
  Male Sprague-Dawley rats self-administered morphine (0.5 mg/kg/infusion) or saline for 3 weeks (4-6 h/day, 5 days/week) and drug intake and body weight were monitored daily. The ASR and the PPI (baseline, 1 day and 1 week) and drug seeking (1 week) were measured during spontaneous withdrawal.
  
  
  RESULTS
  Morphine animals did not gain weight (101 % ± 0.69), while the control animals did (115 %  ± 1.06) after 3 weeks of self-administration. The ASR and the PPI were not significantly different between morphine and saline animals in 1-day or 1-week withdrawal. However, individual differences in initial (first 10 min), but not total (4-6 h), morphine intake of the daily sessions were positively correlated with weight change (r = 0.437, p = 0.037) and drug seeking (r = 0.424, p = 0.035) while inversely correlated with the ASR (r = -0.544, p = 0.005) in 1-week withdrawal from chronic morphine.
  
  
  CONCLUSIONS
  A subgroup of animals that self-administered a larger amount of morphine at the beginning of the daily sessions exhibited subsequent weight gain, reduced ASR, and enhanced drug seeking in morphine withdrawal. Thus, individual differences in initial morphine intake may reveal a novel behavioral phenotype in opioid addiction.
authors.affiliation: Department of Psychiatry and Program in Neuroscience, Center for the Study of Traumatic Stress, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, MD, 20814, USA.
authors.forename: Thien
authors.initials: T
authors.lastname: Le
authors.forename: Mercedes
authors.initials: M
authors.lastname: Xia
authors.forename: Min
authors.initials: M
authors.lastname: Jia
authors.forename: Nathan
authors.initials: N
authors.lastname: Sarkar
authors.forename: Jerry
authors.initials: J
authors.lastname: Chen
authors.forename: He
authors.initials: H
authors.lastname: Li
authors.forename: Gary H
authors.initials: GH
authors.lastname: Wynn
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Ursano
authors.forename: Kwang H
authors.initials: KH
authors.lastname: Choi
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage*; D000818:Animals; D001835:Body Weight / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D058999:Drug-Seeking Behavior / Q000187:drug effects*; D008297:Male; D009020:Morphine / Q000008:administration & dosage*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013216:Reflex, Startle / Q000187:drug effects*; D012646:Self Administration
chemical_list: D000701:Analgesics, Opioid; D009020:Morphine