title: Failure of rewarding and locomotor stimulant doses of morphine to promote adult rat 50-kHz ultrasonic vocalizations.
abstract: BACKGROUND
  Frequency-modulated 50-kHz ultrasonic vocalizations (USVs) are emitted by adult rats in response to psychostimulants and non-pharmacological appetitive stimuli and thus have been proposed to model positive affect.
  
  
  OBJECTIVE
  The main aim was to determine whether rewarding doses of morphine increase 50-kHz call rate or alter the relative prevalence of the trill call subtype.
  
  
  METHODS
  In experiment 1, USVs were recorded from adult male Long-Evans rats after subchronic morphine (1 mg/kg subcutaneous (SC)) administration, acute challenge with morphine (1 and 3 mg/kg SC) or amphetamine (1 mg/kg IP, positive control), and in conjunction with locomotor activity tests with morphine (1 and 3 mg/kg SC). In experiments 2 and 3, the USV altering, rewarding, and locomotor effects of morphine were examined using a conditioned place preference (CPP) procedure.
  
  
  RESULTS
  In experiment 1, morphine (1 mg/kg) initially suppressed calling; rats became tolerant to this effect with repeated exposure. Tested subsequently in singly- and pair-tested rats, morphine markedly decreased USVs but significantly increased locomotor activity. In experiments 2 and 3, morphine produced a significant CPP without increasing either unconditioned or conditioned USV emission. Morphine did not detectably alter the relative prevalence of 50-kHz call subtypes.
  
  
  CONCLUSIONS
  Although 50-kHz calls, and the trill call subtype in particular, have been proposed as an animal model of positive mood, not all euphoriant drugs acutely increase the rate of 50-kHz calling or consistently promote trill calls.
authors: Department of Pharmacology and Therapeutics, McGill University, McIntyre Medical Building Rm. 1320, 3655 Promenade Sir William Osler, Montreal, QC H3G 1Y6, Canada.
authors: Jennifer M
authors: JM
authors: Wright
authors: Lan
authors: L
authors: Deng
authors: Paul B S
authors: PB
authors: Clarke
mesh_terms: D000661:Amphetamine / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D003213:Conditioning, Psychological / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects*; D051381:Rats; D020318:Rats, Long-Evans; D012201:Reward; D014465:Ultrasonics; D014828:Vocalization, Animal / Q000187:drug effects*
chemical_list: D009020:Morphine; D000661:Amphetamine