Failure of rewarding and locomotor stimulant doses of morphine to promote adult rat 50-kHz ultrasonic vocalizations
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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.
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.
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.
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.
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.
KeywordsUltrasonic vocalizations Rat Morphine Opioid Amphetamine Reward Conditioned place preference Locomotor activity
This study was supported by a Natural Science and Engineering Research Council of Canada (NSERC) discovery grant (155055, to P.B.S.C), and an NSERC Postgraduate Scholarship D (to J.M.W). The authors thank Tina Scardochio for constructive comments on the manuscript. P.B.S.C. is a member of the Center for studies in Behavioral Neurobiology at Concordia University in Montreal. The authors have no financial relationship with the organizations that sponsored this research. All experiments comply with the current laws of Canada.
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