Failure of rewarding and locomotor stimulant doses of morphine to promote adult rat 50-kHz ultrasonic vocalizations

Jennifer M Wright,Lan Deng,Paul B S Clarke

doi:10.1007/s00213-012-2776-z

Abstract

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 (1mg/kg subcutaneous (SC)) administration, acute challenge with morphine (1 and 3mg/kg SC) or amphetamine (1mg/kg IP, positive control), and in conjunction with locomotor activity tests with morphine (1 and 3mg/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 (1mg/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.

Highlights

Adult laboratory rats emit two broad categories of vocalizations in the ultrasonic range, commonly designated as “50kHz” and “22-kHz” calls (Brudzynski 2009; Wohr and Schwarting 2010)
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
We have shown that the acoustic profile can be modulated by drugs or social context (Wright et al 2010, 2012), adding to the existing evidence that distinct information may be contained within the repertoire of 50-kHz ultrasonic vocalizations (USVs) (e.g., Burgdorf et al 2008; Simola et al 2009; Wohr et al 2008)

Summary

Introduction

Adult laboratory rats emit two broad categories of vocalizations in the ultrasonic range, commonly designated as “50kHz” and “22-kHz” calls (Brudzynski 2009; Wohr and Schwarting 2010). Adult rat 50-kHz calls (and FM calls in particular) have been proposed to be a measure of hedonia (Burgdorf and Moskal 2009) and have been described as a reliable indicator (Brudzynski 2007) and validated model (Burgdorf et al 2010) of positive affective states. Consistent with this notion, the psychomotor stimulants amphetamine (AMPH) and cocaine both increase the 50-kHz

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Conclusion