Morphine attenuates ultrasonic vocalization during agonistic encounters in adult male rats

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Abstract
Ultrasonic vocalizations (USV) in rats may communicate “affective” states during pain, sex and aggression. This proposal was evaluated in an experiment with adult male Long-Evans rats during agonistic encounters; specifically, morphine and naltrexone effects were studied on different types of USV by intruder rats exposed to resident attacks and to “threat of attacks” (i.e., intruder residing within the home cage of the resident but prevented from physical contact by a wire mesh cage). Intruders readily emitted USV during agonistic encounters. These calls consisted primarily of two distinct distributions of pure tone whistles: 0.3–3 s, 19–32 kHz (“low”) calls and 0.02–0.3 s, 32–64 kHz (“high”) calls. Sonographic analysis revealed a considerable repertoire of frequency modulated calls. Different types of vocalizations proved to be differentially sensitive to the opiate treatments: morphine (1–10 mg/kg SC) dose-dependently decreased the rate, duration and pitch of both low and high frequency USV during the threat of attack; this decrease in rate and duration measures was naltrexone-reversible (0.1 mg/kg IP). Interestingly, audible vocalizations were also emitted but were unaffected by morphine in this dose range. Concomitant with the decrease in USV after morphine was a dose-dependent decrease in rearing, walking and nasal contact behavior with increases in submissive crouch behavior and tail flick analgesia. The decreases in rate and duration of both low and high USV and the pitch of specific frequency modulated calls after morphine administration may reflect an attenuation of affective aspects of pain, and the many characteristics of US (rate, duration, pitch, frequency modulation, pre-and suffix attributes and temporal structure) point to potentially diverse functions. Morphine's pervasive effects on ultrasonic but not audible vocalizations, in addition to reflexive and submissive responses, provides evidence for opioid influences on “affective” as well as somatomotor responses to socially aversive situations.

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