MORPHINE ATTENUATES ULTRASONIC VOCALIZATION DURING AGONISTIC ENCOUNTERS IN ADULT MALE-RATS

Ultrasonic vocalizations (USV) in rats may communicate ''affective'' s tates during pain, sex and aggression. This proposal was evaluated in an experiment with adult male Long-Evans rats during agonistic encount ers; specifically, morphine and naltrexone effects were studied on dif ferent types of USV by intruder rats exposed to resident attacks and t o ''threat of attacks'' (i.e., intruder residing within the home cage of the resident but prevented from physical contact by a wire mesh cag e). Intruders readily emitted USV during agonistic encounters. These c alls consisted primarily of two distinct distributions of pure tone wh istles: 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 repertoi re of frequency modulated calls. Different types of vocalizations prov ed 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 d ecrease in USV after morphine was a dose-dependent decrease in rearing , walking and nasal contact behavior with increases in submissive crou ch behavior and tail flick analgesia. The decreases in rate and durati on of both low and high USV and the pitch of specific frequency modula ted calls after morphine administration may reflect an attenuation of affective aspects of pain, and the many characteristics of US (rate, d uration, pitch, frequency modulation, preand suffix attributes and tem poral structure) point to potentially diverse functions. Morphine's pe rvasive effects on ultrasonic but not audible vocalizations, in additi on to reflexive and submissive responses, provides evidence for opioid influences on ''affective'' as well as somatomotor responses to socia lly aversive situations.