ULTRASOUNDS EMITTED BY FEMALE RATS DURING AGONISTIC INTERACTIONS - EFFECTS OF MORPHINE AND NALTREXONE

Ultrasonic vocalizations may be an expression of the affective pain re sponse in laboratory rodents. The present experiment compared morphine 's effects on high (33-60 kHz) and low (20-32 kHz) frequency ultrasoni c vocalizations to its effects on a range of unconditioned behavioral responses to aversive stimuli; the influence of estrous cyclicity on m orphine sensitivity was also investigated. In experiment 1, naive fema le Long-Evans rats, selected during estrus or diestrus, received cumul ative morphine (1, 3, 6, 10 mg/kg SC) or saline, and in experiment 2, rats were pretreated with naltrexone (0.1 mg/kg IF) 5 min before morph ine (17, 30, 60, 100 mg/kg SC), The following endpoints were measured 20-25 min post-injection: (1) tail flick latency; (2) ultrasonic and a udible vocalizations; (3) the behavioral response to aggressive attack ; and (4) locomotor activity. Following a brief exposure to an attack, rats were threatened by an aggressor but protected from further attac k by a wire mesh cage (30 x 21.5 x 20 cm), thereby allowing for contin ued behavioral and vocal measurement without the risk of physical inju ry; video and audio recordings were made of the attack encounter and a subset of the protected encounter (1 min). The endpoint most potently and specifically modulated by morphine was high frequency ultrasounds . The rate of high frequency calling varied as a function of the estro us cycle, supporting gonadal hormone modulation of ultrasonic vocaliza tions. Low frequency ultrasounds, by contrast, were relatively insensi tive to opiate manipulation and were less influenced by estrous cyclic ity. High frequency vocalizations may be a more sensitive indication o f the affective response to an attacking conspecific than low frequenc y calls. The attenuation of high frequency ultrasonic calls at doses t hat do not affect any other behavioral or vocal responses may correspo nd to human descriptions of morphine analgesia, in which the affective component to pain is more potently modulated than the sensory compone nt.