DELTA-OPIOID RECEPTORS, REFLEXIVE, DEFENSIVE AND VOCAL AFFECTIVE RESPONSES IN FEMALE RATS

Ultrasonic vocalizations may be an expression of the affective pain re sponse in laboratory animals. The present experiment compares the effe cts of morphine to the delta agonist, DPDPE (D-Pen(2), D-Pen(5) enkeph alin) on a range of reflexive, behavioral and affective responses duri ng an aggressive interaction. In experiment 1, naive female Long-Evans rats received morphine (0, 1, 3, 6, 10 mu g ICV), or DPDPE (0, 30, 60 , 100 mu g ICV). In experiment 2, female rats were treated with naltri ndole (1.0 mg/kg IP) 20 min before DPDPE (0, 60, 100 mu g ICV). The fo llowing endpoints were measured: (1) latency to tail flick in response to heat stimuli; (2) high (33-65 kHz) and low (20-32 kHz) frequency u ltrasonic and audible vocalizations; (3) defensive behavior; and (4) m otoric activity. Following a brief exposure to attack, rats were threa tened by the aggressor but protected from further attack by a large, w ire mesh cage, thereby allowing for continued behavioral and vocal mea surement without the risk of physical injury; video and audio recordin gs were made during the attack and then during a portion of the protec ted encounter (2 min). Morphine suppressed pain reactions varying in c omplexity from a spinal reflex, to an organized escape reaction, to an affective vocal response. The delta agonist, DPDPE, attenuated high f requency ultrasonic calling and tail flick responding. Defensive behav iors were also modulated by DPDPE at doses that had no effect on walki ng or rearing, indicating behavioral specificity. By contrast, doses o f morphine that decreased defensive upright and escape also decreased motor activity. In female rats, morphine and DPDPE share a common prof ile of effects on a range of functional end-points, but DPDPE appears to modulate more selectively the reactions related to aversiveness wit hout exerting sedative effects. These data demonstrate a possible phys iological role for delta receptors in affective and defensive reaction s.