M. Haney et Ka. Miczek, DELTA-OPIOID RECEPTORS, REFLEXIVE, DEFENSIVE AND VOCAL AFFECTIVE RESPONSES IN FEMALE RATS, Psychopharmacology, 121(2), 1995, pp. 204-212
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.