Ja. Vivian et Ka. Miczek, EFFECTS OF MU-OPIOID AND DELTA-OPIOID AGONISTS AND ANTAGONISTS ON AFFECTIVE VOCAL AND REFLEXIVE PAIN RESPONSES DURING SOCIAL STRESS IN RATS, Psychopharmacology, 139(4), 1998, pp. 364-375
The present experiments evaluated the influence of intraventricular mu
and delta opioid receptors on affective vocal and reflexive responses
to aversive stimuli in socially inexperienced, as well as defensive a
nd submissive responses in defeated, adult male Long-Evans rats. Defea
t stress consisted of: (1) an aggressive confrontation in which the ex
perimental intruder rat exhibited escape, defensive and submissive beh
aviors [i.e., upright, supine postures and ultrasonic vocalizations (U
SV)], and subsequently, (2) protection from the resident stimulus rat
with a wire mesh screen for 10-20 min. Defeat stress was immediately f
ollowed by an experimental session with tactile startle (20 psi), The
mu opioid receptor agonists morphine (0.1-0.6 mu g ICV) and [D-Ala(2)-
N-Me-Phe(4)-Gly(5)-ol]-enkephalin (DAMGO; 0.01-0.3 mu g ICV), and the
delta opioid receptor agonist [D-Pen(2,5)]-enkephalin (DPDPE; 10-100 m
u g ICV) dose-dependently decreased startle-induced USV and increased
tail-flick latencies in socially inexperienced and defeated rats. Of g
reater interest, morphine, DAMGO and DPDPE increased the occurrence of
the submissive crouch posture, and defeated rats were more sensitive
than socially inexperienced rats to the startle-induced USV-suppressiv
e and antinociceptive effects of morphine and DPDPE. The antinocicepti
ve effects of DAMGO were likewise obtained at lower doses in defeated
rats. Finally, the USV-suppressive effects of morphine and DAMGO were
reversed with the mu receptor antagonist naltrexone (0.1 mg/kg IP), bu
t the USV-suppressive effects produced by DPDPE were not reversed with
the delta receptor antagonist naltrindole (1 mg/kg IP). These results
confirm mu, but not delta opioid receptor activation as significant i
n affective vocal, passive-submissive behavior, as well as reflexive a
ntinociception. Furthermore, similar to previous studies with restrain
t and electric shock stress, the facilitation of mu opioid effects on
vocal responses and antinociception is consistent with the proposal th
at defeat stress activated endogenous opioid mechanisms.