Chronic naltrindole administration does not modify the inhibitory effect of morphine on vocalization responses in the tail electric stimulation test in rats

To address the existence of possible functional interactions between delta- and mu- receptors in relation to the affective component of pain, we have studied the effects of functional blockade of delta-receptors by a chronic treatment with naltrindole (1 mg/kg, 8 consecutive days) on antinociceptive responses to morphine (2 and 5 mg/kg) in the tail electric stimulation tes t, in adult male rats. The thresholds for the motor response (tail withdraw al), vocalization during stimulus and vocalization afterdischarge were asse ssed. These responses are considered to be integrated at spinal, medulla ob longata and diencephalon-rhinencephalon levels, respectively. The results s how that the vocalization during stimulus and the vocalization afterdischar ge were significantly affected by morphine in a dose dependent manner, the latter response being the most sensitive to the effects of the mu-opioid ag onist. However, no significant effect was observed on motor responses at th e doses used in this study. Chronic naltrindole treatment did not modify th e inhibitory effect of morphine on the vocalization responses. Since the vo calization afterdischarge is related to the affective component of pain, th e data suggest that the delta-opioid receptor is not involved in the supras pinal mechanisms at which these responses are organized and that there is n ot a mu-delta interaction in the modulation of the affective responses to n oxious electrical stimulation. (C) 1999 Elsevier Science Ireland Ltd. All r ights reserved.