EFFECTS OF PREWEANLING CHRONIC NALTRINDOLE ADMINISTRATION ON STRESS-INDUCED ANTINOCICEPTIVE RESPONSES IN RATS

The effect of a daily injection of the delta-selective opioid antagoni st naltrindole (1 mg/kg), from birth to postnatal day 19, on the devel opment of stress-induced-antinociception (SIA) and on the antinocicept ive response to the mu-selective agonist alfentanil (65 mu g/kg) in fe male rats was investigated. Functional blockade of the delta-receptor during the preweanling period markedly reduced the antinociceptive res ponse to swim-stress in 25-day-old rats, and SIA was only mediated by delta-receptors at this age. In 20-day-old rats and in adults, SIA was predominantly mu-receptor mediated and unaffected by delta-receptor b lockade. The lack of interference with mu-receptor function was confir med as alfentanil responses were unaffected by preweanling naltrindole treatment. The data show independence of mu- and delta-receptors in t he control of SIA during development and an impairment of delta- but n ot mu-mediated SIA after chronic delta-antagonist treatment. (C) 1998 Elsevier Science B.V. All rights reserved.