Effects of preweanling chronic naltrindole administration on stress-induced antinociceptive responses in rats

Abstract

The effect of a daily injection of the δ-selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on the development of stress-induced-antinociception (SIA) and on the antinociceptive response to the μ-selective agonist alfentanil (65 μg/kg) in female rats was investigated. Functional blockade of the δ-receptor during the preweanling period markedly reduced the antinociceptive response to swim-stress in 25-day-old rats, and SIA was only mediated by δ-receptors at this age. In 20-day-old rats and in adults, SIA was predominantly μ-receptor mediated and unaffected by δ-receptor blockade. The lack of interference with μ-receptor function was confirmed as alfentanil responses were unaffected by preweanling naltrindole treatment. The data show independence of μ- and δ-receptors in the control of SIA during development and an impairment of δ- but not μ-mediated SIA after chronic δ-antagonist treatment.