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

The effect of a daily injection of the delta-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 mu-selective agonist alfentanil (65 microg/kg) in female rats was investigated. Functional blockade of the delta-receptor during the preweanling period markedly reduced the antinociceptive response 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 blockade. The lack of interference with mu-receptor function was confirmed as alfentanil responses were unaffected by preweanling naltrindole treatment. The data show independence of mu- and delta-receptors in the control of SIA during development and an impairment of delta- but not mu-mediated SIA after chronic delta-antagonist treatment.