Effects of neonatal naltrindole treatment on antinociceptive and behavioral responses to mu, and kappa agonists in rats

The effects of a daily injection of the delta selective opioid antagonist n altrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive an d behavioral responses to the mu selective agonist alfentanil (65 mu g/kg) and the kappa selective agonist CI-977 (50 mu g/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion t est and behavioral testing was performed by employing an open field. The fu nctional blockade of the delta receptor by naltrindole blocked the antinoci ceptive response to alfentanil but did not affect the antinociception induc ed by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal nal trindole treatment. The results suggest a functional interaction between de lta and mu receptors in the postnatal period but not between delta and kapp a receptors. The data also suggest differences in the delta and mu receptor s interacting in the modulation of antinociception and those involved in be havioral responses in the open field. (C) 1998 Elsevier Science Inc.