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

FERNANDEZ, B., M. T. ANTELO, I. KITCHEN AND M. P. VIVEROS. Effects of neonatal naltrindole treatment on antinociceptive and behavioral responses to μ and κ agonists in rats. PHARMACOL BIOCHEM BEHAV 62 (1) 145–149, 1999.—The effects of a daily injection of the δ selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive and behavioral responses to the μ selective agonist alfentanil (65 μg/kg) and the k selective agonist CI-977 (50 μg/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion test and behavioral testing was performed by employing an open field. The functional blockade of the δ receptor by naltrindole blocked the antinociceptive response to alfentanil but did not affect the antinociception induced by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal naltrindole treatment. The results suggest a functional interaction between δ and μ receptors in the postnatal period but not between δ and k receptors. The data also suggest differences in the δ and μ receptors interacting in the modulation of antinociception and those involved in behavioral responses in the open field.