Neonatal naltrindole and handling differently affect morphine antinociception in male and female 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 re sponses to morphine (2 mg/kg) in 20-day-old rats of both sexes were investi gated. The effects of postnatal handling were studied by including two cont rol groups-one group receiving daily injections of saline, and a naive unha ndled group. Antinociception was assessed using the tail-immersion test and time-response curves (5, 10, 15, and 30 min) were carried out for all expe rimental groups. In all treatment groups females showed greater sensitivity to the noxious stimuli compared to males. No significant effect of naltrin dole treatment on baseline latencies was found. Postnatal handling increase d sensitivity to thermal pain in both sexes, and reduced the effect of morp hine in males. No significant effect of chronic naltrindole administration on morphine antinociception was found in this sex. Naltrindole-treated fema les showed an increased antinociception when compared to unhandled animals of the same gender. The results indicate that preweanling handling stress a nd chronic naltrindole treatment differentially affected morphine antinocic eption in male and female neonatal rats. (C) 1999 Elsevier Science Inc.