Repeated neonatal maternal separation alters morphine-induced antinociception in male rats.

Recent studies indicate that Long-Evans rats separated from their dam for 3 h daily over the first 2 weeks of life (maternally separated [MS] rats) exhibit exaggerated behavioral and neuroendocrine responses to stress as adults compared to handled (H) or non-handled (NH) control animals. Our aim was to determine whether repeated neonatal maternal separation results in altered sensitivity to the opioid agonist morphine in male and female adult rats. Sensitivity to morphine was assessed using hot-plate and tail-flick tests. Morphine was less potent inducing antinociception in MS males compared to same-sex controls in the hot-plate, but not in the tail-flick test. Decrease in sensitivity to morphine in MS females compared to same-sex controls was present only as a trend in the hot-plate, but not in the tail-flick test. These results suggest that neonatal maternal separation results in long-lasting changes in opioid responsiveness primarily in male rats.