Protracted behavioral effects of neonatal morphine-pellet implantation in male rats.

Male rat pups (Charles Rivers CD albino strain) were implanted subcutaneously with a morphine (M), placebo (P), or no pellet (NP) on Day 5 or 11 of postnatal life. Body weights of both Day 5 and 11 drug-treated males were depressed below those of controls through Day 63 (p < 0.01). On Day 72, compared to P and NP controls, M-treated males showed impaired learning in a Lashley III maze. Day 5, but not Day 11 drug-treated animals showed greater activity-wheel behavior (p < 0.01) than controls on Days 85--86, and less suppression (p < 0.025) of a conditioned emotional response on Day 145. On Day 150, Day 5 M-treated animals showed a reduced steroid response (p < 0.05) to a challenge dose of M (18 mg/kg), suggesting protracted tolerance to the pituitary-adrenal activating effects of M. Members of each treatment group were successfully bred with females on Day 90; no group differences were found in litter size, sex ratios, or birth weights of the offspring. The results of this study indicate that early exposure to M causes long-term impairment of several behaviors in male rats.