Ontogenesis of morphine-induced behavior in the rat.

Ten, seventeen and twenty-four day old rats were observed using a behavioral-time sampling procedure following injection of saline, 0.1, 0.5, 1.0 or 5.0 mg/kg/5cc morphine sulphate. At Day 10, the predominant response to morphine was a depression of locomotor activity. At the 5 mg/kg dose, catalepsy was also seen. In animals of this age, 0.1 mg/kg morphine, which was not sufficient to depress activity, also had no effect on stereotyped gnawing/mouthing behavior, nor did it produce any increase in locomotor activity. A morphine-induced increase in locomotion was first seen on Day 17 (after 0.5 mg/kg morphine). As with Day 10 animals, Day 17 animals given 5 mg/kg morphine showed a depression of locomotion and catalepsy. Stereotypic gnawing/mouthing behavior was first seen in Day 24 animals (after 5 mg/kg morphine), although no dose of morphine produced significant differences in activity at this age. Possible mechanisms resulting in these marked alterations in behavioral response patterns to morphine during this two week period of ontogeny are discussed.