Effects of daily cocaine on milk intake and motor activity in cannula-fed and bottle-fed rats

The purpose of this experiment was to determine if tolerance to cocaine-induced hypophagia involves learning to suppress stereotyped movements that interfere with feeding. Milk intake and motor activity were measured in rats fed via bottles or intraoral cannulas. On dose-response 1, the bottle group was more hypophagic than the cannula group at the 8, 16, and 32 mg/kg doses. After 60d of chronic cocaine (16 mg/kg), only the bottle-fed group showed tolerance, indicated by a shift to the right on dose-response 2. Tolerance was accompanied by a decrease in activity, while activity in the cannula-fed groups given 16 or 32 mg/kg showed sensitization. These results suggest that moderate doses of cocaine interfere with feeding primarily by producing incompatible behaviors. Tolerance involves learning to inhibit these behaviors in order to feed.