The effects of feeding conditions on drug-reinforced behavior: Maintenance at reduced body weight versus availability of food

Recent research has shown that food deprivation increases drug self-administration in rats and rhesus monkeys. The purpose of the present study was to examine two variables related to this food-deprivation effect: maintenance of rats at reduced body weights and the absence of food. Etonitazene HCl was established as a reinforcer orally for 12 rats according to procedures previously used in experiments reported by this laboratory. Lever-pressing behavior was maintained under fixed-ratio (FR) schedules during daily 1-h sessions by etonitazene or water, which were available either concurrently or on alternating days. In the first experiment, six rats were maintained at 75% of their free-feeding weights. The effect of presenting the daily food allotment at 23, 4, 2, 1, or 0 h before their daily drug or water self-administration session was studied. When the rats were fed 23, 4, or 2 h before the session, etonitazene dipper presentations were at maximum levels and were substantially higher than for water. When the rats were fed during (0) or 1 h before the session, the number of etonitazene dipper presentations was lower, but it exceeded those for water. Under conditions of complete food satiation (0 h deprived-100% body weight), etonitazene and water dipper presentations were both low, and there were no differences between them. In the second experiment, six rats maintained at 75% of their free-feeding weights were trained to respond for etonitazene or water on alternating days. When they were subsequently food satiated (100% body weight), drug- and water-maintained behavior decreased to low levels. These rats were then deprived of food for 4 or 16 h before their daily 1-h session, and responding did not increase. Body weight did not decrease below 100%. These results suggest that maintenance at reduced body weight rather than the absence of food is the determinant of increased rates of drug-reinforced behavior.