The interaction of a fixed time food delivery schedule and body weight on self-administration of narcotic analgesics.

Experiment 1 reported the effects of the interaction of a fixed 1 min delivery schedule and body weight, using schedule-induced self-injection paradigm, in the rate of acquisition of methadone and heroin. Eighty-one rats were assigned to 100% and 80% reduced body weight conditions with and without a schedule. The findings show that: (a) voluntary heroin and methadone intake was enhanced when a schedule was introduced to animals at 80% but not at 100% body weight; (b) high intake of heroin and methadone was accompanied by increased levels of plasma 11-OHCS. Experiment 2 showed that the high rate of self-injection was due to the interaction of pharmacological properties of opiates and environmental variables rather than to a general increase in activity arising from the deprivation state or the effects of the schedule. The results are discussed in terms of a stress factor arising from an interaction between environmental and pharmacological factors.