Effects of tolerance on intravenous morphine self-administration behavior

Abstract

After their operant level of leverpressing was determined, four groups of rats were given intravenous injections of either saline or morphine sulfate in daily dosages of 50, 100, or 300 mg/kg/day (divided among three equal doses for 3 days. Following these tolerance-producing treatments, all subjects were allowed to acquire a leverpressing behavior on a CRF schedule for intravenous injections (1.0 mg/kg) of morphine. In the 10-h acqusition sessions, both the number of infusions received and the amount of drug self-administered were directly proportional to the dosage used to induce tolerance.