Intracranial self-stimulation in rats as a function of various stimulus parameters. VI. Influence of fentanyl, piritramide, and morphine on medial forebrain bundle stimulation with monopolar electrodes.

The effects of different subcutaneous doses of fentanyl (0.02, 0.04, 0.08, and 0.16 mg/kg), piritramide (0.63, 2.50, 10.0 and 40.0 mg/kg), and morphine (2.50, 5.00, 10.0 and 20.0 mg/kg) on self-stimulation in rats were studied. Different stimulus parameter combinations (SPC) inducing low, high or intermediate control response rates (CRR) were applied during the same experimental sessions. The three narcotic analgesics induced response depression (RD) and response stimulation (RS). RS was mostly observed at low dose levels; RD was dose-related. SPC's inducing low CRR were more sensitive than those inducing high CRR. Fentanyl was more potent than piritramide and than morphine. The RD is related to motor incapacitation, as the doses needed to effectively reduce self-stimulation also induced obvious catatonia. The RS probably is a more specific effect reflecting sensitization of structures involved in reinforcement of behavior.