Effects of abused drugs on thresholds and breaking points of intracranial self-stimulation in rats.

The present study aimed to compare the effects of various abused drugs on threshold current intensities and the breaking points of intracranial self-stimulation. Effects of morphine (1-10 mg/kg, s.c.), d-amphetamine (0.3-3.2 mg/kg, i.p.), nicotine (0.1-3.2 mg/kg, s.c.), ethanol (0.6-2.4 g/kg, p.o.), caffeine (1-20 mg/kg, i.p.) and phencyclidine (0.3-5.6 mg/kg, i.p.) were studied in male Wistar rats trained to lever-press for electrical stimulation of ventral tegmental area. Morphine and d-amphetamine were the only two drugs that both decreased threshold currents and increased the maximal ratio of reinforced and non-reinforced responses. Nicotine (1 mg/kg) and ethanol (1.2 g/kg) lowered threshold currents while both decreases and increases in threshold current were seen after administration of low (5 mg/kg) and high (20 mg/kg) doses of caffeine, respectively. Nicotine, ethanol and caffeine had no effects in the progressive ratio procedure. Effects of phencyclidine did not reach levels of statistical significance in either procedure although high doses of phencyclidine disrupted performance in the progressive ratio procedure.