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 t hreshold current intensities and the breaking points of intracranial self-s timulation. 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 s tudied in male Wistar rats trained to lever-press for electrical stimulatio n of ventral tegmental area. Morphine and d-amphetamine were the only two d rugs 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 i n threshold current were seen after administration of low (5 mg/kg) and hig h (20 mg/kg) doses of caffeine, respectively. Nicotine, ethanol and caffein e had no effects in the progressive ratio procedure. Effects of phencyclidi ne did not reach levels of statistical significance in either procedure alt hough high doses of phencyclidine disrupted performance in the progressive ratio procedure. (C) 1999 Elsevier Science B.V. All rights reserved.