An analysis of the effects of amphetamine on brain self-stimulation behavior.

Rats were implanted with stimulating electrodes in the medial forebrain bundle-lateral hypothalamus, and trained in one of two brain self-stimulation (ICSS) procedures. One group of animals was trained in the auto-titration procedure which measured response rates and reinforcement thresholds for ICSS. When trained, the animals were tested with D-amphetamine (0.03-1.0 mg/kg) and saline. The second group of animals was trained in a procedure which established the discriminative properties of brain stimulation and which also measured the detection threshold for ICSS. When trained, the animals were tested with D-amphetamine and saline, and were subsequently tested with amphetamine and saline in an activity monitor. A graded decrease in mean reinforcement thresholds was produced by D-amphetamine. The two lower doses (0.03, 0.10 mg/kg) increased the rate of lever-pressing, but the highest dose (1.0 mg/kg) decreased it. No reliable changes in detection threshold occurred, although at the 0.3 mg/kg dose reductions were seen in two of five animals. Animals also showed a graded increase in motor activity with dose as well as increases in several performance measures. These results demonstrated that although amphetamine lowers reinforcement thresholds, it does not reliably alter detection thresholds, suggesting that the motivation to respond for ICSS is changed without any corresponding changes in the capacity to detect the presence of the stimulus.