Differential effects of morphine and D-amphetamine on self-stimulation from closely adjacent regions in rat midbrain

The effects of morphine were investigated on self-stimulation from numerous electrode placements in the area of the substantia nigra or in the ventral half of mesencephalic central gray matter. Before pharmacological testing, current intensity was reduced to yield stable, submaximal rates of self-stimulation. Rats were then injected daily with morphine for 10 days, and were tested three hours after injection. Between days 5 and 10 of treatment, many rats self-stimulated at more than 150% of baseline, but some others reduced self-stimulation to as little as 3% of baseline. Histological evaluation revealed that morphine facilitated self-stimulation when the electrode tip was located more than 0.3 mm from substantia nigra or more than 0.2 mm from the midline of central gray. In rats with electrode tips closer to substantia nigra or to the midline of central gray, morphine often reduced or failed to alter self-stimulation rates. The effects of a low dose of D-amphetamine (0.1 mg/kg) were investigated on electrode placements in the substantia nigra area. Placements close to the dorsal border of substantia nigra yielded less facilitation of self-stimulation by D-amphetamine than did placements located more dorsally or medially. Possible catecholaminergic substrates of these differential effects are discussed.