Locomotor hyperactivity in neonatal rats following electrolytic lesions of mesocortical dopamine neurons.

These experiments examined the effects on locomotor activity of brain lesions that destroyed either mesocortical or nigrostriatal dopamine (DA) neuronal projections in neonatal rats. Electrolytic lesions of the medial ventral tegmental area in 4-day-old rats reduced the content of DA within the frontal cortex and septum by 42-57% and resulted in a 2-fold increase in locomotor activity during days 22-24 of life. In contrast, bilateral electrolytic lesions of the substantia nigra in 4-day-old rats reduced the content of DA within the caudate putamen by 68%, but failed to alter locomotor activity during days 22-24 of life. These results suggest that loss of mesocortical DA neurons may underlie the locomotor hyperactivity seen following brain DA-depleting 6-hydroxydopamine injections in neonatal rats and that these mesocortical DA neurons may normally influence the ontogeny of locomotion in the rat.