The development of ventral tegmental area (VTA) projections to the visual cortex of the rat

The development of the ventral tegmental area (VTA) projections to the rat visual cortex was studied with the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing technique. Large injections of WGA-HRP in the visual cortex of newborn, early postnatal, and adult rats resulted in a substantial number of retrogradely labelled neurons in the VTA showing the same distribution pattern at all ages examined. Contrary to other reports, labelled cells were never found in the pars compacta of the substantia nigra but occasionally were seen in the contralateral VTA near the midline. These neurons showed a continuous growth from the day of birth to the end of the second postnatal week, when they acquired morphological features comparable to the adult; they subsequently showed a substantial decrease in soma size. The present results clearly demonstrate that there exists a substantial mesocortical projection to the rat visual cortex which arises exclusively from the VTA. This projection is already established at birth, but the neurons which give rise to it appear morphologically immature during the first two postnatal weeks.