Regulation of developing dopaminergic axonal arbor size in three-dimensional reaggregate tissue culture

The role of target availability in determining axonal size of the developing dopaminergic neuron was examined in three-dimensional reaggregate culture in which these neurons undergo normal neurochemical and morphological development. The ratio of striatal target cells to dopaminergic neurons was systematically increased from 2:1 to 50:1 (striatal to mesencephalic cells). These ratios provide sufficient target for quantitative survival of dopaminergic neurons similar to that seen in vivo. After 3 weeks in culture, dopamine levels increased linearly and proportionally to dopaminergic cell number over a 25-fold relative increase in target cell availability. The amount of neurotransmitter per dopaminergic cell remained constant, suggesting that the axonal arbor did not increase as a function of target availability. This conclusion was directly tested by morphological analysis of the dopaminergic axonal arbor in single aggregates containing between 0 and 45 dopaminergic cells under conditions in which sufficient target was not a factor in dopaminergic cell survival. As was the case with respect to dopamine concentration, axonal arbor size per cell remained constant in the face of increasing target availability. Thus, availability of target cells under these conditions, is not, per se, the limiting factor on the rate of growth of the developing dopaminergic axonal arbor.