Developmental time course of the effect of nerve growth factor on the parasympathetic ciliary ganglion

Neurite outgrowth in the presence and absence of nerve growth factor (NGF) was compared in neuronal cultures from the parasympathetic ciliary ganglion and from a traditional target of NGF, the sensory dorsal root ganglion. Both ciliary and dorsal root ganglion cultures exhibited a developmental time window during which the effect of NGF on neurite length was maximal. Although neuronal cultures from embryonic day 4 and 5 ganglia exhibited considerable neurite outgrowth in the absence of NGF, there was no significant increase in neurite outgrowth in the presence of NGF. After embryonic day 6, there was a steady increase in the effect of NGF in both types of ganglia. With ciliary ganglia, the effect of NGF increased until day 8, plateaued, then fell off significantly after day 11. With dorsal root ganglia, the effect of NGF continued to increase until day 12, plateaued, then fell off significantly after day 17. Thus, the period of maximal responsiveness of chick ciliary ganglia to NGF occurs earlier in development than for dorsal root ganglia. At the ages when the effect of NGF was maximal, approximately 20% of ciliary ganglion neurons exhibited substantial increases in neurite length compared to approximately 40% of dorsal root ganglion neurons. The effect of NGF was maximal at or below 1 ng/ml (4 X 10(-11) M) for both types of ganglia. These results support previous evidence that NGF does not simply boost ciliary ganglionic neurite growth non-specifically: the effect of NGF is already maximal at low, physiological concentrations and it appears at a specific time in development.