Morphometric analysis of embryonic rat trigeminal neurons treated with different neurotrophins

In whole-mount explant cultures of the trigeminal ganglion (TG) with intact peripheral and brainstem targets, exogenous application of nerve growth factor (NGF) and neurotrophin-3 (NT-3) leads to elongation and precocious arborization of embryonic trigeminal axons, respectively. In addition, neurotrophins play a major role in survival and differentiation of distinct classes of TG neurons. In the present study, we conducted morphometric analyses of trigeminal neurons exposed to exogenous NGF or NT-3 in whole-mount explant cultures. Explants dissected from embryonic day (E) 13 and E15 rats were cultured in the presence of serum-free medium (SFM) or in SFM supplemented with NGF or NT-3 for 3 days. TG neurons were then retrogradely labeled with lipophilic tracer DiI and their soma size distributions were compared following different treatments. The mean diameters of E13 and E15 trigeminal neurons grown in the presence of NT-3 were similar to those grown in SFM. On the other hand, in cultures supplemented with NGF, the mean diameters of neurons were larger at E13, but smaller at E15. Double immunolabeling with TrkA and TrkC antibodies confirmed the presence of large-diameter TrkA-positive neurons in E13 TG, but not in E15 TG. At both ages, other large-diameter neurons expressed only TrkC. These results show that exposure to NGF leads to phenotypic changes in TrkA-expressing trigeminal neurons at early embryonic development, but selective survival of small diameter neurons at later ages.