Regenerative capacity of bulbar projection neurons during development: a quantitative neuronal analysis with functional correlation.

Critical periods and degrees of regeneration in injured olfactory bulbar projection neurons (mitral cells) were examined in adult rats whose lateral olfactory tracts (LOTs) were transected at different postnatal (P) days. After the LOTs were transected at P7, P10, and P14, a retrograde fluorescent tracer, Fluoro-Gold (FG), was injected into the posterior olfactory cortex (the olfactory tubercle and the piriform cortex), a target brain region of mitral cells, 5 weeks after the transection. FG (+) mitral cells were observed in P7 LOT-transected bulbs and some of P10 LOT-transected bulbs but not in P14 LOT-transected bulbs. Neuron numbers of regenerated FG (+) mitral cells in P2 LOT-transected adult rats decreased to approximately 70% of the normal values (actually counted number: 804±46; stereologically estimated number: 49 700±4300), and 100% of these rats were demonstrated to exhibit olfactory discriminative ability in our previous study. Meanwhile, the numbers in P7 LOT-transected adult rats further decreased to approximately 40% of the normal values, and 78% of these rats showed olfactory discriminative ability. We conclude that the critical periods of spontaneous regeneration of the LOT are between P0 and P10 and that the proportions of regenerated mitral cells decreased as rats became older.