The relationship of the birth date of rat sympathetic neurons to the target they innervate.

In many parts of the nervous system, neurons with the same function often have similar "birth dates" (the time their precursor withdrew from the cell cycle). We investigated the birth dates of eight functional classes of rat sympathetic postganglionic neurons by injecting bromodeoxyuridine during embryonic development, while retrograde tracing and immunohistochemistry were used to identify postganglionic neurons of different functional classes in the mature animals. The times of withdrawal from the cell cycle overlapped, but there were significant differences in the peak time of withdrawal for most of the classes. Furthermore, sympathetic cholinergic postganglionic neurons had a significantly greater proportion of their total population labelled with bromodeoxyuridine than did any of the noradrenergic classes of neurons, indicating prenatal class-specific differences in the handling of bromodeoxyuridine. Together, our findings indicate that, prior to extending axons to their targets, different functional classes of sympathetic neurons show differences in phenotype.