SELECTIVE SPARING OF LATER-BORN GANGLION-CELLS AFTER NEONATAL TRANSECTION OF THE INFRAORBITAL NERVE

A combination of [H-3]thymidine labelling and retrograde tracing with either horseradish peroxidase (HRP) or true blue (TB) was used to determine whether V primary afferent neurons born on different embryonic (E) days were differentially susceptible to neonatal transection of the infraorbital nerve (ION). In one experiment, rat fetuses were exposed to [H-3]thymidine on E-8.5, 9.5, 10.5, 11.5, 12.5, 13.5, 14.5, or 15.5, the left infraorbital nerve (ION) was transected on the day of birth, and both the regenerate and intact IONs were labelled with HRP when the animals reached adulthood. The percentage of HRP labelled cells that were also heavily labelled by [H-3]thymidine was calculated for both the intact ganglion and that ipsilateral to the damaged nerve for each animal. A consistently higher percentage of double labelled cells on the lesioned rather than on the intact side for a given E-day was taken as an indication that cells born on the day in question had an increased probability of survival relative to the entire population of V ganglion cells that contributed axons to the ION. Cells born late in gestation on E-12.5 through 14.5 were significantly more likely than early born (E-9.5 through 11.5) cells to survive neonatal axotomy. In a second experiment, fetuses were exposed to [H-3]thymidine on either E-9.5, E-10.5, or E-14.5, the vibrissa pads on both sides of the face were injected with TB within 6 hours of birth, and the ION was transected 6-8 hours later. When these rats reached at least 60 days of age, ganglia were processed for the visualization of both TB and [3H]thymidine labelled neurons. Cells labelled with both tracers would have been born on a given E-day, projected to the vibrissa pad via the ION at the time of nerve transection, and survived any naturally occurring or lesion-induced cell death. As in the HRP tracing experiment, ganglion cells born on E-14.5 were significantly more likely to survive neonatal ION transection than those born on either E-9.5 or E-10.5.