Neonatal infraorbital nerve transection in rat results in peripheral trigeminal sprouting

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Abstract
Retrograde tracing techniques were employed to determine whether transection of the infraorbital (IO) nerve in either newborn or adult rats resulted in peripheral sprouting by undamaged trigeminal (V) axons. The IO nerve was sectioned just behind the vibrissa pad, either on the day of birth or when animals reached at least 60 days of age. After an additional 60 days, the same nerve was retransected in the orbit; horseradish peroxidase (HRP) or diamidino yellow (DY) was injected into the central portion of the vibrissa pad; and animals were killed 2–3 days later. In the neonatally nerve‐damaged rats, this procedure invariably labelled primary afferent neurons in both the ipsilateral and contralateral V ganglia. On the ipsilateral side, these cells were located in the caudal portion of the ophthalmicmaxillary region and, less often, in the mandibular division. Their average diameter was 22.6 μm (s.d.=5.6). On the contralateral side, most labelled ganglion cells were visible in the anteromedial part of the ophthalmicmaxillary region but a few could also be seen in the mandibular division. Their average diameter was 21.1 μm (s.d.=5.5). No labelled ganglion cells were observed in adult rats subjected to the same series of manipulations. In a separate series of neonatally nerve‐damaged animals, the above‐described procedures were combined with neonatal injection of capsaicin in an effort to determine whether the observed sprouting was dependent upon the presence of large numbers of unmyelinated axons. The addition of this treatment reduced the number of labelled cells in both the ipsilateral and contralateral ganglia, but it did not alter either their distribution or average soma diameter. In a final experiment, sequential double‐labelling techniques were used to determine whether the V axons that projected to the vibrissa pad via non‐IO nerve branches were the result of sprouting by undamaged ganglion cells or arose from neurons that had originally projected into the IO nerve, were axotomized by our lesions, and regenerated to the vibrissa pad via another V branch. Here, the long‐lived retrograde tracer true blue (TB) was injected into the vibrissa pad 6–8 hours before the neonatal nerve cut and DY was deposited into the pad after transection of the regenerate IO nerve in adulthood. Double‐labelled cells in this experiment would have projected to the vibrissa pad via the IO nerve at birth and regenerated to it via another V branch in adulthood. Nearly 55% of the DY‐labelled cells in this experiment also contained TB. This result indicated that about one‐half of the innervation of the vibrissa pad by non‐IO V branches was the result of axonal sprouting by undamaged ganglion cells.