Sympathetic nerve sprouting fails to occur in the trigeminal ganglion after peripheral nerve injury in the rat

Peripheral nerve injury induces sprouting of sympathetic nerve fibers in do rsal root ganglia after spinal nerve injury. In the present study, we sough t to determine the extent of intraganglionic noradrenergic sprouting in the trigeminal system. The inferior alveolar nerve, a major branch of the mand ibular division, or the infraorbital nerve of the maxillary division was ei ther ligated or chronically constricted in Sprague-Dawley rats and recovery permitted for either 2-3 or 6-9 weeks. In some animals both nerves were in jured. Using immunohistochemistry with tyrosine hydroxylase antibodies, we found no signs of sympathetic nerve fiber sprouting in the trigeminal gangl ion after injury. In contrast, sciatic nerve injury in rat littermates indu ced a widespread autonomic nerve outgrowth in affected DRGs. Thus, sensory ganglion sympathetic nerve sprouting does not seem to be a general outcome of PNS injury, but is restricted to certain specific locations. Sympathetic nerve fiber networks that surround primary sensory neurons have been sugge sted to form a structural basis for interactions between the sympathetic an d sensory nervous systems after PNS injury. Such interactions, sometimes re sulting in paraesthesia or dysaesthesia in patients, appear to be less comm on in territories innervated by the trigeminal nerve than in spinal nerve r egions. The lack of injury-induced intraganglionic sympathetic sprouting in the trigeminal ganglion may help to explain this observation. (C) 1999 Int ernational Association for the Study of Pain. Published by Elsevier Science B.V.